A 55-year-old man is admitted to the hospital with severe chest pain. The ECG tracing shows an ST segment depression that indicates subendocardial infarction in the left ventricular wall. The greatest reduction in the coronary blood flow to the subendocardial region of the left ventricular wall is most likely to occur during which one of the following phases of the cardiac cycle?

a. Atrial systole

b. Rapid filling period

c. Rapid ejection period

d. Isovolumetric relaxation

e. Ventricular diastole

The correct answer is c.

The coronary blood flow is also directly affected by the external physical compression of the cardiac muscle in each beat of the heart (ie, each cardiac cycle). The cardiac cycle is divided into 7 periods or phases, the rapid ejection period is the only option that occurs during ventricular systole where the blood flow to the coronary vessels will be at its lowest level. Rapid ventricular filling and isovolumetric relaxation both occur during ventricular diastole when the coronary blood flow is at its highest level and allows the myocardium to receive nourishment.