If oxygen consumption and the arterial-venous oxygen content difference are unknown, which of the following can be calculated?

• A. Deadspace-to-tidal-volume ratio
• B. Shunt percentage
• C. Cardiac output
• D. Oxygen content

Answer: (C)

The key idea is the Fick principle: oxygen consumption equals cardiac output times the difference between arterial and venous oxygen content (VO2 = CO × (CaO2 − CvO2)). When the oxygen consumption and the arterial-venous oxygen content difference are unknown, you can still determine cardiac output because it is the quantity that links how much oxygen is delivered to tissues with how much oxygen is used. In practice, cardiac output can be inferred from other measurements (for example, arterial oxygen content and oxygen delivery measured by other means, or by noninvasive hemodynamic methods), whereas the other options require the O2 content difference or oxygen consumption data that aren’t provided.