What are the causes of third-degree (complete) atrioventricular block with a wide QRS complex?

Causes of Third-Degree AV Block with Wide QRS

Third-degree AV block with wide QRS complexes indicates that the site of block is typically below the His bundle (infra-Hisian), with the escape rhythm originating from the distal conduction system or ventricles. 1, 2

Anatomical Location and Pathophysiology

The wide QRS morphology in complete AV block provides critical information about the anatomical site:

• When the escape rhythm shows wide QRS complexes (≥120 ms), the block is usually located below the His bundle in the distal His-Purkinje system or bilateral bundle branches 1, 2
• The site of block may be in the AV node, within the His bundle, or below the His bundle, but infra-Hisian location is most common with wide QRS 1
• Pathologic studies demonstrate that wide QRS in complete AV block correlates with lesions in the branching portion of the His bundle (Hisb), combined Hisb and bundle branch lesions, or bilateral bundle branch involvement 3

Primary Etiologies

Degenerative/Fibrotic Disease

• Idiopathic fibrosis of the conduction system is the most common cause, particularly affecting the His bundle and bilateral bundle branches 3
• Progressive sclerosis and calcification of the cardiac skeleton, especially involving the central fibrous body 3

Ischemic Heart Disease

• Myocardial infarction causes up to 20% of conduction disturbances, with complete heart block occurring in 8% of post-MI patients 4
• Ischemic damage to the His-Purkinje system 1, 4

Structural Heart Disease

• Calcific aortic stenosis with extension into the conduction system 3
• Hypertrophic cardiomyopathy (rare but documented cause) 5
• Non-ischemic cardiomyopathy 4

Inflammatory/Infiltrative Conditions

• Myocarditis 4
• Infectious endocarditis 4
• Infiltrative cardiac diseases (sarcoidosis, amyloidosis, hemochromatosis) 1
• Rheumatoid arthritis and systemic lupus erythematosus 1

Iatrogenic Causes

• Cardiac surgery, especially valve surgery 1
• Transcatheter aortic valve replacement (TAVR) 1
• Catheter ablation procedures 1
• Alcohol septal ablation 1

Medication-Related

• Beta blockers, calcium channel blockers (verapamil, diltiazem), digoxin 1
• Antiarrhythmic drugs (particularly procainamide, which can cause progressive QRS widening and increased AV block) 6

Congenital/Genetic

• Familial AV block (autosomal dominant trait) characterized by adult onset, progressive conduction disease, and widened QRS complexes 7
• Congenital complete heart block 1

Neuromuscular Disorders

• Myotonic dystrophy 1
• Kearns-Sayre syndrome 1
• Erb's dystrophy 1

Metabolic/Endocrine

• Electrolyte disturbances (hyperkalemia, hypercalcemia) 1, 4
• Endocrine disorders (hypothyroidism, pheochromocytoma, hypoaldosteronism) 1

Critical Clinical Distinctions

The wide QRS in third-degree AV block carries significantly worse prognosis than narrow QRS complete block because:

• The ventricular escape rhythm is slower (typically 20-40 bpm vs 40-60 bpm for junctional escape) 4
• The escape mechanism is more unpredictable and unreliable 1
• Progression is often rapid and unexpected 1
• The rhythm does not respond to atropine (though may respond to catecholamines) 1
• Risk of asystole and sudden death is substantially higher 4, 8

Important Caveats

• In cases with wide QRS and AV block, electrophysiological study may be required to definitively determine the exact level of block 1, 2
• Histologic studies show that lesions in the branching portion of the His bundle can produce either narrow or wide QRS complexes depending on the severity and extent of involvement in adjacent bundle branches 3
• Type II second-degree block with wide QRS commonly progresses to complete heart block, making it a harbinger of third-degree block 1