What Does a Wide QRS Complex Indicate?
A wide QRS complex (>120 ms) on ECG indicates one of three primary mechanisms: ventricular tachycardia (most critical to identify), supraventricular tachycardia with bundle branch block or aberrant conduction, or supraventricular tachycardia conducting over an accessory pathway. 1
Primary Diagnostic Categories
The wide QRS complex fundamentally represents abnormal ventricular depolarization through one of these mechanisms:
1. Ventricular Tachycardia (VT)
- This is the most dangerous diagnosis and must be assumed until proven otherwise 1, 2
- VT shows ventricular-atrial dissociation with the ventricular rate exceeding the atrial rate, though this is only visible in 30% of cases 3
- Fusion complexes (merger of conducted supraventricular impulses with ventricular depolarization) are pathognomonic for VT 3
- History of prior myocardial infarction strongly suggests VT as the mechanism 1, 2
2. Supraventricular Tachycardia with Bundle Branch Block
- Bundle branch block may be pre-existing or rate-related (occurring when one bundle branch becomes refractory at rapid rates) 3, 1
- Most rate-related bundle branch blocks occur due to a long-short sequence of initiation 3
- This can occur with any supraventricular arrhythmia 3
3. Supraventricular Tachycardia with Accessory Pathway Conduction
- Antidromic AVRT involves anterograde conduction over the accessory pathway and retrograde conduction over the AV node 3, 1
- Wide QRS with left bundle branch block morphology may indicate conduction over atriofascicular, nodofascicular, or nodoventricular tracts 3
Critical ECG Criteria Favoring Ventricular Tachycardia
When evaluating a wide QRS complex, these features strongly suggest VT:
- QRS width >140 ms with right bundle branch block pattern or >160 ms with left bundle branch block pattern 3, 1, 2
- RS interval >100 ms in any precordial lead 1, 2
- Negative concordance pattern in precordial leads (all QRS complexes predominantly negative across V1-V6) 1
- AV dissociation with ventricular rate faster than atrial rate 3, 1, 2
- Irregular cannon A waves in jugular venous pulse and variable loudness of first heart sound on physical examination 3
Wide QRS in Non-Tachycardic Rhythms
When the wide QRS occurs without tachycardia:
Conduction System Disease
- In first-degree AV block with wide QRS, the conduction delay may be in either the AV node or His-Purkinje system 3
- In second-degree AV block type II with wide QRS, the block is usually in the His-Purkinje system and progression to complete heart block is common 3
- Wide QRS in this context indicates infra-Hisian disease requiring pacemaker consideration 3
Metabolic and Structural Causes
- Electrolyte abnormalities, particularly hyperkalemia, can cause QRS widening 1
- Fragmented wide QRS (>2 notches on R or S wave in ≥2 contiguous leads) indicates myocardial scar with 86.8% sensitivity and 92.5% specificity in coronary artery disease patients 4
- Fragmented wide QRS is an independent predictor of mortality 4
Critical Management Principle
If the diagnosis cannot be definitively established as supraventricular in origin, treat the patient as having ventricular tachycardia 3, 1, 2. This is because:
- Intravenous verapamil or diltiazem given for presumed SVT may precipitate hemodynamic collapse if the rhythm is actually VT 3, 2
- Stable vital signs do NOT help distinguish SVT from VT 3
- The consequences of misdiagnosing VT as SVT are potentially fatal 2
Common Pitfalls to Avoid
- Never assume hemodynamic stability excludes VT - patients with VT can maintain stable blood pressure 3
- Avoid calcium channel blockers in undifferentiated wide QRS tachycardia - this can be lethal if the rhythm is VT 3, 2
- Do not rely solely on QRS width criteria when class I antiarrhythmic drugs are present - these medications can widen the QRS in SVT beyond typical thresholds 3
- Remember that QRS width criteria are not helpful for differentiating VT from SVT with accessory pathway conduction 3