How does heart rate influence coronary perfusion pressure and myocardial oxygen demand?

• A. Higher heart rate increases diastolic filling time and reduces oxygen demand.
• B. Higher heart rate shortens diastole, reducing coronary perfusion time and supply, while increasing oxygen demand.
• C. Heart rate has no effect on coronary perfusion.
• D. Lower heart rate reduces oxygen demand but worsens perfusion.

Answer: (B)

The main idea is how heart rate changes the balance between oxygen supply to the heart muscle and its oxygen demand. Coronary blood flow mostly happens in diastole, when the heart muscle is relaxed. As heart rate goes up, diastole gets shorter, so there’s less time for the coronaries to deliver blood to the myocardium. That can reduce coronary perfusion, especially to the inner heart layers. At the same time, a higher heart rate increases the work the heart does—more contractions per minute mean more wall tension and ATP use—so oxygen demand rises.

So, when the heart beats faster, the heart needs more oxygen, but the window to supply that oxygen through the coronaries narrows. That combination is captured by the statement that higher heart rate shortens diastole, reducing coronary perfusion time and supply, while increasing oxygen demand. In contrast, increasing heart rate does not increase diastolic filling time, and perfusion isn’t unaffected by rate, nor does lowering heart rate inherently worsen perfusion.