Upload
amaryogi
View
151
Download
2
Embed Size (px)
Citation preview
Physics & Measurement
Peter C A KamPeter C A KamProfessor of Anaesthesia, UNSWProfessor of Anaesthesia, UNSW
St George HospitalSt George Hospital
The Gas Laws
Boyle’s LawBoyle’s Law
Charles’ LawCharles’ Law
The Third Perfect Gas LawThe Third Perfect Gas Law
The Ideal Gas EquationThe Ideal Gas Equation
Henry’s LawHenry’s Law
Dalton’s LawDalton’s Law
Boyle’s Law or First Gas Law
Boyle's LawBoyle's Law at a constant at a constant temperaturetemperature,,
the volume of a given mass of gasthe volume of a given mass of gasvaries inversely with its absolute pressure,varies inversely with its absolute pressure,
or,or,
PV = k1PV = k1
Charles’ Law or Second Gas Law
Charles' LawCharles' Law at a constant at a constant pressurepressure,,
the volume of a given mass of gasthe volume of a given mass of gasvaries proportionately to its absolute temperature,varies proportionately to its absolute temperature,
or,or,
V/T = k2V/T = k2
The Third Gas Law
The Third Perfect Gas LawThe Third Perfect Gas Law at a constant at a constant volumevolume,,
the absolute pressure of a given mass of gasthe absolute pressure of a given mass of gasvaries proportionately to its absolute temperature,varies proportionately to its absolute temperature,
or,or,
P/T = k3P/T = k3
Ideal Gas Equation
for 1 mol of any perfect gas,for 1 mol of any perfect gas,the the universal gas constantuniversal gas constant
R = PV/TR = PV/T
or, where n = number of mol of gas,or, where n = number of mol of gas,
PV = nRTPV = nRT
A Mole
the quantity of any substance containing the the quantity of any substance containing the
same number of particles as there are atoms in same number of particles as there are atoms in
0.012 kg of 0.012 kg of 1212CarbonCarbon
1 mol ~ 6.0223 x 101 mol ~ 6.0223 x 102323
Avogadro's numberAvogadro's number
Avogadro’s Hypothesis
equal volumes of gases, at the same equal volumes of gases, at the same temperature and pressure contain equal temperature and pressure contain equal numbers of moleculesnumbers of molecules
STPSTP T = T = 273.15 K273.15 K (0°C)(0°C) P = P = 101.325 kPa101.325 kPa (760 mmHg)(760 mmHg) for any gas at STP, 1 mol ~ for any gas at STP, 1 mol ~ 22.4 litre22.4 litre
Henry’s Laws
Henry's LawHenry's Law
at a constant at a constant temperaturetemperature, the amount of a gas , the amount of a gas dissolveddissolved in a liquid is directly proportional to the in a liquid is directly proportional to the partial pressurepartial pressure of that gas in equilibrium with that of that gas in equilibrium with that liquidliquid
Dalton's Law of Partial Pressures
Dalton's Law of Partial PressuresDalton's Law of Partial Pressures
in a mixture of gases, the pressure exerted by each in a mixture of gases, the pressure exerted by each
gas is equal to the pressure which would be exerted if gas is equal to the pressure which would be exerted if
that gas alone were presentthat gas alone were present
Solubility
Bunsen Solubility CoefficientBunsen Solubility Coefficient
the the volumevolume of gas, corrected to of gas, corrected to STPSTP, which dissolves , which dissolves
in one in one unit volumeunit volume of the liquid at the temperature of the liquid at the temperature
concerned, where the partial pressure of the gas concerned, where the partial pressure of the gas
concerned is concerned is 1 atmosphere1 atmosphere
Solubility
Ostwald Solubility CoefficientOstwald Solubility Coefficient the the volumevolume of gas which dissolves in one of gas which dissolves in one unit unit
volumevolume of the liquid at the of the liquid at the temperaturetemperature concerned concerned the temperature must be specifiedthe temperature must be specified it is it is independentindependent of pressure of pressure
as the pressure rises the number of molecules of gas as the pressure rises the number of molecules of gas in the liquid phase increases,in the liquid phase increases, however, when measured at the higher pressure the however, when measured at the higher pressure the volume is the samevolume is the same
Partition Coefficient
Partition CoefficientPartition Coefficient the the ratioratio of the amount of a substance present in one of the amount of a substance present in one
phase as compared with than in anotherphase as compared with than in another
the two phases being of the two phases being of equal volumeequal volume
the the temperaturetemperature must be specified, and must be specified, and
the phases being in the phases being in equilibriumequilibrium eg. blood:gas and tissue:bloodeg. blood:gas and tissue:blood
Diffusion
the spontaneous movement of molecules or the spontaneous movement of molecules or other particles in solution, owing to their random other particles in solution, owing to their random thermal motion, to reach a uniform concentration thermal motion, to reach a uniform concentration throughout the solventthroughout the solvent
the constant random thermal motion of the constant random thermal motion of molecules, in gaseous or liquid phases, which molecules, in gaseous or liquid phases, which leads to the net transfer molecules from a region leads to the net transfer molecules from a region of higher concentration to a region of lower of higher concentration to a region of lower concentration (concentration (thermodynamic activitythermodynamic activity))
Fick’s Law of Diffusion
the the rate of transferrate of transfer of a gas through a sheet of of a gas through a sheet of tissue is,tissue is, proportional to the proportional to the areaarea available for transfer available for transfer proportional to the gas proportional to the gas tension differencetension difference inversely proportional to the tissue inversely proportional to the tissue thicknessthickness
V gas = k.A (P gas 1 - Pgas 2) T
Determinants of diffusion
Characteristics of the GasCharacteristics of the Gas
Pressure GradientPressure Gradient
Membrane CharacteristicsMembrane Characteristics
Gas Characteristics
Molecular WeightMolecular Weight VV 1/ 1/MWMW
Graham's LawGraham's Law: relative rate of diffusion is inversely : relative rate of diffusion is inversely proportional to the square root of the gas molecular proportional to the square root of the gas molecular weightweight thus, lighter gases diffuse faster in gaseous media thus, lighter gases diffuse faster in gaseous media
than heavier gasesthan heavier gases lighter molecules for given energy have faster lighter molecules for given energy have faster
velocitiesvelocities therefore, Otherefore, O22 diffuses more rapidly than CO diffuses more rapidly than CO22 in the in the
gas phase (1.17 : 1)gas phase (1.17 : 1)
Gas Characteristics: Solubility
Henry's LawHenry's Law the amount of a gas which dissolves in unit the amount of a gas which dissolves in unit
volume of a liquid, at a given temperature, volume of a liquid, at a given temperature, is directly proportional to the partial is directly proportional to the partial pressure of the gas in the equilibrium phase pressure of the gas in the equilibrium phase
Gas Characteristics: Solubility
relative solubilities of COrelative solubilities of CO22 & O & O22 in water ~ in water ~
24:124:1 combining this with Graham's Law from combining this with Graham's Law from
above,above,the relative rates of diffusion the relative rates of diffusion
from alveolus to rbc for COfrom alveolus to rbc for CO22:O:O22 ~ ~ 20.7 : 120.7 : 1 solubility determines the limitation to the rate of solubility determines the limitation to the rate of
diffusion, gases being eitherdiffusion, gases being either
• diffusion limiteddiffusion limited, as for CO, as for CO
• perfusion limitedperfusion limited, as for N, as for N22OO
Diffusion, k
further, the diffusion of gas across a further, the diffusion of gas across a membrane, or into or out of a liquid, is membrane, or into or out of a liquid, is proportional to the gases proportional to the gases solubilitysolubility in the in the liquidliquid COCO22 being more soluble than O being more soluble than O22 diffuses far more diffuses far more
rapidly across the alveolar membrane and into the rapidly across the alveolar membrane and into the RBCRBC
NN22O being far more soluble than NO being far more soluble than N22 may diffuse may diffuse
into and expand closed cavities during induction of into and expand closed cavities during induction of anaesthesiaanaesthesia
Osmotic Pressure
Osmosis & pressure
OsmosisOsmosis : the movement of solvent across : the movement of solvent across a semipermeable membrane, down a a semipermeable membrane, down a thermodynamic activity gradient for that thermodynamic activity gradient for that solventsolvent
Osmotic PressureOsmotic Pressure : the pressure which : the pressure which would be required to prevent the movement would be required to prevent the movement of solvent across a semipermeable of solvent across a semipermeable membrane, down a thermodynamic activity membrane, down a thermodynamic activity gradient for that solventgradient for that solvent
Osmolality
the number of osmotically active particles the number of osmotically active particles (osmoles) per (osmoles) per kilogramkilogram of solvent of solvent
depression of depression of freezing pointfreezing point of a solution is of a solution is directly proportional to the osmolalitydirectly proportional to the osmolality 1 mol of a solute added to 1 kg of water 1 mol of a solute added to 1 kg of water
depresses the freezing point by depresses the freezing point by 1.86°C1.86°C presence of increased amounts of solute presence of increased amounts of solute
also lowers the also lowers the vapour pressurevapour pressure of the of the solvent, viz…….solvent, viz…….
Raoult’s Law
the depression or lowering of the the depression or lowering of the vapour vapour pressurepressure of a solvent is proportional to the of a solvent is proportional to the molar concentration of the solutemolar concentration of the solute
as the presence of a solute decreases the as the presence of a solute decreases the vapour pressure, making the solvent less vapour pressure, making the solvent less volatile, so the volatile, so the boiling pointboiling point is raised is raised
Raoult’s Law
These phenomena, These phenomena, depression of freezing point, depression of depression of freezing point, depression of
vapour pressure vapour pressure and elevation of boiling point, being related and elevation of boiling point, being related
to osmolarity to osmolarity are termed are termed colligative propertiescolligative properties of a of a
solutionsolution
Osmotic Pressure
1 mol1 mol of any solute dissolved in of any solute dissolved in 22.4 litres22.4 litres of of solution at solution at 0°C0°C will generate an osmotic will generate an osmotic pressure of pressure of 1 atmosphere1 atmosphere
in mixed solutions the osmotic pressure is the in mixed solutions the osmotic pressure is the sum of the individual molalitiessum of the individual molalities
Osmotic Pressure
> 99% of the plasma osmolality is due to electrolytes> 99% of the plasma osmolality is due to electrolytes
contribution of the plasma proteins: contribution of the plasma proteins: 1 mosmol/l 1 mosmol/l
normal rbc's lyse at osmolalities ~ 200 mosmol/lnormal rbc's lyse at osmolalities ~ 200 mosmol/l
as capillaries are relatively impermeable to protein, as capillaries are relatively impermeable to protein,
this generates an osmotic pressure difference this generates an osmotic pressure difference
between the plasma and the interstitial fluid, the between the plasma and the interstitial fluid, the
plasma oncotic pressure plasma oncotic pressure ~ 26 mmHg~ 26 mmHg
COLLIGATIVE PROPERTIES OF A SOLVENT
• Presence of solute stabilises solvent molecules
• More stable solvent molecules cause
(a) Increase in boiling point
(b) Increase in osmotic pressure
(c) Decrease in freezing point
(d) Decrease in vapour pressure
of solvent.
