Cardiac output is usually expressed in which units?

• A. Liters per minute
• B. Milliliters per minute
• C. Beats per minute
• D. Milliliters per beat

Answer: (A)

Cardiac output measures how much blood the heart pumps each minute. It’s the product of how fast the heart beats (heart rate) and how much blood is pumped with each beat (stroke volume). Mathematically, that gives a volume per time, and because stroke volume is typically measured in milliliters per beat and heart rate in beats per minute, CO would be milliliters per minute. In practice this is most commonly expressed as liters per minute for clarity and because resting values are several liters per minute. For example, with a stroke volume around 70 mL/beat and a heart rate around 70 beats/min, CO ≈ 4900 mL/min ≈ 4.9 L/min. Thus liters per minute is the standard unit. The other options describe heart rate alone (beats per minute) or volume per beat (milliliters per beat) or mix a rate without the per-time context, which don’t represent cardiac output.