Complete Heart Block

Complete (third degree) heart block is total failure of AV conduction, with independent atrial and ventricular activity (AV dissociation), maintained by a junctional (narrow QRS, 40 to 60 beats per minute) or ventricular escape rhythm (wide QRS, 20 to 40 beats per minute).

• Causes:
  • Degenerative conduction system disease (Lenègre, Lev), most common in the elderly.
  • Ischaemia: inferior myocardial infarction (transient, nodal level, narrow escape), anterior infarction (infranodal, wide escape, poor prognosis).
  • Drugs: beta blockers, calcium channel blockers, digoxin, amiodarone.
  • Infiltrative or inflammatory: sarcoidosis, amyloidosis, Lyme carditis, Chagas disease, rheumatic fever, endocarditis with aortic root abscess, myocarditis.
  • Congenital (maternal anti Ro/La antibodies, 1 in 20,000), after cardiac surgery or TAVI, hyperkalaemia, ablation.
• Complications: syncope (Stokes Adams attacks), heart failure, polymorphic VT, asystole, sudden death.

• Fatigue, dizziness, syncope, dyspnoea, chest pain, confusion; sudden cardiac arrest.
• Bradycardia (30 to 50 beats per minute), regular pulse, variable intensity first heart sound, cannon a waves in the jugular venous pulse, wide pulse pressure, hypotension.
• Congenital: fetal bradycardia, neonatal heart failure.

• ECG: P waves unrelated to QRS complexes, atrial rate faster than ventricular rate, QRS narrow (junctional) or wide (ventricular escape).
• Telemetry or Holter monitoring for paroxysmal block.
• Reversible causes: electrolytes (potassium), digoxin level, Lyme serology, troponin, thyroid tests, drug review.
• Echocardiography, cardiac MRI or PET (sarcoid, amyloid), coronary angiography for ischaemia.
• Electrophysiology study for level of block (His bundle recording): infra Hisian block is high risk.

Immediate

• Unstable (hypotension, syncope, heart failure, ischaemia): atropine 0.5 to 1 mg IV (often ineffective in infranodal block), dopamine 5 to 20 micrograms/kg/min or epinephrine 2 to 10 micrograms/min, transcutaneous pacing as a bridge, then transvenous pacing.
• Treat reversible causes: stop AV nodal blocking drugs, correct hyperkalaemia, digoxin immune Fab for toxicity, antibiotics for Lyme carditis (ceftriaxone 2 g IV daily for 14 to 21 days; block often resolves in 1 week).
• Inferior MI block is often transient; reperfusion and observation (temporary pacing only for symptoms).

Definitive

• Permanent pacemaker for symptomatic or persistent complete heart block regardless of symptoms in most cases (acquired, non reversible). Dual chamber pacing (DDD) preferred to maintain AV synchrony; conduction system pacing (His bundle or left bundle branch area pacing) or biventricular pacing if ejection fraction reduced or high ventricular pacing burden expected.
• Congenital complete heart block: pacing for symptoms, heart rate below 50, wide QRS, long QT, ventricular dysfunction; fetal management with dexamethasone 4 mg daily (limited evidence) and monitoring in anti Ro/La positive mothers (hydroxychloroquine 200 mg twice daily for prevention in prior affected pregnancies).
• Sarcoid and giant cell myocarditis: pacing with ICD plus immunosuppression.
• Post TAVI: pacemaker when persistent high grade block over 24 to 48 hours.
• Follow up device interrogation every 6 to 12 months.