COLLIGATIVE PROPERTIES
1 Osmole of solute leads to ;
a) Boiling point of water increase by 0.52oC
b) Osmotic pressure increase by 2267kPa (17000)
c) Freezing point 1.85oC depression
d) Vapour pressure 0.04kPa (0.3 mmHg)
FREEZING POINT OSMOMETER
- 7OC
Sample
Thermometer
EthyleneGlycol
Thermocouple
StirringWire
MEASUREMENT OF OSMOLALITY
Time
Supercooling
True freezing pointTemperature
GAS OR LIQUID FLOW
Hagen-Poiseuille
where flow is where flow is laminarlaminar,, etaeta (h) (h) = = viscosityviscosity of the fluid in pascal seconds of the fluid in pascal seconds there are no eddies or turbulencethere are no eddies or turbulence flow flow
• is greatest at the is greatest at the centrecentre, being ~ twice the mean, being ~ twice the mean
• near the wall near the wall 0 0
• is directly proportional to the driving pressureis directly proportional to the driving pressure
Q = r4P
8l
Laminar Flow
but as R = dP/Q, sobut as R = dP/Q, so
thus, resistance in inversely proportional thus, resistance in inversely proportional to the to the (radius)(radius)44
R = 8nl r4
Turbulent Flow
the velocity profile across the lumen is lostthe velocity profile across the lumen is lost flow becomes directly proportional to the flow becomes directly proportional to the
square rootsquare root of the driving pressure of the driving pressure pressure flow is not linear and resistance is not pressure flow is not linear and resistance is not
constantconstant flow at which R is measured must be specifiedflow at which R is measured must be specified
other factors in turbulent flow follow,other factors in turbulent flow follow,
((rho) = densityrho) = density of the fluid in kg.m of the fluid in kg.m-3-3
Q = k r2 P l
Turbulent Flow
thus, radius has less effect, cf. laminarthus, radius has less effect, cf. laminar likelihood of the onset of turbulent flow is likelihood of the onset of turbulent flow is
predicted by the predicted by the Reynold's numberReynold's number
d =d = the the diameterdiameter of the tube of the tube v =v = the the velocityvelocity of flow of flow == rho, the rho, the densitydensity of the fluid in kg.m of the fluid in kg.m-3-3
== eta, the eta, the viscosityviscosity of the fluid in of the fluid in pascal secondspascal seconds
Re = vd
Turbulent Flow
empirical studies show that for empirical studies show that for cylindrical tubes, if cylindrical tubes, if Re > 2000Re > 2000 turbulent turbulent flow becomes more likelyflow becomes more likely
for a given set of conditions there is a for a given set of conditions there is a critical velocitycritical velocity at which Re = 2000 at which Re = 2000
the breakpoint for turbulent flow versus the breakpoint for turbulent flow versus Re also varies with the nature of the Re also varies with the nature of the fluidfluid eg. for blood turbulent flow at eg. for blood turbulent flow at Re > 1000Re > 1000
Viscosity
for a given set of conditions, flow is inversely proportional for a given set of conditions, flow is inversely proportional to viscosityto viscosity
blood viscosity increases with,blood viscosity increases with, low temperatureslow temperatures increasing ageincreasing age cigarette smokingcigarette smoking increasing haematocritincreasing haematocrit abnormal elevations of plasma proteinsabnormal elevations of plasma proteins
the viscosity of blood is anomalous due to the presence of the viscosity of blood is anomalous due to the presence of cellscells behavior is said to be behavior is said to be non-newtoniannon-newtonian
Tension
Laplace's LawLaplace's Law
P = T.h.(1/rP = T.h.(1/r11 + 1/r + 1/r22))
T T = the tangential force in N/m= the tangential force in N/m acting along a length of wallacting along a length of wall
h h = the thickness of the wall (usually = the thickness of the wall (usually smallsmall))
Laplace’s Law
thus, for straight tubes,thus, for straight tubes,
P = T.h./rP = T.h./r
and, for and, for spheresspheres,,
P = 2T.h/rP = 2T.h/r
Laplace’s Law
thus, as vessel diameter becomes thus, as vessel diameter becomes smaller, the collapsing force becomes smaller, the collapsing force becomes greater greater
this can lead to vessel closure at low this can lead to vessel closure at low pressures, the pressures, the critical closing critical closing pressurepressure
seen in alveoli, leading to instability with seen in alveoli, leading to instability with small alveoli tending to fill larger ones small alveoli tending to fill larger ones major action of surfactant is to maintain major action of surfactant is to maintain
alveolar stabilityalveolar stability
Measurement of Gas Volumes and Flows
Direct methodsDirect methods
Indirect methodsIndirect methods
WET SPIROMETER
••
••
••
••
••
••
Recorder
CO2 Absorber
Disadvantages1. High inertia2. Inaccurate at high respiratory rate or
FVC
VITALOGRAPH
Bellows
Recorder
Patient
Disadvantage : Patient effort dependent Bellows collect expired gas.
Only measures forced expiratory volumes and flows.
WRIGHT RESPIROMETER
Vane
Channels
Gas Flow
1. Gas stream directed by tangential slits to vane
2. Gas flow drives spinning vane
3. Spinning vane activates gears to record flow
4. Over reads at peak flow
Under reads at continuous flow
DRAGER VOLUMETER
Gasflow
1. Consists of 2 interlocking dumb bell rotors2. More accurate3. Affected by water vapour
INDIRECT MEASUREMENT OFGAS VOLUMES
1. Magnetometers
2. Pneumographs
3. Capacitance spirometry
4. Respiratory inductance plethysmograph
MAGNETOMETERS
1. Electromagnets attached to chest wall andabdomen
2. Electromagnetic field generated .
3. Chest and abdominal diameter changes –alter magnetic filed.
4. Disadvantage : Inaccurate ++
PNEUMOGRAPH
Pressure Transducer
PressureTransducer
Disadvantage : frequent recalibration required
Chest wall
CAPACITANCE SPIROMETRY
C
Top plate
Bottom plate
Used for apnoeic monitoring
Chest wall
INDUCTANCE SPIROMETRY
Oscillator
RECORDER COMPUTER
Chest wall
GAS FLOW MEASUREMENT
1. Variable orifice (constant pressure drop)flowmeter eg. rotameter
2. Variable pressure – fixed orifice flowmeter
ROTAMETER
1. Variable orifice flowmeter
2. Gas flow controlled by control value at bottom of rotameter
3. Vertical tube with tapering internal diameter- wider at the top- narrower at the bottom
4. Bobbin - acts as indicator of flow
5. Pressure drops across the annular space around bobbin opposes downward pressure produced by weight of bobbin.
ROTAMETER
W
Pressure drop [P1 – P2] balances weight [W] of bobbin
Bobbin
Rotameter tube
Gas Flow
P1
P2
ROTAMETER
1. Non – linear scale
2. At lower flow;- bobbin length > distance between bobbin and glass (d)- Laminar flow
3. At high flows;- bobbin length < d- turbulent flow
4. Accuracy + 2%
PNEUMOTACHOGRAPH
1. Measure gas flows.
2. Types(a) Fixed Resistance
Gas flow across fixed resistance differentialpressure signal & flow eg. screen and fleisch pneumotachograph.
(b) Hot wireSignal produced by gas flow cooling a heatedresistance wire.
(c) Pitot tube
SCREEN PNEUMOTACHOGRAPH
ScreenGas
P1 P2
Pressure difference flow
FLEISCH PNEUMOTACHOGRAPH
HEATING COIL
HEATING COIL
Fine bore parallel tubeEnsure laminar flow
P1 - P2
Heating coil to prevent water condensation
GAS FLOW
PITOT TUBE PNEUMOTACHOGRAPH
Upstream Downstream P2
P1 (total) (static P)
P1 - P2 velocity of gas
GAS FLOW
Heat & Temperature
Heat & Temperature
HeatHeat : a form of energy, being the state : a form of energy, being the state of of thermal agitationthermal agitation of the molecules of of the molecules of a substance, which may be transferred a substance, which may be transferred by,by,
conductionconduction through a substance through a substance
convectionconvection by a substance, and by a substance, and
radiationradiation as electromagnetic waves as electromagnetic waves
Heat & Temperature
TemperatureTemperature : is the physical state of a : is the physical state of a
substance which determines whether or not the substance which determines whether or not the
substance is in thermal equilibrium with its substance is in thermal equilibrium with its
surroundings, surroundings, heat energyheat energy being transferred being transferred
from a region of higher temperature to a region of from a region of higher temperature to a region of
lower temperaturelower temperature
Heat & Temperature
KelvinKelvin
the SI unit of thermodynamic temperaturethe SI unit of thermodynamic temperature
equal to 1/273.16 of the absolute temperature of equal to 1/273.16 of the absolute temperature of
the the triple pointtriple point of water of water
the temperature at which ice, water and water the temperature at which ice, water and water
vapour are all in equilibriumvapour are all in equilibrium Celsius scaleCelsius scale
Temperature (K) = Temperature (°C) + 273.15Temperature (K) = Temperature (°C) + 273.15
in Celsius the triple point of water is in Celsius the triple point of water is 0.01 °C0.01 °C
Critical Temperature
the temperature above which a gas cannot be the temperature above which a gas cannot be
liquified by pressure aloneliquified by pressure alone NN22OO = 36.5 °C= 36.5 °C
OO22 = -119 °C= -119 °C
GasGas:: a substance in the gaseous phasea substance in the gaseous phaseaboveabove its critical T its critical T
VapourVapour:: a substance in the gaseous phasea substance in the gaseous phasebelowbelow its critical its critical TT
Critical Pressure
the pressure at which a gas liquifies at the pressure at which a gas liquifies at its critical Tits critical T
NN22O ~ O ~ 73 bar 73 bar @@ 36.5 °C36.5 °C NN22O ~ 52 bar O ~ 52 bar @ 20.0 °C@ 20.0 °C
Pseudo-Critical TemperaturePseudo-Critical Temperature for a mixture of gases at a specific pressure, the for a mixture of gases at a specific pressure, the
specific temperature at which the individual gases specific temperature at which the individual gases may separate from the gaseous phase may separate from the gaseous phase
NN22O 50% / OO 50% / O22 50% 50% = - 5.5 °C= - 5.5 °C
• for cylinders (most likely at 117 bar)for cylinders (most likely at 117 bar) NN22O 50% / OO 50% / O22 50% 50% = - 30 °C= - 30 °C
• for piped gasfor piped gas
Adiabatic Change
the change of the change of physical statephysical state of a gas, without the of a gas, without the transfer of heat energy to or from the surrounding transfer of heat energy to or from the surrounding environmentenvironment rapid rapid expansionexpansion & energy required to overcome Van & energy required to overcome Van
der Waal's forces of attraction, as this energy cannot be der Waal's forces of attraction, as this energy cannot be
gained from the surroundings, it is taken from the kinetic gained from the surroundings, it is taken from the kinetic
energy of the molecules energy of the molecules basis of the basis of the cryoprobecryoprobe
rapid rapid compressioncompression, the energy level between molecules , the energy level between molecules
is reduced, as this energy cannot be dissipated to the is reduced, as this energy cannot be dissipated to the
surroundings, it is transferred to the kinetic energy of the surroundings, it is transferred to the kinetic energy of the
moleculesmolecules
T Measurement: Non-electrical
mercurymercury thermometers thermometers
accurate, reliable, cheapaccurate, reliable, cheap
readily made into a thermostat, or max. reading readily made into a thermostat, or max. reading
formform
Angulated constriction at base of stem prevents Hg Angulated constriction at base of stem prevents Hg
column returning to bulb via surface tension forcescolumn returning to bulb via surface tension forces
requires 2-3 mins to reach thermal equilibriumrequires 2-3 mins to reach thermal equilibrium
unsuitable for insertion in certain orificesunsuitable for insertion in certain orifices
T Measurement: Non-electrical
alcoholalcohol thermometers thermometers
cheaper than mercurycheaper than mercury
useful for very low T, mercuryuseful for very low T, mercury solid at solid at -39°C-39°C
unsuitable for high T, alcohol boils at unsuitable for high T, alcohol boils at 78.5°C78.5°C
expansion also tends to be less linear than expansion also tends to be less linear than
mercurymercury
Bimetallic stripsBimetallic strips
Bourdon gauge Bourdon gauge pressurepressure
T Measurement: Electrical
resistanceresistance thermometer thermometer
metalsmetals R R linearlylinearly with with TT
frequently use a platinum wire resistor, or frequently use a platinum wire resistor, or
similarsimilar
accuracy improved with a Wheatstone accuracy improved with a Wheatstone
bridgebridge
Resistance Thermometer
Platinum Wire T α R (linear) Disadv. R Not sensitive α R α T wire Battery T
T Measurement: Electrical
thermistorthermistor metal oxidesmetal oxides R R exponentiallyexponentially with with TT made exceeding smallmade exceeding small rapid thermal equilibrationrapid thermal equilibration narrow reference rangenarrow reference range different thermistors for different scalesdifferent thermistors for different scales accuracy improved with a Wheatstone bridgeaccuracy improved with a Wheatstone bridge Accuracy reduced with exposure to severe T, Accuracy reduced with exposure to severe T,
egeg. . sterilisationsterilisation
Thermistor
Thermistor oxide of R exponentially with Temp. metal Adv : small - rapid change
- accessible to remote location Disadv : Drift in calibration
R To
T Measurement: Electrical
thermocouplethermocouple based on the Seebeck effectbased on the Seebeck effect at the junction of two dissimilar metals a small at the junction of two dissimilar metals a small
voltage is produced, the magnitude of which is voltage is produced, the magnitude of which is
determined by the temperature determined by the temperature metals such as copper and constantan (Cu+Ni alloy)metals such as copper and constantan (Cu+Ni alloy) requires a constant reference temperature at the requires a constant reference temperature at the
second junction of the electrical circuitsecond junction of the electrical circuit may be made exceeding small and introduced may be made exceeding small and introduced
almost anywherealmost anywhere
Thermocouple – “Seebeck effect”
Reference Junction Copper Constantan Junction Potential
mV Measuring junction Temp
Thermocouple
Junction of 2 different metals P. Diff (α To) Seebeck effect Metal 1 eg Cu V Metal 2 eg. Constantin
Ref Measured Adv. Large linear range Temp Temp can be very small (eg.Ice) Disadv. Small output 40μv
Specific Heat Capacity
heat required to raise the temperature of 1 kg heat required to raise the temperature of 1 kg
of a substance by 1 K (J/kg/K)of a substance by 1 K (J/kg/K) water SHCwater SHC = 4.18 kJ/kg/K or, 1 kcal/kg/K= 4.18 kJ/kg/K or, 1 kcal/kg/K blood SHCblood SHC = 3.6 kJ/kg/K= 3.6 kJ/kg/K
infusion of 2000 ml of blood at 5°C, requiring infusion of 2000 ml of blood at 5°C, requiring
warming to 35°C, warming to 35°C, require require 2 kg x 3.6 kJ/kg/°C x (35-5)°C = 216 kJ2 kg x 3.6 kJ/kg/°C x (35-5)°C = 216 kJ
would result in the person's temperature falling by would result in the person's temperature falling by
~ 1°C~ 1°C
Specific Latent Heat
the heat required to convert 1 kg of a substance the heat required to convert 1 kg of a substance from one phase to another at a given temperaturefrom one phase to another at a given temperature latent heat of vaporisation latent heat of vaporisation (from liquid to vapour)(from liquid to vapour)
LHV of waterLHV of water at 100°C = 2.26 MJ/kgat 100°C = 2.26 MJ/kg at 37°Cat 37°C = 2.42 MJ/kg = 2.42 MJ/kg the lower the T the greater the latent heat requiredthe lower the T the greater the latent heat required
as T rises, the latent heat falls until ultimately it as T rises, the latent heat falls until ultimately it reaches reaches zerozero at a point which corresponds with the at a point which corresponds with the critical temperaturecritical temperature = 373°C = 373°C
Humidity
ABSOLUTE HUMIDITY
• Mass of water vapour (g) in a given
volume of air (m3)
numerically = mg / 1L
• Fully saturated air
@ 20oC contains 17mg/L water
@ 37oC contains 44mg/L water
RELATIVE HUMIDITY
• Defined as ratio of mass of water vapour in a given volume of air to the mass required
to fully saturate that volume of air at a given temperature. (%).
