Complete Heart Block: Third-Degree AV Block on ECG
In complete (third-degree) heart block no atrial impulse reaches the ventricles. P waves and QRS complexes each keep their own regular rate, there are more P waves than QRS complexes, and an escape rhythm from the junction or the ventricles keeps the heart beating. Treat it as an emergency until the escape rhythm is stable and the cause is known.
At a glance
- P waves regular, QRS complexes regular, no fixed relation between them, and the atrial rate faster than the ventricular rate.
- A narrow escape, often 40 to 60 bpm, comes from the junction and usually means block at the AV node. A wide escape, often 20 to 40 bpm, comes from the ventricles and can fail without warning.
- If the ventricles beat as fast as or faster than the atria, it is AV dissociation from another cause, not complete block.
- Apply pacing pads, look for reversible causes (ischemia, drugs, hyperkalemia, Lyme carditis) and arrange temporary, then permanent, pacing.
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Criteria
AV dissociation
P waves and QRS complexes each regular, with PR intervals that vary at random because no P wave conducts.
Atrial rate faster than ventricular rate
More P waves than QRS complexes. If the QRS rate is the faster one, look for an accelerated junctional or ventricular rhythm instead.
Escape rhythm
A narrow QRS at about 40 to 60 bpm from the junction, or a wide QRS at about 20 to 40 bpm from the ventricles.
What it looks like
How to measure it
- March out the P waves: they keep their own regular rate through the QRS complexes and T waves.
- March out the QRS complexes: they are regular too, and slower.
- Check the PR intervals. If they vary with no pattern and the atrial rate is faster, the block is complete.
- Measure the QRS width and the ventricular rate to place the escape rhythm.
- In atrial fibrillation, complete heart block shows up as a slow, regular ventricular rhythm despite the chaotic atrial activity.
Causes
Often reversible
- Inferior myocardial infarction, usually nodal and transient
- AV nodal blocking drugs and digoxin toxicity
- hyperkalemia
- Lyme carditis
- high vagal tone
Structural or degenerative
- Fibrosis of the conduction system with age
- anterior myocardial infarction
- after cardiac surgery, TAVR or septal reduction
- cardiac sarcoidosis and amyloidosis
- endocarditis with an aortic root abscess
- neuromuscular disease
Congenital
- Maternal anti-Ro/SSA antibodies (neonatal lupus)
- congenitally corrected transposition of the great arteries
What to do
- Check perfusion first. In an unstable patient give atropine (it may fail when the block is below the AV node), then start transcutaneous pacing or a dopamine or epinephrine infusion, and arrange urgent transvenous pacing.
- Stable patient: continuous monitoring, pacing pads on, and admission to a monitored bed.
- Treat reversible causes: stop AV nodal blockers, correct potassium, give digoxin-specific antibody fragments for digoxin toxicity, treat ischemia, and test for Lyme disease in endemic areas.
- Lyme carditis usually recovers with antibiotics. Temporary pacing bridges the recovery and a permanent pacemaker is rarely needed.
- Without a reversible cause, a permanent pacemaker is indicated, with or without symptoms.
Pitfalls and mimics
- Calling 2:1 or high-grade block complete while some P waves still conduct with a constant PR.
- Calling AV dissociation from a faster junctional or ventricular rhythm, for example in digoxin toxicity or VT, complete heart block.
- Deciding P waves are absent before marching them out through the QRS complexes and T waves.
Key studies
Kusumoto 2019, 2018 ACC/AHA/HRS bradycardia guideline
Permanent pacing is recommended for acquired third-degree AV block that isn't caused by a reversible or physiologic problem, with or without symptoms.
Forrester 2014, Clin Infect Dis
In 45 published US cases of third-degree block from Lyme carditis, the median age was 32, 84% were male and 39% needed temporary pacing.
Panchal 2020, AHA adult life support guidelines
Atropine first for symptomatic bradycardia, then transcutaneous pacing or a dopamine or epinephrine infusion if it fails.
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Questions clinicians ask
What does complete heart block look like on an ECG?
Regular P waves and regular, slower QRS complexes that have no fixed relation to each other: the PR changes from beat to beat and there are more P waves than QRS complexes.
What is the difference between complete heart block and AV dissociation?
Complete heart block is one cause of AV dissociation. In complete block the atria beat faster than the ventricles and no P wave conducts. AV dissociation without block happens when a junctional or ventricular rhythm runs as fast as or faster than the sinus rate.
Is complete heart block an emergency?
Yes, until proven otherwise. The escape rhythm can slow or stop, especially when it is wide. Monitor the patient with pacing pads on and arrange pacing.
Can complete heart block go away?
Yes, when the cause is reversible: block from inferior MI, drug toxicity, hyperkalemia or Lyme carditis often resolves. Degenerative block does not, and needs a permanent pacemaker.
Sources
- Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Circulation. 2019;140(8):e382-e482. doi.org/10.1161/CIR.0000000000000628
- Panchal AR, Bartos JA, Cabanas JG, et al. Part 3: Adult basic and advanced life support: 2020 American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care. Circulation. 2020;142(16 suppl 2):S366-S468. doi.org/10.1161/CIR.0000000000000916
- Forrester JD, Mead P. Third-degree heart block associated with Lyme carditis: review of published cases. Clin Infect Dis. 2014;59(7):996-1000. doi.org/10.1093/cid/ciu411
- Barold SS, Hayes DL. Second-degree atrioventricular block: a reappraisal. Mayo Clin Proc. 2001;76(1):44-57. doi.org/10.4065/76.1.44
Written in our own words by 2 Minute Medicine's physician editors from the sources listed here. The diagrams are schematic drawings made for 2MM, not patient tracings. This page supports, and does not replace, a full ECG reading and clinical judgment.
For health care professionals. Values are for adults unless a page says otherwise.
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