Left ventricular subendocardial blood flow occurs predominantly during which phase of the cardiac cycle, and why?

• A Systole, because rising aortic pressure drives flow into the coronaries
• B Systole, because sympathetic vasodilatation peaks with contraction
• C Diastole, because intramyocardial pressure during systole compresses the subendocardial vessels ✓
• D Diastole, because coronary vascular resistance is highest during systole in the epicardial layer only

Explanation

During systole the contracting left ventricle generates intramyocardial pressure that exceeds coronary luminal pressure in the subendocardium, squeezing its vessels shut, so left ventricular perfusion, especially of the subendocardium, occurs mainly in diastole. This explains why tachycardia shortening diastole predisposes to subendocardial ischemia. Option A describes the pressure source but misses the compression mechanism, and the right ventricle, with low wall pressure, is perfused throughout the cycle.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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