Myocardial oxygen consumption increases as which of the following parameters increase?
A. Preload, afterload, and cerebral blood flow.
B. Preload, afterload, and renal blood flow.
C. Preload, afterload, contractility, and heart rate.
D. Preload, afterload, cerebral blood flow, and heart rate.
Correct Answer: C. Preload, afterload, contractility, and heart rate.
Myocardial oxygen consumption increases as preload, afterload, renal contractility, and heart rate increase. Cerebral blood flow doesn’t directly affect myocardial oxygen consumption. Myocardial oxygen consumption is equal to coronary blood flow multiplied by the arterial-venous oxygen difference. During diastole, the ventricles are receiving blood before systolic contraction. This filling phase of the cardiac cycle allows the coronary arteries to provide maximum blood flow to the heart.
Option A: Since the heart operates solely under aerobic metabolism, myocardial mitochondria must maintain an abundance of oxygen to continue oxidative phosphorylation. Heart rate, contractility, and ventricular-wall tension are the three factors that determine myocardial oxygen demand. An increase in any of these variables requires the body to adapt to sustain adequate oxygen supply to the heart.
Option B: Heart rate is thought to be the most important factor affecting myocardial oxygen demand. With an increased heart rate, the myocardium must work harder to complete the cardiac cycle more efficiently. With a shortened cardiac cycle, the time spent in diastole decreases.
Option D: Contractility or inotropism is the rate of increase in the intraventricular pressure during contraction at a given muscle fiber length. Interestingly, myocytes have the innate ability to exert a contraction at any muscle length. This force is measured after the closure of the mitral valve and before the opening of the aortic valve during which time the intraventricular volume remains constant.
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