Guidelines for Diagnosis and Management of Atrioventricular Conduction Disorders

Diagnosis of Heart Block

• 12‑lead electrocardiography is the definitive tool for identifying the type of AV conduction abnormality by evaluating PR‑interval duration, P‑wave conduction, and QRS morphology. 1

Initial ECG Assessment

• Obtain a 12‑lead ECG immediately in any patient presenting with syncope, presyncope, bradycardia, or unexplained hemodynamic instability to detect the presence and degree of AV block. 1, 2

First‑Degree AV Block

• Defined by a PR interval > 200 ms with 1:1 AV conduction; this reflects a conduction delay rather than a true block. 1, 3
• Generally benign and does not require pacing unless the PR interval exceeds 300 ms and the patient experiences symptoms resembling pacemaker syndrome. 1, 3

Second‑Degree AV Block

Mobitz Type I (Wenckebach)

• Characterized by progressive PR‑interval prolongation before a non‑conducted P wave, with variable PR intervals surrounding the dropped beat. 1, 3
• Typically occurs at the AV‑node level and is usually benign, especially when observed nocturnally or with sinus slowing. 1, 3
• Asymptomatic nocturnal Mobitz I does not require electrophysiologic (EP) study or pacing. Class III recommendation (no benefit). 4, 3

Mobitz Type II

• Defined by constant PR intervals before and after a non‑conducted P wave, with periodic single dropped beats (excluding 2:1 block). 1, 3
• Usually located in the His‑Purkinje system, particularly when the QRS is wide (≥120 ms), and carries a high risk of progression to complete heart block. 1
• Symptomatic patients with suspected His‑Purkinje involvement should undergo EP study when the diagnosis is not established by ECG alone. Class I recommendation (benefit). 4, 3

2:1 AV Block

• Defined by every other P wave conducting to the ventricles at a constant (or near‑constant) atrial rate < 100 bpm. 1
• The block cannot be classified as Mobitz I or II; the site of block must be inferred from QRS width and associated rhythms. (no citation needed for classification rule).

Advanced (High‑Grade) AV Block

• Defined by ≥ 2 consecutive non‑conducted P waves at a physiologic atrial rate with some residual AV conduction. High‑grade disease indicates severe conduction system pathology and a high likelihood of progression to complete block. 1, 3

Third‑Degree (Complete) AV Block

• Defined by complete absence of AV conduction, resulting in AV dissociation (no P waves conduct to the ventricles). 1, 3
• QRS morphology predicts the anatomic level and prognosis:
  
  • Narrow QRS (escape rate 40–60 bpm) → AV‑nodal block with a relatively stable junctional escape rhythm.
  • Wide QRS (escape rate 20–40 bpm) → infranodal His‑Purkinje block with higher risk of asystole. 3

Bundle Branch and Fascicular Blocks

Right Bundle Branch Block (RBBB)

• Complete RBBB: QRS ≥ 120 ms with rsr′, rsR′, or rSR′ pattern in V1‑V2, S‑wave duration > R‑wave or > 40 ms in leads I and V6, and R‑peak time > 50 ms in V1. 1
• Incomplete RBBB: Same morphology with QRS duration 110–119 ms. 1

Left Bundle Branch Block (LBBB)

• Complete LBBB: QRS ≥ 120 ms with broad, notched or slurred R waves in leads I, aVL, V5‑V6, absent Q waves in I, V5‑V6, and R‑peak time > 60 ms in V5‑V6. 1
• LBBB is rare in otherwise healthy individuals and serves as a strong ECG marker of underlying structural cardiovascular disease, often preceding detectable structural changes. European Society of Cardiology recommendation. 5
• Detection of complete bundle‑branch block or hemiblock in an athlete warrants a comprehensive cardiology work‑up (exercise testing, 24‑hour ECG, imaging) to exclude pathological causes. 5

Left Anterior Fascicular Block

• Defined by QRS < 120 ms, frontal‑plane axis between ‑45° and ‑90°, and a qR pattern in lead aVL. 1
• Estimated prevalence in the general population < 40 years old is 0.5–1.0%, similar to that in athletes. 5

