Nitroglycerin reduces myocardial oxygen demand primarily by which mechanism?

• A. Relaxes vascular smooth muscle causing systemic vasodilation and decreased preload and afterload
• B. Increases myocardial contractility
• C. Promotes bronchoconstriction
• D. Inhibits heart rate

Answer: (A)

The main idea is that nitroglycerin lowers the heart’s oxygen demand by reducing the work the heart has to do, primarily through venodilation that lowers preload. By relaxing veins, it reduces venous return, which lowers left-ventricular end-diastolic volume and wall tension. With less wall stress, the myocardium uses less oxygen. There is also some arterial dilation that can reduce afterload, but the preload reduction is the dominant effect for decreasing oxygen demand.

As a result, increasing contractility would raise oxygen use, not lower it. Nitroglycerin does not cause bronchoconstriction as its primary action, and while it can trigger a reflex increase in heart rate, it does not inhibit heart rate.