Describe the principles, and limitations, of the measurement of cardiac output using an indicator dilution technique
Most candidates chose to describe the thermodilution technique of cardiac output
measurement. Descriptions of other techniques and indicators such as dye dilution
using indocyanine green were acceptable alternatives.
Better answers included a description of the Fick Principle and the fact that it is
based on the law of conservation of matter. For thermodilution, heat lost from the
blood = heat gained from the injectate. Also required were an accurate description of
the technique, a description of the indicator-time curve and errors encountered in the
technique. For thermodilution these included the requirement for a Swan Ganz
catheter, nature and temperature of the injectate, temperature measurement using a
thermistor in the pulmonary artery and an appreciation that it is the curve of a
decrease in temperature versus time that is being analysed.
Recommended sources: Anaesthesia, Miller, Chp 40
"Better answers included a description of the Fick Principle", they say. One might raise an eyebrow at this, as in fact the college examiners appear to have penalised any reference to the Fick method in Question 19 from the first paper of 2014, where an explanation of thermodilution was called for. However, this makes sense, as the thermodilution (or any other indicator dilution) method relies on the Fick principle, but does not involve the Fick method.
The Fick method is where you collect the exhaled oxygen in a bag to calculate the VO2, and measure the arterio-venus oxygen difference to calculate the cardiac output. The Fick principle is the theoretical basis of this measurement, which simply points out the relationship between the cardiac output and the concentration difference of a marker substance between an upstream and downstream points in the blood flow. If you know the dose of injected marker, the principle states, you can calculate the cardiac output from the concentration difference - which is basically what indicator dilution does.
- Rate of bloodflow can be determined from the rate of change in the concentration of substance after a known amount of it has been added to the bloodstream
- V̇ = m/Ct,
- V̇ = flow, or cardiac output
- C = concentration
- m = dose of the indicator, and
- t = time
- or, Cardiac output = indicator dose / area under the concentration-time curve
Advantages and limitations:
- Access to mixed venous blood and arterial blood is not essential
- It is convenient: with electronic calculations, cardiac output measurement can be automated and continuous
- Good correlation with gold standard measurements of cardiac output
- Use of dye limits the frequency and repeatability of measurements, as it produces recirculation, and even the most rapidly cleared dyes are cleared after some minutes.
- Manual integration of the area under the concentration/time curve is laborious
- Automated calculation of cardiac output involves the use of correction factors and coefficients, which reduces its accuracy
- The method relies on uniform mixing of blood and unidirectional flow
- Thermodilution measurements have numerous potential sources of error
- Under laboratory conditions, agreement between this method and the direct Fick method is within a margin of 25%
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