Measurement of flow&Volume of Blood.ppt

7/27/2019 Measurement of flow&Volume of Blood.ppt

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Flow & Volume of Blood


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Measurement of

Flow & Volume of Blood

A measurement of paramount importance: concentration of O2 and other

nutrients in cells Very difficult to measure

Second-class measurement: blood flow and changes in blood volumecorrelate well with concentration

Third-class measurement: blood pressure correlates well with blood flow Fourth class measurement: ECG correlates adequately with blood pressure

How to make blood flow / volume measurements? Standard flow meters,

such as turbine flow meters, obviously cannot be used!

Indicator-dilution method: cont./rapid injection, dye dilution, thermodilution

Electromagnetic flowmeters

Ultrasonic flowmeters / Doppler flowmeters

Plethysmography: Chamber / electric impedance / photoplethysmography


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Indicator Dilution with Continuous

Injection

Measures flow / cardiac output averaged over several heart beats

Ficks technique: the amount of a substance (O2) taken up by an organ /

whole body per unit time is equal to the arterial level of O2 minus the

venous level of O2 times the blood flow

va CC

dtdm

Cdtdm

dtdVF

Blood flow, liters/min

(cardiac output)

Consumption of O2 (mL/min)

Arterial and venous

concentration of O2 (mL/L of blood)

dtdV

dtdmC


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Indicator Dilution with

Rapid Injection

A known amount of a substance, such as a dye or radioactive isotope,

is injected into the venous blood and the arterial concentration of the

indicator is measured through a serious of measurements until the

indicator has completely passed through given volume.

The cardiac output (blood flow) is amount of indicator injected,divided by average concentration in arterial blood.

t

dttC

mF

0

)(


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Dye Dilution

In dye-dilution, a commonly used dye is indocyanine green

(cardiogreen), which satisfies the following

Inert

Safe

Measurable though spectrometry Economical

Absorption peak is 805 nm, a wavelength at which absorption of blood isindependent of oxygenation

50%of the dye is excreted by the kidneys in 10 minutes, so repeat

measurements is possible


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Thermodilution

The indicator is coldsaline, injected into the right atrium using a

catheter

Temperature change in the blood is measured in the pulmonary

artery using a thermistor The temperature change is inversely proportional to the amount of

blood flowing through the pulmonary artery


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Measuring Cardiac Output

Several methods of measuring cardiac output In the Fick method, the indicator is O2; consumptionis measured by a spirometer. The arterial-venous concentration difference is measure by drawingsimples through catheters placed in an artery and in the pulmonary artery. In the dye-dilutionmethod, dye is injected into the pulmonary artery and samples are taken from an artery. In the

thermodilution method, cold saline is injected into the right atrium and temperature is measured inthe pulmonary artery.


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Electromagnetic

Flowmeters

Based on Faradays law of induction that a conductor that movesthrough a uniform magnetic field, or a stationary conductor placed

in a varying magnetic field generates emfon the conductor:

When blood flows in the vessel withvelocityu and passes through the magneticfield B, the induced emf emeasured at theelectrodes is.

L

de

0

LBu

For uniform B and uniform velocity profile u,the induced emf is e=BLu. Flow can be obtainedby multiplying the blood velocityuwith thevessel cross sectionA.


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Electromagnetic

Flowmeter Probes

Comes in 1 mm increments for

1 ~ 24 mm diameter blood vessels

Individual probes cost $500 each

Made to fit snuggly to the vessel

during diastole

Only used with arteries, not veins,

as collapsed veins during diastole

lose contact with the electrodes

Needless to say, this is an

INVASIVE measurement!!! A major advantage is that it can

measure instantaneous blood

flow, not just average flow


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Ultrasonic Flowmeters

Based on the principle of measuring the time it takes for an acoustic

wave launched from a transducer to bounce off red blood cells and

reflect back to the receiver.

All UT transducers, whether used for flowmeter or other

applications, invariably consists of a piezoelectric material, whichgenerates an acoustic (mechanical) wave when excited by an

electrical force (the converse is also true)

UT transducers are typically used with a gel that fills the air gaps

between the transducer and the object examined


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Transit time flowmeters

Effective velocity of sound in blood: velocity of sound (c)+

velocity of flow of blood averaged along the path of the ultrasound ()

=1.33 for laminar flow, =1.07 for turbulent flow: velocity of blood averaged over the cross sectional area, this is differentthan because the UT path is along a single line not over an averaged of

cross sectional area

Transit time in up/down stream direction:

Difference between upstream and downstream directions

cosvelocityconduction

distance

uc

Dt

2222

cos2

)cos(

cos2

c

uD

uc

uDt


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Transit Time

Flowmeters

The quantity T is typically very smalland very difficult to measure,

particularly in the presence of noise.

Therefore phase detection techniques

are usually employed rather then

measuring actual timing.


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Doppler

Flowmeters

The Doppler effect describes the change in the frequency of a

received signal , with respect to that of the transmitted signal, when

it is bounced off of a moving object.

Doppler frequency shift

c

uf

f

o

d

cos2

Speed of sound in blood

(~1500 m/s)

Angle between UT beam

and flow of blood

Speed of blood flow(~150 cm/s)

Source signalfrequency


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Directional Doppler

Directional Doppler borrows the quadrature phase detector

technique from radar in determining the speed and direction of an

aircraft.

Two carrier signals at 90 phase shift are used instead of a single

carrier. The +/- phase difference between these carriers after thesignal is bounced off of the blood cells indicate the direction,

whereas the change in frequency indicate the flowrate

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Measurement of flow&Volume of Blood.ppt