Spinal Anaesthesia-Induced Bradycardia

Spinal anaesthesia induced bradycardia is a heart rate under 50/min (or fall of over 20% to 30%) after subarachnoid block. Severe cases progress to asystole.

Epidemiology. Bradycardia affects about 10% to 15% of spinal anaesthetics, and cardiac arrest occurs in about 1 in 1,500 to 1 in 3,000 spinal anaesthetics (higher in some reports).

Risk factors. Baseline heart rate under 60/min, age under 50 years, male sex, ASA 1, block higher than T6, beta blockers, calcium channel blockers, first degree heart block, young fit patients, opioid or dexmedetomidine sedation, and venous return reduction (hypovolaemia, head up position).

Pathophysiology.

Block of cardiac accelerator fibres (T1 to T4) leaves vagal tone unopposed. Reduced venous return activates the Bezold-Jarisch reflex (ventricular mechanoreceptors and chemoreceptors) and the right atrial stretch (Bainbridge) response falls, producing profound bradycardia and vasodilation.

Slow pulse, dizziness, nausea, sweating, hypotension, loss of consciousness, and sinus arrest or asystole in severe cases, often abrupt.

Red flags.

• Heart rate under 40/min.
• Pauses over 3 seconds.
• Loss of consciousness.

ECG shows sinus bradycardia, junctional rhythm or atrioventricular block. Check block height, blood pressure and drug history. Exclude hypoxia, vagal stimulation (surgical traction), and electrolyte disturbance.

Differential diagnosis.

• High spinal.
• Hypoxia.
• Vagal reflex from surgery.
• Drug effect.
• Anaphylaxis.
• Hypovolaemia.

Stop vagal stimulation, give oxygen, lay the patient flat or head down, and give fluid.

• Atropine: 500 microgram IV, repeated every 3 to 5 minutes; maximum 3 mg; monitor heart rate and rhythm; use caution in glaucoma and tachyarrhythmia.
• Glycopyrronium: 200 to 400 microgram IV; repeat once; maximum 800 microgram; reduce in renal impairment.
• Ephedrine: 6 to 12 mg IV every 2 to 3 minutes; maximum 50 mg; monitor blood pressure; tachyphylaxis.
• Adrenaline: 10 to 20 microgram IV early for refractory bradycardia or asystole, then 0.05 to 0.5 microgram/kg/min; for arrest 1 mg IV every 3 to 5 minutes; monitor ECG and blood pressure.

Use transcutaneous pacing if drug treatment fails, and begin cardiopulmonary resuscitation for arrest.

Prevention. Give atropine or glycopyrronium early in high risk patients, avoid drugs that slow heart rate, treat hypotension quickly and avoid sudden positional change.

Monitoring. Continuous ECG and blood pressure during and for at least 30 minutes after the block.

Referral and escalation. Senior help and critical care for arrest. Cardiology review for persistent bradycardia after block resolution.

Special groups. Young, fit men are at greatest risk of arrest.