What is the common mechanism by which nitroglycerin benefits patients with angina?

• A. Arterial dilation increasing afterload.
• B. Vasoconstriction increasing blood pressure.
• C. Increases heart rate to improve perfusion.
• D. Venodilation reducing preload and myocardial oxygen demand.

Answer: (D)

Nitroglycerin helps angina mainly by lowering the heart’s oxygen demand through venodilation. When veins dilate, more pooling of blood occurs in the venous system, and venous return to the heart decreases. This reduces the end-diastolic volume and pressure, which in turn lowers the wall tension of the left ventricle. With less wall stress, the heart uses less oxygen for the same amount of work, alleviating chest pain even if coronary blood flow hasn’t changed much. At antianginal doses, nitroglycerin’s venous effect dominates; higher doses can dilate arteries too and may lower blood pressure, but the key mechanism for reducing angina is decreased preload and myocardial oxygen demand. The other choices would either raise oxygen demand (like increasing heart rate) or worsen perfusion (vasoconstriction or increased afterload), which would not relieve angina.