Pharmacology  ·  Cardiovascular

Nitrates — Mechanism, Use & Tolerance

Nitric oxide pathway, formulation selection, tolerance prevention, and key contraindications

• Organic nitrates are prodrugs converted in the body to nitric oxide, which activates soluble guanylyl cyclase, raises cyclic guanosine monophosphate, and relaxes vascular smooth muscle — reproducing the same signal healthy endothelium normally provides.
• The vascular effect is dose-dependent: at standard doses nitrates are primarily venodilators that reduce preload; at higher doses they also dilate arteries and reduce afterload; coronary arteries are sensitive at virtually any dose, explaining the rapid relief of sublingual nitroglycerin in vasospasm.
• Nitrate tolerance develops within twenty-four to forty-eight hours of continuous exposure as bioactivating enzyme activity falls; the only reliable fix is a daily nitrate-free interval, which is why long-acting nitrate regimens are never dosed at evenly spaced intervals around the clock.
• Three conditions share a preload-dependent contraindication: right ventricular infarction, severe aortic stenosis, and hypertrophic obstructive cardiomyopathy — in each, nitrate-induced venodilation removes the filling pressure the heart is relying on to maintain output.
• Phosphodiesterase type 5 inhibitors are absolutely contraindicated with nitrates: both raise cyclic guanosine monophosphate through converging pathways, and the combination can produce severe, potentially life-threatening hypotension.