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Arrhythmias · Atrioventricular conduction disease

Complete (third-degree) AV block

Lesson 3 of 214 <2% of the exam 4 min 7 sources

What the exam asks

Diagnosis (telling it from other AV dissociation), the cause that changes management (inferior MI, Lyme disease, cardiac sarcoidosis, drugs), and the choice of acute and permanent therapy, including when not to implant a pacemaker yet and when to implant a defibrillator instead.

The one-minute version

  • No atrial impulse reaches the ventricles: P waves and QRS complexes are unrelated, the atrial rate is faster than the ventricular rate, and an escape rhythm drives the ventricles [1][2].
  • Without a reversible cause it needs a permanent pacemaker; untreated, five-year survival can be as low as 37% [1].
  • After an inferior MI it usually recovers (median about 5 days after reperfusion), so permanent pacing waits [2][3].
  • In Lyme carditis it usually resolves on antibiotics, so pace temporarily, not permanently [4].
  • In a younger patient with unexplained high-grade block, think of cardiac sarcoidosis; if a device is needed there, it is usually a defibrillator [5].
Evidence (6)
  • "AV conduction is absent in third-degree or complete heart block, and the P waves are never related to the QRS complexes." [1] PMID 29083636
  • "Untreated patients diagnosed with third-degree AV block face a 5-year survival rate as low as 37%." [1] PMID 29083636
  • "The atrial rate, demonstrated by the P wave, should be faster than the ventricular rate, as demonstrated by the QRS complex." [2] PMID 31424783
  • "The median time of AVB recovery from AMI onset was 5d" [3] PMID 40021997
  • "Routinely permanent pacemaker implantation, however, is contraindicated because of the frequent transient nature of the condition." [4] PMID 35631104
  • "Symptomatic high-grade atrioventricular block (AVB) is the most common and often the only presenting manifestation (lone AVB) of cardiac sarcoidosis." [5] PMID 30354309

Recognising it

The escape rhythm tells you the level: a narrow QRS is a junctional escape (block at the node), a wide QRS a ventricular escape (block below it) [2].

Not all AV dissociation is complete block. If the ventricles are faster than the atria (an accelerated junctional rhythm) or the two rates are nearly the same (isorhythmic dissociation), P waves may be blocked only because they arrive while the node is refractory [1].

At the bedside, atria contracting against closed AV valves give cannon a waves in the neck veins [2].

Evidence (3)
  • "Depending on the location of the block, the QRS complex might be a narrow morphology (junctional escape QRS complex) or a wide morphology (ventricular escape QRS complex)." [2] PMID 31424783
  • "JVP examination often demonstrates cannon A-waves owing to the simultaneous contraction of the atria and ventricles." [2] PMID 31424783
  • "A 12-lead ECG should be examined closely to differentiate an AV block from an isorhythmic AV dissociation and a junctional escape rhythm." [1] PMID 29083636

Causes that change the plan

Cause What to do differently
Degenerative fibrosis (the most common) Permanent pacemaker [1]
Inferior MI (right coronary artery supplies the node) Reperfuse and wait: it usually recovers [2][3]
Lyme carditis Antibiotics, temporary pacing if needed [4]
Cardiac sarcoidosis Defibrillator whenever pacing is needed [5]
Drugs (beta-blockers, calcium channel blockers, digoxin, antiarrhythmics) Stop or reverse the drug first [1]
After valve surgery or TAVR Mechanical injury near the conduction system [1]
Evidence (7)
  • "age-related degeneration of conduction system diseases is the most common cause of acquired AV block" [1] PMID 29083636
  • "Patients with suspected reversible causes of an AV block, such as digoxin toxicity, Lyme myocarditis, or an overdose of AV node-blocking drugs, often require medical therapy." [1] PMID 29083636
  • "AV blocks occurring after cardiac surgery and transcatheter aortic valve replacement are primarily caused by the mechanical effects exerted on the surrounding conduction system." [1] PMID 29083636
  • "About 5 to 10% of patients with an inferior wall MI will develop a complete heart block" [2] PMID 31424783
  • "The median time of AVB recovery from AMI onset was 5d" [3] PMID 40021997
  • "Fortunately, in most cases, AVB subsides within the first week of antibiotic treatment, so temporary pacing is the preferred modality and permanent pacemaker placement is not indicated for long-term therapy" [4] PMID 35631104
  • "The consensus recommendation to implant an intracardiac cardioverter defibrillator whenever permanent pacing is needed seems well-founded." [5] PMID 30354309

Acute treatment

Atropine is the first drug for symptomatic bradycardia, but it works on the AV node and rarely raises the rate in complete heart block, least of all with a wide escape [1][2].

If it fails: transcutaneous pacing, then a temporary transvenous wire, chosen by haemodynamics and the escape rhythm, not as a reflex; dopamine, adrenaline or isoproterenol can bridge [1][2].

