Makindo Medical Notes

⚡ Isoprenaline (Isoproterenol) is a non-selective β-adrenergic agonist with potent chronotropic and inotropic effects, but minimal α-adrenergic activity. It is primarily used to treat bradyarrhythmias or heart block while awaiting pacing. Because of its strong β₁ and β₂ stimulation, it increases both heart rate and myocardial oxygen demand — so close monitoring is essential. 🩺 Always correct hypovolaemia before commencing an infusion.

📘 About

• Always check the NSW guideline here or equivalent source for up-to-date infusion advice.
• Also check BNF (Isoprenaline) for formulation and safety information.
• Once widely used for cardiac support, isoprenaline is now reserved for temporary pacing support or β-blocker toxicity.
• Acts on both β₁ (heart) and β₂ (vessels, bronchi) receptors → increased heart rate, contractility, and vasodilation.

⚙️ Mode of Action

• Non-selective β-adrenergic agonist — stimulates both β₁ and β₂ receptors.
• β₁ effect: ↑ heart rate (positive chronotropy), ↑ contractility (positive inotropy), ↑ conduction through AV node.
• β₂ effect: peripheral vasodilation → ↓ diastolic BP, ↓ mean arterial pressure.
• Minimal α activity → little vasoconstrictive effect.
• Overall: ↑ cardiac output but risk of hypotension and tachyarrhythmia.

💊 Indications & Dosing

• Bradyarrhythmias / Heart block (pending pacing): 2–10 micrograms/min by IV infusion, titrated to heart rate and rhythm response.
• β-blocker overdose: start at 2–4 micrograms/min, titrate cautiously.
• Adjunct in cardiogenic or septic shock: only under specialist supervision when β-stimulation desired.
• Dilution: Add 2 mg (10 mL) to 490 mL 5% dextrose → concentration 4 micrograms/mL.
• Monitor continuously: ECG, blood pressure, and urine output.

⚗️ Pharmacokinetics

• Rapid onset, short duration (half-life ≈ 2 minutes).
• Metabolised by COMT (catechol-O-methyltransferase) in the liver and other tissues.
• Administer only by controlled IV infusion — not suitable for bolus use.

🔄 Interactions

• Do not administer with adrenaline (epinephrine) or digoxin — risk of severe arrhythmia.
• Potentiated by tricyclic antidepressants, MAOIs, or thyroid hormones.
• Opposed by non-selective β-blockers.

⚠️ Cautions

• Correct hypovolaemia before use — response is poor in volume-depleted patients.
• Use cautiously in ischaemic heart disease, aortic stenosis, or hypertension — may precipitate angina or infarction.
• Can worsen myocardial oxygen demand → risk of arrhythmia or myocardial injury.
• Continuous ECG and blood pressure monitoring mandatory.

🚫 Contraindications

• Ventricular or supraventricular tachyarrhythmias.
• Tachycardia or heart block due to digitalis intoxication.
• Recent myocardial infarction or unstable angina.
• Concurrent adrenaline or digoxin therapy.
• Known or suspected phaeochromocytoma.
• Hypersensitivity to isoprenaline.

💥 Adverse Effects

• ❤️ Palpitations, tachycardia, arrhythmias, angina, hypotension, or hypertension.
• 🧠 Headache, restlessness, nervousness, insomnia.
• 💨 Tremor, flushing, and peripheral vasodilation.
• GI upset: nausea, dry mouth.
• Allergic rash or bronchospasm (rare).

🧠 Teaching Note

Isoprenaline is a classic teaching example of dose-dependent β-receptor selectivity and the physiological effects of non-selective adrenergic stimulation. It has largely been replaced by safer and more selective agents such as dobutamine and adrenaline for inotropic support, and by temporary pacing for bradycardia. However, it remains important for understanding cardiac autonomic pharmacology and β-adrenergic receptor physiology.

📚 References

• NSW Health: Isoprenaline Infusion Protocol
• BNF: Isoprenaline Hydrochloride
• Resuscitation Council UK (2023) — Bradycardia Algorithm
• Rang & Dale’s Pharmacology (10th ed.)