Explain the Fick principle used to calculate cardiac output.

• A. Cardiac output equals heart rate times stroke volume.
• B. Cardiac output equals mean arterial pressure divided by systemic vascular resistance.
• C. Cardiac output equals oxygen content difference divided by arterial oxygen content.
• D. Cardiac output equals oxygen consumption divided by the arteriovenous oxygen content difference (CO = VO2 / (CaO2 − CvO2)).

Answer: (D)

The Fick principle links oxygen use to blood flow: the amount of oxygen the body consumes each minute (VO2) equals the cardiac output (CO) times the amount of oxygen extracted from the blood per liter (the arteriovenous O2 content difference, CaO2 − CvO2). In other words, VO2 = CO × (CaO2 − CvO2), so CO = VO2 / (CaO2 − CvO2). Here CaO2 is the oxygen content in arterial blood and CvO2 is the oxygen content in mixed venous blood; these contents reflect both hemoglobin-bound oxygen and dissolved oxygen. Because the contents are typically expressed per 100 mL of blood, you adjust to obtain CO in liters per minute (the difference times 10 gives the per-liter amount). For example, if the body uses 250 mL of O2 per minute and the arterial–venous O2 content difference is 50 mL per liter (i.e., 5 mL per 100 mL), then CO = 250 / 50 = 5 L/min. This principle hinges on conservation of mass: the oxygen delivered to tissues equals the oxygen consumed, distributed according to the flow of blood.