What is the mechanism of action of nitroglycerin?

• A. Vasoconstrictor
• B. Vasodilator
• C. Anti-hypertensive
• D. Inotrope

Answer: (B)

Nitroglycerin works by releasing nitric oxide, which activates the enzyme that increases cyclic GMP in vascular smooth muscle. This causes the muscles to relax, leading to vasodilation. In practice, it primarily dilates veins at typical EMS doses, which lowers preload (the amount of blood filling the heart before it contracts) and reduces wall stress on the left ventricle. That drop in wall stress lowers the heart’s oxygen demand. At higher doses, arteries also dilate, which reduces afterload (the pressure the heart has to work against). So the overall effect is a decrease in myocardial oxygen demand and can improve blood flow to ischemic areas.

It is not a vasoconstrictor, so it doesn’t cause the opposite effect of tightening vessels. It also doesn’t increase heart muscle contractility (inotropy), and while it can lower blood pressure, its defining action is vasodilation through the NO–cGMP pathway that reduces the heart’s workload.