The thermodilution measurement of cardiac output is notoriously error-prone, and the values generated by this method are only trustworthy if various conditions are met. This came up in Question 19 from the second paper of 2007, and has never been asked about again, which suggests that CICM are moving away from this subject matter. However, one cannot exclude the possibility that some Swan Ganz fanboys among the examiners are biding their time, waiting to release a curveball at the Fellowship candidates. Moreover, it would appear that PA catheters are still in use around the world, even in this echosonography-obsessed age, and so conceivably CICM trainees and fellows might at some stage find themselves faced with thermodilution cardiac output measurements in their actual patients, agonising over whether or not they are reliable.
The raw awesomeness of the PA catheter is discussed in greater detail elsewhere. For some theoretical background, thermodilution measurement of cardiac output by the PA catheter and the PiCCO device is described and critiqued in the CICM First Part revision section. For something even more theoretical, a discussion of the foundational principles behind indicator dilution measurement of blood flow is also available, irrelevant though it might be to the practical needs of the end-stage CICM trainee.
In summary:
- Errors of technique:
- Temperature of the injectate (should be consistent between measurements)
- Volume of the injectate (should be consistent between measurements)
- Concurrent infusion of IV fluids (underestimates CO)
- Timing with breath cycle (measurement should be performed at end-expiration)
- Catheter tip malposition (should have good blood flow)
- Misplaced thermistor (against the vessel wall, or near to the source of injectate)
- Concurrent use of electrocautery
- Technical errors
- Use of estimated rather than measured coefficients in the Stewart-Hamilton equation
- Sensitivity of the thermistor (device error range)
- Sampling rate of the thermistor (for integration of area under the curve)
- Cardiac abnormalities
- Extremely low cardiac output (ends up being overstimated)
- AF or other arrhythmias (interferes with pulse contour CO monitoring)
- Intracardiac shunt (underestimates CO)
- Tricuspid valvular regurgitation (underestimates CO)
- Extracardiac abnormalities
- Erratic respiration (renders spot measurements of CO unreliable)
- Haematocrit changes (interferes in the Stewart-Hamilton equation)
- Small body mass (requires adjustment of injectate volume)
- Hypothermia (requires a correction factor)
- Pulmonary oedema (minor source of interference)
One might notice that a lot of these sources of error have been eliminated by improvements in measurement technique and equipment, to say nothing of the near-total abandonment of invasive cardiac output monitoring in the Age of Echo. The reader will notice that most of the studies quoted below are from the early to mid-1980s. Modern devices and strategies have banished most of these concerns to the pages of exam papers and the hallways of major teaching institutions, where academic mastodons stomp and trumpet. For the rest of us, the contents of this grey box is probably enough, at least for a standard CICM question answer. If for whatever more detail is required, one may be referred to any number of excellent articles, such as Nishikawa & Dohi (1993) or Nadeau & Noble (1986).
There's the right way and there's the wrong way of performing a thermodilution measurement, with the wrong way generally generating numbers which will not be reproducible and which can give rise to errors in clinical decisionmaking. Much of this, over decades, has been ameliorated by the advent of automated thermodilution catheters and standardised techniques.
In contrast to the errors of technique, these are errors inherent in the measurement process. No matter how you standardise and control the technique, these errors will always be present.
These are unexpected or unaccounted-for changes in patient physiology which can create errors in the thermodilution measurement of cardiac output. This could be because they interfere in some way with the empirically derived (i.e. guessed) estimated variables plugged into the Stewart-Hamilton equation, or because they introduce an unexpected or unpredictable variation which derails the reliability of the measurement process.
Nishikawa, Toshiaki, and Shuji Dohi. "Errors in the measurement of cardiac output by thermodilution." Canadian Journal of Anaesthesia 40.2 (1993): 142-153.
Nadeau, Sophie, and William H. Noble. "Limitations of cardiac output measurements by thermodilution." Canadian Anaesthetists’ Society Journal 33.6 (1986): 780-784.
Gefen, Nurit, et al. "Experimental assessment of error sources in thermodilution measurements of cardiac output and ejection fraction." Proceedings of the First Joint BMES/EMBS Conference. 1999 IEEE Engineering in Medicine and Biology 21st Annual Conference and the 1999 Annual Fall Meeting of the Biomedical Engineering Society (Cat. N. Vol. 2. IEEE, 1999.
Stevens, John H., et al. "Thermodilution cardiac output measurement: Effects of the respiratory cycle on its reproducibility." Jama 253.15 (1985): 2240-2242.
WETZEL, RANDALL C., and TERRY W. LATSON. "Major errors in thermodilution cardiac output measurement during rapid volume infusion." Anesthesiology: The Journal of the American Society of Anesthesiologists 62.5 (1985): 684-687.
Elkayam, Uri, et al. "Cardiac output by thermodilution technique: Effect of injectate’s volume and temperature on accuracy and reproducibility in the critically ill patient." Chest 84.4 (1983): 418-422.
van Grondelle, Albertus et al. "Thermodilution method overestimates low cardiac output in humans." American Journal of Physiology-Heart and Circulatory Physiology 245.4 (1983): H690-H692.
Alfieri, O., J. Agosti, and S. Subramanian. "Thermodilution cardiac output measurement in infants and small children following intracardiac surgery." Journal of pediatric surgery 10.5 (1975): 649-656.
Wyse, S. D., et al. "Measurement of cardiac output by thermal dilution in infants and children." Thorax 30.3 (1975): 262-265.
Keller, R., N. Goettel, and Karim Bendjelid. "Transpulmonary thermodilution curve and the cross-talk phenomenon." Medicina intensiva 36.6 (2012): 446-448.
Mackenzie, J. D., N. E. Haites, and J. M. Rawles. "Method of assessing the reproducibility of blood flow measurement: factors influencing the performance of thermodilution cardiac output computers." Heart 55.1 (1986): 14-24.+
Stetz, Christian W., et al. "Reliability of the thermodilution method in the determination of cardiac output in clinical practice." American Review of Respiratory Disease 126.6 (1982): 1001-1004.
Merrick, Scot H., Eugene A. Hessel II, and David H. Dillard. "Determination of cardiac output by thermodilution during hypothermia." The American Journal of Cardiology 46.3 (1980): 419-422.
Nishikawa, T., and S. Dohi. "Haemodynamic changes associated with thermodilution cardiac output determination during myocardial ischaemia or pulmonary oedema in dogs." Acta anaesthesiologica scandinavica 36.7 (1992): 679-683.
Weyland, A., et al. "The effect of an intracardiac left-right shunt on thermodilution measurements of cardiac output. An extracorporeal circulation model." Der Anaesthesist 44.1 (1995): 13-23.
Freed, Michael D., and John F. Keane. "Cardiac output measured by thermodilution in infants and children." The Journal of pediatrics 92.1 (1978): 39-42.
Pearl, Ronald G., and Lawrence C. Siegel. "Thermodilution cardiac output measurement with a large left-to-right shunt." Journal of clinical monitoring 7.2 (1991): 146-153.