• By ideal gas equation, mass is proportional to number of moles present.
Relative humidity = actual vapour pressure saturated vapour pressure
HAIR HYGROMETER
Principle : Hair lengthens as humidity increases
Accuracy : - Low - Accurate between RH 15-85% - very simple & cheap
RH LHair
WET AND DRY BULB HYGROMETER
WetGauze
Water
T1 T2
Air
- - - - - - - -- - - -
WET AND DRY BULB HYGROMETER
T1 = temperature of wet bulb decreases because of evaporation in wet gauze.
Lower humidity causes more evaporation and T1 decreases more.
Humidity T1 – T2
δ - % humidity from tables
DEW POINT
• Defined as temperature at which ambient air is fully saturated
• At this point condensation occurs
REGNAULT’S HYGROMETER
Thermometer
Silver Tube
Ether
Bubble
Condensation at “ dew point”
AIR
RH = SVP at dew point SVP at ambient temperature
OR from tables
HUMIDITY TRANSDUCERS
• Principle : When a substance absorbs water, its resistance or capacitance changes.
• Substance is incorporated into electrical circuit as resistor or dielectric portion of a capacitor.
HUMIDITY TRANSDUCERS
Advantages :1. Extremely sensitive2. Rapid response - can be used as
servo-systems.
Disadvantages :1. Display hysteresis – unsuitable for critical applications where high degrees of accuracy required.
MASS SPECTROMETER for measuring humidity
- Used to measure water vapour pressure
- Rapid response - can be used to measure breath –by- breath changes.
- Disadvantage - very expensive
WEIGHING TECHNIQUES
a) Weighing quantity of water vapour that has condensed in a known volume of air
or
b) Warming air so that all water droplets areevaporated & then weighing volume of air.
c) Absorption techniquesAbsorption of water vapour in either
concentrated sulphuric acid, silica gel or anhydrous CaCl2
PRESSURE – Physics and Measurement
PRESSURE
• Defined as Force per unit area• Units : Pascal (Pa) or Newton per square meter ( N.m-2).
Newton = Force that will accelerate 1 kg (N) mass at 1ms-2.
1 Pascal = 1 N acting on area of 1m2
• Gravity = 9.81m.s-2
PRESSURE UNITS
1 kPa = 10.2 cm H2O1 kPa = 10.2 cm H2O
= mm Hg= mm Hg
[mercury = 13.6 times as dense as water][mercury = 13.6 times as dense as water]
1 bar = 100 kPa1 bar = 100 kPa
= mm Hg= mm Hg
GAUGE PRESSURE
• Pressure relative to atmospheric pressure. i.e. zero at atmospheric pressure
• Gauge Pressure may be determined by how much pressure is above or below atmospheric pressure.
ABSOLUTE PRESSURE
•Pressure relative to a true zero pressure(i.e.vacuum)
Therefore,
Zero gauge pressure = 1 atmosphere absolute
• Gauge pressure 1 atmosphere = 760 mmHg
= 101.325 kPa
= 2 atmosphere
absolute
MEASUREMENT OF BLOODPRESSURE
UNITS OF PRESSURE
Unit Value
Pascal ( Pa) N/m2 SI Unit
mmHg 133.3Pa 1 mmHg = 7.5kP
bar 105Pa
Torr almost = 1 mmHg
cmH2O ~ 1-Pa
PRESSURE
P = p x g x h
p = density of fluid
g = acceleration due to gravity
H = Height of column
Conversions 10cmH2O = 7.4 mmHg 10mmHg = 13.6 cmH2O
Mercury 13.6 x water density
INDIRECT BLOOD PRESSUREMEASUREMENT
Principlea) Utilise cuff - occlude pulse
a) Detection of return of pulse or blood flow distal to cuff.
OCCLUDING CUFF
• Cuff pressure transmitted to tissues surrounding artery.
• Cuff width = 40% limb circumference
• Bladder - at least half circumference - Centred over artery
• Cuff level with heart
ERRORS WITH CUFF
a) Too narrow or to loose cuff overestimate
SP and DP.
b) Too wide
underestimate SP and DP
(Pressure = F/A)
CUFF – AHA STANDARDS
Small Adult 10 x 24 cm 22 – 26 cm
Adult 13 x 30 cm 27 – 34 cm
Large adult 16 x 38 cm 35 x 44 cm
Adult thigh 20 x 42 cm 45 – 52 cm
Bladder sizeArm
Circumference
DEVICES MEASURING CUFFPRESSURE
Mercury manometers - Used to be standard
- Now phased out
- Column must be vertical
- Air vent on top of column
Aneroid gauges - Convenient
- Commonly under-read BP
- Calibrated 6 monthly
FLOW DETECTION DISTALTO CUFF
Palpation Finger palpation
Finger Photoplethysmograph
Auscultation Audible range – Korotkov sound
Ultrasound (5MH2) range – Arteriosounde
Subaudible (10-40mH2) range – Infrasound
Oscillometry Defection of oscillations
von Recklinghausen’s oscillations
NO2 invasive BP
KOROTKOV SOUNDS
Phase I: First appearance of tapping sounds
Phase II: Brief softening of sounds
Auscultation gap : disappearance of sounds
Phase III: Return of sounds
Phase IV: Muffing of sounds
Phase V: Disappearance of all sounds
KOROTKOV SOUNDS
Cuff deflation rate = 2-3 mm Hg per sec.
Rapid deflation = underestimate BP
Note auscultatory gap may be present
DBP = point of disappearance of sound
Difference between phase IV & V ~ 5 mmHg
OSCILLOMETRY
• Basis of NIBP
• Only one cuff acts as (i) occluding cuff
(ii) Sensor using microprocessor
• Cuff both actuator & transducer
OSCILLOMETRY
Oscillations begins at SBP
Maximal at MAP
Abruptly diminishes at DBP
OSCILLOMETRY - NATURE OFOSCILLATIONS
• Diamond shaped pattern
• Pressures at oscillation between 2 heart beats compared.
• Average is recorded
• MAP most accurate
OSCILLOMETRY
Advantage : 1. Not operator dependent
2. “hands free”
3. Automated
Disadvantage :
1. Inaccurate in shock or
arrhythmia.
2. Cuff placement important
3. Bruising + skin damage
4. Venous congestion
5. Nerve compression
FINAPRES
• Cuff placed around finger
• Changes in volume of arterial blood in
finger detected by plethysmography
• MAP - cuff inflated to maximal
- cuff pressure approximates
arterial pressure waveform
ARTERIAL TONOMETER
• Force transducer placed over artery with under lying bone.
• Electrical signal - reproduces arterial waveform
• Needs to be calibrated 5 – 10 min against oscillometric measurements
INVASIVE (DIRECT) BP MEASUREMENT
Advantages
1. Continuous monitoring
2. Trends observed
3. Accuracy over wide range
4. Enables visual analysis of pulse
pressure
VISUAL ANALYSIS OF WAVEFORM
Myocardial Contractility
Upstroke of pulse pressure ~ LV dp/dtSteep upstroke = strong LV contraction
Stroke Volume
Area under systolic ejection ~ LV stroke volume
Systemic Vascular Resistance
Low diastolic notch = Rapid run off & Steep down stroke low SVR
CIRCULATING BLOOD VOLUME
Exaggerated beat to beat variation with
ventilation = hypovolaemia
INDICATIONS FOR INVASIVE BP
1. Rapid changes in BP
2. Monitor effects of potent hypotensive or
vasopressor agents
3. During CP bypass
4. Operation with volume shifts eg. AAA or
phaeochromocytoma.
5. Shock
6. Difficult access eg. Morbid obesity
VARIATION OF BP
• Waveform distorted the further away from the heart.
• High frequency components eg. incisura damped out / disappear.
• Systolic BP increases distally.
• Hump becomes more prominent in diastolic part of waveform
VARIATION OF BP
• SBP increases towards periphery
• DBP decreases towards periphery
• MAP - only slight drop
eg. MAP radial 5% < MAP aorta
• Pulse Pressure increases towards periphery
Causes : a) Reflection of pressure wave from peripheral arterioles
b) Resonance
BP AND POSTURE
- 42 mmHg
0
+80 mmHg
COMPONENTS OF INVASIVE BP
• Mechanical coupling of blood to transducerintravascular catheterconnecting tubingstop lock
• TransducerConverts pressure changes to voltage changes
• Electronic processing
• Display + recorder
REQUIREMENTS FOR ACCURACYINVASIVE BP
1. STATIC Accuracy - Ability to measure stationary events- No baseline/sensitivity drifts
- Input & output linearity - No hysteresis
2. DYNAMIC Accuracy - Ability to accurately record over rapid changes.
3. PHYSIOLOGICAL Reactance - measuring system must have effect on event recorded / measured.
TRANSDUCER
• Coverts pressure energy movement electrical signal
• Commonly diaphragm resistance movement change Wire stretch resistance
• Wheatstone bridge used to convert resistance change to voltage signal
STATIC CALIBRATION
• Linear response (straight line) between pressure and output voltage.
OutputGain = slope of line
offset
Pressure
• Offset - Transducer output at zero pressure• Gain - Change in output for given change in pressure
must be constant.• Sensitivity (factory set) at 5V/V/mmHg
MEAN BLOOD PRESSURE
a
bMAP
• Average Pressure• Equal to pressure when a = b• Electronically averaged instantaneous measurements• Highly damped system eg. aneroid gauge gives MAP• MAP = diastolic pressure + 1/3 pulse pressure = SBP + 2DBP
3
DYNAMIC RESPONSE
• Basic or Fundamental Harmonic (1st) Heart Rate 60 beats/min = 1 Hz Heart Rate 120 beats/min = 2 Hz
• By Fourier Analysis Fundamental = f = 2Hz (HR 120) 2nd Harmonic = 2 x f = 4Hz 3rd Harmonic = 3 x f = 6Hz 10th Harmonic = 10 x f = 20Hz
• Accurate waveform without amplitude distortion achieved with 10th harmonic ~ 20Hz
PRACTICAL ASPECTS OF DYNAMIC RESPONSE
• Natural frequencies of clinical systems approx, 30Hz
• Acceptable if damping ratio is optimal
• To achieve dynamic accuracy - fundamental frequency (fo) must be maximised.
