Thermodilution is a medical technique used to measure blood flow in the heart. It involves injecting a known amount of a cold fluid into the bloodstream and then measuring the temperature change downstream. This temperature change helps calculate the cardiac output, which is the volume of blood the heart pumps in a minute. The procedure is often performed during a cardiac catheterization or in intensive care settings. It provides valuable information about heart function and can help diagnose conditions like heart failure or assess the effectiveness of treatments.