Beta-adrenergic blocking agents decrease the oxygen demands of the heart by what mechanism?

The correct answer is based on the physiological effects of beta-adrenergic blocking agents, which are used primarily in the management of conditions like angina and hypertension. These agents work by interfering with the action of catecholamines, such as adrenaline, on beta-adrenergic receptors in the heart.

The key mechanism through which beta-blockers decrease the oxygen demands of the heart is by reducing the heart rate. A lower heart rate allows for a longer diastolic filling time, which is the period when the chambers of the heart fill with blood. This increased filling time facilitates enhanced coronary perfusion and, consequently, a more effective supply of oxygen-rich blood to the myocardium. With the heart beating more slowly, it also means that the myocardial oxygen consumption is reduced, as energy expenditure is directly related to heart rate.

Furthermore, by decreasing the heart's workload, beta-blockers can alleviate symptoms of angina and prevent myocardial ischemia. This is crucial in patients who experience angina pectoris because the heart can often demand more oxygen than what is supplied through the coronary arteries, particularly during stress or exertion. By reducing heart rate and contractility, beta-blockers effectively lower the heart’s oxygen requirements.

This understanding highlights the importance