In Fick's original method, the following variables are measured:
* V̇O, oxygen consumption in ml of pure gaseous oxygen per minute. This may be measured using a spirometer within a closed rebreathing circuit incorporating a CO absorber
* C, the oxygen content of blood taken from the pulmonary vein (representing oxygenated blood)
* C, the oxygen content of blood from an intravenous cannula (representing deoxygenated blood)