โก Isoprenaline (Isoproterenol) is a non-selective ฮฒ-adrenergic agonist with potent chronotropic and inotropic effects, but minimal ฮฑ-adrenergic activity.
It is primarily used to treat bradyarrhythmias or heart block while awaiting pacing.
Because of its strong ฮฒโ and ฮฒโ stimulation, it increases both heart rate and myocardial oxygen demand - so close monitoring is essential.
๐ฉบ Always correct hypovolaemia before commencing an infusion.
๐ 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 ฮฒ-blocker toxicity.
- Acts on both ฮฒโ (heart) and ฮฒโ (vessels, bronchi) receptors โ increased heart rate, contractility, and vasodilation.
โ๏ธ Mode of Action
- Non-selective ฮฒ-adrenergic agonist - stimulates both ฮฒโ and ฮฒโ receptors.
- ฮฒโ effect: โ heart rate (positive chronotropy), โ contractility (positive inotropy), โ conduction through AV node.
- ฮฒโ effect: peripheral vasodilation โ โ diastolic BP, โ mean arterial pressure.
- Minimal ฮฑ activity โ little vasoconstrictive effect.
- Overall: โ cardiac output but risk of hypotension and tachyarrhythmia.
๐ Indications & Dosing
- Bradyarrhythmias / Heart block (pending pacing): 2โ10 micrograms/min by IV infusion, titrated to heart rate and rhythm response.
- ฮฒ-blocker overdose: start at 2โ4 micrograms/min, titrate cautiously.
- Adjunct in cardiogenic or septic shock: only under specialist supervision when ฮฒ-stimulation desired.
- Dilution: Add 2 mg (10 mL) to 490 mL 5% dextrose โ concentration 4 micrograms/mL.
- Monitor continuously: ECG, blood pressure, and urine output.
โ๏ธ Pharmacokinetics
- Rapid onset, short duration (half-life โ 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.
๐ 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 ฮฒ-blockers.
โ ๏ธ 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 โ risk of arrhythmia or myocardial injury.
- Continuous ECG and blood pressure monitoring mandatory.
๐ซ 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.
๐ฅ Adverse Effects
- โค๏ธ Palpitations, tachycardia, arrhythmias, angina, hypotension, or hypertension.
- ๐ง Headache, restlessness, nervousness, insomnia.
- ๐จ Tremor, flushing, and peripheral vasodilation.
- GI upset: nausea, dry mouth.
- Allergic rash or bronchospasm (rare).
๐ง Teaching Note
Isoprenaline is a classic teaching example of dose-dependent ฮฒ-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 ฮฒ-adrenergic receptor physiology.
๐ References
