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.
Syllabus: S2c
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.
Thus:
Principle:
Stewart-Hamilton equation:
Advantages and limitations:
However:
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Valentinuzzi, M. E., L. A. Geddes, and L. E. Baker. "A simple mathematical derivation of the Stewart-Hamilton formula for the determination of cardiac output." Medical and biological engineering 7.3 (1969): 277-282.
Kinsman, J. Murray, J. We Moore, and W. F. Hamilton. "Studies on the circulation: I. Injection method: physical and mathematical considerations." American Journal of Physiology-Legacy Content 89.2 (1929): 322-330.
Profant, M., K. Vyska, and U. Eckhardt. "The Stewart–Hamilton equations and the indicator dilution method." SIAM Journal on Applied Mathematics 34.4 (1978): 666-675.
Argueta, Erwin E., and David Paniagua. "Thermodilution Cardiac Output: A Concept Over 250 Years in the Making." Cardiology in Review 27.3 (2019): 138-144.
Frone, A., and V. Ganz. "Measurement of flow in single blood vessels including cardiac output by local thermodilution." Circulation Research 8.1 (1960): 175-182.
Harvey, Sheila, et al. "Assessment of the clinical effectiveness of pulmonary artery catheters in management of patients in intensive care (PAC-Man): a randomised controlled trial." The Lancet 366.9484 (2005): 472-477.
Hamilton, W. F., et al. "Comparison of the Fick and dye injection methods of measuring the cardiac output in man." American Journal of Physiology-Legacy Content 153.2 (1948): 309-321.
Laperche, Yannick, Marie-Claire Oudea, and Danielle Lostanlen. "Toxic effects of indocyanine green on rat liver mitochondria." Toxicology and applied pharmacology 41.2 (1977): 377-387.