Atrioventricular Block
Definition
Atrioventricular (AV) block is a disorder of conduction between the atria and ventricles, classified by site (AV nodal, intra-Hisian, infra-Hisian) and by ECG severity (first-degree through third-degree). The two axes are not interchangeable: severity describes what the ECG shows today, site describes what happens next. Together they determine symptom burden, the risk of progression to complete block, and the indication for pacing. Unlike sinus node dysfunction, where the symptom–bradycardia correlation is the whole indication, infranodal AV block warrants pacing regardless of symptoms — progression can be abrupt and the available escape rhythm is slow or absent. (sources/bradycardia-acc-aha-hrs-2018, rating: very high)
Key Concepts
Etiology
| Cause | Typical clue | Notes |
|---|---|---|
| Degenerative | Age, hypertension, diabetes | Most common in practice; fibrosis of the AV node and His-Purkinje system |
| Ischaemic | Inferior MI → nodal; anterior MI → infranodal | Inferior: usually transient. Anterior: extensive damage, worse prognosis |
| Infectious | Lyme carditis | Advanced AVB in 80–90% of cardiac Lyme, usually transient; also perivalvular abscess, Chagas (RBBB + LAFB in >50%), rheumatic fever |
| Infiltrative / inflammatory | Unexplained AVB under 60 | ~30% of under-60s with unexplained AVB have undiagnosed cardiac sarcoidosis; also amyloidosis and myocarditis |
| Autoimmune | Rheumatoid arthritis, SLE | RA → nodal infiltration, RBBB in ~35%; SLE → sinus or AV nodal dysfunction in up to 70% |
| Iatrogenic | Cardiac surgery, ablation | PPM in 1–3% after CABG, 5% after mitral replacement, up to 12% after aortic valve replacement; TAVR, myectomy, catheter ablation |
| Neuromuscular | Family history, myopathy | Myotonic dystrophy type 1, Kearns-Sayre, Emery-Dreifuss, limb-girdle dystrophies |
| Genetic | Young, familial, progressive | SCN5A (76% of familial conduction disease), LMNA, KCNJ2, PRKAG2, GLA (Fabry) |
| Congenital | Maternal anti-SSA/Ro; L-TGA | See below |
(sources/conduction-disorders-jaha-2025, rating: high; sources/bradycardia-acc-aha-hrs-2018, rating: very high)
- The differential is age-dependent, which is the reason to take an age. Under 50, conduction disorders are ~13% of all conduction disease and the ranking inverts: post-cardiac surgery 15.3%, congenital AV block 9.0%, cardioinhibitory reflex 5.0%, congenital heart disease 4.2%, post-ablation 3.4%, cardiomyopathy 3.0% — and ischaemic heart disease only 1.4%. The cause is unknown in about half. (sources/conduction-disorders-jaha-2025)
- Congenital complete AV block is an antibody disease. Maternal anti-SSA/Ro antibodies cross the placenta by neonatal Fc receptor and inflame the fetal AV node, with peak onset at 18–24 weeks' gestation. Mortality is 17.5%; 93% of affected newborns have complete block; two-thirds require a pacemaker before adulthood and 10% develop cardiomyopathy-related heart failure. Transplacental corticosteroid plus IV immunoglobulin reverses second-degree block in ~25% — the only reversible window in the disease. (sources/conduction-disorders-jaha-2025)
Classification by Site
| Site | Characteristics |
|---|---|
| AV nodal | Slower progression; faster and more reliable junctional escape (40–60 bpm); responds to atropine and catecholamines |
| Intra-Hisian | Within the His bundle itself; often narrow-QRS escape; response to atropine variable |
| Infra-Hisian | Rapid and unpredictable progression; slow or unreliable ventricular escape (<40 bpm); does not respond to atropine; may respond to catecholamines |
(sources/bradycardia-acc-aha-hrs-2018)
- Site is a physiological inference from the surface ECG, not a finding on it. Mobitz II and a wide-QRS third-degree block point infranodal; Mobitz I with a narrow QRS points nodal. The surrogates mislead often enough that 2:1 block is explicitly unclassifiable without an electrophysiology study.