MAXIMISING Fo
Note : fo = 1 E 2 M
To minimize fo Minimize M (mass)
- Minimal volume of fluid in transducer- Short tubing
Maximise E, modulus of elasticity- Stiff tubing- Stiff diaphragm- Eliminate air bubbles
FACTORS THAT INCREASE DAMPING
• Increase fluid viscosity eg. blood clots
• Narrow tubing eg. kinked catheter
• Increased tubing length
• Decreased stiffness of tubing
LIQUID MANOMETERS(Absolute Pressure)
Mercury Barometer
••
••
•
• ••
••
•
••
•
••••
•• •
•
•
••
•
•
• Mercury
Torricellian Vaccum
hP
Measures Absolute Pressure
LIQUID MANOMETERS (Gauge Pressure)
P
h - Amount by which pressure exceeds atmospheric
Note tube open at both ends
LIQUID MANOMETERSMethods to increase sensitivity
• Use low density liquid
• Amplify vertical movement of
meniscus.
a) inclined plane manometer
b) differential liquid manometer
MECHANICAL PRESSURE GAUGE
Bourdon Gauge
Wheel
Pointer
Coiled tube unwinds at high pressure
Pressure
Fixed Point Low
Pressure
HighPressure
Cross Section
• Usually for measuring high pressure• Can be adapted for temperature or flow measurement
MECHANICAL PRESSURE GAUGE
Aneroid Gauge
P
BellowsExpands with pressure
Pointer
Lever System - amplifies change
Uses : BP, Airway Pressures on IPPV
DIAPHRAGM GAUGE
• Pressure measurement made by sensing movement of flexible diaphragm.
• Diaphragm movement sensed by a) Direct movement of levers etc. (not sensitive) b) Optical method pressure Diaphragm Mirror
stretched more rotated & curved
c) Electromechanical transducers
OPTICAL ELECTROMECHANICALTRANSDUCERS
Principles :
1. Increased pressure Diaphragm more convex
2. Light beam reflected off silvered surface of diaphragm on to photoelectric cell.
3. Reflected light beam more divergent.
4. Light intensity sensed by photo-electric cell decreases and electrical output falls
OPTICAL ELECTROMECHANICALTRANSDUCERS
P1 P2
Mirror Mirror
Photoelectric
Cell
Photoelectric
CellSlivered surface
Divergent reflected light
Convergent
Reflected light
Sliveredsurface
LightSource
STRAIN GAUGE ELECTROMECHANICALTRANSDUCERS
Principles :
Wire stretched or compressedChange in length and diameteratomic stricture change
Resistance change
STRAIN GAUGE TRANSDUCER
Wired compressed
Movable block
Fixed pointDiaphragm
P
Resistance wire stretch
1. Resistance wires arranged in 2 sets2. When pressure increases, one set stretches & other set compresses3. Difference in resistance is measured by wheatstone bridge system.
BONDED STAIN GAUGE
PStrain gaugeBonded todiaphragm
Single Bond Double Bond
1. Resistance wires in zig-zag patter cemented to diaphragm surfaces.
2. Robust but subject to hysteresis.3. Resistance wire – low temperature coefficient.4. Double bonded strain gauge one stretched & other
compressed.
WHEATSTONE BRIDGE ARRANGEMENT OF RESISTANCE WIRES
OUTPUT OUTPUT
StrainGaugeelement
StrainGaugeelement
Half bridge circuit Full bridge circuit
CAPACITANCE TRANSDUCER
2ND plateDiaphragmas one plate
Charge
Characteristics : 1. Very sensitive 2. High natural frequency 3. Temperature drift
4. Unstable
VARIANCE INDUCTANCE TRANSDUCER
P Iron Core
Diaphragm
Coil magnetic field
WHEATSTONE BRIDGE
Wheatstone bridge is a special arrangement of resistors designed to amplify change in resistance.
Balanced Wheatstone bridge
A
R1R2
R adjust R measure
Ammeter readszero
R measure R adjust
= R2
R1
FREQUENCY RESPONSE
1. Measurement systems respond to restricted range of frequencies.
2. Input signals of same amplitude at different frequencies will produce output over a limited range of frequencies.
3. Within this frequency range, response may be more sensitive to some frequencies than others.
4. Response of system (system gain) plotted against signal frequency is called “ Frequency Response of System”.
FREQUENCY RESPONSE OFA SYSTEM
Bandwidth
Lower cut off Upper cut off
Frequency
Systemgain
DETERMINANTS OFFREQUENCY RESPONSE
MECHANICAL SYSTEM
Inertial elements (eg. mass)Compliance elements (eg. spring)
ELECTRICAL CIRCUIT
InductanceCapacitance
NATURAL OR RESONANT FREQUENCY
1. When a constant amplitude waveform is applied at increasing amplitude occurs at resonant ornatural frequency (fo) of the system.
2. Beyond fo (higher frequencies), amplitude of oscillations increase and then fall to zero.
3. Fo depends on inertial and compliance etc.
AMPLITUDE AS FREQUENCYINCREASES
Natural or resonantFrequency (fo)(maximal oscillation)
Increasing frequency
Amplitude decreasedBeyond fo
Amplitude of oscillation
ENERGY INTERCHANGE INOSCILLATING SYSTEM
1. Continental interchange between kinetic energy of mass in motion and potential energy.
2. Kinetic energy = ½ mv2.
3. Narrow Tube
(a) More energy required to make given mass of fluid to oscillate because it has to reach higher velocity.
(b) Catheter fluid velocity > fluid velocity at diaphragm.
(c) E, Effective mass catheter > E, diaphragm.
(d) Larger effective mass lower fo.
RESONANT FREQUENCY
OUTPUT
FREQUENCY FO Resonantfrequency
UNDAMPED NATURAL FREQUENCY
Fo = 1 2
S M
Fo = undamped natural frequency
S = stiffness of transducer diaphragm
M = Effective mass
HIGH UNDAMPED NATURALFREQUENCY
Catheter - Transducer System
- Needs high fo
- Occurs when fluid velocity is minimized
- Achieved by;
(a) Stiff diaphragm
(b) Short and wide catheter.
DAMPING
• Defined as tendency of a system to resist oscillations caused by a sudden change.
• In mechanical devices, damping arises from frictional effects on mechanical moving parts.
• In fluid operated devices, damping is caused by vicious forces that oppose fluid movement.
• In electrical devices, electrical resistance oppose passage of electrical currents.
EXTENT OF DAMPING
Underdamping - Results in oscillation and over-estimation of measurement (overshoot ofoutput) .
Overdamping - Results in slow response and underestimation of measurement.
Critical damping - No overshoot of output signal but speed of response is too slow.
SIGNAL AMPLITUDE & DAMPING
2
0.50.1 0.5 1.0 1.5
1
0.2
0.5
0.64D=1.0
RelativeAmplitude
OPTIMAL DAMPING
1. State of damping in which(a) Minimal overshoot(b) Response speed only slightly reduced
2. D = 0.64 (i.e. 64% critical damping)
(a) 7% overshoot
(b) Response speed only minimally reduced
PHASE SHIFT RESPONSE
1. Waveform or signal composed of series of component frequencies.
2. Each component waveform undergoes different time delay or phase shift.
3. Phase shift is time delay expressed as an angle(radians).
4. At fo, waveform delayed by 90%
5. Other frequencies, phase lag is linearly related at D = 0.64 I.e. phase distortion minimal at D = 0.6.
SPECIFICATIONS OF TRANSDUCERS
1. To avoid waveform, amplitude and phase distortion, catheter-transducer system should have undamped natural frequency 25-40Hz .
2. Standard transducer – undamped natural frequency of 100 Hz or more.
3. Catheter - tap - cannula arrangement reduces natural frequency of the system.
DIRECT BP MEASUREMENT SPECIFICATIONS
1. Transducer : - Frequency response > 100 Hz i,.e. resonant frequency > 100Hz.
2. Tubing & cannula : - Lowers fo and adds damping.
Length increase - lower fo, more damping
Compliant tube - lower fo, more damping
Small bore tube - lower fo, more damping
air/clot in tube - lower fo, more damping
Factors that increases fo tend to lower damping.
CARDIAC OUTPUT MEASUREMENT
USES OF CARDIAC OUTPUT MEASUREMENT
1. GENERAL ICU
- Cardiac performance assessment in shocked patients.
- Management of inotropes and vasoconstrictors
- Optimisation of PEEP Therapy.
2. OPERATING THEATRES
- Major Anaesthetic eg. AAA, Liver transplant
- Anaesthesia in severe cardiac disease (eg. L V failure, Recent MI)
CARDIAC OUT MEASUREMENT USES
3. Post-cardiac Surgery Intensive Care Units4. Coronary Care Units / Laboratories
- Assessment of severity of ischaemic & valvular disease
- Management of inotropes vasoconstrictors and vasodilators.
INFORMATION FROM C.O. MEASUREMENT
Cardiac output L/min
Cardiac Index = CO Surface area L/min/m2
Systemic vascular resistance = MAP-RAP mmHg/L/min CO or PRU
dyne.sec.cm-5
Pulmonary Vascular Resistance = MPAP-LAP CO
mmHg/L/min or PRU
dyne.sec.cm-5
INFORMATION FROM C.O. MEASUREMENT
LV stroke work = MAP x SV gm.m
RVstroke work = MPAP x SV gm.m
Oxygen Consumption = CO x (CaO2-Cv-O2) ml/min
Oxygen Delivery = CO x CaO2 ml/min (D O2)
DIRECT CO MEASUREMENT
1. ELECTROMAGNETIC FLOWMETER PROBE
a) Periaorticb) Intraaortic
2. Ultrasonic Flow Probe
3. Intravascular thermal velocity transducer
INDIRECT CO MEASUREMENT
A. INVASIVE METHODS1. Fick method (1970) i) Direct (O2 Consumption) ii) Indirect (CO2 production)2. Dye dilution (Stewart, 1894,
Hamilton, 1979)3. Thermodilution (Fegler, 1954)
NON-INVASIVE METHODS1. Radioactive tracer dilution (radiocardiography)2. Bollisto cardiography3. Pneumocardiography4. Impedance Plethysmography
ELECTROMAGNETIC FLOW PROBE
Faraday’s Law states that :-
“When a conductor moves with a given velocity across theLines of force of a uniform magnetic field, an electromotiveForce will be induced at right angles to the flow, and will be Proportional to the velocity of the conductor”
MAGNETIC FIELD
Blood flow
Magnetic H
V
E
a
Magnetic field is held at right angles to blood flowElectromotive force induced at right angles to moving conductor, the blood flow.
NOTE : EMF technique measures blood velocity
ELECTROMOTIVE FORCE, E
+a E = v. H 2a 10 -8
-a
V = Velocity of bloodH = strength of magnetic field (gauss)2a = length of conductor or diameter of blood vessel.
