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Isoprenaline Infusion - Clinical Guideline

Pharmacology

• β1 effects: Increased heart rate (chronotropy), increased contractility (inotropy), increased automaticity and conduction velocity
• β2 effects: Vasodilation (skeletal muscle, renal, mesenteric), bronchodilation, smooth muscle relaxation
• Net hemodynamic effect: Increased cardiac output, decreased diastolic BP, decreased systemic vascular resistance, mean arterial pressure typically falls
• Onset: Immediate (IV)
• Half-life: 2.5-5 minutes
• Duration of action: 10-15 minutes
• Metabolism: Primarily by COMT in the liver; not significantly taken up by sympathetic neurons (unlike epinephrine/norepinephrine), so duration may be slightly longer than epinephrine

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Indications

Primary (Emergency) Indications

1. Symptomatic life-threatening bradycardia not responding to atropine (hypotension, syncope, myocardial ischaemia, heart failure, reduced consciousness)
2. Mobitz Type II AV block (second-degree heart block)
3. Complete (third-degree) heart block with broad QRS complexes
4. Ventricular pauses > 3 seconds
5. Recent asystole
6. Refractory torsades de pointes (TdP) - used as "pharmacological overdrive pacing" to shorten the QT interval and suppress bradycardia-dependent TdP

Other Uses

• Bridge therapy to temporary or permanent cardiac pacing
• Post-cardiac transplant (denervated heart)
• Beta-blocker overdose (high doses may be needed - consult toxicologist)

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Contraindications

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Preparation & Dilution

Standard Concentration (Peripheral or Central)

• Dilute: 2 mg (10 mL) isoprenaline HCl in 500 mL Glucose 5%
• Concentration: 4 mcg/mL
• Use Glucose 5% only - isoprenaline is acidic and unstable at higher pH; NaCl 0.9% increases vascular irritation and may cause decomposition

Concentrated (Fluid Restriction - Central Line Only)

• Dilute: 2 mg (10 mL) isoprenaline HCl in 50 mL Glucose 5%
• Concentration: 40 mcg/mL

Peripheral use: Maximum concentration 4 mcg/mL. Prefer central access when available.

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Dosing

Adults - Standard Protocol

• Starting dose: 1-2 mcg/min
• Titration: Increase by 1-2 mcg/min every 2-3 minutes
• Usual range: 2-10 mcg/min
• Maximum dose: 10 mcg/min (higher doses used in beta-blocker overdose under toxicologist guidance)

Weight-Based Dosing (Paediatric/ICU)

• 0.05-1 mcg/kg/min (up to 2 mcg/kg/min in beta-blocker overdose)

Titration Goal

• Aim for haemodynamic stability and end-organ perfusion - NOT a specific heart rate target
• Do NOT rapidly up-titrate - poorly tolerated and increases adverse effects
• If haemodynamically stable, even a low heart rate is acceptable

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Monitoring Requirements

• Continuous 3-lead cardiac monitoring (ECG)
• Automated blood pressure monitoring
• Pulse oximetry
• 12-lead ECG of underlying rhythm before starting
• Urine output assessment (end-organ perfusion marker)
• Blood glucose (beta-2 stimulation can cause hyperglycaemia)
• Electrolytes (hypokalaemia worsens arrhythmia risk)

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Special Situations

Torsades de Pointes

• Used alongside IV magnesium sulfate (1-2 g IV rapid bolus, repeat as needed)
• Acts as "pharmacological overdrive pacing" - increases intrinsic rate, shortens QT interval, suppresses bradycardia-dependent TdP
• Discontinue any QT-prolonging drugs
• Definitive treatment is ventricular pacing when available

Complete Heart Block

• Use as a bridge to definitive pacing (temporary or permanent)
• Most patients tolerate low heart rates; titrate only enough to maintain perfusion

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Adverse Effects

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Pre-Administration Checklist

• Confirm indication: life-threatening bradycardia / TdP not responding to first-line treatment
• Review contraindications
• Correct electrolyte abnormalities (especially K⁺, Mg²⁺)
• Correct acidosis prior to starting
• Ensure adequate circulating blood volume
• Confirm appropriate monitoring in place
• Confirm high-dependency environment and senior clinician supervision
• Discuss with cardiologist re: urgent pacing if in working hours

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Key Clinical Pearls

1. Isoprenaline is a bridge, not a destination - always plan for definitive pacing in complete heart block
2. Atropine is first-line for bradycardia; isoprenaline is used when atropine fails or is contraindicated
3. In torsades de pointes, the goal is to suppress bradycardia - overdrive pacing (external or transvenous) is preferred if available
4. Never give simultaneously with adrenaline (epinephrine) - combined use can cause serious ventricular arrhythmias
5. Isoprenaline infusion is NOT suitable for cardiogenic shock with hypotension from causes other than bradycardia, as it may worsen hypotension via vasodilation