ECG Classification and Definitions
| Type | Definition |
|---|---|
| First-degree (AV delay) | PR interval >200 ms with 1:1 conduction; not true block |
| Second-degree Mobitz I (Wenckebach) | Gradual PR prolongation → periodic single non-conducted P wave; usually AV nodal |
| Second-degree Mobitz II | Constant PR before a periodic single non-conducted P wave; usually infranodal |
| 2:1 AV block | Every other P wave not conducted; cannot be classified as Mobitz I or II |
| High-grade/advanced AV block | ≥2 consecutive non-conducted P waves at a physiological rate, with some AV conduction preserved |
| Third-degree (complete) AV block | No AV conduction whatsoever; paroxysmal or persistent; junctional or ventricular escape |
| Vagally mediated AV block | Any type driven by heightened parasympathetic tone; identified by concomitant sinus slowing (P-P prolongation) |
(sources/ecg-bbb-aha-2009, rating: high; sources/bradycardia-acc-aha-hrs-2018)
- 2:1 block is a severity, not a mechanism — an EP study may be needed to establish the level, and therefore the indication.
- Complete AV block in AF is suspected when the ventricular response is slow (<50 bpm) and regular; AF with a slow irregular response is not block.
- Isorhythmic dissociation and atrial bigeminy mimic AV block; VT produces AV dissociation without block. Both errors change management, so the strip deserves scrutiny before the diagnosis is made.
Clinical Presentation
- Determined by whether block is fixed or intermittent, the ventricular rate, and the aetiology. (sources/bradycardia-acc-aha-hrs-2018, rating: very high)
- Vagally mediated nocturnal block may be asymptomatic, because the sinus node slows simultaneously — which is also why it is benign.
- Symptomatic block produces syncope, presyncope, dyspnoea and heart failure symptoms, or confusional states from cerebral hypoperfusion (sources/syncope-aha-acc-hrs-2017, rating: very high).
- LBBB may present with heart failure from dyssynchrony or from the underlying cardiomyopathy rather than from the block itself.
Diagnostic Workup for Unexplained AV Block
- Under 60, unexplained AV block is a diagnostic problem rather than a diagnosis. About 30% have cardiac sarcoidosis, and roughly half have no identifiable cause at all. (sources/conduction-disorders-jaha-2025, rating: high)
- Serology for Lyme disease when there is an exposure history, and for Trypanosoma cruzi in endemic regions or migrant populations.
- CMR and FDG-PET for sarcoidosis — particularly with AV block plus LV dysfunction, ventricular arrhythmia, or conduction disease that does not fit the patient's age.
- Genetic testing in progressive conduction disease under 50 is recommended by European guidelines and remains underused. The gene that changes management is LMNA, because it moves the device decision from pacemaker to ICD.
- Family history and cascade ECG screening belong in every young proband — conduction disease is frequently the first manifestation of a cardiomyopathy that has not yet remodelled.
- Electrophysiology study when the level of block determines the device, chiefly in 2:1 block.
Acute Management
| COR | LOE | Recommendation |
|---|---|---|
| I | C-EO | Evaluate and treat reversible causes (Lyme carditis, medications, AMI, electrolytes) |
| IIa | C-LD | Atropine for symptomatic or haemodynamically significant AV nodal block |
| IIa | B-NR | Beta-agonists (dopamine, epinephrine, isoproterenol) for refractory AV nodal block, or infranodal block unresponsive to atropine |
| I | — | Temporary pacing for medically refractory, haemodynamically significant AV block |
(sources/bradycardia-acc-aha-hrs-2018, rating: very high)
- Atropine does not work on infranodal block — a vagolytic cannot restore conduction below the node — and it does not work after cardiac transplantation, where there is no vagal innervation to withdraw.
Natural History
- Untreated Mobitz II and third-degree AV block progress to recurrent syncope, heart failure and excess mortality, and pacing improves survival — the evidence being 1970s–80s cohort studies that would not be repeated today. (sources/bradycardia-acc-aha-hrs-2018, rating: very high)
- AV block in inferior MI is usually transient (vagal and AV nodal ischaemia); in anterior MI it marks extensive myocardial damage, and long-term outcome is set by infarct size rather than by the block.
- A first pacemaker before age 50 carries a 3–4-fold increased risk of adverse outcomes within five years — an argument for getting the aetiology right and for choosing the pacing strategy deliberately in the young. (sources/conduction-disorders-jaha-2025, rating: high)
Permanent Pacing — the Short Version
| Situation | Action |
|---|---|
| Mobitz II, high-grade or third-degree AV block not from a reversible cause | Pace, regardless of symptoms |
| Symptomatic block at nodal level, or essential GDMT with no alternative | Pace |
| Asymptomatic first-degree or Mobitz I | Observe |
Full detail: concepts/Permanent-Pacing-Indications.