BLOOD VELOCITY
Blood velocity = Flow rate (cm3 / sec) Cross section area of vessel (cm2)
Flow rate = Blood velocity x cross section area
TYPES OF EMF FLOW PROBES
NOTE : Peri or intra-aortic flow probes measure CO (less coronary blood flow)
(a) Periaortic Flow Probe• used in open heart surgery• rarely used clinically
(b) Intravascular Flow Probe• introduced via peripheral artery• invasive• velocity depends on exact site (maximal in centre of blood vessel)
ULTRASONIC FLOW PROBE
Measures velocity of flowing fluid
(a) Pulsed Ultrasound- Cuff transducer around artery- Pulse of 5 MHz from piece-electric crystal
in cuff.- receiver crystal downstream- Transit time between
ULTRASONIC FLOW PROBE
(B) Doppler EffectPrinciple : frequency shift of emitted wave frombarium titanate crystal when it is reflected frommoving fluid column
Disadvantage :Cannot detect difference between forward and backward flow.eg. aortic blood flow : mean velocity = 40 cm/sec
systolic velocity = 120 cm/sec
ULTRASONIC FLOW PROBE
1. Probe placed at suprasternal notch with beam directed to aortic arch or
2. Transoesophageal probe with beam directed at descending aorta
3. Velocity of blood flow and cross – sectional area of aorta determined.
ACCUCOM C.O. MONITOR
1. Continuous C.O reading
2. Oesophageal probe containing dual crystal Doppler probe transducer to measure velocity in descending aorta.
3. Second Doppler probe placed at suprasternal notch used to calibrate instrument
4. Aortic diameter determined by echocardiogram or monogram
2 D COLOUR FLOW – ECHO-DOPPLER
1. Pulse ultrasound for imaging of cardiac flow
2. L V outflow tract imaged and cross sectional area determined.
3. Velocity of blood flow in LV – outflow tractmeasured.
4. CO = Cross Section Area x Velocity.
5. Colour Doppler to demonstrate direction of flow
INTRAVASCULAR THERMAL VELOCITY TRANSDUCER
1. Heated thermistor placed in moving liquid stream dissipates heat as a function of flow velocity.
2. Probe maintained at fixed position in blood stream within vessel of fixed diameter.
3. Velocity signal translated into flow.
INDIRECT METHODS OF C.O. MEASUREMENT
I INVASIVE METHODS Direct Fick Method Indirect Fick Method Dye dilution Method Thermodilution technique
II NON INVASIVE METHODS
Radioactive tracer dilution Ballisto cardiography Thoracic impedance plethysmography
FICK PRINCIPLE(Adolph Fick – 1870)
States that : “ the flow of a liquid in a given period ofTime is equal to the amount of substance entering orLeaving the stream / or organ) in the same period of Time, divided by the difference between the concentration of the substance before and after thePoint of entry or exit “
DERIVATION OF FICK PRINCIPLE
Substance added ( M mg)
o
Amount ofIndicatorAt entry ()
+Amount of IndicatorAdded (M)
=Amount ofIndicator atExit ( o )
Concentration = Amount VolumeAmount = Volume x concentration
DERIVATION OF FICK PRINCIPLE
ConcentrationAt entry ( ) x
Volume at () +
Mass added (M)
= CE VE
Dividing at entry = Vol at exit
Conc. At entry x flow rate + M (amount added/min)
=ConcAtExit
x Flowrate
Therefore Flow Rate
=Amount indicator added per min Exit conc. – entry conc.
HYDRAULIC ANALOGUE MODEL
M mgs-1
Q ml sec –1
C mg / m
V ml
INDICATOR CONCENTRATION CHANGE AT CONSTANT INFUSION WITH NO INDICATOR INPUT
Conc.(mg ml-1)
Time
C1
C max
INDICATOR CONCENTRATION(known concentration Co at input with constant infusion)
Conc
C max
Co
Time
INDICATOR CONCENTRATION vs TIME (Bolus Injection)
yo
y1
y1 = yoe-ke Conc
Time
FICK METHOD
Used in 3 ways
1. Direct Fick - uses oxygen uptake as indicator Co = ___Vo2____
CaO2 - CVO2
2. Indirect Fick – uses CO2 production as indicator
Co = VCO2
CVCO2 – CaCO2
3. INERT GAS METHODeg. N2O xe137, K85, K7a
- Used for specific organ blood flow measurement- basis of Kety – Schmidt Method
DIRECT FICK METHOD
Assumptions ;
1. Steady State of both flow (Q) and oxygen consumption.
2. CaO2 and CVO2 constant.
3. Closed systemI.e. blood is only source of substance taken up.
DIRECT FICK METHOD
Measurement of O2 consumption
1. Breathe O2 (FIO2) via one-way valve
2. Expires into Douglas bag or Tissot Spirometer
E = Volume measured Time
.V
DIRECT FICK METHOD
Calculation of Oxygen consumption
VO2 = InspiredGas Vol
x FiO2 - ExpiredGas Vol
x FEO2
DIRECT FICK METHODOxygen consumption calculations
Nitrogen is in steady state
V inspired N2 = V expired N2
VI x FIN2 = VE x FEN2
VI = VE x FEN2
FIN2
FIN2 = I - FIO2
FEN2 = 1 – FEO2 – FECO2
VI = VE x 1 – FEO2 – FECO2
1 – FIN2
VO2 = (VI x FIO2) – VE x FEO2
DIRECT FICK METHOD
CaO2 = Hb (g/L) x SaO2 x 1.34 ( mIO2) + 0.003 x PaO2
SaO2 measured using arterial blood and calibrated oximeter
DIRECT FICK METHOD
( Mixed Venous Oxygen Content)
Need pulmonary artery blood for mixed venousSample.
CVO2 = Hb x SVO2 x 1.34 dissolved component usually ignored
DIRECT FICK METHOD
CO = 250 ml / min_____ 200ml/L - 150 ml/L
= 5 L / Min
Features : 1. Steady state of CO, VO2, CO2 production N2 balance arterial; and venous O2 concentration.
2. Accuracy + 10% used as reference.
3. Slow, cumbersome, unsuitable for rapid measurements.
4. Unsuitable during GA - Not a steady state - Uptake of volatile agents and N2 washout.
INDIRECT FICK METHOD
1. CO2 used as indicator
2. Theoretical advantage :Mixed venous CO2 estimated by rebreathingtechnique - No need for CVP.
3. Problem :Large CO2 stores - steady state not easily
achieved.
DYE DILUTION METHOD
1. Applies to bolus of indicator
2. Also known as Stewart – Hamilton principle.
3. Basis of dye dilution and thermodilution techniques.
DYE DILUTION METHOD
ExitConc.
Ct
0 T 00time
Cmax
C (t)
Ct = C maxe-kt
Where e = 2.718K = decay constant of exponential
DYE DILUTION METHOD: CALCULATIONS
At any moment c = M V M = C x V
Integrating :
Mdt = Cdt x Vdt
o o oWhere
oMdt = Original injected
o
= total volume flow
STEWART – HAMILTON FORMULA
M =
o
cdt x Q
Q = ___M___
ocdt
DYE – DILUTION INFUSION TECHNIQUE
C exit
C entry
t o
t1
Q = __M__
t = 1
t - 0
Cdt
= __M__Ct1 - Ct0
= _______M________C exit - C entry
DYE – DILUTION METHOD
Dye - Indocyanine green (ICG)
- peak absorption = 805 nm(isobestic point of HbO2)
- non toxic
- rapid removal 18-24% per min
- 2.5-5mg injected into right atrium
DYE DILUTION – GRAPH ANALYSIS
recirculation
t
C
o
Cdt = area under curve
Methods used (a) Trapezoid Method (b) Forward method
Triangle area - Peak Height x width at halve peak height (c) Computer integrator
DYE DILUTION METHOD
EQUIPMENT
1. Withdrawal Pump/syringe ~ 20ml/min
2. Optical densitometer ~ 805 nm
3. Chart Recorder
THERMODILUTION TECHNIQUE
Bolus of “Negative Heat”
Sensor = Thermistor in pulmonary artery
Cardiac Output = ___Amount of –ve Heat (M)__
Tdto
THERMODILUTION METHOD
Negative Heat M = V1 x (TB – T1) D S
DBSB
Where M = Amount of negative heat VI = Volume injectable ( 10ml 5% D) TB = Patient’s blood temperature TI = Injectable temperature DI = Injectable density DB = Blood density SI = Specific heat of injectate
SB = Specific heat of blood
For 5% Dextrose = __DS SI__ = 1.08 DB - SB
THERMODILUTION ADJUSTMENTS
(a) Correction factor
(b) AVC = Calculated to 1.22
t
0
Tdt
t where C = _30 Cmax 100
(c) Final Formula
CO = VI x (TB – TI) x 1.08 x CT x 60
1.22
o
Tdt
THERMODILUTION
1. Room temperature D 5% used
2. Inject within 1.5 sec.
3. Inject at end-expiration
ICED vs ROOM TEMPERATURE INJECTATE
1. Random error greater with injectateat room temperature
2. Iced Injectate slightly overestimates atlow cardiac output.
3. Room temperature significantly overestimatesat CO < 2-3L/min by 20-50%.
FICK METHOD
ADVANTAGES
Correlates with direct measurements referenceMethod.
DISADVANTAGES
Slow and cumbersomeNot suitable for rapid repeated measurements.
DYE DILUTION
ADVANTAGES 1. Correlates well with direct and Fick method
DISADVANTAGES 1. Arterial cannulation needed 2. Limited to 3 measurements 3. Recirculation “Noise” 4. Unsuitable for rapid, repeated measurements
THERMODILUTION METHODS
ADVANTAGES
1. Simple and convenient
2. No blood withdrawal
3. Limited recirculation
4. Unlimited number of measurements
5. Rapid repeated measurements possible
RADIOACTIVE TRACER DILUTION
Risa washout over heart
Scintillation Counter
Difficult to calibrate
Radiation hazard.
BALLISTOCARDIOGRAPHY
Patient coupled to light bed
Ultralow frequency recording
Body acceleration (recoil) measures aortic
Acceleration ( dQ/dt) and stroke volume
F
H
L
G
I
Lo
t
I and J waves = dQ/dt
IMPEDANCE PLETHYSMOGRAPHY
12
3
4
-100Yz sinusoidal current 4 mA through chest
-Wheatstone bridge to measure resistance
-Voltage change with constant current I
-R = Voltage change I
-Resistance change reflect pulmonary blood flow
LITHIUM DILUTION CARDIAC OUTPUT
- Indicator = Lithium chloride (150 mM)
- Dose ~ 0.3 mmol via any venous line
- Artyerial litium plasma concentration measured by lithium sensitive electrode aspirated at 4 ml/min.
- Co = LiCl dose x 6- Area (1 – haemocrit)
PULSE PRESSURE ANALYSIS
- Arterial pulse pressure waveform analysed
- Cardiac output ~ Area under systolic portion of arterial waveform from diastole to end-systole
- Calibrated initial using lithium dilution technique.
Measurement of pH
Measurement of pH
pH = - logpH = - log1010 of the hydrogen ion activity (~ []'n) of the hydrogen ion activity (~ []'n) at 37°C, normal blood at 37°C, normal blood pH = 7.4 ± 0.04pH = 7.4 ± 0.04 circuit consists of,circuit consists of,
capillary tube of pH sensitive glass ® dVcapillary tube of pH sensitive glass ® dV reference buffer solution the other side of the glassreference buffer solution the other side of the glass
+ a silver/silver chloride electrode + a silver/silver chloride electrode an electrolyte solution (KCl) in contact with bloodan electrolyte solution (KCl) in contact with blood
+ a silver/silver chloride electrode+ a silver/silver chloride electrode surrounding water jacket at 37°Csurrounding water jacket at 37°C voltmetervoltmeter
Measurement of pH
MEASUREMENT OF pHpH electrode
- Depends on ion selective electrode
- pH sensitive Glass Electrode
- Utilises glass membrane which is
selectively permeable to hydrogen ions.
- Glass electrode - placed in series with 2 half cells which generate a constant potential gradient
pH ELECTRODE SYSTEMS
• Electrode consists of:
metal – conducts - electrons
electrolyte – conducts ions.
• Ag:AgCl + Hydrochloric Acid
Hg:Hg2Cl2 + saturated KCl Solution
• EMF generated at interface of 2 electrodes.
SCHEMATIC ARRANGEMENT OF pH ELECTRODE
V
HCl
Ag/AgClReferenceElectrodeSAMPLE
KCl Saltbridge
Hg / Hg2Cl2
Calomel Reference Electrode
Porous Plug
pH sensitiveglass
Potential Constant Constant Variable Constant
Voltmeter
pH ELECTRODE
Saturated KCl
• Provides salt bridge
• Completes circuit between blood sample and calomel electrode.
