Blood flow to the left ventricular subendocardium is greatest during which phase of the cardiac cycle, and why?

• A Systole, because aortic root pressure is highest then
• B Isovolumetric contraction, because oxygen demand peaks at that moment
• C Diastole, because intramyocardial compressive forces are minimal ✓
• D Flow to the subendocardium is equal throughout the cycle

Explanation

During systole the contracting left ventricle compresses the intramyocardial vessels supplying the subendocardium, so perfusion there falls almost to zero. In diastole the muscle relaxes, compression disappears, and coronary driving pressure from the aortic diastolic pressure produces maximal subendocardial flow. Although aortic pressure is highest in early systole, option A fails because extravascular compression overrides the higher perfusion pressure in this layer.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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