USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

During which phase of the cardiac cycle does coronary blood flow peak?

1. Isometric contraction

2. Mid-systole

3. Early diastole

4. Late diastole

The correct answer is: Early diastole

During the isometric contraction phase, the cardiac muscle is not actively receiving blood flow, so it cannot be the peak for coronary blood flow. Mid-systole is the peak of ventricular contraction, which does not directly affect coronary blood flow. Late diastole is when the ventricles are relaxed and filling with blood, so the peak of coronary blood flow has already occurred during early diastole. This is when the ventricles are fully relaxed and the aortic and pulmonary valves are still open, allowing for maximum blood flow to the coronary arteries. Therefore, early diastole is when coronary blood flow peaks.