Measurements & Monitoring

Prof. Abdulhamid Al-Saeed, FFARCSIChairman Anaesthesia Department

College of MedicineKing Saud University

Pulse Oximetry

Physical PrincipleWithin the probe are two light emitting diodes (LED's), one in the visible red

spectrum (660nm) and the other in the infrared spectrum (940nm). The beams of light pass through the tissues to a photodetector. During passage through the tissues, some light is absorbed by blood and soft tissues depending on the concentration of haemoglobin. The amount of light absorption at each light frequency depends on the degree of oxygenation of haemoglobin within the tissues

Microprocessor can select out the absorbance of the pulsatile fraction of blood

Within the oximeter memory is a series of oxygen saturation values obtained from experiments performed in which human volunteers were given increasingly hypoxic mixtures of gases to breath. The microprocessor compares the ratio of absorption at the two light wavelengths measured with these stored values, and then displays the oxygen saturation digitally as a percentage and audibly as a tone of varying pitch. As it is unethical to desaturate human volunteers below 70%, it is vital to appreciate that oxygen saturation values below 70% obtained by pulse oximetry are unreliable.

A pulse oximeter gives NO information on any of these other variables:

• The oxygen content of the blood • The amount of oxygen dissolved in the blood • The respiratory rate or tidal volume i.e.

ventilation • The cardiac output or blood pressure

Incomptencies

Critically ill with poor peripheral circulation Hypothermia & VC Dyes ( Nail varnish ) Lag Monitor Signalling 5-20 sec

PO2 Cardiac arrhythmias may interfere with the oximeter

picking up the pulsatile signal properly and with calculation of the pulse rate

Abnormal Hb ( Met., carboxy)

Capnography

• Capnography is the graphic display of instantaneous CO2 concentration versus time (Time Capnogram)

• Or expired volume (Volume Capnogram) during a respiratory cycle.

• Methods to measure CO2 levels include infrared spectrography, Raman spectrography, mass spectrography, photoacoustic spectrography and chemical colorimetric analysis

Physical Principle

• The infrared method is most widely used and most cost-effective.

• Infrared rays are given off by all warm objects and are absorbed by non-elementary gases (i.e. those composed of dissimilar atoms), while certain gases absorb particular wavelengths producing absorption bands on the IR electromagnetic spectrum.

• The intensity of IR radiation projected through a gas mixture containing CO2 is diminished by absorption; this allows the CO2 absorption band to be identified and is proportional to the amount of CO2 in the mixture.

Types

Side stream Capnography• The CO2 sensor is located in the main unit itself (away

from the airway) and a tiny pump aspirates gas samples from the patient’s airway through a 6 foot long capillary tube into the main unit.

• The sampling tube is connected to a T-piece inserted at the endotracheal tube or anaesthesia mask connector Other advantages of the side stream capnograph

• No problems with sterilisation, ease of connection and ease of use when patient is in unusual positions like the prone position

Main stream Capnograph • Cuvette containing the CO2 sensor is inserted between the

breathing circuit and the endotracheal tube. • The IR rays traverse the respiratory gases to an IR detector within

the cuvette.• To prevent condensation of water vapour, which can cause falsely

high CO2 readings, all main stream sensors are heated above body temperature to about 40oC.

• It is relatively heavy and must be supported to prevent endotracheal tube kinking.

• Sensor’s window must be kept clean of mucus and particles to prevent false readings.

• Response time is faster

The Alpha angle• The angle between phases II and III, which

has• increases as the slope of phase III

increases. • The alpha angle is an indirect indication of

V/Q• status of the lung. • Airway obstruction causes an increased• slope and a larger angle. • Other factors that affect the angle are the

response time of the capnograph, sweep speed, and the respiratory cycle time.

The Beta angle• The nearly 90 degrees angle between phase

III and the descending limb in a time capnogram has been termed as the beta angle.

• This can be used to assess the extent of rebreathing. During rebreathing, there is an increase in beta angle from the normal 90 degrees.

Clinical Applications

Monitoring NMJ

DEPOLARISING BLOCK

• Fasiculation

• No tetanic fade

• No post-tetanic potentiation

• Anticholinesterases increase block

• Potentiation by other depolarisers May develop Phase 2 block

NON-DEPOLARISING BLOCK

• No fasiculation

• Tetanic fade

• Post-tetanic facilitation

• Anticholinesterases decrease block

• Antagonism by other depolarisers No change in character of block

Train of four (TO4) • Fade is prominent with non-depolarising blockers and at 0.5 Hz is greatest

by the 6th twitch. Using four twitches at 0.5 second intervals (TO4) was popularised by Ali and from these the ratio of T4/T1 (the "TO4 Ratio") can be derived. The degree of paralysis is estimated from the number of twitches present, or if four are present the TO4 ratio.

• Counting the number of palpable twitches is quite a good guide to deeper levels of paralysis; two or more twitches usually implies reasonably easy reversal and some return of muscle tone, while virtually no response suggests difficulty with reversal, weak cough at best, and very little muscle tone.

• TO4 ratios around 0.25 are commonly estimated at between 0.1 and 0.7, while at 0.5 some 40% of and at 0.7 fewer than 10% of observers can reliably detect any fade at all. Consequently the presence of any detectable fade indicates the presence of some paralysis and furthermore even if all four twitches appear normal many patients are in fact partly paralysed.

• It cannot be used to assess very deep levels of block (no T1!) and is not very sensitive to assessing adequacy of reversal.

