If heart rate doubles while stroke volume remains constant, what happens to cardiac output?

• A. Cardiac output doubles
• B. Cardiac output remains the same
• C. Cardiac output halves
• D. Cardiac output increases by four times

Answer: (A)

Cardiac output is the amount of blood the heart pumps per minute and is calculated as heart rate times stroke volume. If the heart rate doubles while stroke volume stays the same, the total blood pumped per minute doubles as well. For example, with stroke volume at 70 mL per beat, increasing heart rate from 60 to 120 beats per minute raises cardiac output from about 4.2 L/min to about 8.4 L/min. The key idea is that doubling heart rate directly doubles cardiac output when stroke volume is constant.