Dexmedetomidine-Induced Bradycardia

Dexmedetomidine induced bradycardia is a fall in heart rate, with or without hypotension, from alpha-2 adrenergic agonist effects. It is the commonest cardiovascular adverse effect of the drug and is dose and speed dependent.

Epidemiology. Bradycardia occurs in about 5% to 15% of patients in intensive care and anaesthetic use, with hypotension in about 20% to 30%. Asystole and sinus arrest are rare. A loading dose can cause a biphasic response: transient hypertension followed by hypotension and bradycardia.

Risk factors. Loading dose, rapid infusion, doses over 0.7 microgram/kg/h, older adults, hypovolaemia, beta blockers, digoxin, calcium channel blockers, other sedatives, high vagal tone, high spinal or epidural block, heart block, athletic patients and hepatic impairment.

Pathophysiology.

Stimulation of central alpha-2 receptors reduces sympathetic outflow, and heart rate and blood pressure fall. Peripheral alpha-2B receptor stimulation causes early vasoconstriction.

Baroreflex mediated vagal activity adds bradycardia, and the sinus node and conduction tissue are directly slowed.

Sinus bradycardia, hypotension, sinus pauses, first degree or second degree atrioventricular block, dizziness, reduced cardiac output and, rarely, asystole. Sedation is arousable.

Red flags.

• Heart rate under 40/min or pauses over 3 seconds.
• Hypotension with end organ signs.
• High grade atrioventricular block.
• Chest pain or ECG ischaemia.

Continuous ECG and blood pressure. Document timing against loading dose or rate change. Check electrolytes (potassium, magnesium, calcium), glucose, oxygenation, concurrent drugs and, if persistent, troponin and echocardiography.

Differential diagnosis.

• Hypoxia.
• Vagal stimulation.
• Drug interaction.
• High neuraxial block.
• Raised intracranial pressure.
• Myocardial ischaemia.
• Electrolyte disturbance.
• Hypothyroidism.
• Intrinsic conduction disease.

Prevention. Avoid or omit the loading dose in older adults, haemodynamic instability, beta blockade and heart block. If used, give 0.5 to 1 microgram/kg IV over 10 minutes or longer. Start maintenance at 0.2 to 0.7 microgram/kg/h and titrate every 30 minutes. Correct hypovolaemia before starting.

Treatment. Reduce the rate or stop the infusion. Half life is about 2 hours, so effects resolve slowly, but clinical recovery is often in 30 to 60 minutes after stopping.

• Atropine: 500 microgram IV, repeated every 3 to 5 minutes; maximum 3 mg; monitor heart rate and rhythm; use caution in glaucoma and tachyarrhythmia; may be less effective with deep sedation.
• Glycopyrronium: 200 to 400 microgram IV; repeat once; maximum 800 microgram; monitor heart rate; reduce in renal impairment.
• Fluid bolus 250 to 500 mL crystalloid for hypotension from hypovolaemia.
• Ephedrine: 3 to 6 mg IV every 3 to 4 minutes for bradycardia with hypotension; monitor blood pressure and heart rate; tachyphylaxis with repeated doses.
• Adrenaline (refractory): 10 to 20 microgram IV boluses or 0.02 to 0.1 microgram/kg/min infusion; titrate to rate; monitor ECG and blood pressure; avoid excess in ischaemia.

Use transcutaneous pacing for persistent unstable bradycardia, and start cardiopulmonary resuscitation for asystole.

Monitoring. Continuous ECG and blood pressure during infusion and for 1 to 2 hours after stopping when bradycardia occurred.

Referral and escalation. Critical care or cardiology review for high grade block, recurrent pauses or structural heart disease. Reassess the need for dexmedetomidine.

Special groups. Older adults and those with hepatic impairment need a lower dose (reduce by 50% in severe impairment). Avoid in advanced heart block unless paced.