Infra‑Hisian Third‑Degree AV Block with Wide QRS Complexes

Anatomical Location & Pathophysiology

• A QRS duration ≥ 120 ms in complete AV block indicates that the conduction interruption is located below the His bundle (infra‑Hisian), with the ventricular escape rhythm arising from the distal His‑Purkinje system or ventricular myocardium. – American College of Cardiology 1
• Among patients with wide‑QRS complete AV block, an infra‑Hisian site of block is the most common anatomic pattern. – American College of Cardiology 1

Etiology

Ischemic Injury

• Damage to the His‑Purkinje network caused by myocardial infarction can produce a wide‑QRS third‑degree AV block. – American Heart Association 2

Infiltrative & Inflammatory Disorders

• Systemic infiltrative diseases such as sarcoidosis, amyloidosis, and hemochromatosis may involve the distal conduction system and result in wide‑QRS AV block. – American Heart Association 2
• Autoimmune conditions rheumatoid arthritis and systemic lupus erythematosus are recognized precipitants of infra‑Hisian block with wide QRS. – American Heart Association 2

Iatrogenic Causes

• Cardiac valve surgery, especially prosthetic valve implantation, frequently leads to infra‑Hisian block with a wide QRS escape rhythm. – American Heart Association 2
• Transcatheter aortic valve replacement (TAVR), catheter ablation, and alcohol septal ablation are procedural risk factors for developing wide‑QRS complete AV block. – American Heart Association 2

Medication‑Induced Block

• Beta‑blockers, non‑dihydropyridine calcium‑channel blockers (verapamil, diltiazem), and digoxin can depress conduction through the His‑Purkinje system, precipitating a wide‑QRS third‑degree block. – American Heart Association 2

Congenital & Genetic Factors

• Congenital complete heart block (including familial autosomal‑dominant forms) may present with a wide QRS escape rhythm due to distal conduction disease. – American Heart Association 2

Neuromuscular Disorders

• Neuromuscular diseases such as myotonic dystrophy, Kearns‑Sayre syndrome, and Erb’s dystrophy are associated with infra‑Hisian block and wide QRS complexes. – American Heart Association 2

Metabolic & Endocrine Disturbances

• Electrolyte abnormalities (e.g., hyperkalemia, hypercalcemia) and endocrine disorders (hypothyroidism, pheochromocytoma, hypoaldosteronism) can impair His‑Purkinje conduction, leading to a wide‑QRS complete block. – American Heart Association 2

Clinical Implications

• Prognosis: A wide QRS in third‑degree AV block is linked to a significantly worse outcome than a narrow‑QRS block because the ventricular escape rhythm is slower, more unpredictable, and less reliable. – American Heart Association 2
• Pharmacologic response: The wide‑QRS escape rhythm does not respond to atropine, although it may improve with catecholamine infusion. – American Heart Association 2
• Disease course: Progression from a wide‑QRS block to hemodynamic collapse is often rapid and unexpected. – European Society of Cardiology 3

Diagnostic & Management Recommendations

• Electrophysiological study should be considered to definitively localize the level of block when a wide QRS and AV block are present. – European Society of Cardiology 3
• Type II second‑degree AV block with a wide QRS is a recognized harbinger of imminent progression to complete heart block and warrants close monitoring and early pacing consideration. – European Society of Cardiology 3

QRS Complex Characteristics in AV Block

Understanding AV Block and QRS Morphology

• First-degree AV block is characterized by prolonged PR interval (>200 ms) with all atrial impulses conducted to the ventricles, according to the European Society of Cardiology 4
• If the QRS complex is narrow, the conduction delay is usually in the AV node, as stated by the European Society of Cardiology 4, 5
• If the QRS is wide, the conduction delay may be either in the AV node or in the His-Purkinje system, as noted by the European Society of Cardiology 4, 5
• Only a His bundle electrogram can precisely locate the site of delay when QRS is wide, according to the European Society of Cardiology 5
• Second-degree AV block occurs when one or more atrial stimuli are not conducted to the ventricles, as described by the European Society of Cardiology 4, 6
• In Mobitz type I (Wenckebach), progressive PR interval prolongation until a P wave is not conducted, usually associated with narrow QRS complexes, as stated by the American College of Cardiology 7, 8
• In Mobitz type II, PR interval remains constant before and after blocked P waves, usually associated with wide QRS complexes, as noted by the American College of Cardiology 7, 8
• No atrial stimuli are conducted to the ventricles in third-degree AV block, with ventricles depolarized by an escape rhythm, as described by the European Society of Cardiology 4, 6
• QRS morphology depends on the site of the block in third-degree AV block, with narrow QRS complexes if the escape rhythm originates above or within the His bundle, and wide QRS complexes if it originates below the His bundle, as stated by the American College of Cardiology 8, 9

Clinical Implications and Anatomical Correlations

• The width of the QRS complex provides important clues about the anatomical location of the block, with narrow QRS suggesting AV nodal block and wide QRS suggesting infra-nodal block, as noted by the European Society of Cardiology 6, 8
• In third-degree AV block with wide QRS, the site of block is usually below the His bundle, according to the American College of Cardiology 8, 9

Important Clinical Considerations

• Type II second-degree block, especially with wide QRS, has higher risk of progressing to complete heart block, as stated by the American Academy of Family Physicians 7
• Permanent pacing is generally recommended for symptomatic third-degree AV block and type II second-degree block, particularly with wide QRS, as recommended by the American Academy of Family Physicians 7
• The prognosis of AV block depends on the site of block, with AV nodal blocks generally having better prognosis and infra-Hisian blocks having poorer prognosis, as noted by the American College of Cardiology 9

