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

• A. Cardiac output increases
• B. Cardiac output decreases
• C. Cardiac output remains the same
• D. Cardiac output becomes unpredictable

Answer: (A)

Cardiac output is the product of heart rate and stroke volume (CO = HR × SV). If stroke volume remains constant and heart rate increases, the product increases, so cardiac output rises. For example, with a stroke volume of about 70 mL per beat, a heart rate of 60 beats per minute gives roughly 4.2 L/min, and increasing the rate to 90 bpm with the same stroke volume yields about 6.3 L/min. So CO increases, not decreases or stays the same, and it isn’t unpredictable—the relationship is a straightforward multiplication. (In real life, very high heart rates can reduce filling time and SV, but that’s outside the stated condition.)