Cardiac Output, Thermodilution Measurement of

Abstract

If a substance is introduced at a constant rate into the flowing bloodstream and allowed to “mingle” with the blood, then the measured steady‐state concentration of that substance downstream of the site of introduction is inversely proportional to the flow rate. If a small amount of the substance is introduced rapidly, then the concentration integrated over time is inversely proportional to flow rate. In thermodilution the indicator substance is heat (or cold) whose concentration is measured using a thermistor on a pulmonary artery catheter. The indicator is typically 10 mL of iced or room temperature saline introduced rapidly as a bolus into the right atrium, producing a temperature transient that rises rapidly and then decay exponentially. In practice, the indicator recirculates before the transient returns to baseline and an extrapolation needs ato be made. There are other practical considerations, such as indicator warming during injection, baseline temperature fluctuations, variations in curve shape, variation in thermal properties of blood, and anatomical abnormalities that can affect the accuracy and reproducibility of the measurement. All told, the accuracy is to within 2–7%, when compared to other measurement techniques. Continuous measurements are possible using a filament heater on the catheter to produce small thermal fluctuations and sophisticated signal processing to extract the signal from the background noise. Pulmonary artery catheters have many uses and are placed in patients for hemodynamic and cardiac assessments beyond cardiac output. Catheters and cardiac output computers are available from a number of manufacturers. The bolus thermodilution measurement of cardiac output is convenient and practicle and is considered the “gold” standard by which other techniques are compared.