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Rhythm

Third-degree (complete) AV block

Content last updated · 2 min read ·

This is the date the draft was written, not a medical review date.

In complete heart block no atrial impulse reaches the ventricles. Atria and ventricles beat independently (AV dissociation) and the ventricles are driven by a slow junctional or ventricular escape rhythm.

ECG criteria

  • P waves regular and QRS complexes regular, but with no consistent relationship between them (AV dissociation).
  • P-P interval shorter than R-R interval; the PR interval varies randomly.
  • Ventricular rate slow, typically 20 to 50 beats/min: about 40 to 60 for a junctional escape, 20 to 40 for a ventricular escape.
  • Narrow QRS with a junctional escape; wide QRS with a ventricular escape.
  • Some P waves may fall on QRS or T waves and be hard to see.

Mechanism

Complete interruption of conduction at the AV node, His bundle or both bundle branches leaves a lower pacemaker to take over. Junctional escape arises above the bifurcation of the His bundle and is faster and more stable than a ventricular escape, which arises distal to the block and is slower and less reliable.

Common causes

  • Degenerative conduction system disease (the commonest cause in older adults).
  • Acute myocardial infarction: inferior (usually transient, nodal) or anterior (infranodal, worse prognosis).
  • Drug toxicity (digoxin, beta-blockers, calcium channel blockers), hyperkalaemia.
  • Myocarditis, Lyme disease, endocarditis with aortic root abscess, infiltrative disease, cardiac surgery; congenital in rare cases.

Management principles

Complete heart block is a medical emergency when symptomatic: patients follow the local bradycardia and advanced life support protocols, with temporary pacing and correction of reversible causes such as drugs or electrolytes. Permanent pacing is indicated for most non-reversible cases; follow local guidelines.

High-level principles for study. Treatment decisions follow local guidelines and the clinical situation.

Pitfalls and mimics

  • A regular slow rhythm with P waves is not necessarily complete block: sinus bradycardia with 2:1 block or a junctional rhythm with isorhythmic dissociation can resemble it; look for a variable PR with P-P shorter than R-R.
  • Atrial fibrillation with a very slow and regular ventricular rate also indicates complete block.
  • Digoxin toxicity and hyperkalaemia are reversible causes that must be sought before committing to permanent pacing.

Practise in the ECG trainer — Recognise this pattern on generated tracings.

References

  1. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. American College of Cardiology / American Heart Association / Heart Rhythm Society (2018) — Tier 2 (guidelines / primary literature)
  2. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. European Society of Cardiology (2021) — Tier 2 (guidelines / primary literature)
  3. Wagner GS, Strauss DG. Marriott's Practical Electrocardiography. Wolters Kluwer, 12th edition (2013) — Tier 3 (textbook)

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