• Porous plug prevents diffusion of KCl into blood sample.
pH ELECTRODE
• Measures activity of H+; not concentration
• Calibrated against 2 standard buffers;
(a) pH 6.841 = Zero (b) pH 7.383
pH ELECTRODE
HCl
Ag : AgClelectrode
SENDING CIRCUITAND DISPLAY
SAMPLECUVETTE
Platinum Wire
Mercurous chloride
Mercury
Saturated KCl
Porous Plug
pH sensitive glass
Measurement of Gases
GAS ANALYSIS
CHEMICAL METHODSAbsorption in chemicals using Haldane apparatusCO2 : 10 – 20% KOH or NaOHO2 : Alkaline pyrogallol or sodium anthraquinone
PHYSICAL METHODS :• Mass spectrometers• Infra-red absorption• Polarography• Galvanic fuel cell• Ultra violet absorption• Paramagnetism• Thermal conductivity
Spectrophotometry
first used to determine the [Hb] the 1930's, by first used to determine the [Hb] the 1930's, by application of the application of the Lambert-Beer LawLambert-Beer Law
IIii = the incident light= the incident light IItt = the transmitted light= the transmitted light DD = the distance through the medium= the distance through the medium CC = the concentration of the solute= the concentration of the solute = the = the extinction coefficientextinction coefficient of the of the
solutesolute
ITrans = ISource x e- DC
Spectrophotometry
the the extinction coefficientextinction coefficient is specific for is specific for a given solute at a given wavelength of a given solute at a given wavelength of lightlight
therefore, for each wavelength of light therefore, for each wavelength of light used an independent Lambert-Beer used an independent Lambert-Beer equation can be writtenequation can be written
if the number of equations = the number if the number of equations = the number of solutes, then the concentration for of solutes, then the concentration for each one can be solvedeach one can be solved
Spectrophotometry
by convention oxyhaemoglobin by convention oxyhaemoglobin concentration, HbOconcentration, HbO22 is the fractional is the fractional
concentration as measured by concentration as measured by cooximetrycooximetry
a 4 wavelength device, and includes a 4 wavelength device, and includes COHbCOHb and and MetHbMetHb in the denominator in the denominator
%HbO2 = 100 [ HbO2 ]
Hb + HbO2 + COHb + Met Hb
ULTRA-VIOLET ABSORPTION
• Halogenated vapours absorb uv light
• used for measuring halothane
• Disadvantage : Slow response time produce toxic product
THERMAL CONDUCTIVITY (KATHAROMETERS)
1. High thermal conductivity gas - more rapid heat conductioneg. Helium 600%CO2 35% compared with air
2. Gas passed over heated wire which cools.
3. Decreased wire temperature – depends on flow rate and thermal conductivity of gas.
4. Temperature leads to wire resistance
5. Advantages : Simple and inexpensive
6. Disadvantage : Slow response time ( ~ 5s)
Measurement: Methods
Mass spectrometryMass spectrometry
Raman spectrographyRaman spectrography
Photo-acoustic spectrographyPhoto-acoustic spectrography
Infra-red spectrographyInfra-red spectrography
RAMAN LIGHT SCATTERING
1. Photon of light passes thro’ gas
2. Photon energy partly given to gas molecule
3. Light is re-emitted at longer wavelengthcharacteristic to gas.
Measurement: Raman Spectrography
Raman scatteringRaman scattering occurs with illumination with high intensity occurs with illumination with high intensity argon laserargon laser light light
absorbed light energy produces unstable energy states absorbed light energy produces unstable energy states (rotational & vibration)(rotational & vibration)
emitted low energy light, Raman lightemitted low energy light, Raman light measured at 90° to the laser pathmeasured at 90° to the laser path
can be used to identify all types of molecules in the gas can be used to identify all types of molecules in the gas sample, and has been incorporated into new monitors sample, and has been incorporated into new monitors (RASCAL) which instantaneously identify & quantify CO(RASCAL) which instantaneously identify & quantify CO22 and and inhalational agentsinhalational agents
Measurement: Photo-acoustic spect.
relies on the absorbance of IR light by COrelies on the absorbance of IR light by CO22 gas expansiongas expansion
IR light is pulsed at IR light is pulsed at acoustic frequenciesacoustic frequencies and the and the energy absorbed is detected by a microphoneenergy absorbed is detected by a microphone
amount of light absorbed is measured amount of light absorbed is measured directlydirectly without the need for a reference chamberwithout the need for a reference chamber no zero point driftno zero point drift
other claimed advantages over IR spectrometry,other claimed advantages over IR spectrometry, higher accuracyhigher accuracy increased reliabilityincreased reliability reduced maintenance & reduced need for reduced maintenance & reduced need for calibrationcalibration
MASS SPECTROMETER
PRINCIPLE :
1. Gas passed into ionizing chamber
2. Electron beam ionizes gas
3. Ions diffuse thro’ slit in chambers
4. Negatively charge plate accelerate ions
5. Different particles streams separate according to mass & charge.
6. Detector plate
MASS SPECTROMETER
Detector
Low charge / mass ratio
Deflection Angle
High charge / mass ratio
GAS
AcceleratorPotentialOn screen electrode
Magnetic field
MASS SPECTROMETER
ADVANTAGES
1. Rapid response time ( < 0.1s)2. Can measure variety of gases (May be affected by water vapour)
DISADVANTAGES
1. Complex2. Expensive
Capnometry
capnometrycapnometry is the measurement and display of is the measurement and display of COCO22 concentrations on a digital or analogue concentrations on a digital or analogue displaydisplay
capnographycapnography is the graphic recording of is the graphic recording of instantaneous respired COinstantaneous respired CO22 concentrations concentrations during the respiratory cycleduring the respiratory cycle
Capnometry
first IR COfirst IR CO22 measuring and recording apparatus measuring and recording apparatus was introduced by was introduced by LuftLuft in 1943 in 1943
expensive, bulky and principally only used for expensive, bulky and principally only used for researchresearch
widespread use within the last 10-15 years with widespread use within the last 10-15 years with cost and size reductioncost and size reduction
ASA closed claims ASA closed claims 93%93% of anaesthetic of anaesthetic mishaps preventable by mishaps preventable by ETCOETCO22 / SpO / SpO22
INFRA-RED ABSORPTION
PRINCIPLE :
1. Molecule composed of 2 or more dissimilar atoms absorb infra red light.
2. Absorption of 2.5 - 25 m cause covalent bonds to bend and vibrate; increasing rotational speed.
3. Different gas molecules absorb specific of infra red light.
4. Detecting increased absorption allows their concentrations to be determined
Measurement: IR Spectroscopy
Lambert-Beer lawLambert-Beer law applies, (cf. Hb) applies, (cf. Hb) more compact and less expensivemore compact and less expensive assymetric, assymetric, polyatomicpolyatomic gases gases of two or more of two or more
molecules, absorb IR radiation (> 1.0 µm)molecules, absorb IR radiation (> 1.0 µm) HH22O, NO, N22O, COO, CO22
absorbance peak is characteristic for a gasabsorbance peak is characteristic for a gas COCO22 ~ 4.28 µm ~ 4.28 µm
Measurement: IR Spectroscopy
glass absorbs IR radiationglass absorbs IR radiation chamber windows must be made of a crystalchamber windows must be made of a crystal
sodium chloride or sodium bromidesodium chloride or sodium bromide
calibration may be achieved by filling the chamber calibration may be achieved by filling the chamber with a COwith a CO22 free gas, or by splitting the incident beam free gas, or by splitting the incident beam
and passing this through a and passing this through a reference chamberreference chamber
Measurement: IR Spectroscopy
the use of a reference beam also allows for the use of a reference beam also allows for compensation for variations in the output of the compensation for variations in the output of the IR sourceIR source
the sample chamber is made small, so that the sample chamber is made small, so that continuous analysis is possible continuous analysis is possible
the the response timeresponse time ~ 100 ms~ 100 ms enabling end-tidal COenabling end-tidal CO22 estimations and real-time estimations and real-time
graphical analysisgraphical analysis
INFRA-RED GAS ANALYSER(SPECTROPHOTOMETER)
1. LED split infra-red into different .
2. Sample chamber is transilluminated and IR absorption measured.
3. Reference chamber transilluminated & absorption allowscalibration.
4. IR absorption in sample chamber compared with reference chamber.
INFRA-RED SPECTROPHOTOMETER
REFERENCE
Known CO2
SAMPLE CELL
Light splitterChopper
Detector
ADVANTAGES : Fast response for CO2 N2O and volatile anaesthetic agentDISADVANTAGES : Rapid respiratory rates
decrease accuracy
ETCO2 : Classification 1
side-streamside-stream sensor is located within the main unit and gas is aspirated sensor is located within the main unit and gas is aspirated
from the circuitfrom the circuit sampling flow rate may besampling flow rate may be
highhigh > 400 ml/min, or> 400 ml/min, or lowlow < 400 ml/min< 400 ml/min
optimal gas flowoptimal gas flow is considered to be 50-200 ml/min, is considered to be 50-200 ml/min, ensuring reliability with both adults and childrenensuring reliability with both adults and children
exhaust gases contain anaesthetic agentsexhaust gases contain anaesthetic agents & should be & should be routed to the routed to the scavengingscavenging unit unit
ETCO2 : Classification 2
mainstreammainstream sensor is located at the patient, with a curvette placed sensor is located at the patient, with a curvette placed
within the circuitwithin the circuit
these are heated to > 39° to prevent occlusion by water these are heated to > 39° to prevent occlusion by water vapourvapour
no mixing of gases occurs during sampling and the no mixing of gases occurs during sampling and the response time is more rapidresponse time is more rapid
curvettes tend to be bulky, add dead space, are heated, curvettes tend to be bulky, add dead space, are heated, and are expensive if dropped & brokenand are expensive if dropped & broken
ETCO2 : Sources of Error
Atmospheric pressure differencesAtmospheric pressure differences NN22OO HH22OO OthersOthers
OO22
alinearityalinearity volatile agentsvolatile agents
ETCO2 : PAtm
direct effectsdirect effects gas densitygas density
for a given chamber thickness, no. of molecules for a given chamber thickness, no. of molecules increasesincreases
eliminated by calibration against a known Peliminated by calibration against a known PCO2CO2 (% (%
x Atm.)x Atm.) units calibrated against Cunits calibrated against CCO2CO2 require correction require correction
(1%:1%)(1%:1%) IR absorbanceIR absorbance
intermolecular forces ® IR absorbance for a given [COintermolecular forces ® IR absorbance for a given [CO22]]
PPAtmAtm ~ 1% ~ 1% absorbance ~ 0.5-0.8%absorbance ~ 0.5-0.8%
ETCO2 : PAtm
direct effects (continued)direct effects (continued) sampling sampling flow rateflow rate may reduce sample chamber may reduce sample chamber
pressurepressure units should be calibrated for a given sample rateunits should be calibrated for a given sample rate
PEEPPEEP maymay PPCO2CO2 reading (some unit compensate reading (some unit compensate automatically)automatically)
PEEP ~ 20 cmHPEEP ~ 20 cmH22O O PPCO2CO2 ~ 1.5 mmHg ~ 1.5 mmHg
ETCO2 : PAtm
indirect effectindirect effect : volume percent : volume percent, ,
where Pwhere PCO2CO2 = F = FCO2CO2 x Atm. x Atm.
where Pwhere PAtmAtm at calibration is different to the time of measurement at calibration is different to the time of measurement
ETCO2 : N2O
absorbs IR at absorbs IR at 4.5 µm4.5 µm (cf. CO(cf. CO22 ~ 4.28 µm) ~ 4.28 µm)
NN22OO falsely elevatedfalsely elevated CO CO22 readings readings effect minimised by a narrow bandwidth filtereffect minimised by a narrow bandwidth filter
however, presence of Nhowever, presence of N22O molecules results in O molecules results in collision broadeningcollision broadening of the absorbance peak of of the absorbance peak of COCO22
resulting in apparently resulting in apparently elevated COelevated CO22 readings readings
ETCO2 : N2O
simplest correction is to calibrate the monitor with simplest correction is to calibrate the monitor with the same background gas as is to be used during the same background gas as is to be used during anaesthesiaanaesthesia
alternatively correction factors may be applied,alternatively correction factors may be applied, 50% N50% N22OO P'P'CO2CO2 ~ P ~ PCO2CO2 x 0.9 x 0.9 70% N70% N22OO P'P'CO2CO2 ~ P ~ PCO2CO2 x 0.94 x 0.94
ETCO2 : H2O
condensed watercondensed water
result in falsely result in falsely highhigh readings readings
prevented in mainstream units by heating the sensorprevented in mainstream units by heating the sensor
side-stream units use water trapsside-stream units use water traps
some units use semipermeable Nafionsome units use semipermeable Nafion®® tubing tubing
ETCO2 : H2O
water vapourwater vapour mainstream analysers measure breathing circuit gasmainstream analysers measure breathing circuit gas
generally saturated at body T. but may be affected by the generally saturated at body T. but may be affected by the
use of humidifiers, FGF's, and the ambient T.use of humidifiers, FGF's, and the ambient T.
