Introduction
Nitroglycerin is a vasodilator used to treat acute angina, myocardial infarction, and heart failure. It relaxes smooth muscle by increasing the availability of nitric oxide (NO), leading to venodilation, reduced preload, and improved myocardial oxygen supply.
Pharmacokinetics
Nitroglycerin is rapidly absorbed when administered sublingually or intravenously. It undergoes extensive first-pass metabolism in the liver, with a short half-life of about 1-4 minutes. Continuous infusion is often required for longer-term effects.
Mechanism of Action
Nitroglycerin converts to nitric oxide, which activates guanylate cyclase and increases cyclic GMP, causing smooth muscle relaxation and vasodilation, primarily affecting veins. This reduces preload and myocardial oxygen demand.
Pharmacodynamics
Nitroglycerin reduces venous return (preload) and, at higher doses, can also reduce systemic vascular resistance (afterload). These effects decrease myocardial oxygen consumption, making it beneficial in ischemic heart conditions.
Adverse Effects
Hypotension: Significant vasodilation can cause profound hypotension.
Headache: Nitroglycerin commonly causes headaches due to vasodilation.
Tachyphylaxis: Tolerance to nitroglycerin can develop with continuous use, requiring dose escalation.
Drug Interactions
PDE5 inhibitors: Concurrent use with PDE5 inhibitors (e.g., sildenafil) can cause severe hypotension.
Antihypertensives: The combination of nitroglycerin with other antihypertensive drugs may result in excessive blood pressure reduction.
Dosages
Adults (Angina): Sublingual tablets: 0.3-0.6 mg every 5 minutes as needed, up to 3 doses.
Intravenous: 5-10 mcg/min, titrated to response.