A patient pops a sublingual nitroglycerin tablet but gets no relief after three doses — the next action isn't a fourth tablet, it's calling 911. Knowing why separates safe care from disaster.
Nitroglycerin is a nitrate vasodilator that works by releasing nitric oxide, which relaxes vascular smooth muscle. Its predominant effect is venous dilation, which reduces preload — the volume of blood returning to the heart. By decreasing preload, myocardial oxygen demand drops, relieving anginal chest pain. At higher doses, arterial dilation also reduces afterload. Sublingual (SL) onset is 1–3 minutes; IV onset is immediate. For acute angina in a patient with an existing NTG prescription and known stable angina pattern, the patient takes one SL tablet (0.4 mg) every 5 minutes for a maximum of 3 doses. If pain persists after 3 doses, activate EMS — this signals possible MI. Patients experiencing new or unexpected chest pain without a current NTG prescription should call 911 immediately rather than borrowing someone else's NTG. Before administering, check blood pressure: hold if systolic BP is below 90 mmHg. Nitroglycerin causes significant hypotension, so the patient must sit or lie down during administration. Headache is the most common side effect; reassure the patient it is expected and treat with acetaminophen. SL tablets should produce a slight tingling or burning under the tongue, confirming potency. Tablets degrade with light, heat, and air — store in the original dark glass container, replace every 6 months. IV nitroglycerin requires glass bottles and special non-PVC tubing because the drug adsorbs to standard plastic, reducing delivered dose. Nitroglycerin is contraindicated in right ventricular infarction — the right ventricle is preload-dependent, and reducing preload can cause severe hypotension. Suspect right ventricular involvement with inferior MI plus hypotension, JVD, and clear lungs.
Key Distinctions
Don't confuse nitroglycerin (reduces preload via venous dilation) with nitroprusside (reduces both preload AND afterload aggressively, used in hypertensive emergencies). Students often think nitroglycerin primarily dilates coronary arteries — it does modestly, but the major benefit is decreasing myocardial oxygen demand through preload reduction. Never give nitroglycerin to a patient who has taken a PDE-5 inhibitor — sildenafil or vardenafil within 24 hours; tadalafil within 48 hours due to its longer half-life — the combination causes life-threatening hypotension.
Clinical Pearl
Tingle means it's working — if the SL tablet doesn't burn under the tongue, the medication has likely lost potency and needs replacing.