Atrioventricular Block

Definition

Atrioventricular (AV) block is a disorder of conduction between the atria and ventricles. It is classified anatomically by site (AV nodal, intra-Hisian, or infra-Hisian) and by electrocardiographic severity (first-degree through third-degree). The site and severity determine symptom burden, risk of progression, and indication for permanent pacing. Unlike SND, infranodal AV block warrants pacemaker regardless of symptoms because of risk of sudden complete AV block.

Key Concepts

Etiology

• Degenerative (most common in clinical practice): age-dependent fibrosis of the AV node/His-Purkinje system; associated with hypertension, diabetes mellitus (sources/bradycardia-acc-aha-hrs-2018, very high)
• Infectious: Lyme carditis (reversible with antibiotics — important to exclude); bacterial endocarditis with perivalvar abscess; Chagas disease; acute rheumatic fever
• Inflammatory/infiltrative: cardiac sarcoidosis, amyloidosis, myocarditis; rheumatologic diseases (systemic sclerosis, SLE, RA)
• Ischemic: inferior wall MI (often transient, vagal/AV nodal ischemia); anterior MI with AV block → extensive myocardial damage, worse prognosis
• Iatrogenic: cardiac surgery (aortic valve, TAVR, septal myectomy, TAVI), catheter ablation
• Neuromuscular: myotonic dystrophy type 1, Kearns-Sayre syndrome, Emery-Dreifuss, limb-girdle muscular dystrophies
• Genetic: SCN5A mutations, lamin A/C mutations (LMNA), KCNJ2
• Congenital: maternal systemic lupus erythematosus (neonatal lupus → SSA/anti-Ro antibody mediated); L-transposition of great arteries

Classification by Site

ECG Classification and Definitions

• 2:1 AV block cannot be classified as Mobitz I or II — EPS may be needed to determine level of block
• Complete AV block in AF: suspected when ventricular response is slow (<50 bpm) and regular
• Isorhythmic dissociation and atrial bigeminy can mimic AV block — careful ECG analysis required

Clinical Presentation

• Depends on whether AV block is fixed or intermittent, the ventricular rate, and the etiology
• Vagally mediated AV block (nocturnal): may be asymptomatic (sinus node also slows simultaneously)
• Symptomatic AV block: syncope, presyncope, dyspnea, HF symptoms, confusional states from cerebral hypoperfusion
• LBBB: may present with HF from cardiac dyssynchrony or underlying cardiomyopathy

Acute Management

• Atropine is NOT effective for infranodal block (vagolytic agents cannot bypass infranodal block)
• Atropine is NOT effective after cardiac transplantation (no vagal innervation)

Permanent Pacing Indications

• Key principle: Infranodal AV block (Mobitz II, high-grade, third-degree) → PPM regardless of symptoms (risk of sudden complete heart block with slow or absent ventricular escape)
• Natural history studies (1970s–80s): untreated second-degree Mobitz II and third-degree AV block → recurrent syncope, HF, poor prognosis; mortality benefit with pacing demonstrated
• Up to 20% of myotonic dystrophy type 1 patients have AV block on ECG or ambulatory monitoring; >50% with normal ECG may have infra-Hisian block at EPS

Pacing Mode for AV Block

• Systematic review showed physiologic pacing (CRT or His bundle) preferred over RV pacing in LVEF 36–50% when ventricular pacing >40% expected — prevents pacing-induced cardiomyopathy
• RV pacing >40% causes abnormal ventricular activation → LV dysfunction → adverse remodeling
• Patients with AV block require ventricular pacing for rate support regardless of pacing technique — hence physiologic activation becomes critical

Special Population: AV Block After TAVR

• New LBBB: 19–55% of TAVR cases; new high-degree AV block: ~10%; up to half can resolve before discharge
• Pre-TAVR predictors of PPM: preexisting RBBB, increased prosthesis-to-LVOT ratio, increased LV end-diastolic diameter
• Class I (B-NR): PPM before discharge for new persistent AV block with symptoms/hemodynamic instability after TAVR
• Class IIa (B-NR): New persistent BBB after TAVR → surveillance for bradycardia
• Class IIb (B-NR): New persistent LBBB after TAVR → PPM may be considered (early PPM not protective against increased mortality associated with new LBBB)
• 29% of patients with new LBBB after TAVR experience first high-degree AV block episode after discharge (sources/bradycardia-acc-aha-hrs-2018, very high)

