Physics and Monitoring in ICU – Part 2
Dr Swapnil Pawar
MEASUREMENT AND EQUIPMENT 2
Written by – Dr Madhuri Anupindi
Describe how the values for pao2, paco2, ph and bicarbonate are determined on a blood gas sample
All three of these electrodes must be kept at 37 degrees.
Define and explain damping, resonance, critical damping, optimum damping and natural frequency.
Outline the components required to measure BP from an intra-arterial catheter. List other information (other than blood pressure) may be obtained from an arterial line trace?
The components required to measure blood pressure from an intra-arterial catheter include:
Other information that can be obtained from an arterial line trace includes:
What is the mechanism of oscillometric non-invasive BP measurement. What factors affect accuracy and what are the advantages and disadvantages?
Oscillometric devices use the fact that blood flowing through an artery between systolic and diastolic pressures causes vibrations in the arterial wall which can then be detected and transduced into electrical signals.
Factors affecting accuracy
Describe principles and limitations of measurement of cardiac output using a thermodilution technique.
The thermodilution technique is a variation on the indicator dilution technique of cardiac output measurement, and is generally thought to be the gold standard for measurement of cardiac output. Traditionally this technique requires a pulmonary artery catheter but this technique is also now utilised by PiCCO. A known volume of cold fluid with a known temperature is injected rapidly into the right atrium port of the PAC, this should then decrease the temperature in the pulmonary artery. The temperature of the blood is measured within the pulmonary artery by the thermistor at the end of the PAC, and a temperature over time curve is produced. The area under the curve is then used to calculate the cardiac output using the modified Stewart-Hamilton equation. The change in temperature in the pulmonary artery over time should be inversely proportional to the rate of blood flow, that is the cardiac output. Thus, the mean decrease in temperature over time is inversely proportional to the cardiac output.
Limitations of measurement technique include:
Briefly outline the principles behind CO measurement using pulse wave contour analysis
Pulse wave contour analysis analyses the arterial waveform and uses the principle that aortic pulse pressure is proportional to stroke volume and inversely proportional to aortic compliance. It integrates the area under the systolic part of the arterial pressure waveform which is assumed to be proportional to stroke volume. Cardiac output is then calculated as it equals stroke volume multiplied by heart rate. PiCCO system uses a combination of pulse wave contour analysis and calibration with thermodilution to calculate cardiac output
All pulse contour analysis systems rely on an optimal arterial signal so under or over damped traces will affect the accuracy. Similarly, arrhythmias, aortic regurgitation and the use of intra-aortic balloon pumps will affect the contour of the arterial waveform and thus the calculation of cardiac output.
Why did the sceptic develop hypertension? Because he took everything with a pinch of salt.