Barron/Elsevier Critical Care Registered Nurse (CCRN) practice test

The majority of myocardial oxygen consumption occurs during isovolumetric contraction. This phase takes place after the ventricles have filled and before they begin to eject blood into the aorta and pulmonary artery. During this time, the ventricles are contracting, and the myocardium is generating tension against a closed system, which requires significant energy and oxygen.

During isovolumetric contraction, the heart muscle is working hard to build up pressure necessary to open the aortic and pulmonary valves for ejection of blood. The increased myocardial workload results in elevated oxygen demand. This is the point in the cardiac cycle where the heart is metabolically active and consuming the most oxygen as it prepares for the next phase of systolic ejection.

In contrast, the other phases of the cardiac cycle—like systolic ejection, diastolic filling, and isovolumetric relaxation—while they do involve myocardial work, do not have the same intensity of oxygen consumption as isovolumetric contraction. During systolic ejection, for example, the valves are open, and the myocardium is not generating as high of a tension as it does in isovolumetric contraction, leading to lower physiological oxygen demand.