If the heart rate is too fast, ventricles don't have time to fill, reducing cardiac output.

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

Answer: (A)

When the heart rate becomes excessively fast, there isn’t enough time in diastole for the ventricles to fill properly. The amount of blood that returns to the heart and fills the ventricles (preload) drops, so stroke volume falls. Cardiac output is the product of heart rate and stroke volume, so if the rate is very high and stroke volume decreases, overall output goes down. So, yes—the statement is true: a tachycardic state that shortens filling time can reduce cardiac output.

It’s worth noting that at moderate increases in rate, cardiac output may rise briefly because the heart is beating faster, but once filling time is too limited, the benefit reverses and CO declines. The idea hinges on filling time limiting preload and thus stroke volume, which determines the overall output.