This is possible since human anatomy places the veins close to the surface of the body. Once the venous flow is occluded, arterial inflow will change the volume of the extremity. In the chamber, temperature is constant. Chamber Plethysmography technique applied to a leg to measure the blood flow below the knee. Thermodilution Method for Flow Measurements Thermodilution method allows an accurate measurement of average flow in a vessel.
Technique is simple.
A heater element and a temperature sensor are placed in blood vessel, usually using a catheter. Heater element is upstream, and the temperature sensor is downstream. Operator first allows the temperature sensor to stabilize and obtains a baseline measurement. Afterwards, the heater is turned, which produces a known amount of heat per unit time. Changes in the measured temperature is inversely related to the blood flow.