What is the recommended dose of Isoprenaline (isoproterenol) for bradycardia and asthma?

Isoprenaline Dosing

For Bradycardia

For symptomatic bradycardia, administer isoprenaline as an intravenous infusion at 2-20 mcg/min (or 1-20 mcg/min per some protocols), titrated to heart rate response, but only after atropine has failed and in patients without coronary ischemia risk. 1, 2

Treatment Algorithm

• First-line therapy: Atropine 0.5-1 mg IV, repeated every 3-5 minutes up to a maximum total dose of 3 mg 1, 3

  • Avoid doses <0.5 mg as they may paradoxically slow heart rate further 1, 3

• Second-line therapy (if atropine fails): Isoprenaline infusion 1, 2

  • Standard dosing: 2-20 mcg/min IV infusion, titrated to hemodynamic response 1, 2
  • Alternative formulation: 1 mg in 100 mL normal saline infused to maximum of 75 mL/h (equivalent to 2-12 mcg/min) 1
  • Bolus dosing: 20-60 mcg IV bolus, followed by 10-20 mcg increments, or continuous infusion of 1-20 mcg/min 4

Critical Contraindications and Warnings

Avoid isoprenaline in any setting where coronary ischemia is suspected or likely. 1, 2 The drug increases myocardial oxygen demand through beta-1 effects while simultaneously decreasing coronary perfusion through beta-2 vasodilatory effects, potentially worsening ischemia. 1

• Isoprenaline is predominantly used in the electrophysiology laboratory and has only a second-line role in bradycardia resuscitation 1
• Two randomized trials showed no improvement in return of spontaneous circulation or survival to hospital discharge when used as adjunctive therapy in cardiac arrest 1

When Isoprenaline is Most Appropriate

• Sinus bradycardia, AV nodal block, or sinus arrest (likely to respond) 3
• Type II second-degree or third-degree AV block with wide QRS (unlikely to respond, as block is in non-nodal tissue) 3
• Heart transplant patients (preferred over atropine, which can cause paradoxical high-degree AV block) 1, 3

Monitoring Requirements

• Continuously monitor heart rate, blood pressure, and ECG during administration 2
• Higher doses (>20 mcg/min) may cause vasoconstriction or arrhythmias 2

Alternative Agents

If isoprenaline is unavailable or contraindicated, consider:

• Dopamine: 5-10 mcg/kg/min IV, titrated to response 1, 3
• Epinephrine: 2-10 mcg/min IV infusion 1, 4, 3
• Transcutaneous or transvenous pacing for medication-refractory bradycardia 1, 3

Important Pitfall

Paradoxical bradycardia can occur with isoprenaline infusion in approximately 7% of patients, particularly in young patients with hypervagotonia or those with infraHisian AV block. 5 This phenomenon may reveal underlying organic conduction disturbances, especially in patients with exercise-related syncope. 5

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For Asthma

For refractory bronchial asthma, isoprenaline can be administered as an IV infusion at 0.0375 to 0.225 mcg/kg/min, with maximal bronchodilator effect occurring within 2-5 minutes of starting or stopping the infusion. 6

Dosing Details

• Dose range: 0.0375-0.225 mcg/kg/min IV infusion 6
• Onset of action: 2-5 minutes 6
• Offset of action: 2-5 minutes after stopping infusion 6
• Bronchodilation improves with each dose increment 6

Monitoring and Safety

• Heart rate reaches maximum at approximately 0.075 mcg/kg/min 6
• At lower doses, alveolar-arterial oxygen gradients may widen and heart rate increases 6
• At higher doses, as heart rate plateaus, bronchodilation continues and oxygen gradient narrows 6
• Requires continuous monitoring of cardiac rhythm, blood pressure, and adequate oxygenation 6

Clinical Advantages

• Delivers bronchodilator to airways not reached by inhalation 6
• Prompt onset and offset of effects allows rapid titration 6
• Reversibility of undesirable side effects 6
• May prevent need for intubation and mechanical ventilation 6

Critical Warning for Asthma Use

Isoprenaline is cardiotoxic during hypoxia and can cause fatal cardiac asystole rather than the expected tachycardia. 7 In hypoxemic conditions (PaO2 reduced from 84 to 38 mmHg), doses that normally produce tachycardia (10-50 mcg/kg) instead caused bradycardia, reduced arterial pressure, and death from cardiac asystole without ventricular fibrillation. 7 This effect can be prevented by pretreatment with propranolol, though this would negate the bronchodilator effect. 7 Therefore, adequate oxygenation must be ensured before and during isoprenaline administration in asthmatic patients.