Isoprenaline

Isoprenaline (isoproterenol) is a non-selective beta adrenergic agonist. It raises heart rate and conduction speed and lowers peripheral resistance. Its main uses are as a bridge in symptomatic bradycardia or heart block until pacing, in torsades de pointes with bradycardia, and in selected specialist indications. Supply is limited in many African settings, where adrenaline or dopamine infusion and pacing are the alternatives.

Classification. Catecholamine; non-selective beta 1 and beta 2 agonist.

Mechanism. Beta 1 stimulation increases sinus rate, atrioventricular conduction and contractility. Beta 2 stimulation dilates bronchial and skeletal muscle vessels, lowering diastolic pressure.

Pharmacodynamics. Onset within 1 to 2 minutes IV; duration 10 to 15 minutes after stopping the infusion. Cardiac output rises with heart rate; mean arterial pressure may fall. Increases myocardial oxygen demand while lowering diastolic coronary perfusion.

Pharmacokinetics. Inactive orally (sublingual preparations are erratic). Half life 2.5 to 5 minutes. Metabolised by catechol O methyltransferase; excreted in urine.

Symptomatic bradycardia or high grade atrioventricular block unresponsive to atropine, adult (bridge to pacing).

• Start 1 to 2 micrograms/minute IV infusion.

• Titrate every 5 to 10 minutes to a heart rate of 50 to 60/min and adequate perfusion; usual range 2 to 10 micrograms/minute.

• Maximum about 20 micrograms/minute; avoid long infusions.

• Set up temporary pacing at the same time. It is not a long term treatment.

Torsades de pointes with bradycardia or pause dependent onset, adult. Magnesium sulphate 2 g IV over 10 to 15 minutes first, correct potassium to 4.5 to 5 mmol/L, then isoprenaline 2 to 10 micrograms/minute to raise the heart rate to about 90/min if pacing is not available. Not for congenital long QT syndrome (beta agonists provoke arrhythmia).

Brugada syndrome electrical storm, specialist. Dosing is individualised under cardiology direction and is not given here.

Children, bradycardia or heart block. 0.05 to 2 micrograms/kg/min; specialist.

Historic uses. Bronchodilation and shock are no longer advised.

Contraindications. Tachyarrhythmias; ventricular arrhythmias other than torsades with bradycardia; digoxin toxicity; congenital long QT syndrome; hypertrophic obstructive cardiomyopathy; angina or acute myocardial infarction (relative); hypersensitivity.

Warnings. Angina, myocardial infarction, ventricular fibrillation, hypotension (especially with volume depletion) and hypokalaemia. Raises myocardial oxygen demand. May worsen shock by vasodilation. Lactic acidosis and hyperglycaemia on prolonged use.

Adverse effects. Palpitations, tachycardia, flushing, tremor, headache, sweating, chest pain, hypotension.

Interactions. Beta blockers antagonise. Other sympathomimetics (adrenaline) cause excessive stimulation and arrhythmia. Halogenated anaesthetics: arrhythmia. Theophylline: tachycardia and hypokalaemia. Digoxin: arrhythmia.

Special groups.

• Pregnancy: limited data; use if no safe alternative; may relax the uterus.

• Breastfeeding: no data.

• Children: dose by weight.

• Older adults: start at 1 microgram/minute; ischaemia risk.

• Kidney impairment: no adjustment.

• Liver impairment: no adjustment.

Formulations. Injection 200 micrograms/mL (1 mL ampoule).

Preparation. For example 1 mg in 100 mL of 5% glucose (10 micrograms/mL), through a pump. Use a central or large vein. Protect from light; discard if pink or brown. Do not mix with alkaline solutions.

Baseline. 12 lead ECG with QTc, potassium, magnesium, blood pressure, indication for pacing.

Monitoring. Continuous ECG, heart rate, blood pressure every 5 minutes during titration, then every 15 minutes; chest pain; potassium and glucose.

Stopping. Stop as soon as pacing is established or the underlying cause (drug toxicity, electrolyte disturbance) is corrected; wean slowly.

Referral and escalation. Any patient needing isoprenaline needs urgent cardiology review and temporary or permanent pacing assessment.

Common prescribing errors. Using it for long term control; running it with adrenaline; ignoring contraindication in tachyarrhythmia; mixing up units (micrograms/minute against micrograms/kg/min).