Key Facts

Complete heart block (third-degree AV block) is characterised by complete absence of conduction between the atria and ventricles. The atria and ventricles beat independently, with the ventricles relying on an escape rhythm at a rate slower than the sinus rate.

Epidemiology

• Incidence increases with age: rare in young adults, common in elderly
• Leading indication for permanent pacemaker implantation worldwide
• Congenital CHB: incidence ~1 in 15,000-20,000 live births
• Accounts for ~5-10% of acute MI cases with conduction disturbance

Aetiology

Acquired (most common):

• Degenerative: idiopathic fibrosis of the conduction system (Lenegre/Lev disease) — commonest cause in elderly
• Ischaemic: inferior MI (usually transient, AV nodal), anterior MI (usually permanent, infra-nodal)
• Drugs: beta-blockers, CCBs, digoxin toxicity, amiodarone
• Post-surgical: aortic valve replacement, TAVI, septal myectomy
• Inflammatory: myocarditis, endocarditis, rheumatic fever, Lyme disease
• Infiltrative: sarcoidosis, amyloidosis, haemochromatosis
• Autoimmune: SLE (anti-Ro antibodies)

Congenital:

• Maternal anti-Ro (SS-A) and anti-La (SS-B) antibodies (neonatal lupus)
• Structural congenital heart disease (corrected TGA, AV canal defects)

Pathophysiology

• Complete interruption of electrical conduction between atria and ventricles
• Atrial rhythm driven by SA node; ventricular rhythm driven by escape pacemaker
• Junctional escape (narrow QRS, 40-60 bpm): block at AV node, more reliable
• Ventricular escape (broad QRS, 20-40 bpm): block below His bundle, unreliable and prone to asystole
• Cardiac output reduced due to loss of AV synchrony and slow ventricular rate

Typical Presentation

• Symptomatic bradycardia: dizziness, presyncope, syncope (Stokes-Adams attacks)
• Fatigue, exercise intolerance, dyspnoea
• Heart failure symptoms if prolonged

Examination Findings

• Slow regular pulse (escape rate)
• Variable intensity of S1 (due to changing PR relationship)
• Cannon 'a' waves in JVP (atria contracting against closed tricuspid valve)
• Wide pulse pressure

Red Flags

• Syncope or Stokes-Adams attacks
• Haemodynamic instability: hypotension, shock
• Broad QRS escape rhythm (unreliable, risk of asystole)
• New complete heart block in anterior MI
• Heart failure or pulmonary oedema
• Associated with VT/VF (long QT from slow rate may trigger TdP)

Bedside

• 12-lead ECG: AV dissociation, escape rhythm rate and QRS width
• Continuous cardiac monitoring: essential — risk of asystole
• Observations: HR, BP, SpO₂

Bloods

• U&Es: potassium, magnesium, calcium
• Troponin: if MI suspected
• Digoxin level: if applicable
• TFTs: hypothyroidism
• FBC, CRP: if inflammatory cause suspected
• Lyme serology: if epidemiological risk
• Anti-Ro/La antibodies: if congenital or young patient with SLE features

Imaging

• Echocardiography: structural heart disease, LV function, valve disease
• Cardiac MRI: sarcoidosis, myocarditis
• Chest X-ray: cardiac size, pulmonary oedema

Special Tests

• Electrophysiology study: rarely needed for complete heart block (diagnosis usually clinical)
• Genetic testing: in congenital cases if familial

Non-pharmacological

• Admit to monitored bed (CCU/HDU)
• External transcutaneous pacing pads applied as standby
• Lie patient flat if hypotensive

Pharmacological

Acute management (per Resuscitation Council UK bradycardia algorithm):

• Atropine 500 mcg IV (repeat up to 3mg): may be effective for nodal block, often ineffective for infra-nodal
• Isoprenaline infusion 1-10 mcg/min: increases ventricular rate; bridging to pacing
• Adrenaline infusion 2-10 mcg/min: if isoprenaline unavailable
• Treat reversible causes: correct electrolytes, stop culprit drugs, treat MI

Surgical/Interventional

• Temporary transvenous pacing: urgently if haemodynamically compromised or broad QRS escape
• Transcutaneous pacing: bridge to transvenous pacing in emergency
• Permanent pacemaker implantation: definitive treatment for acquired symptomatic CHB
  • Dual-chamber pacemaker (DDD) is standard
  • Rate-responsive pacing for chronotropic incompetence
  • NICE recommends within 24-48 hours if no reversible cause
• Congenital CHB: pacemaker if symptomatic, resting HR <50 bpm, broad QRS escape, or associated structural defect

Referral Criteria

• All acquired complete heart block: urgent cardiology referral
• Temporary pacing as bridge to permanent pacemaker
• Post-MI CHB: assess need for permanent pacing before discharge

• Without pacing: significant mortality, especially with broad QRS escape (risk of asystole)
• With permanent pacemaker: near-normal life expectancy
• Post-inferior MI CHB: often transient (resolves in ~70% within 7 days), rarely needs permanent pacing
• Post-anterior MI CHB: poor prognosis (extensive infarction), even with pacing; mortality ~60-80%
• Congenital CHB with narrow QRS and no structural defect: generally favourable with monitoring; pacing if symptomatic
• Pacemaker complications: infection (~1-2%), lead displacement (~2-3%), generator replacement every 7-15 years

Resuscitation Council UK Bradycardia Algorithm Summary

Escape Rhythm Characteristics