Fick Principle for Oxygen Consumption

Adolf Fick set this out in 1870, and the reasoning is pure conservation. Whatever oxygen the body consumes must have been carried there by blood, so the consumption equals the flow multiplied by the oxygen the blood gives up on the way through. A resting adult with a cardiac output of 5 L/min and an arteriovenous difference of 5 mL O₂ per decilitre consumes $5 \times 5 \times 10 = 250$ mL of oxygen a minute, which is the standard resting figure.

The factor of 10 is the only fussy part, and it is a unit conversion: oxygen contents are quoted per decilitre while flow is quoted per litre, and there are ten decilitres in a litre. Leave it out and the answer is off by a factor of ten, which is the single most common error on this equation.

In practice the relation is almost always run backwards, to get cardiac output from a measured oxygen consumption and two blood gases. That is the reference method against which thermodilution and echocardiography are compared, and it remains the most accurate bedside estimate available. Its weakness is that a true consumption measurement requires a metabolic cart, and many catheter labs substitute an assumed value from a table based on age, sex and body size. Those assumed values can be off by 25 percent in a critically ill or sedated patient, and the error passes straight through into the cardiac output. The venous sample also has to be genuinely mixed venous, from the pulmonary artery, since a peripheral vein reports only the tissue bed it drained.