The Fick Principle Cardiac Output calculation is the physiological gold standard for determining exactly how many liters of blood the heart is pumping per minute.
The Logic of the Equation
Imagine a delivery truck dropping off packages. If you know exactly how many packages were dropped off (Oxygen Consumption, VO2), and you know how many packages were in the truck when it left the warehouse (Arterial O2 Content) minus how many were left when it returned (Venous O2 Content), you can calculate exactly how many trips the truck made.
Cardiac Output = VO2 / (Arterial O2 Content - Venous O2 Content)
The Variables
- VO2: The body's total oxygen consumption per minute.
- CaO2 (Arterial Content): The amount of oxygen leaving the lungs, heavily dependent on Hemoglobin and SaO2.
- CvO2 (Venous Content): The amount of oxygen returning to the heart. If the heart is failing (low cardiac output), the blood moves very slowly, and the tissues extract almost all the oxygen, resulting in a drastically low CvO2.
By measuring these parameters during a cardiac catheterization, cardiologists obtain an indisputable measurement of the heart's pumping capability.