Nitrates
Nitrates are vasodilators used primarily to treat angina. They work by donating nitric oxide, which relaxes smooth muscle in blood vessels. The result is reduced oxygen demand on the heart and improved oxygen supply to the myocardium. These medications range from fast-acting sublingual tablets to long-lasting transdermal patches, and understanding the differences between formulations is a big part of using them safely.
Mechanisms of Nitrates for Angina
Nitrates relax vascular smooth muscle through a specific biochemical pathway:
- The nitrate is converted to nitric oxide (NO) in the body.
- NO activates the enzyme guanylate cyclase inside vascular smooth muscle cells.
- Guanylate cyclase increases levels of cyclic guanosine monophosphate (cGMP).
- Elevated cGMP decreases intracellular calcium, which causes the smooth muscle to relax and the vessel to dilate.
This vasodilation happens in two key places:
- Venodilation (veins): This is the primary effect. Dilating veins reduces preload, meaning less blood returns to the heart. Lower preload means the heart doesn't have to work as hard, which directly decreases myocardial oxygen demand.
- Arteriolar dilation (arteries): This reduces afterload and systemic vascular resistance, so the heart pumps against less pressure. This also lowers oxygen demand.
On top of reducing demand, nitrates cause coronary vasodilation, which dilates the coronary arteries themselves. This improves blood flow to ischemic (oxygen-starved) heart tissue, increasing oxygen supply and relieving angina symptoms.
Hemodynamic Effects and Myocardial Oxygen Balance
The therapeutic benefit of nitrates comes down to rebalancing myocardial oxygen supply and demand. They reduce oxygen demand through:
- Decreased preload (less venous return)
- Decreased afterload (less resistance to pump against)
- Reduced ventricular wall tension (the heart doesn't stretch as much)
At the same time, they increase oxygen supply by improving coronary blood flow. This combination is what makes them effective for ischemia in patients with coronary artery disease.
The main organic nitrates you'll see in clinical practice are nitroglycerin (NTG), isosorbide dinitrate (ISDN), and isosorbide mononitrate (ISMN).
Comparison of Nitrate Medication Forms
| Formulation | Onset | Duration | Clinical Use |
|---|---|---|---|
| Sublingual NTG | 1–3 min | 20–30 min | Acute angina episodes; prophylaxis before exertion |
| NTG spray | 1–3 min | 20–30 min | Same as sublingual; may be easier for patients with dry mouth or difficulty handling small tablets |
| Transdermal NTG patch | 30–60 min | 8–12 hrs | Chronic angina management; requires a 10–12 hour patch-free interval daily to prevent tolerance |
| Isosorbide dinitrate (ISDN) | 15–30 min | 4–6 hrs | Oral therapy for chronic angina; available in extended-release forms |
| Isosorbide mononitrate (ISMN) | 30–60 min | 6–8 hrs | Oral therapy with longer duration than ISDN; available as once-daily dosing |
| The key distinction: sublingual NTG and the spray are your acute, fast-acting options. The patch, ISDN, and ISMN are for chronic, preventive management. |
Adverse Effects of Nitrate Therapy
- Headache is the most common side effect, caused by dilation of cerebral blood vessels. It often decreases with continued use.
- Hypotension can result from excessive vasodilation, sometimes causing a significant blood pressure drop.
- Orthostatic hypotension is a sudden drop in blood pressure when standing, which puts patients at risk for falls.
- Reflex tachycardia occurs because the body compensates for the drop in blood pressure by increasing heart rate.
- Flushing of the skin results from peripheral vasodilation.
- Dizziness or lightheadedness is related to the hypotensive effects.
- Nitrate tolerance can develop with continuous, uninterrupted use. This is why patch-free intervals and asymmetric dosing schedules (for oral nitrates) are so important.
Critical drug interactions:
- PDE-5 inhibitors (sildenafil, tadalafil, vardenafil): Concurrent use is contraindicated. Both drugs cause vasodilation, and together they can produce severe, life-threatening hypotension. Ask patients about PDE-5 inhibitor use before administering nitrates.
- Antihypertensives (beta-blockers, calcium channel blockers): Additive hypotensive effects are possible. Monitor blood pressure closely when these are used together.
- Alcohol: Potentiates the hypotensive effects of nitrates. Patients should limit or avoid alcohol.
Patient Education for Nitrate Use
Administration by formulation:
- Sublingual NTG: Place the tablet under the tongue and let it dissolve completely. Do not chew or swallow it. The patient may feel a slight tingling, which indicates the tablet is active. If angina is not relieved after one dose, a second tablet can be taken after 5 minutes, and a third after another 5 minutes. If pain persists after three doses (15 minutes total), call 911.
- NTG spray: Spray onto or under the tongue. Do not inhale the spray.
- Transdermal patch: Apply to clean, dry, hairless skin. Rotate application sites (chest, upper arm, back) to prevent skin irritation. Remove the patch for 10–12 hours each day (typically overnight) to prevent tolerance.
Safety and self-monitoring:
- Sit or lie down before taking short-acting nitrates to prevent falls from hypotension.
- Keep a log of angina episodes, including what triggered them and how many NTG doses were needed. Share this with your healthcare provider.
- Headache is common and often indicates the medication is working. Over-the-counter acetaminophen can help, but patients should report severe or persistent headaches.
- Store sublingual NTG in its original dark glass container, away from heat and moisture. Replace the supply every 6 months or by the expiration date, since potency decreases over time.
- Avoid PDE-5 inhibitors and limit alcohol while on nitrate therapy.
- Inform all healthcare providers (including dentists and surgeons) about nitrate use before starting new medications or undergoing procedures.