What does the Fick principle relate to cardiac output measurement?

• A. Cardiac output equals heart rate multiplied by stroke volume
• B. Cardiac output equals oxygen consumption divided by the arteriovenous oxygen content difference
• C. Cardiac output equals right atrial pressure minus left atrial pressure
• D. Cardiac output equals pulmonary artery systolic pressure

Answer: (B)

The Fick principle ties cardiac output to how much oxygen the body uses and how much oxygen is extracted from the blood. It says tissues take up a certain amount of oxygen from the blood as it passes through the circulation, so the rate of oxygen consumption (VO2) equals the product of cardiac output (CO) and the arteriovenous difference in oxygen content (CaO2 − CvO2). Rearranging gives CO = VO2 / (CaO2 − CvO2). In practice, this means you measure the body's oxygen consumption and the oxygen content of arterial blood and mixed venous blood to calculate flow.

The other statements don’t use the oxygen-content difference to determine flow, so they aren’t the Fick calculation. For example, linking cardiac output to heart rate times stroke volume is a fundamental relationship for flow, but it’s not the Fick method of measuring CO; and relying on pressure values alone (such as atrial or pulmonary artery pressures) does not directly give cardiac output.