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