Nitroglycerin Injection (Intravenous)

Mechanism of Action

• Morgan & Mikhail's Clinical Anesthesiology, 7e, p.470

Preparations (IV)

• Harriet Lane Handbook, 23rd ed., p.1266

Clinical Indications for IV Use

1. Acute Coronary Syndrome / Unstable Angina

• IV nitroglycerin is used when sublingual nitroglycerin fails, or when there is ongoing or recurrent chest pain at rest.
• It also has anti-platelet aggregation effects that may be beneficial in ACS.
• Start at 5-10 mcg/min, escalate by 10 mcg/min increments every 5 min until symptoms resolve or adverse effects develop. Max: 400 mcg/min for acute angina.
• Goldman-Cecil Medicine, p.645; Washington Manual of Medical Therapeutics

2. Hypertension

• IV nitroglycerin controls significant hypertension associated with cardiac ischemia.
• Titrate up to a max of 200 mcg/min for hypertension.
• Harriet Lane Handbook, 23rd ed.

3. Acute Decompensated Heart Failure (Cardiogenic Pulmonary Edema)

• As a venodilator, IV nitroglycerin reduces preload and lowers pulmonary capillary wedge pressure, relieving pulmonary congestion.
• Best suited for patients with volume overload, hypertension, coronary ischemia, or severe mitral regurgitation - without systemic hypotension.
• Use is limited to dyspnea relief in hospitalized patients with intact blood pressure; no survival benefit has been established.
• Fuster and Hurst's The Heart, 15th ed., p.1585

4. Obstetric Use (uterine relaxation)

• IV nitroglycerin bolus (50-100 mcg) is an effective, short-lived uterine relaxant during procedures such as retained placenta or uterine inversion.
• Morgan & Mikhail's Clinical Anesthesiology, 7e, p.471

Dosing

Adults

• Continuous IV infusion: Start at 5 mcg/min, increase by 5 mcg/min every 3-5 min up to 20 mcg/min. If no response, increase by 10-20 mcg/min every 3-5 min.
• Maximum: 200 mcg/min (hypertension); 400 mcg/min (acute angina)

Pediatric / Infant

• Continuous IV infusion: Begin at 0.25-0.5 mcg/kg/min; increase by 0.5-1 mcg/kg/min every 3-5 min as needed.
• Usual dose: 1-5 mcg/kg/min; max: 20 mcg/kg/min
• Harriet Lane Handbook, 23rd ed.

Hemodynamic Effects

How it reduces myocardial oxygen demand / increases supply:

1. Venous pooling in capacitance vessels reduces preload and ventricular end-diastolic pressure - reduces O2 demand and improves endocardial perfusion
2. Arteriolar dilation reduces afterload and end-systolic pressure
3. Redistributes coronary blood flow to ischemic subendocardial areas
4. Relieves coronary artery spasm

Metabolism

• Methemoglobinemia is rare but can be treated with IV methylene blue (1-2 mg/kg over 5 min).

Administration Notes

• Use polypropylene (non-PVC) infusion sets - nitroglycerin adsorbs to polyvinylchloride (PVC) tubing, which can reduce delivered dose.
• Decrease dose gradually in patients on prolonged therapy to avoid withdrawal reaction.
• IV nitroglycerin may antagonize the anticoagulant effect of heparin - monitor PTT.

Tolerance (Tachyphylaxis)

• Develops within 24-48 hours of continuous IV administration.
• A 10-12 hour nitrate-free period per day is recommended to prevent tolerance.
• Goldman-Cecil Medicine, p.645

Contraindications

Adverse Effects

• Headache (most common - from cerebral vasodilation)
• Nausea, dizziness, flushing
• Hypotension (dose-dependent)
• Reflex tachycardia
• Methemoglobinemia (rare, with prolonged high-dose infusion)
• Blurred vision
• Rebound hypertension is less likely after stopping nitroglycerin than after stopping sodium nitroprusside

Key Clinical Pearls

• Always rule out RV infarction before starting IV nitroglycerin in ACS - it can precipitate profound hemodynamic collapse.
• No studies have shown survival benefit with IV nitroglycerin in acute MI; recent prospective trials found no mortality advantage.
• In acute heart failure, IV nitroglycerin is for symptom relief (dyspnea) only - not for improving outcomes.
• It does not cause the coronary steal phenomenon seen with sodium nitroprusside, making it preferable in patients with CAD.
• Prophylactic low-dose nitroglycerin in high-risk surgical patients has no proven benefit.