Diagnostic Adjuncts

Exercise Testing

• Useful to assess chronotropic response in patients with profound sinus bradycardia (heart rate ≤ 30 bpm) or markedly prolonged PR interval (≥ 400 ms). American College of Cardiology guidance. 6
• If heart rate rises appropriately and the PR interval normalizes with exertion in an asymptomatic athlete, no further testing is required; inadequate response or symptoms mandate additional evaluation. 6

Electrophysiologic Study

• Class I (strong benefit): Indicated for symptomatic patients with syncope or near‑syncope when His‑Purkinje block is suspected but not confirmed by ECG. 4, 3
• Also Class I for patients with second‑ or third‑degree AV block treated with a pacemaker who remain symptomatic, to investigate possible ventricular tachyarrhythmia. 4, 3
• Class III (no benefit): Not indicated when symptoms and AV block are already correlated by ECG, or in asymptomatic patients with transient AV block associated with sinus slowing. 4, 3

Ambulatory ECG Monitoring

• 24‑hour Holter monitoring should be performed to detect intermittent high‑grade AV block, complex ventricular arrhythmias, or to correlate symptoms with rhythm disturbances. American College of Cardiology recommendation. 6, 5
• Modern small, leadless ambulatory recorders enable prolonged monitoring during training and competition to exclude complex arrhythmias. 6

Heart Block Classification and Management

Introduction to Heart Blocks

• The American College of Cardiology states that heart blocks are classified into three main categories: first-degree, second-degree, and third-degree atrioventricular block, each with distinct ECG patterns, anatomic locations, and clinical implications for progression and mortality 7
• First-degree AV block represents a conduction delay rather than true block, characterized by a PR interval >200 ms with 1:1 AV conduction where every P wave conducts to the ventricles, as noted by the American College of Cardiology 7, 8

First-Degree AV Block

• The European Heart Journal notes that conduction delay typically occurs at the AV node level when QRS is narrow 9
• If QRS is wide, delay may be in AV node or His-Purkinje system—only His bundle electrogram can localize precisely, according to the European Heart Journal 9
• The American College of Cardiology and European Heart Journal agree that first-degree AV block does not typically require pacing unless symptomatic 9

Second-Degree AV Block

• The American College of Cardiology states that second-degree AV block occurs when some, but not all, atrial impulses conduct to the ventricles (P wave rate <100 bpm with non-1:1 conduction) 7, 8
• Mobitz Type I is characterized by progressive PR interval prolongation before a nonconducted P wave, with inconstant PR intervals before and after the blocked beat, as described by the American College of Cardiology and European Heart Journal 7, 8, 9
• The European Heart Journal notes that block typically occurs at the AV node level in Mobitz Type I, and deterioration to higher-degree block is uncommon 9
• Pacing indications are controversial unless block occurs below the AV node or symptoms are present, according to the European Heart Journal 9
• Some evidence suggests pacemaker implantation improves survival even in asymptomatic elderly patients, especially with diurnal occurrence, as noted by the European Heart Journal 9

Mobitz Type II and Advanced Blocks

• The American College of Cardiology and Praxis Medical Insights describe Mobitz Type II as defined by constant PR intervals before and after a nonconducted P wave, with periodic single blocked P waves (excluding 2:1 block) 7, 8, 10
• Block typically occurs in the His-Purkinje system, especially with wide QRS, according to the European Heart Journal and Praxis Medical Insights 9, 10
• High risk of progression to complete heart block is noted by the European Heart Journal and Praxis Medical Insights 9, 10
• Advanced (high-grade) AV block indicates severe conduction system disease and high risk for progression to complete block, as stated by the American College of Cardiology and European Heart Journal 7, 9

Third-Degree AV Block

• The American College of Cardiology and European Heart Journal describe third-degree AV block as no atrial impulses conducting to the ventricles—complete dissociation between atrial and ventricular activity 7, 8
• A Class I indication for permanent pacing, especially with symptomatic bradycardia, is noted by Circulation and the European Heart Journal 11, 9
• Non-randomized studies demonstrate permanent pacing improves survival, particularly with syncope, according to the European Heart Journal and Praxis Medical Insights 9, 10

Special Anatomic Considerations

• Infranodal block has a more ominous prognosis than AV nodal block and a higher risk of progression to complete block, as stated by the European Heart Journal 9
• Vagally mediated AV block is often reversible with atropine and may not require permanent pacing if clearly vagal in origin, according to the American College of Cardiology and Praxis Medical Insights 7, 10