Glyceryl trinitrate (GTN)

How it works

It is metabolised to nitric oxide, which raises vascular smooth-muscle cyclic GMP to cause vasodilatation, predominantly venous, reducing cardiac preload and myocardial oxygen demand and relieving coronary spasm.

Prescribing in practice

• It is contraindicated with phosphodiesterase-5 inhibitors such as sildenafil because the combination can cause profound, life-threatening hypotension.
• Sublingual tablets and sprays act within minutes for acute angina; if chest pain is not relieved after repeated dosing, emergency help should be sought as this may indicate myocardial infarction.
• Continuous nitrate exposure causes tolerance, so a nitrate-free interval is needed with longer-acting use, and abrupt withdrawal of intravenous nitrate should be avoided.

Monitoring

With intravenous use monitor blood pressure and heart rate closely; for oral and sublingual use review symptom control and tolerance.

Counselling the patient

• Sit down before using a sublingual dose as it can cause headache and dizziness.
• Never use with erectile-dysfunction medicines such as sildenafil.
• If angina is not relieved after repeated doses, call emergency services.

Evidence & guidelines

Sublingual and intravenous GTN are long-standing guideline-recommended treatments for angina and acute cardiogenic pulmonary oedema.