The Fick principle is commonly applied to cardiac output calculations. It essentially states that blood flow to an organ can be calculated from a marker substance if the following is known:
- Amount of marker substance taken up by organ per unit time
- Concentration of marker in arterial blood supplying organ
- Concentration of marker in venous blood leaving the organ
This can be applied to the measure cardiac output where the marker substance is oxygen.
In other words the amount of a substance taken up by an organ per unit time is equal to the arterial concentration minus the venous concentration, divided by the production or uptake.
Therefore the cardiac output is inversely proportional to the arteriovenous difference of the marker substance.
In reality this method is cumbersome and rarely used as measuring oxygen uptake is difficult in a clinical situation. The Fick determination is a derived version of the Fick principle where a standard oxygen uptake is used based on the assumption that the oxygen consumption is 125ml of O2 per minute perm² of body surface area. The average body surface area for an adult male is 2m², giving a rough oxygen consumption of 250mls per minute.
This method can also be used to estimate renal blood flow, but instead of oxygen a marker such as para-aminohippuric acid or inulin is used.