What is true about coronary perfusion pressure and myocardial oxygen supply, and when is supply greatest?

• A. Perfusion pressure equals LV systolic pressure minus RA pressure.
• B. Coronary blood flow is greatest during diastole.
• C. Higher LV end-diastolic pressure improves supply.
• D. Coronary blood flow is greatest during systole.

Answer: (B)

Coronary perfusion depends on a pressure gradient and on whether the heart muscle is actively squeezing the vessels. The myocardium receives most of its blood flow when the heart is relaxed, so coronary blood flow is greatest during diastole. During systole, the contracting left ventricle generates high intraventricular pressure that compresses the intramyocardial vessels, increasing resistance and limiting flow.

Perfusion pressure is the driving gradient for coronary flow. For the left-side circulation, it’s approximated by the aortic diastolic pressure minus the left ventricular end-diastolic pressure; for the right side, it’s roughly aortic diastolic pressure minus right atrial pressure. If the LV end-diastolic pressure is high, the gradient drops and myocardial oxygen delivery falls. Hence, the best time for supply is diastole, when the heart is relaxed and the pressure gradient favors blood entering the coronary arteries.