During which part of the cardiac cycle is coronary blood flow to the myocardium greatest?

• A. Systole
• B. Diastole
• C. Atrial systole
• D. Isovolumetric contraction

Answer: (B)

Coronary blood flow to the heart muscle is greatest when the heart is relaxed, because the vessels are no longer squeezed by muscle contraction. During systole the heart walls actively contract and build high intramyocardial pressure, especially in the left ventricle, which compresses the coronary vessels and sharply reduces flow. In diastole, the heart relaxes, intramyocardial pressure falls, and the coronary arteries are open, allowing blood to flow from the aorta into the coronaries. The aortic diastolic pressure still provides the driving force, so early diastole often yields the highest perfusion. Atrial systole and isovolumetric contraction aren’t the phases that maximize coronary flow because the myocardium is still under compression during those times, limiting perfusion.