- Where pacing is required, the mode matters as much as the decision. Dual-chamber pacing is preferred to VVI; where ventricular pacing will exceed 40% in a patient with LVEF 36–50%, conduction system pacing or CRT is preferred to right ventricular pacing, because RV pacing at that burden produces pacing-induced cardiomyopathy. (sources/bradycardia-acc-aha-hrs-2018)
Special Populations
- TAVR: new LBBB in 19–55%, new high-grade AV block in ~10%, and up to half of these resolve before discharge — but 29% of patients with new LBBB have their first high-grade episode after discharge, which is the surveillance problem. (sources/bradycardia-acc-aha-hrs-2018)
- Acute MI: a waiting period precedes any permanent device decision, because inferior-territory block usually resolves; pacing is reserved for block that persists or is infranodal.
- Both have their own graded recommendations in concepts/Permanent-Pacing-Indications.
Paroxysmal AV Block
- PAVB is the sudden onset of complete AV block with prolonged ventricular asystole, distinct from gradual block, and a specific high-risk subset — the mechanism of syncope in a patient whose resting ECG looks unremarkable. (sources/PAVB-HR-2009, rating: high)
- Two forms: TD-PAVB (tachycardia-dependent, from repetitive concealed conduction in diseased His-Purkinje tissue at faster atrial rates) and PD-PAVB (pause-dependent, from source-sink mismatch or phase 4 depolarisation after a long diastolic interval).
- The His bundle is the commonest site (7 of 10 cases at EPS in one series), even when bundle branch block dominates the surface ECG, and intra-Hisian disease is frequently mislabelled as nodal or infra-Hisian unless split His potentials are carefully dissected.
- The terms "phase 3" and "phase 4 block" are mechanistic misnomers; TD-PAVB and PD-PAVB are preferred. Full mechanism: concepts/Paroxysmal-AV-Block.
Contradictions / Open Questions
- AV block in AF cannot be diagnosed with confidence. Whether pauses >3 s in AF are symptomatic or incidental rests on conflicting retrospective data, and the decision to pace may turn on a correlation that no monitor established. (sources/bradycardia-acc-aha-hrs-2018, rating: very high)
- TAVR has no evidence-based timing. The optimal duration and method of monitoring after TAVR are unstandardised, there are no randomised trials of PPM timing, and early pacing for new LBBB is not protective against the mortality associated with that LBBB.
- The guideline predates the pacing trials that answer its own question. Physiologic pacing versus RV pacing in LVEF >50% was not addressed by the 2018 document, and the conduction system pacing literature has since moved further than it.
- 2:1 block is the classification's blind spot, and the test that resolves it is invasive, operator-dependent and not always available. A substantial group is managed on inference.
- Sarcoid-related block may reverse on corticosteroids, yet pacing is still recommended because reversibility is unpredictable — which leaves no way to identify the patients who could safely avoid a device.
- Lyme AV block is usually transient but occasionally needs pacing, and no clinical or ECG feature reliably separates the two at presentation.
- Two-thirds of congenital complete AV block are paced before adulthood with no randomised evidence behind it, and the thresholds (rate, symptoms, LV dysfunction) are consensus rather than trial-derived.
- Half of young adults with conduction disease have no identifiable cause, which is simultaneously an argument for universal genetic testing and a reason it will usually be negative — and the yield-versus-cost question has not been settled.
- PD-PAVB mechanism remains unresolved: the relative contributions of phase 4 depolarisation and source-sink mismatch are disputed, and isolated Purkinje fibre data may not translate to the intact conduction system. (sources/PAVB-HR-2009, rating: high)
Connections
- Related to concepts/Sinus-Node-Dysfunction — the same fibrotic substrate; the two frequently coexist
- Related to concepts/Paroxysmal-AV-Block — the high-risk paroxysmal subtype and its mechanism
- Related to concepts/Permanent-Pacing-Indications — the full graded indications, including TAVR and AMI
- Related to concepts/Conduction-System-Pacing — His bundle and left bundle branch pacing for AV block
- Related to concepts/Cardiac-Resynchronization-Therapy — the CRT option when pacing burden is high
- Related to concepts/ECG-Conduction-Disturbances — ECG definitions and bundle branch block criteria
- Related to concepts/Cascade-Family-Screening — family ECG screening in young probands
- Related to entities/ATTR-Amyloidosis — AV block as a manifestation of infiltrative cardiomyopathy
- Related to entities/LMNA — laminopathy conduction disease, where the device is an ICD
- Related to entities/Atrial-Fibrillation — diagnosing block in AF, and pacing for AF with symptomatic bradycardia