In Fick's original method, the "organ" was the entire human body and the marker substance was oxygen.
[citation needed] In Fick's original method, the following variables are measured:[2] From these values, we know that: where This allows us to say and hence calculate cardiac output.
[citation needed] In reality, this method is rarely used due to the difficulty of collecting and analysing the gas concentrations.
[citation needed] A commonly used value for O2 consumption at rest is 125 mL O2 per minute per square meter of body surface area.
Using this value in the above equation, the oxygen concentration of mixed venous blood is approximately 150 mL of O2 per L. Therefore, using the assumed Fick determination, the approximated cardiac output for an average man (1.9 m3) is: Cardiac output may also be estimated with the Fick principle using production of carbon dioxide as a marker substance.