Overview
This lecture covers the pharmacology, indications, administration, contraindications, and precautions of glyceryl trinitrate (GTN), with a focus on GTN spray use in emergency care.
Mechanism of Action
- GTN is a potent vasodilator that works mainly on veins and also affects arteries.
- It acts by forming nitric oxide, which relaxes vascular smooth muscle.
- GTN reduces venous return (preload) and arterial resistance (afterload), decreasing myocardial oxygen demand.
- Coronary artery dilation may increase blood supply, but this effect is usually minor.
Indications for Use
- GTN spray treats myocardial ischaemia, cardiogenic pulmonary oedema, and hypertension in specific acute care settings.
- In STEMI, it's used cautiously for hypertension, especially pre-fibrinolysis or during transfer.
- IV GTN is reserved for use by clinical care paramedics for hypertension and CPO.
Contraindications
- Severe allergy to GTN.
- Systolic blood pressure <110 mmHg.
- Heart rate <40 or >150 bpm.
- Ventricular tachycardia.
Cautions
- Use with caution in STEMI, especially right ventricular involvement, frail patients, and those with shock or dysrhythmia.
- Avoid if a phosphodiesterase inhibitor (e.g., sildenafil) was used in the last 24 hours.
- Caution in known aortic or mitral stenosis.
Dosage & Administration
- Myocardial ischaemia: 0.4 mg SL every 5 minutes; increase interval if caution present.
- Cardiogenic pulmonary oedema: 0.8 mg SL every 5 minutes; increase dose/frequency if not improving.
- Hypertension: 0.4 mg SL every 5 minutes.
- Administer sublingually; if caution is present, patient should be flat and have IV access.
Adverse Effects
- Hypotension, flushing, headache, tachycardia, and light-headedness.
Pharmacokinetics & Preparation
- Onset: 1–2 minutes (spray).
- Duration: 15–30 minutes (spray).
- Metered-dose bottle delivers 0.4 mg per spray.
- Rapidly absorbed under the tongue; not effective if swallowed.
- Mostly metabolised in the liver.
Common Interactions
- Enhanced effects with antihypertensives.
- Severe hypotension with phosphodiesterase inhibitors taken within 24 hours.
Administration Precautions
- Check indication, take safety precautions, explain and get consent, verify medicine (5Rs, 4Cs, Expiry), and document administration.
Key Terms & Definitions
- Preload — Volume of blood in ventricles at end of diastole.
- Afterload — Resistance the heart must overcome to eject blood.
- Myocardial Ischaemia — Inadequate blood supply to heart muscle.
- Cardiogenic Pulmonary Oedema (CPO) — Fluid in lungs due to heart failure.
- Sublingual (SL) — Under the tongue administration.
Action Items / Next Steps
- Review CPGs for current protocols on GTN, STEMI, and autonomic dysreflexia.
- Complete associated quiz and skill sheets on sublingual GTN administration.
- Listen to listed podcasts for case examples.