thermodilution

Thermodilution is a hemodynamic method used to measure cardiac output and other circulatory parameters by tracking how a known amount of indicator, typically cold saline, changes temperature as it passes through the cardiovascular system. The technique relies on the Stewart-Hamilton equation to relate the temperature-time curve measured downstream to the volume of blood pumped by the heart. It is most often performed with a catheter that includes a temperature sensor located in a central circulation bed, frequently the pulmonary artery.

Bolus thermodilution involves injecting a single, known bolus of cold saline through a central venous or right atrial access point. The temperature change is recorded by a nearby downstream sensor, usually in the right atrium or pulmonary artery. The resulting thermodilution curve is integrated to yield a cardiac output value, which can be repeated to assess trends or average measurements. Accuracy depends on the injectate temperature, injection speed, and proper catheter position.

Continuous thermodilution uses a catheter with a temperature sensor that enables ongoing monitoring, often in ICU settings. A controlled injection or intermittent thermal changes are analyzed in real time to provide a continuously updated cardiac output and trend data. Some systems also derive additional parameters related to preload and volume status, such as intrathoracic blood volume or global end-diastolic volume, though availability and interpretation vary by technology.

Applications include perioperative hemodynamic monitoring and critical care for patients with shock, heart failure, or complex fluid management needs. Limitations include invasiveness, risk of infection or thrombosis, catheter misplacement, and measurement inaccuracies from temperature variations, intracardiac shunts, severe valvular disease, or rapid hemodynamic changes.