Dual Burst Stimulation (DBS) • 50Hz train of 3 repeated 0.75 seconds later by an identical train of

three. Each group of three twitches results in one twitch, and hence only two twitches available for comparison. Since the first twitch sums T1, T2 and T3, while the second sums T4, T5, and T6, it is easy to see how the presence of fade would be easier to notice and there is data to support this. As the level of block increases, response to the second burst is lost as the third twitch of TO4 is lost; the first burst is retained until a little after you lose all response to TO4. Surgical paralysis is generally OK if only one response is present; the patient is reversible if two are present, particularly if the second is strong. TO4 is better for quantifying the intensity of "surgical" paralysis, whereas DBS is better for noting persistance of fade after reversal. If you use NMB's so that there is just no response to DBS, the patient will be a little more paralysed than if there was just no response to TO4.

• Tetanic stimulation • Continuous stimulation at either 50 or 100 Hz is so painful as to

preclude its use in conscious patients, and is difficult to quantify, but is probably the most useful and emulates physiological maximal responses. Tetany is more sensitive to both residual and deep paralysis than any other form of monitoring. The presence of any persisting strength during tetany is a good indicator of the patient's ability to maintain muscle tone.

• Comparing two bursts of tetany (each 3-5 seconds long) with a gap of 3 seconds results in post-tetanic potentiation of the response to the second burst. When assessing adequacy of reversal the initial part of the second response (potentiated) can be compared to the last part of the first (faded).

• If fade is present it is becomes more obvious with this rather than any other method.

• Post-Tetanic Count (PTC) • This consists of counting 1 Hz twitches 3 seconds after 5

seconds of 50Hz tetany and can give an approximate time to return of response to single twitches and hence permits assessment of block too deep for any other technique. A Post-Tetanic Count (PTC) of 2 by palpation suggests no twitch response for about 20-30 minutes, PTC of 5 about 10-15 minutes.

• This is clearly the best method for monitoring paralysis for patients in whom you seek to prevent diaphragmatic movement, ie micro-neurosurgery; it is best to use infusions of drugs and aim for PTC of 2.

Arterial Blood Pressure

Damping is the tendency of the system to resist oscillations caused by sudden changes

• Overdamping The waves tend to faltten thus underestimating systolic reading and Overestimating diastolic reading

• Underdamping magnify the waves with overshooting, thus overestimating systolic reading and uinderestimating diastolic reading

Factors causing Overdamping

1- Narrow tubing

2- Long elastic tubings(Compliant )

3- High density fluid

4- Air bubbles

5- Clot formation

Central Venous Pressure

Pulmonary Artery Catheter

Haemodynamic Profiles Obtained from PA Catheters

• SV = CO / HR (60-90 mL/beat)

• SVR = [(MAP – CVP) / CO] 80 (900-1500 dynes-sec/cm5)

• PVR = [(MPAP – PCWP) / CO] 80 (50-150 dynes-sec/cm5)

• O2 delivery (DO2)

= C.O. O2 content• Arterial O2 content (CaO2)

= ( Hb 1.38 ) (SaO2)• Mixed venous O2 content (CvO2)

= ( Hb 1.38 ) (SvO2)• O2 consumption (VO2)

= C.O. (CaO2-CvO2)

SvO2 = SaO2 – [VO2 / (Hb 13.8)(CO)]

ECG

Electrocardiogram

• Displays the overall electrical activities of the myocardial cells– Heart rate & dysrhythmias– Myocardial ischaemia– Pacemaker function– Electrolyte abnormalities– Drug toxicity

• Does NOT indicate mechanical performance of the heart:– Cardiac output– Tissue perfusion

Full (12)-lead ECG– Standard limb leads (bipolar)– Precordial leads (unipolar)

5-lead system– Unipolar + bipolar– RA, LA, RL, LL, C3- lead system

Bipolar with RA, LA, LLV5 usually used

– Best compromise between detecting ischaemia and diagnosing arrhythmia

May come with ST-segment analysis

ECG

Standard Limb Leads

Unipolar Chest Leads

Artifacts in ECG Monitoring

• Loose electrodes or broken leads• Misplaced leads• Wrong lead system selected• Emphysema, pneumothorax,

pericardial effusion• Shivering or restlessness• Respiratory variation and

movement• Monitor Pulse Oximetry, Invasive

ABP

Question NO. 8• 1- Identify the monitor Tracing?•

………………………………………

• 2- What is the Name & Cause of the Notch on the descending limb of the trace?

•………………………………………………………………………………

• 3- Name two different Clinical informations could be interpreted from this tracing?

• a) ……………………………..• b) ……………………………..

Question NO. 10

• 1- Identify the Rhythm in the shown ECG Strip?

• ------------------------------------------------------• 2- What is your first line of management in case

of Unstable patient•

…………………………………………………………

• 3- What is the normal QRS duration•

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• 1- Identify the tracing•

………………………………………………………………………………………

• 2- Name the different phases of the trace• I ………………………• II ……………………..• III …………………….• IV ……………………..• 3- What different clinical informations

could be interpreted from the trace• a)

………………………………………………..• b)

………………………………………………..

Question NO. 14

• 1- Name the different waves on the trace?• ------------------------------------------------• 2- Define Central Venous Pressure?•

……………………………………………………•

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• 3- What are the main determinants regulating CVP?

• A-………………………………….• B- ………………………………...

Question NO. 15

• brief the mechanism of action of this monitor :• ……………………………………………………………………………

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• Name 4 factors affecting the accuracy of this monitor?• ……………………………………………………………………………

…• ……………………………………………………………………………

…• ……………………………………………………………………………

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• If P50 of oxyhemoglobine dissociation curve is 40; is this curve shifted to the right or left; mention 3 possible causes?

• …………………………………………………………………………• …………………………………………………………………………• …………………………………………………………………………..

Question NO. 19

• 36-Each of the following factors may lead to error in readings using pulse oximetry EXCEPT:A. electrocauteryB. high cardiac output statesC. infrared lights near the sensorD. intravenous dyesE. severe hemodilution