Evidence (5)
  • "Unfortunately, atropine acts at the AV node and, as such, is rarely effective in raising the heart rate in patients with complete heart block." [2] PMID 31424783
  • "Transcutaneous pacing is more rapid, although both electrical and mechanical capture must be assured." [2] PMID 31424783
  • "Intravenous atropine should be considered as initial therapy when the AV block is suspected to be supra-Hisian." [1] PMID 29083636
  • "It should be carefully considered after evaluating the risk-benefit ratio in clinical settings." [1] PMID 29083636
  • "Dopamine and isoproterenol can be used in symptomatic patients with second or third-degree AV block with evidence of acute myocardial ischemia." [1] PMID 29083636

Permanent treatment

Complete block without a reversible cause gets a permanent pacemaker, with or without symptoms; a dual-chamber device is preferred over a single-chamber one unless atrial fibrillation is persistent [1][6].

With infiltrative cardiomyopathy or muscular dystrophy, a defibrillator is implanted together with pacing [1].

Evidence (3)
  • "Permanent pacemaker implantation must be considered in patients with symptomatic second-degree AV block, high-degree AV block, or third-degree AV block who do not have a reversible cause of the AV block." [1] PMID 29083636
  • "According to the current guidelines, a dual-chamber pacemaker is recommended over a single-chamber pacemaker." [1] PMID 29083636
  • "Patients who exhibit second-degree or third-degree atrioventricular (AV) block and have conditions such as infiltrative cardiomyopathy or muscular dystrophy should receive both a defibrillator and a permanent pacemaker implantation." [1] PMID 29083636

Board traps

  • A permanent pacemaker in the first days after an inferior MI is usually the wrong answer: the block mostly recovers after reperfusion [3].
  • A permanent pacemaker for Lyme carditis is wrong unless conduction has not returned after about two weeks of antibiotics [4].
  • Atropine is not the answer for a wide-complex escape rhythm: pace [2].
  • A plain pacemaker for heart block due to cardiac sarcoidosis is wrong: the sudden death risk calls for a defibrillator [5].
  • AV dissociation with a ventricular rate faster than the atrial rate is not complete heart block [2].
Evidence (5)
  • "The median time of AVB recovery from AMI onset was 5d" [3] PMID 40021997
  • "If 1:1 atrioventricular conduction is not re-established after two weeks of antibiotic therapy, permanent pacemaker implantation should be considered." [4] PMID 35631104
  • "Unfortunately, atropine acts at the AV node and, as such, is rarely effective in raising the heart rate in patients with complete heart block." [2] PMID 31424783
  • "The atrial rate, demonstrated by the P wave, should be faster than the ventricular rate, as demonstrated by the QRS complex." [2] PMID 31424783
  • "The risk of sudden cardiac death is significant in cardiac sarcoidosis presenting with high-grade AVB" [5] PMID 30354309

Europe differs

They agree on pacing complete block whatever the symptoms when no reversible cause is found [6][7].

Not yet checked against the source's own words: the guideline's full text could not be fetched by our build. The citation is right; the wording check is pending.

Think it through

Case. A 29-year-old man from Connecticut is seen in July with fatigue and light-headedness. Three weeks ago he had a spreading rash after a hike. ECG: complete heart block with a narrow escape at 44/min. Blood pressure is 112/70.

Question. What is the most appropriate management?

Show the answer

Answer. Treat it as Lyme carditis: admit to telemetry, give intravenous ceftriaxone, and use temporary pacing only if he becomes unstable. The block usually resolves within a week of antibiotics, so a permanent pacemaker is not placed now [4].

  • "First-line therapy is represented by intravenous ceftriaxone 2 g, once a day" [4] PMID 35631104
  • "Fortunately, in most cases, AVB subsides within the first week of antibiotic treatment, so temporary pacing is the preferred modality and permanent pacemaker placement is not indicated for long-term therapy" [4] PMID 35631104

References

  1. 1. Ahmed I, Goyal A, Chhabra L. Atrioventricular Block. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2024. StatPearls PMID 29083636
  2. 2. Knabben V, Chhabra L, Slane M. Third-Degree Atrioventricular Block. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. StatPearls PMID 31424783
  3. 3. Feng J, Zhang Y, Fan X. Recovery time of atrioventricular conduction and its influencing factors in patients presenting late with inferior wall acute myocardial infarction and atrioventricular block. BMC Cardiovasc Disord. 2025;25(1):138. Article PMID 40021997 doi:10.1186/s12872-025-04589-9
  4. 4. Radesich C, Del Mestre E, Medo K, et al. Lyme Carditis: From Pathophysiology to Clinical Management. Pathogens. 2022;11(5). Review PMID 35631104 doi:10.3390/pathogens11050582
  5. 5. Nordenswan HK, Lehtonen J, Ekström K, et al. Outcome of Cardiac Sarcoidosis Presenting With High-Grade Atrioventricular Block. Circ Arrhythm Electrophysiol. 2018;11(8):e006145. Article PMID 30354309 doi:10.1161/CIRCEP.117.006145
  6. 6. 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: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. Circulation. 2019;140(8):e382-e482. Guideline PMID 30586772 doi:10.1161/CIR.0000000000000628
  7. 7. Glikson M, Nielsen JC, Kronborg MB, et al. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. Eur Heart J. 2021;42(35):3427-3520. Guideline PMID 34455430 doi:10.1093/eurheartj/ehab364

After the lesson

Flashcards

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