termodilution

Thermodilution is a physiological measurement method used to determine cardiac output and related hemodynamic parameters by tracking the dilution of an indicator as it mixes with the bloodstream. The technique relies on the indicator-dilution principle: a known quantity of a cold indicator is injected into the circulation, and a downstream temperature sensor records the resultant change in blood temperature over time. The recorded temperature-time curve, often called a thermodilution curve, is analyzed with the Stewart-Hamilton equation to derive cardiac output. In general, a larger injection temperature difference and a smaller area under the thermodilution curve indicate a higher cardiac output.

Procedure and calculation are typically performed with a catheter-based setup. A small volume of cold saline (commonly 5 ml at 0–4°C) is rapidly injected into a central vein, usually via a venous catheter that leads to the right side of the heart. A thermistor or temperature sensor placed in the pulmonary artery or near the catheter tip measures the temperature change downstream. The resulting curve is used to calculate cardiac output, which is expressed in liters per minute. To improve accuracy, multiple injections are often averaged, and the measurement may be repeated under different hemodynamic conditions.

Clinical use and limitations are well established. Thermodilution is widely employed in critical care, anesthesiology, and cardiac surgery to monitor cardiac output and other dynamic parameters. It provides information on preload and ventricular performance and can guide fluid management and vasoactive therapy. Limitations include potential inaccuracies in the presence of intracardiac shunts, significant tricuspid regurgitation, rapid injections, arrhythmias, or line/air-bubble issues. Invasive requirements include a catheter, typically a pulmonary artery catheter, which carries risks such as infection or vascular injury. Continuous thermodilution variants exist, using repeated injections to provide ongoing estimates of cardiac output.