Terbutaline

Terbutaline is a short-acting selective beta-2 agonist used for asthma and COPD relief when inhaled, and by injection in severe asthma when inhaled therapy is not possible. Onset is within 5 to 15 minutes inhaled, about 15 minutes subcutaneous, duration 4 to 6 hours.

Classification. Short-acting beta-2 agonist.

Mechanism. Beta-2 stimulation raises cyclic AMP, relaxing bronchial and uterine smooth muscle.

Pharmacokinetics. Oral bioavailability about 15% from extensive first-pass sulphation. Half-life about 3 to 4 hours. Renal excretion of unchanged drug and conjugate.

First-line, relief. Dry powder 500 micrograms as needed, maximum 4 times daily outside acute care. In adults and adolescents with asthma, use only with a regular inhaled corticosteroid.

Alternative, nebulised. 5 to 10 mg two to four times daily.

Emergency, severe asthma when inhaled therapy is not possible. Subcutaneous or intramuscular 250 to 500 micrograms, repeated every 20 minutes for up to 3 doses if needed; maximum 2 mg in 24 hours.

Specialist, intravenous. Slow injection of 250 to 500 micrograms over at least 5 minutes, up to 4 times daily, or infusion 1.5 to 5 micrograms/minute titrated to response, with cardiac and potassium monitoring.

Oral. 2.5 to 5 mg three times daily. Rarely needed and not advised when the inhaled route is possible.

Children. Subcutaneous 10 micrograms/kg (maximum 300 micrograms) up to 4 times daily. Dry powder 500 micrograms as needed from 6 years of age. Nebulised dosing under 6 years is not stated here with confidence and salbutamol is preferred in this age group.

Titration and loading. No loading or maintenance steps apply except for the infusion, which is titrated to heart rate and symptom control.

Boxed-warning-level risk. Must not be used for prolonged tocolysis (over 48 to 72 hours) or for oral or maintenance tocolysis, because of serious maternal cardiac events including death.

Contraindications. Hypersensitivity, tachyarrhythmia, unstable ischaemic heart disease. Use with caution in thyrotoxicosis, hypertension, diabetes and epilepsy.

Serious adverse effects. Hypokalaemia, arrhythmia, myocardial ischaemia, pulmonary oedema (especially in pregnancy), lactic acidosis, hyperglycaemia.

Common adverse effects. Tremor, palpitations, headache, cramps.

Interactions. Non-selective beta-blockers; potassium-lowering drugs (loop and thiazide diuretics, corticosteroids, theophylline); digoxin toxicity with hypokalaemia; monoamine oxidase inhibitors and tricyclic antidepressants.

Common prescribing errors. Using terbutaline for preterm labour. Confusing 250 micrograms with 2.5 mg. Giving repeated injections without potassium monitoring.

Special groups.

• Pregnancy: acceptable for asthma by inhalation; avoid injections near term unless needed for severe asthma. Breastfeeding: compatible at usual doses.

• Older adults: start at the lower end of the range.

• Kidney impairment: for oral use, reduce to about 75% of the usual dose at GFR 10 to 50 mL/min and 50% under 10 mL/min. Liver impairment: no specific adjustment.

• East Africa: subcutaneous terbutaline is a practical option in severe asthma where nebulisers are unavailable.

Formulations. Dry powder inhaler 500 micrograms/dose, nebuliser solution 2.5 mg/mL, injection 500 micrograms/mL, oral tablets.

Intravenous preparation. Dilute in sodium chloride 0.9% or glucose 5%, for example 5 mg in 500 mL gives 10 micrograms/mL.

Baseline. Pulse, potassium, glucose, SpO2, peak flow, ECG if giving injection or infusion.

Monitoring. Heart rate, SpO2 and peak flow after each dose. Potassium and lactate after repeated parenteral doses.

Stop and reduce rules. Stop for arrhythmia, chest pain or persistent marked tachycardia. Replace potassium when under 3.5 mmol/L. Therapeutic drug monitoring is not used.

Referral and escalation. Escalate to high-dependency care for failure to improve after parenteral therapy or any silent chest.