Nitroglycerin
Nitroglycerin
Generic Name
Nitroglycerin
Mechanism
Nitroglycerin is a nitric oxide (NO) donor that undergoes enzymatic bioactivation (aldoketoreductase) in vascular smooth muscle to release NO.
• NO activates guanylate cyclase ➜ ↑ cyclic GMP → protein kinase G‑mediated phosphorylation of myosin light‑chain phosphatase → relaxation of vascular smooth muscle.
• Predominantly dilates pre‑capillary venous vessels → ↓ preload, reducing left‑ventricular end‑diastolic pressure and myocardial oxygen demand.
• At higher doses, dilation occurs → ↓ afterload, improving cardiac output.
• Shunts blood from myocardial ischemic regions to healthier myocardium.
Indications
- Acute angina pectoris and unstable angina (sublingual tablets or spray)
- ST‑segment elevation myocardial infarction (IV bolus + infusion) as adjunct to reperfusion therapy
- Pre‑operative coronary vasodilatory support
- Hypertensive emergencies (IV infusion)
- Heart failure (transdermal patch for chronic pulmonary edema)
- Pre‑operative vasodilator therapy for diastolic dysfunction
Contraindications
- Absolute: Severe thrombocytopenia, hypovolemia, uncontrolled hyperthyroidism, myocardial infarction within 7 days of coronary artery bypass grafting, pregnancy (category X)
- Relative:
- Caution in sickle cell disease (ischemia risk)
- Severe aortic stenosis (↓ preload may precipitate collapse)
- Elevated intracranial pressure (after brain injury)
- Recent use of sildenafil or other PDE5 inhibitors (risk of severe hypotension)
Warnings
• Rebound angina if therapy abruptly discontinued; taper dosage.
• Headache is the most common adverse event; advise dosing schedule to mitigate.
• Hypotension—monitor blood pressure closely, especially in the elderly or volume‑depleted patients.
• Erythrocytosis with chronic use; monitor hematocrit every 4–6 weeks.
Dosing
| Form | Dosage | Frequency | Special Instructions |
| Sublingual tablets | 0.3 mg → 0.6 mg → 0.9 mg (max 3 × 0.3 mg) | At onset of chest pain | Max 4 × 0.3 mg within 24 h; avoid within 24 h of nitrate meds. |
| Sublingual spray | 0.4 mg (4 puffs) | At onset | Ideal for rapid onset; monitor BP. |
| Intravenous infusion | Loading: 20–30 µg/min → Infusion 5–10 µg/min | Continuous | Adjust by increments of 5 µg/min every 5 min until BP >100 mmHg or symptoms resolved. |
| Transdermal patch | 5 mg (20 cm²) | 12–24 h, then 12 h off | For chronic angina; patch area may cause skin irritation. |
| Oral | 0.3–0.6 mg TID | For selected outpatient cases | Not preferred due to hepatic first‑pass. |
Adverse Effects
- Common
- Headache (vasodilatory) → treat with acetaminophen or indomethacin.
- Flushing, dizziness, nausea.
- Hypotension (orthostatic).
- Serious
- Severe hypotension (SBP 54%.
Monitoring
- Vital signs: BP & HR every 5 min during IV administration, then hourly.
- Cardiac: Continuous telemetry for arrhythmias.
- Laboratory: Hematocrit every 4–6 weeks; serum methemoglobin if symptoms.
- Pain/Symptom diary: Document anginal episodes and nitrate response.
- Skin: Inspect patch area daily for dermatitis or ulceration.
Clinical Pearls
- Avoid Giving Nitroglycerin to patients with severe anemia — low blood volume less responsive to preload reduction.
- Rebound phenomenon is dose‑dependent; for chronic therapy, use a 10‑day taper rather than abrupt withdrawal.
- Tablet” vs “Spray”: Spray yields faster onset in uncontrolled conditions (e.g., severe tachycardia) because surface area contacts the mucosa more freely.
- Beta‑blocker interaction: If hypotension persists on nitrate therapy, consider adding a short‑acting beta‑blocker (e.g., propranolol) to blunt reflex tachycardia but watch for bronchospasm in asthmatics.
- Sickle cell patients: Use caution as nitrate‑induced vasodilation can worsen tissue ischemia if nitro‑induced vasoconstriction occurs via reflex mechanisms.
- Nitrate tolerance: Implement at least 12‑hour nitrate‑free interval; cyclic dosing protocols (3 days on, 1 day off) are effective.
- Transdermal use in heart failure: Promote BNP reduction by venous pooling; check that renin‑angiotensin‑aldosterone system isn’t overly activated.
- Combination with PDE5 inhibitors: Immediate hypotension possible; contraindicate concurrent use unless infusion stopped and BP stabilized >1 h.
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