Diagnostic Pearls

• In cases with wide QRS and first or second-degree AV block, electrophysiological study may be required to determine the exact level of block, as stated by the European Society of Cardiology and the American College of Cardiology 4, 9

Pathophysiology and Clinical Implications of Atrioventricular Heart Blocks

First‑Degree Atrioventricular (AV) Block

• First‑degree AV block is a conduction delay (PR interval > 200 ms) in which all atrial impulses reach the ventricles but transit time is prolonged due to slowed conduction velocity through nodal tissue or the specialized His‑Purkinje system, without complete interruption of impulse transmission. 10
• A wide range of etiologies—including developmental, hereditary/genetic, metabolic, infectious, inflammatory, infiltrative, traumatic, ischemic, malignant, and degenerative processes—can impair AV conduction; the underlying pathophysiology may be static or progressive. 10

Second‑Degree AV Block

General Definition

• Second‑degree AV block occurs when only a subset of atrial impulses are conducted to the ventricles; it is classified as Mobitz Type I (Wenckebach) or Mobitz Type II based on PR‑interval behavior and the anatomical level of block. 11

Mobitz Type II (Infranodal)

• Mobitz Type II block, which reflects an all‑or‑none failure of impulse propagation through diseased His‑Purkinje tissue, is more likely to progress rapidly and unpredictably to complete heart block and Stokes‑Adams arrest. 11
• Because the block is infranodal, pacing is indicated regardless of the presence or severity of symptoms. 11

Third‑Degree (Complete) AV Block

Overall Mechanism

• Third‑degree AV block denotes complete electrical dissociation between atrial and ventricular activity; ventricular depolarization relies on an independent escape rhythm arising distal to the site of block. 11

Narrow‑QRS Complete Block (Intranodal/Proximal)

• When the escape rhythm originates above or within the His bundle (narrow QRS < 120 ms), the block is intranodal; a stable junctional escape rhythm typically runs at 40–60 bpm and confers a relatively favorable prognosis. 11

Wide‑QRS Complete Block (Infranodal/Distal)

• Wide QRS complexes (≥ 120 ms) indicate that the escape rhythm arises from the distal His‑Purkinje system or ventricular myocardium, reflecting an infranodal block; the ventricular escape rhythm is slower (20–40 bpm), more unpredictable, and less reliable. 11
• Wide‑QRS complete block carries a significantly worse prognosis than narrow‑QRS block because of the unreliable escape rhythm. 11
• The wide‑QRS escape rhythm does not respond to atropine, although it may improve with catecholamine infusion. 11
• When a wide QRS complex coexists with AV block, an electrophysiological study is recommended to definitively localize the level of block. 11

Rapid Differentiation of First‑Degree versus Third‑Degree Atrioventricular Block in Clinical Practice

Diagnostic Electrocardiographic Criteria

• First‑degree AV block shows a constant 1:1 P‑QRS relationship with a uniformly prolonged PR interval (>200 ms). This pattern indicates that every atrial depolarization conducts to the ventricles despite the delay. 12
• Third‑degree (complete) AV block is characterized by complete AV dissociation: P waves and QRS complexes occur independently with no fixed temporal relationship. The PR interval is therefore variable because atrial and ventricular activity are uncoupled. 12
• In first‑degree block, both atrial and ventricular rhythms are regular and share the same rate, reflecting synchronized atrial‑ventricular activity. 12

Practical ECG Interpretation Algorithm

Step 1 – Assess PR Intervals

• If every PR interval is equal and prolonged (>200 ms), diagnose first‑degree AV block. Uniform PR prolongation confirms 1:1 conduction. 13

• If PR intervals vary from beat to beat without a reproducible pattern, diagnose third‑degree AV block. Variable PR reflects independent atrial and ventricular cycles. 12

Step 2 – Evaluate P‑to‑QRS Numerical Ratio

• A 1:1 ratio (equal numbers of P waves and QRS complexes) supports first‑degree AV block. Consistent pairing indicates preserved conduction. 12

• A surplus of P waves relative to QRS complexes without a fixed relationship indicates third‑degree AV block. The excess atrial activity is not conducted. 13

Clinical Presentation and Prognosis

• First‑degree AV block is usually asymptomatic; symptoms may appear only when the PR interval is markedly prolonged (e.g., >300 ms). Most patients remain clinically silent. 12

• The prognosis of first‑degree AV block is benign, particularly when the QRS complex is narrow, suggesting a nodal‑level delay. No immediate therapy is required in the majority of cases. 13

• Third‑degree AV block commonly presents with syncope, presyncope, dizziness, or severe fatigue, reflecting inadequate ventricular output. Prompt recognition is essential. 12

Common Pitfalls and Differentiation Tips

• Isorhythmic AV dissociation can mimic first‑degree block when atrial and ventricular rates are similar. The distinguishing feature is a gradually varying PR interval rather than a fixed prolonged PR. 12

• Blocked atrial bigeminy may resemble second‑degree AV block; careful inspection of P‑wave morphology (premature P waves often have a different shape) helps avoid misclassification. 12

• Atrial fibrillation with a slow, regular ventricular response (<50 bpm) can be mistaken for complete AV block. The absence of organized P waves and the presence of fibrillatory waves differentiate the two conditions. 12