Question 10

Compare and contrast two methods of measuring cardiac output.

Methods of measuring cardiac output

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College Answer

Good answers began with a definition of cardiac output. For each method, it was expected that
candidates discuss the theoretical basis, equipment, advantages and disadvantages / sources
of error and limitations. Additional marks were awarded when an attempt was made to compare
and contrast the two methods (often helped by the use of a table).

Discussion

Thought it might seem disrespectfully wasteful of the readers' time to take up this answer space with a rant, somebody has to explain to them (who may be future CICM examiners) how unfair it is to expect something in the answer if you didn't ask for it in the question. Not everybody would immediately start their response to this SAQ with a definition of the cardiac output: most people would just tabulate the differences and similarities between two methods of measurement, as they were asked. Carrying on with the theme of bizarrely misstated expectations, the examiners applauded additional marks being awarded to people who compared and contrasted the two methods, as if it were some secret extra credit assignment, even though the question specifically asks them to "compare and contrast two methods". 

So, what would an answer look like if it answered the actual question? Hopefully, this would have scored enough marks to pass:

Comparison of Direct Fick and Thermodilution Methods

for Cardiac Output Measurement

Method  Fick method Indicator dilution
Theoretical basis

Total uptake of oxygen by the body is equal to the product of the cardiac output and the arterial-venous oxygen content difference:

CO = VO2 / (Ca - Cv)

Cardiac output is calculated from the dose of indicator and the area under the concentration-time curve, measured by a downstream detector:

V̇ = m/Ct

Equipment
  • Flowmeter, mask, collector bag to measure VO2
  • PA catheter for mixed venous blood sampling
  • Arterial catheter for arterial blood sampling
  • Blood gas analyser
  • PA catheter or CVC for injection of indicator
  • Detector (eg. thermistor) in a pulmonary or systemic artery
  • Central processor to perform calculations and report values
Advantages
  • "Gold standard"
  • Good accuracy
  • Necessary invasive devices are often already available in ICU patients
  • Does not require mixed venous blood
  • Numerous indicator options (eg. thermodilution)
  • Reasonable accuracy
Sources of error and limitations
  • Requires stable CO over some minutes
  • Highly invasive (requires PAC and arterial line)
  • Requires cumbersome VO2 measuring equipment
  • Use of estimated instead of measured variables("indirect Fick method")
  • Accuracy is highly technique-dependent
  • Rendered inaccurate by intacardiac shunts and valve disease
  • Accuracy is reduced by estimated coefficients in the equation
  • Must be times with respiratory cycle (measure at end-expiration)

References

Ehlers, Kevin C., et al. "Cardiac output measurements. A review of current techniques and research." Annals of biomedical engineering 14.3 (1986): 219-239.

Lavdaniti, M. "Invasive and non-invasive methods for cardiac output measurement." International Journal of Caring Sciences 1.3 (2008): 112.

Jhanji, S., J. Dawson, and R. M. Pearse. "Cardiac output monitoring: basic science and clinical application." Anaesthesia 63.2 (2008): 172-181.