How does an increase in heart rate affect diastolic filling time?

• A. Increases diastolic filling time
• B. No effect on diastolic filling time
• C. Slightly increases diastolic filling time, preserving preload
• D. Shortens diastole and reduces diastolic filling time, potentially decreasing preload

Answer: (D)

An increased heart rate shortens the diastolic phase, the period when the ventricles fill. With less time for filling, the ventricle ends diastole with less blood—so end-diastolic volume and preload decrease. This reduced preload can in turn lower stroke volume via the Frank-Starling mechanism, even though the heart is beating faster. Higher rates do not increase diastolic filling time, and while atrial contraction can help a bit, the overall effect is a shorter diastolic filling window and potentially reduced preload.