What are the causes of wide QRS (Q wave, R wave, S wave) bradycardia (abnormally slow heart rate)?

Causes of Wide QRS Bradycardia

The most common causes of wide QRS bradycardia include atrioventricular conduction blocks, drug toxicity, electrolyte abnormalities, and sinus node dysfunction with conduction system disease. Understanding these causes is crucial for proper diagnosis and management to reduce morbidity and mortality.

Conduction System Disease

Atrioventricular Blocks

• Third-degree (complete) AV block: No evidence of AV conduction, often presenting with wide QRS complexes when the block is infranodal 1
• Second-degree Mobitz type II AV block: Constant PR interval before dropped QRS, usually associated with wide QRS complexes 1
• Advanced or high-grade AV block: Multiple consecutive P waves that don't conduct to ventricles 1

Bundle Branch Blocks and Fascicular Blocks

• Complete RBBB: QRS duration ≥120 ms with characteristic rSR' pattern in V1/V2 1
• Complete LBBB: QRS duration ≥120 ms with broad notched R waves in leads I, aVL, V5, V6 1
• Bifascicular block: Blockage in two of the three fascicles (right bundle, left anterior fascicle, left posterior fascicle) 1
• Trifascicular block: Evidence of block in all three fascicles, may present as alternating bundle branch block patterns 1

Drug-Induced Causes

Antiarrhythmic Medications

• Class IA drugs (quinidine, procainamide): Can cause QRS widening and bradycardia 1
• Class IC drugs (flecainide, propafenone): Associated with QRS widening >150% and conduction abnormalities, especially in structural heart disease 1
• Class III drugs (amiodarone, sotalol): Can cause sinus node dysfunction and AV block 1, 2
• Combined use of beta-blockers with sodium channel blockers: Can cause severe bradycardia, especially in elderly patients 3

Other Medications

• Digoxin: Can cause various bradyarrhythmias, especially when combined with hyperkalaemia 4
• Beta-blockers: Common cause of drug-related bradycardia 5
• Calcium channel blockers: Can cause sinus node dysfunction and AV block 5

Electrolyte Abnormalities

• Hyperkalaemia: Can cause widened QRS complexes and bradycardia 4
• Hypokalemia/hypomagnesemia: Predispose to drug-induced ventricular proarrhythmia 1

Structural Heart Disease

• Ischemic heart disease/prior myocardial infarction: Common underlying cause of conduction system disease 6
• Cardiomyopathies: Can affect the conduction system
• Infiltrative diseases: Sarcoidosis, amyloidosis can cause progressive conduction system disease 1

Sinus Node Dysfunction (Sick Sinus Syndrome)

• Can present with bradycardia and conduction abnormalities 1
• Often associated with aging and fibrosis of the conduction system

Diagnostic Approach

When evaluating wide QRS bradycardia, consider:

1. QRS morphology: Determine if it resembles RBBB (rSR' in V1) or LBBB (broad notched R waves in I, aVL, V5, V6) 1
2. QRS duration: >120 ms indicates conduction system disease 1
3. PR interval: Prolonged PR suggests AV conduction disease
4. Presence of AV dissociation: Indicates complete heart block 7
5. Drug history: Particularly antiarrhythmics, beta-blockers, calcium channel blockers, and digoxin 5
6. Electrolyte status: Especially potassium and magnesium levels 4

Common Pitfalls

• Misdiagnosis of wide QRS tachycardia as SVT: Wide QRS tachycardia in adults is most commonly ventricular tachycardia, not SVT with aberrancy 6, 8
• Failure to recognize drug combinations causing bradycardia: Particularly beta-blockers with sodium channel blockers 3
• Overlooking reversible causes: Such as electrolyte abnormalities or drug toxicity 4
• Inadequate monitoring: Patients with bifascicular block and prolonged HV interval have increased risk of developing complete heart block 1

Understanding these causes and their clinical presentations is essential for appropriate management of wide QRS bradycardia and prevention of potentially life-threatening complications.