True or False: A heart rate that is too fast reduces cardiac output by shortening ventricular filling time.

• A. False
• B. True
• C. Not sure
• D. Sometimes

Answer: (B)

When the heart beats too fast, there isn’t enough time for the ventricles to fill during diastole. Ventricular filling determines preload and thus stroke volume—the amount of blood ejected with each beat. If filling is limited, end-diastolic volume drops and stroke volume falls. Cardiac output is the product of heart rate and stroke volume, so a very high heart rate can actually lower output because the big increase in rate is offset by a smaller stroke volume. This is why an excessively fast heart rate can reduce cardiac output by shortening ventricular filling time.