How is thermodilution cardiac output measurement performed?

• A. Inject a known temperature saline bolus into the right atrium and measure the temperature change downstream to calculate CO
• B. Inject contrast dye into coronary arteries
• C. Use ultrasound to measure flow
• D. Rely on heart rate to calculate CO

Answer: (A)

Thermodilution cardiac output measurement relies on tracking a known-temperature injectate as it travels with the blood and using the resulting temperature change to calculate flow. A small, known volume of cold saline is injected into the venous system, typically the right atrium through a central venous/PA catheter. As this bolus mixes with the blood and moves downstream to the pulmonary artery, a thermistor downstream in the catheter records the transient drop in temperature over time. The temperature–time curve (the bolus curve) is analyzed with the Stewart–Hamilton equation, which relates the injectate volume and temperature change to the cardiac output. In practice, multiple injections are averaged to reduce variability. This approach is specific to thermodilution; other options describe different imaging or estimation methods rather than the thermodilution calculation.