side-stream units, cooling of the gases results inside-stream units, cooling of the gases results in water vapour pressure, andwater vapour pressure, and
apparent apparent increaseincrease in P in PCO2CO2 ~ ~ 1.5-2%1.5-2%
ETCO2
transit timetransit time creating a creating a phase shiftphase shift, but no distortion, but no distortion gas is subject to gas is subject to mixingmixing with overdamping of a with overdamping of a
square waveformsquare waveform results in underestimation of ETCOresults in underestimation of ETCO22, especially in , especially in
childrenchildren this error increases both with,this error increases both with,
increased width and length of the sample tubingincreased width and length of the sample tubing reduced sample flow rates < 50 ml/minreduced sample flow rates < 50 ml/min higher frequency breathing patternshigher frequency breathing patterns
ETCO2
rise timerise time TT10-9010-90
time to change from 10% to 90% of the final valuetime to change from 10% to 90% of the final value
depends on size of the depends on size of the sample chambersample chamber and and flow rateflow rate
capnographs used clinically ~ 50-600 mseccapnographs used clinically ~ 50-600 msec
prolongation may decrease the slope of phase II, and prolongation may decrease the slope of phase II, and
underestimation of anatomical dead spaceunderestimation of anatomical dead space
ETCOETCO22 in adults at < 30 bpm with in adults at < 30 bpm with ± 5% ± 5% accuracyaccuracy
faster units are required in children, Tfaster units are required in children, T7070 < 80 msec < 80 msec
ETCO2
rise timerise time TT10-90 10-90 (continued)(continued)
response times have been markedly response times have been markedly reduced by,reduced by,
more powerful signal amplifiersmore powerful signal amplifiers minimising the volume of the sample minimising the volume of the sample
chamberchamber use of relatively high sample flow rates > use of relatively high sample flow rates >
150 ml/min150 ml/min
ETCO2 : Other Factors
oxygenoxygen OO22 does not directly absorb IR light does not directly absorb IR light
may affect reading by collision broadeningmay affect reading by collision broadening
results in falsely results in falsely lowlow P PCO2CO2 readings readings
not as great as with Nnot as great as with N22O (some units incorporate O (some units incorporate correction)correction)
ETCO2 : Other Factors
halogenated agentshalogenated agents absorb IR light at absorb IR light at ~ 3.3 µm~ 3.3 µm
interference is not clinically significantinterference is not clinically significant
alinearityalinearity of CO of CO22 analysis analysis
the concentration of the calibration gas should be as close the concentration of the calibration gas should be as close as possible to the measured gas sampleas possible to the measured gas sample
Severinghaus CO2 Electrode
Severinghaus developed the COSeveringhaus developed the CO22 electrode in electrode in
19581958
modern arterial blood gas analysis was bornmodern arterial blood gas analysis was born Essentially a modified pH electrodeEssentially a modified pH electrode
provides a direct measure of Pprovides a direct measure of PCO2CO2 from the from the change in pHchange in pH
Severinghaus CO2 Electrode
circuit consists of,circuit consists of, a closed cylinder of a closed cylinder of pH sensitive glasspH sensitive glass in the centre in the centre 2 electrodes, 1 inside, the other outside the cylinder2 electrodes, 1 inside, the other outside the cylinder a surrounding solution of a surrounding solution of sodium bicarbonatesodium bicarbonate
a thin film of bicarbonate impregnated nylon mesh a thin film of bicarbonate impregnated nylon mesh covering the end of the cylinder covering the end of the cylinder
a thin, a thin, COCO22 permeable membrane permeable membrane covering the end covering the end
of the electrodeof the electrode
Severinghaus CO2 Electrode
Severinghaus CO2 Electrode
COCO22 diffuses from the blood sample through the diffuses from the blood sample through the membrane into the nylon mesh and by the membrane into the nylon mesh and by the formation of formation of carbonic acidcarbonic acid lowers the pH of the lowers the pH of the bicarbonate solutionbicarbonate solution
the change in pH alters the dV across the glass, the change in pH alters the dV across the glass, such that,such that,
pH ~ pH ~ loglog1010PPCO2CO2
CO2 Electrode
output of output of voltmetervoltmeter calibrated in terms of P calibrated in terms of PCO2CO2
electrode accuracy ~ 1 mmHgelectrode accuracy ~ 1 mmHg
response time ~ 2-3 minsresponse time ~ 2-3 mins
as for the pH electrode, the COas for the pH electrode, the CO22 electrode kept at electrode kept at 37°C and regularly calibrated with known 37°C and regularly calibrated with known concentrations of COconcentrations of CO22
Measurement of OXYGEN
OXYGEN MEASUREMENT ELECTROCHEMICAL METHODS
• Based on electrochemical reaction in buffer
solution occurring between 2 electrodes,
involving gas molecules.
• 2 Devices
(a) Polarographic electrode
(b) Fuel cell.
Measurement of Oxygen
Leyland Clarke developed the Leyland Clarke developed the
polarographic oxygen electrode in 1956polarographic oxygen electrode in 1956
prior to this the POprior to this the PO22 had not been measured had not been measured
Other Methods
POPO22 may also be measured by, may also be measured by, Volumetric - van Slyke/NeillVolumetric - van Slyke/Neill Clarke electrodeClarke electrode Fuel cellFuel cell ParamagneticParamagnetic Hummel Cell - paramagneticHummel Cell - paramagnetic Optode - photoluminescence quenchingOptode - photoluminescence quenching Raman scatteringRaman scattering Mass spectrometerMass spectrometer
PARAMAGNETISM
• Paramagnetic : attracted toward magnetic field eg. oxygen
• Diamagnetic : repelled by magnetic field eg. nitrogen
• Paramagnetic molecules = 2 unpaired electrons in outer electron shell spinning in the same direction.
PAULING TYPE OF PARAMAGNETICOXYGEN ANALYSER
MAGNET POLE
MAGNET POLE
MAGNET POLE
Gas O2
Nitrogen In GlassDumb-Bell
Light beam Detector
Slow response ( 5 – 20 s)
RAPID PARAMAGNETIC O2 ANALYSERS
Sample Reference Gas
Magnetic field
Gas Mixture out
Made more compactRapid response time
DifferentialPressure transducer
POLAROGRAPHIC ELECTRODE
PRINCIPLE
• 1 pair of electrodes in electrolyte solution
• Electrodes maintained at potential difference
• Current through electrolyte solution dependent on gas concentration in solution
• Reaction driven by voltage applied to electrodes
CLARKE OXYGEN ELECTRODE
• Cathode - Platinum covered by permeable membrane
• Anode - Silver/Silver chloride covered by membrane
• Electrolyte Solution - KCl
• Electrodes connected to DC voltage 0.6V
• Electrons produced by Ag / AgCl anode migrate to cathode to reduce O2 molecules.
Clarke Electrode
the circuit consists of,the circuit consists of, DC voltage source (0.6 V)DC voltage source (0.6 V)
ammeterammeter
platinum cathodeplatinum cathode
silver/silver chloride anodesilver/silver chloride anode
electrolyte solution (KCl), andelectrolyte solution (KCl), and
OO22-permeable membrane-permeable membrane
Clarke Electrode
Clarke Electrode
Ohm’s Law: for any resistive Ohm’s Law: for any resistive circuit:circuit: I I V V
for the Clarkefor the Clarkeelectrode there is aelectrode there is aplateau voltage rangeplateau voltage range I does not change withI does not change with VV however:however: II POPO22
this occurs as the cathode this occurs as the cathode reaction requires both Oreaction requires both O22
and free electronsand free electrons
Clarke Oxygen Electrodes (Cont’d)
• Platinum Cathode - O2 + 4e 2 O
(reduction) 2 O + 2 H2O 4 OH
reaction at the platinum cathode,reaction at the platinum cathode,OO22 + 2H + 2H22O + 4eO + 4e-- 4OH4OH--
At Ag / AgCl Anode (oxidation) :
4 Ag 4 Ag+ + 4e-
Current flow between both electrodes measured
Clarke Electrode
current flow being in direct proportion to the current flow being in direct proportion to the consumption of oxygenconsumption of oxygen
the platinum electrode cannot be inserted directly the platinum electrode cannot be inserted directly into the blood stream as protein deposits form an into the blood stream as protein deposits form an affect its accuracyaffect its accuracy
CLARKE ELECTRODES
Advantages : Robust
Portable
Disadvantages : Limited life span
Silver anode eventually used
up by current
FUEL CELL
Cathode : Silver - reduces O2 molecules in solution.
Anode : Lead : 2Pb + 40H 2Pbo +2H2O + 4e-
Electrolyte : potassium bicarbonate
SAMPLE
No polarising current required
Lead Anode
M Potassium Bicarbonate Solution
Silver CathodeO2 + 4e + 2H2O 4OH-
FUEL CELL
Advantages :CompactNo power supply requiredUnaffected by N2O
Disadvantage :
Slow response timelife-span 6-12 months
OPTODES - PRINCIPLE
1. Oxygen has the property of “quenching” fluorescenceof certain dyes.
2. Dyes exposed to light – electrons excited and release photons when they return to their original state (fluorescence).
3. Oxygen absorbs energy from excited electrons electrons return to original state without releasing photon
4. Absorption of light and reduction in light emitted is proportional to PO2
OPTODE - MECHANISM
• Optical fibre with dye coated tip
• O2 permeable membrane cover
• Sequential illumination of fibre causes dye to fluorescence
• Intensity of fluorescence depends on oxygen concentration at tip
• Fluorescence measured by photo multiplication.
OPTODE : Uses
• Intravascular PO2 monitoring
• Advantages :Independent of blood flowStableRapid response times
• Disadvantage :expensiveDye deteriorate with timeFibrin deposition
Oximetry
Kramer optically measured the OKramer optically measured the O22 in animals in the early in animals in the early 1930's1930's
Karl Matthes in 1936 was the first to measure OKarl Matthes in 1936 was the first to measure O22 from from transmission of transmission of redred and and blue-greenblue-green light through the human light through the human earear
the term oximeter was coined by Millikan the term oximeter was coined by Millikan et alet al. in the 1940's. in the 1940's they developed a lightweight oximeter, a smaller version they developed a lightweight oximeter, a smaller version
of Matthes' design, which measured SaOof Matthes' design, which measured SaO22 by by transillumination of the earlobe using red & green filters transillumination of the earlobe using red & green filters covering Kramer's barrier layer photocellscovering Kramer's barrier layer photocells
Oximetry
the signal detected from the photocell under the the signal detected from the photocell under the green filter later proved to be in the green filter later proved to be in the IR rangeIR range
there were two technical problems with this approach,there were two technical problems with this approach,
there are many non-Hb light absorbers in tissuethere are many non-Hb light absorbers in tissue
the tissues contain capillary & venous blood in addition to the tissues contain capillary & venous blood in addition to arterial bloodarterial blood
TRANSMISSION OXIMETRY
Based on absorbance laws
Blood consists of a mixture of
Oxyhaemoglobin and Deoxyhaemoglobin
ABSORBANCE CURVES FOR HbO2 AND Hb
Absorbance
660 805 940
RED INFRARED
OXY Hb
DEOXY Hb
wavelength
ISOBESTIC WAVELENGTH
ABSORBANCE CURVES
Secondary Peaks of Absorbance
660 nm - Deoxyhaemoglobin
940 nm - Oxyhaemoglobin
805 nm - Isobestic point
defined as point at which absorbances of HbO2 and Hb are equal.
Depends on haemoglobin concentration
CO - OXIMETER
• Measures Oxygen saturation
• Based on absorbance curves
• Requires haemolysis of blood sample before “sats” measurement
• 2 types - Reflectance
- Transmission
Measure at 4 wavelength to enable measurement of metHb and HbCO
CO-OXIMETER
ADVANTAGES :
Light absorbance measured at several
wavelength enables fraction estimation.
