The injection of a quantity (2.5, 5, or 10 mL) of fluid that is below body temperature (usually room temperature or iced) into the right atrium changes the temperature of blood in contact with the thermistor at the tip of the PA catheter.

The degree of change is inversely proportionate to CO: Temperature change is minimal if there is a high blood flow, whereas temperature change is greater when flow is reduced. After injection, one can plot the temperature as a function of time to produce a thermodilution curve.

Comparison of thermodilution curves after injection of cold saline into the superior vena cava. The peak temperature change arrives earlier when measured in the pulmonary artery (a) than if measured in the femoral artery (b). Thereafter, both curves soon reapproximate baseline. (Redrawn and reproduced, with permission from Reuter D, Huang C, Edrich T, et al: Cardiac output monitoring using indicator dilution techniques: basics, limits and perspectives. Anesth Analg 2010;110:799.)

CO is determined by a computer program that integrates the area under the curve. Accurate measurements of CO depend on rapid and smooth injection, precisely known injectant temperature and volume, correct entry of the calibration factors for the specific type of PA catheter into the CO computer, and avoidance of measurements during electrocautery.

Tricuspid regurgitation and cardiac shunts invalidate results because only right ventricular output into the PA is actually being measured. Rapid infusion of the iced injectant has rarely resulted in cardiac arrhythmias.

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