V̇O may also be calculated by the Fick equation:
, when these values are obtained during exertion at a maximal effort. Here Q is the cardiac output of the heart, CO is the arterial oxygen content, and CO is the venous oxygen content. (CO – CO) is also known as the arteriovenous oxygen difference.
The Fick equation may be used to measure V̇O in critically ill patients, but its usefulness is low even in non-exerted cases. Using a breath-based VO to estimate cardiac output, on the other hand, seems to be reliable enough.