Postoperative Atrial Fibrillation

Postoperative atrial fibrillation (POAF) is new onset atrial fibrillation after surgery, lasting over 30 seconds or needing treatment. It peaks on days 2 to 4.

Epidemiology. About 20% to 40% after cardiac surgery (20% to 30% after bypass grafting, up to 50% after valve surgery or combined procedures), 10% to 25% after lung resection and oesophagectomy, and 1% to 5% after other noncardiac surgery. It raises the risk of stroke, infarction, length of stay and long term atrial fibrillation.

Risk factors. Age over 60 to 65 years (strongest), prior atrial fibrillation, hypertension, valve disease, heart failure, COPD, obesity, sleep apnoea, chronic kidney disease, beta blocker withdrawal, hypokalaemia, hypomagnesaemia, sepsis, pain, adrenergic drugs, hypoxia and volume overload.

Prognosis. Most patients return to sinus rhythm by 6 to 8 weeks, but about a third have recurrence later.

Pathophysiology.

Atrial fibrosis and inflammation form the substrate. Triggers are sympathetic surge, pericardial inflammation, electrolyte disturbance, atrial stretch from volume load, ischaemia and pulmonary vein ectopy.

Palpitations, dyspnoea, chest pain, dizziness, hypotension or syncope, or no symptoms. The pulse is irregularly irregular.

Red flags.

• Systolic pressure under 90 mmHg, acute pulmonary oedema, ongoing chest pain or reduced consciousness.
• Rate over 150/min.
• Pre-excitation.
• Stroke or embolic features.

12-lead ECG shows absent P waves and an irregularly irregular rhythm. Check potassium (target 4.0 to 5.0 mmol/L), magnesium (target 1.0 mmol/L or more), thyroid function, troponin, full blood count and chest radiograph. Perform echocardiography for ventricular function, left atrial size, valves and pericardial effusion. Look for triggers: sepsis, hypoxia, embolism, bleeding, pain and hypovolaemia.

Differential diagnosis.

• Sinus tachycardia.
• Atrial flutter.
• Atrial tachycardia.
• Multifocal atrial tachycardia.
• Supraventricular tachycardia.

Correct triggers. Replace potassium and magnesium, treat pain, sepsis and hypoxia, and optimise volume.

• Magnesium sulphate: 2 g (8 mmol) IV over 10 to 20 minutes; single or repeated dose; maximum 4 g in 24 hours for repletion unless levels are low; monitor reflexes, respiratory rate and magnesium; reduce in renal impairment.

Unstable. Synchronised cardioversion with 120 to 200 J biphasic under sedation, then amiodarone.

Stable, rate control (resting target under 110/min, tighter if symptomatic).

• Metoprolol: 2.5 to 5 mg IV over 2 minutes, repeated every 5 minutes to 15 mg; then 25 to 50 mg orally every 6 to 12 hours; maximum 200 mg daily for most patients; monitor heart rate and blood pressure; avoid in decompensated heart failure, bradycardia, severe asthma; reduce in hepatic impairment.
• Diltiazem (preserved ejection fraction only): 0.25 mg/kg IV over 2 minutes, a second dose 0.35 mg/kg after 15 minutes if needed, then 5 to 15 mg/h infusion; monitor heart rate and blood pressure; avoid with ejection fraction under 40%, hypotension or recent IV beta blocker; reduce in hepatic impairment.
• Digoxin (with heart failure or hypotension): 0.5 mg IV over 30 minutes, then 0.25 mg every 6 hours for up to 2 doses; maintenance 0.0625 to 0.25 mg daily; monitor potassium and levels; reduce in renal impairment and older age.

Rhythm control.

• Amiodarone: 300 mg IV over 20 to 60 minutes (central line preferred for concentrations over 2 mg/mL), then 900 mg over 24 hours; maximum 1.2 g in 24 hours; monitor ECG, blood pressure, liver and thyroid function; avoid in thyroid disease or iodine allergy; reduce warfarin by 30% to 50% and halve digoxin; caution in hepatic impairment.
• Cardioversion after atrial fibrillation lasting over 48 hours needs 3 weeks of anticoagulation or exclusion of thrombus.

Anticoagulation. Use CHA2DS2-VASc and bleeding risk. Start when postoperative bleeding risk allows (typically 24 to 72 hours) if atrial fibrillation persists beyond 48 hours or recurs.

• Apixaban: 5 mg orally twice daily; at least 4 to 8 weeks, then reassess at about 3 months with rhythm monitoring; maximum 10 mg daily; monitor creatinine, haemoglobin and bleeding; use 2.5 mg twice daily if two or more of: age 80 or over, weight 60 kg or under, creatinine 133 micromol/L or over; avoid if creatinine clearance is under 15 mL/min.

Prevention. Continue chronic beta blockers. In high risk cardiac surgery, consider prophylactic amiodarone or beta blocker.

Monitoring. Continuous ECG, electrolytes, renal and thyroid function, and ambulatory rhythm monitoring after discharge.

Referral and escalation. Urgent cardiology review for instability, structural heart disease or recurrent atrial fibrillation. Electrophysiology if persistent beyond 3 months.