Cardiac output: Fick Principle

Developed by Adolf Fick, the Fick Principle can determine cardiac output if an investigator knows what amount of a substance is taken up by an organ (or whole body) in a unit of time, the concentration of the substance in the arterial blood supplying the organ (or whole body), and the concentration of the venous blood leaving the organ (or returning from the body). Most commonly, oxygen is the substance used for calculations. Stated simply, Cardiac output equals organ oxygen consumption divided by the arteriovenous difference in oxygen.

If cardiac output were known, the principle could be applied to solve for any of the other variables.

Stated as an equation, the Fick Principle is thus: VO₂ = (CO x Ca) – (CO x Cv) (CO = Cardiac output, Ca = Oxygen content of arterial blood, Cv = oxygen content of venous blood, and VO₂ = oxygen consumption)

Rearranged to solve for CO, CO = VO₂/(Ca – Cv).

VO₂ can be determined by measuring the difference inspired and expired O₂, which can be done by collecting exhaled gas and comparing oxygen content with inhaled gas.

Given the logistics involved in accurately measuring VO₂, resting VO₂ is often estimated using either 3.5 mL of oxygen per kilogram per minute, or 125 mL of oxygen per square meter of body surface area per minute.

The accuracy of these estimates and their clinical usefulness is often questioned, with Narang et al demonstrating poor agreement between estimated and measured VO₂, particularly in obese study participants.