Special Population: AV Block in Acute MI

• Class I: Temporary pacing for refractory symptomatic/hemodynamically significant AV block in AMI
• Class I: Waiting period before PPM in AMI; PPM for persistent/infranodal Mobitz II/high-grade/third-degree AV block after waiting
• Class IIa: Atropine for AV nodal level block in AMI
• Class III Harm: No PPM for transient AV block resolving in AMI; no PPM for new BBB/fascicular block without second/third-degree AV block in AMI
• Inferior MI: AV block often transient (vagal/AV nodal ischemia); anterior MI + AV block → extensive myocardial damage, worse prognosis; long-term outcome determined by extent of myocardial injury, not AV block itself

Special Form: Paroxysmal AV Block (PAVB)

• PAVB is the sudden onset of complete AV block with prolonged ventricular asystole, distinct from gradual AV block — a specific high-risk subset requiring dedicated recognition
• Two forms: TD-PAVB (tachycardia-dependent — atrial rate acceleration triggers repetitive concealed conduction in diseased His-Purkinje tissue) and PD-PAVB (pause-dependent — long diastolic interval causes block via source-sink mismatch or phase 4 depolarization)
• The His bundle is the most common site of block (7/10 cases with EPS in one series), even when bundle branch block is present on surface ECG (sources/PAVB-HR-2009, high)
• Closely associated with Mobitz type II AV block — the two entities share the same diseased His-Purkinje substrate
• Intra-Hisian block is frequently misidentified as infra-Hisian or AV nodal block at EPS when split His potentials are not carefully dissected
• The terms "phase 3 block" and "phase 4 block" are mechanistic misnomers; TD-PAVB and PD-PAVB are the preferred terms (sources/PAVB-HR-2009, high)
• See concepts/Paroxysmal-AV-Block for full mechanistic detail

Contradictions / Open Questions

• AV block in setting of AF: diagnosis more challenging; conflicting retrospective data on whether pauses >3 s in AF are symptomatic or asymptomatic (sources/bradycardia-acc-aha-hrs-2018, very high)
• TAVR: optimal timing and method of monitoring for post-TAVR AV block/BBB not standardized; no prospective RCTs for PPM implantation timing
• Physiologic pacing (His bundle, CSP) vs RV pacing in LVEF >50%: 2018 guideline predates CSP trials — not addressed here
• Distinction between Mobitz I and II in 2:1 AV block requires EPS or longer ambulatory monitoring strip; may be clinically challenging
• PD-PAVB mechanism: relative contribution of phase 4 depolarization vs source-sink mismatch is unresolved; isolated Purkinje fiber data may not fully translate to intact in vivo conduction system (sources/PAVB-HR-2009, high)

Connections

• Related to concepts/Sinus-Node-Dysfunction — same fibrotic substrate; patients may have both
• Related to concepts/Paroxysmal-AV-Block — high-risk paroxysmal subtype with specific mechanism
• Related to concepts/Permanent-Pacing-Indications — comprehensive AV block pacing indications
• Related to concepts/Conduction-System-Pacing — His bundle pacing for AV block
• Related to concepts/Cardiac-Resynchronization-Therapy — CRT option in LVEF 36–50% + high ventricular pacing
• Related to concepts/ECG-Conduction-Disturbances — ECG definitions and BBB criteria
• Related to entities/ATTR-Amyloidosis — AV block is a manifestation of infiltrative cardiomyopathy
• Related to entities/Atrial-Fibrillation — AV block diagnosis in AF; PPM for AF + symptomatic bradycardia

Sources

• sources/bradycardia-acc-aha-hrs-2018 — primary source; ACC/AHA/HRS 2018 guideline
• sources/PAVB-HR-2009 — El-Sherif/Jalife 2009; mechanistic basis of PAVB (TD and PD forms)