DISADVANTAGES :
Cannot provide continuous monitoring
Expensive cost and maintenance
PULSE OXIMETRY
Pulse Oximetry
early 1970's, Japanese engineer Takuo Aoyagi working on a early 1970's, Japanese engineer Takuo Aoyagi working on a
dye dilution method for CO, using an earpiece densitometerdye dilution method for CO, using an earpiece densitometer noted that the noted that the pulsatile componentspulsatile components of the red & IR of the red & IR
absorbances were related to SaOabsorbances were related to SaO22
prototype, built by Nihon Khoden, was tested clinically in prototype, built by Nihon Khoden, was tested clinically in
1973 and the first commercial prototype available in 19741973 and the first commercial prototype available in 1974 further refinements were required and widespread use did further refinements were required and widespread use did
not eventuate until the early not eventuate until the early 1980's1980's
Pulse Oximetry
the signal detected from the photocell under the the signal detected from the photocell under the green filter later proved to be in the green filter later proved to be in the IR rangeIR range
there were two technical problems with this approach,there were two technical problems with this approach, there are many non-Hb light absorbers in tissuethere are many non-Hb light absorbers in tissue the tissues contain capillary & venous blood in addition to the tissues contain capillary & venous blood in addition to
arterial bloodarterial blood
Pulse Oximetry
these were overcome by first measuring the these were overcome by first measuring the absorbance of the ear while it was compressed absorbance of the ear while it was compressed to remove all bloodto remove all blood
after this after this bloodless "baseline"bloodless "baseline" measurement measurement the ear was heated to the ear was heated to "arterialise""arterialise" the blood the blood
this device was shown to accurately predict this device was shown to accurately predict intraoperative desaturations, however, due to the intraoperative desaturations, however, due to the technical difficulties was never adopted on masstechnical difficulties was never adopted on mass
Nomenclature
SaOSaO22 = 100.(O = 100.(O22 content)/(O content)/(O22 capacity) capacity) arterial blood saturation measured arterial blood saturation measured in vitroin vitro OO22 capacity the amount of O capacity the amount of O22 which can combine with which can combine with
reduced Hb, reduced Hb, withoutwithout removing COHb or MetHb removing COHb or MetHb thus, at high Pthus, at high PaO2aO2 the SaO the SaO22 = 100% = 100%
irrespective of the [COHb + MetHb]irrespective of the [COHb + MetHb] HbOHbO22= = oxyhaemoglobin concentrationoxyhaemoglobin concentration
multiwavelength spectrometers measure all speciesmultiwavelength spectrometers measure all species SaOSaO22 computed from P computed from PO2O2 and pH approximates SaO and pH approximates SaO22, not , not
HbOHbO22
SpOSpO22 = = pulse oximeter saturationpulse oximeter saturation
Methodology
2 wavelengths of light,2 wavelengths of light, redred = 660 nm= 660 nm IRIR = 910-940 nm= 910-940 nm
the signal is divided into two components,the signal is divided into two components, acac == pulsatile arterial bloodpulsatile arterial blood dcdc == non-pulsatile arterial bloodnon-pulsatile arterial blood
+ tissue + capillary blood + venous + tissue + capillary blood + venous bloodblood
NB: all pulse oximeters assume that only the NB: all pulse oximeters assume that only the pulsatile absorbancepulsatile absorbance is arterial blood is arterial blood
AC AND DC SIGNALSRECEIVED BY PULSE OXIMETER
ACVariable absorption due to pulsatile arterial blood
DC
Absorption due to arterial blood
Absorption due to venous blood
Tissue absorptionTISSUE
VENOUS BLOOD
Methodology
for each wavelength, the oximeter for each wavelength, the oximeter determines the ac/dc fractiondetermines the ac/dc fraction independent of the incident light intensityindependent of the incident light intensity
= = pulse added absorbancepulse added absorbance the the ratio (R)ratio (R) of these is calculated, of these is calculated,
R =R = (ac absorbance/dc absorbance)(ac absorbance/dc absorbance)RedRed
(ac absorbance/dc absorbance)(ac absorbance/dc absorbance)IRIR
= A= A660nm660nm / A / A940nm940nm
R and SpO2
this value varies from,this value varies from,
SaOSaO22 = 100% = 100% R = 0.4R = 0.4 (0.3)(0.3)
SaOSaO22 = 85% = 85% R = 1.0R = 1.0
SaOSaO22 = 0% = 0% R = 3.4R = 3.4
R and SpO2
Methodology
the photo-detector diodes of the sensor will also the photo-detector diodes of the sensor will also register register ambient lightambient light
interference is reduced by cycling the lightinterference is reduced by cycling the light red only red only infrared only infrared only both off both off repeated at 480-1000 Hz in an attempt to subtract the repeated at 480-1000 Hz in an attempt to subtract the
ambient light signal, even when this is oscillatingambient light signal, even when this is oscillating this allows accurate estimation of SpOthis allows accurate estimation of SpO22 at arterial at arterial
pulse frequencies ~ 0.5-4 Hz (30-240 bpm)pulse frequencies ~ 0.5-4 Hz (30-240 bpm) data is averaged over several cyclesdata is averaged over several cycles
Uses: Oxygenation
anaesthesia & recoveryanaesthesia & recovery intensive careintensive care emergency care & transportemergency care & transport labourlabour premature & newborn infantspremature & newborn infants home & hospital monitoring for SIDShome & hospital monitoring for SIDS patients in remote locations eg XRay, MRIpatients in remote locations eg XRay, MRI "office" procedures eg. dentistry, endoscopy"office" procedures eg. dentistry, endoscopy
Uses: Circulation
systolic BP & pleth waveform appearancesystolic BP & pleth waveform appearance inflation better than deflationinflation better than deflation
sympathetic blockade with central neuraxis anaesthesiasympathetic blockade with central neuraxis anaesthesia autonomic dysfunction with valsalva manoeuvreautonomic dysfunction with valsalva manoeuvre anecdotally reported usesanecdotally reported uses
patency of the ductus arteriosuspatency of the ductus arteriosus level of ischaemia in PVDlevel of ischaemia in PVD patency of arterial graftspatency of arterial grafts circulation in reimplanted digits or graftscirculation in reimplanted digits or grafts
Uses: Therapy
optimise Foptimise FIIOO22 in ventilated patients in ventilated patients
optimise CPAP or PEEPoptimise CPAP or PEEP
extubation of ventilated patientsextubation of ventilated patients
adjust Oadjust O22 therapy in preterm infants therapy in preterm infants
no consensus on optimal levelsno consensus on optimal levels
optimisation of home Ooptimisation of home O22 therapy therapy
Signal:Noise
Freund Freund et al.et al. 1.12%1.12% failure failure cumulative > 30 mins in 11,046 anaestheticscumulative > 30 mins in 11,046 anaesthetics
Gilles Gilles et al.et al. found a found a 1.1%1.1% incidence incidence 2 x 15 mins in 1,403 anaesthetics2 x 15 mins in 1,403 anaesthetics
automatic gain controlsautomatic gain controls amplification of low signal strengthsamplification of low signal strengths
low signal to noise ratio low signal to noise ratio most new meters give "low signal strength" warnings once most new meters give "low signal strength" warnings once
the ac component falls below an arbitrary fraction of the the ac component falls below an arbitrary fraction of the total transmitted light (0.2% for the Biox-Ohmeda)total transmitted light (0.2% for the Biox-Ohmeda)
Low S:N Causes
low perfusion pressurelow perfusion pressure
motion artefactmotion artefact
ambient lightambient light
skin pigments & dyesskin pigments & dyes
probe positionprobe position the "penumbra effect" the "penumbra effect"
Ventilation - a large paradox may lead to searchingVentilation - a large paradox may lead to searching
venous pressure wavesvenous pressure waves - TI, reflectance operation- TI, reflectance operation
electrocauteryelectrocautery - most unit are now immune- most unit are now immune
MRI interferenceMRI interference - rare, usually lead distorts MRI- rare, usually lead distorts MRI imageimage
Ultrasound and anaesthesia
ULTRASOUND
• Sound = disturbance propagating in material (Air, water, tissue or solid)
• Characterized by frequency and intensity.
• Frequency measured in hertz
• ULTRASOUND = sounds waves > 20 KHz Cannot be perceived by human ear.
WAVELENGTH OF SOUND
• Sound Wavelength = Velocity frequency
• Shorter wavelength higher resolution less penetration
• Compromise between penetration and resolution required.
SOUND PRODUCTION
• Ultra-sound probe = Transducer containing an array of piezo-electric crystals.
• Electrical voltage applied to crystals causes piezo- electric crystals to oscillate at resonant
frequency.
• Electrical energy - converted to sound energy.
ELECTRICAL ENERGY
Electrical Energy
Piezo – electriccrystals oscillate
Sound
Electrical energy
ULTRASOUND PROPAGATION
In homogenous tissues : -
ultrasound is absorbedAbsorption – least in fluids greatest in solid tissues
Absorbed energy converted to heat (small)
Amount of heat dissipated hence useless
ULTRASOUND PROPAGATION
In heterogenous tissues :
• Ultrasound strikes interfaces
• Wave is either a) refracted - transmitted – thro’ interface b) reflected - depends on smooth (specular) or non-smooth
surfaces.
• Bone and calcium more reflective
PULSED SOUND WAVES
• Used to prevent transmitted and reflected sound waves.
• Pulse repetition frequency = 10 – 20 Hz
• Longer path sound wave travels - lower is PRF
PULSES OF ULTRASOUND
Usually 2.5 to 7.5 MHz
Frequency - resolution - penetration
IncidentReflected Wave
Surface
Incident Wave
“Scattering” of Ultrasound
ULTRASOUND REFLECTION
REFLECTED ULTRASOUND (ECHO)
Two quantities measured :
(a) Time delay between sound transmission and reception of reflected echo.
(b) Intensity of reflected signal High echo reflection - whiteLess reflection - greyNo reflection - Black
A - MODE (AMPLITUDE)
Brief ultrasound pulses in one direction.
Reflected ultrasound amplitude plottedAgainst time
Time & distance from probe
Amplitude
Peaks = reflective interface
Time (distance)
B - MODE (BRIGHTNESS)
Brief ultrasound pulse in one direction
Reflected ultrasound measured
Amplitude = Brightness of reflected ultrasound
M – MODE (MOTION)
• Repeated B-Mode pulses graphed against time base.
• > 1000 pulses per second
• Good resolution
• Provides one-dimension image against time.
• useful for value motion
2-D ULTRASOUND
• Multiple crystals (linear or phased array) or moving crystals
• Sequential B-mode pulses across 90o
• Single image displayed
• Real time movement
DOPPLER PRINCIPLE
• Frequency of transmitted sound from a moving object
alters depending on velocity and direction of object.
• Change in frequency proportional to
a) ultrasound frequency
b) Cosine of angle between ultrasound beam
direction and moving object.
DOPPLER SIGNAL
• Maximal signal when sound moves towards probe – higher pitch (frequency).
• Lower pitch when sound moves away from probe.
• Pitch change is due to compression and rarefaction of sound waves.
USES OF DOPPLER
• Examine direction and velocity of blood flow in vessels and heart
• Estimate velocities and therefore measure pressure gradients, using Bernoulli equation
P = 4V2
• Types of Doppler used ;
a) Pulse wave b) Continuous Doppler
PULSED-WAVE DOPPLER
• Depends on Doppler shift
• Doppler shift frequency of reflected waves which depends on velocity or reflected wave.
• Used to measure velocity of red blood cells V = FDC
2fo Cos Q V = Velocity of red blood cells FD = Doppler shift
Fo = Ultrasound frequency Q = angle between flow and sound wave.
PULSED-WAVE DOPPLER- Limitations -
• Large angles - results inaccurate
• High velocity flows > 0.6m/s cannot be accurately
measured by intermittent pulses (causes “aliasing”)
CONTINUOUS WAVE DOPPLER
Separate crystals - emit & receive ultrasound continuously along 1 axis
Frequency Spectrum & velocity of interfaces
Graph of Velocity range vs time plotted
CONTINUOUS WAVE DOPPLER
Advantages :
Can measure fast flows
Calculate valve gradients
Disadvantages :
Small incident angle required
COLOURED DOPPLER
Pulsed wave used on 2 D scan
Velocity depicted as colour
Advantage : Easy visualisation
Disadvantage : high velocity – colour reversal rapid turbulent flow produce colour “jets”
TOE PROBE
Phase array 2 D probe
64 piezo-electric crystals
Mounted on gastroscope (9mm)
Can be monoplane biplane (2 array) multiplane (rotating array)
CLINICAL APPLICATIONS OF ULTRASOUND
1. Examination of structure
Brain
Neck
Chest - pleural fluid
Obstetrics
Abdominal structures
Blood vessels
2. Interventional Procedures
Guide placement of needles
CLINICAL APPLICATIONS - DOPPLER
1. Sense blood flow in blood vessels
e.g. Thrombo-embolism
Thrombosis
2. Measurement of blood pressure
a) sense onset of blood flow
b) sense movement of arterial wall
CLINICAL APPLICATIONS –DOPPLER (CONT’D)
3. Cardiac output measurement
a) mean velocity
b) cross-sectional area of aorta or left ventricular
outflow tract.
4. Fetal Heart movements and heart rate
5. Valve functional, Myocardial wall movement
6. Transcranial Doppler - Velocity of blood flow in
cerebral vessels.