What are the ECG characteristics of ventricular pacing?

ECG Characteristics of Ventricular Pacing

Ventricular pacing produces a wide QRS complex (≥120 ms) with a left bundle branch block (LBBB) morphology when pacing from the right ventricle, characterized by broad, deep S waves with no R waves in leads V1-V3, and the absence of the RSR' pattern that defines right bundle branch block. 1, 2

Fundamental ECG Features of Right Ventricular Pacing

QRS Morphology in Precordial Leads

• Right ventricular apical pacing produces a LBBB pattern with broad, deep S waves and absent R waves in the right precordial leads (V1-V3), which is the opposite pattern of right bundle branch block 1, 2
• Lead V1 typically shows a QS complex (entirely negative deflection) or deep S waves during standard RV pacing 3
• The absence of RSR' pattern in V1-V3 is expected during normal RV pacing, as this pattern would indicate right bundle branch block morphology 1, 4
• Lead V6 demonstrates broad, monophasic R waves consistent with LBBB morphology 5

QRS Duration and Timing

• The paced QRS complex is wide, typically ≥120 ms in adults, reflecting abnormal ventricular activation from the pacing site 6
• R wave peak time in V6 is prolonged during RV pacing due to delayed left ventricular activation 7, 8
• A latency interval (delay from pacemaker stimulus to QRS onset) may be present, particularly with left ventricular pacing, and is more pronounced than with RV pacing 3

Critical Diagnostic Pitfall: RBBB Pattern During RV Pacing

When RBBB Morphology Appears

• If an RSR' pattern or RBBB morphology appears during transvenous RV pacing, myocardial perforation or lead malposition must be ruled out immediately, even in asymptomatic patients 2
• The presence of RBBB pattern (RSR', rsR', rSR' in V1-V2) during intended RV pacing is abnormal and potentially dangerous 4, 2
• Chest X-ray and echocardiography are mandatory to confirm proper lead position and exclude perforation when RBBB pattern is observed 2

Rare Exception

• In extremely rare cases, a "safe" RBBB pattern can occur with confirmed RV pacing due to unusual septal anatomy or lead position, but this diagnosis requires definitive imaging confirmation 2

Biventricular and Left Bundle Branch Pacing Patterns

Biventricular Pacing Characteristics

• Simultaneous BiV pacing with marked LV latency may produce a QRS dominated by RV pacing with LBBB configuration and QS complex in V1 3
• Programming LV-before-RV timing (V-V interval) often restores a dominant R wave in V1, representing visible LV contribution to depolarization 3
• Negative QRS in V1 during BiV pacing with short PR interval most likely indicates ventricular fusion with intrinsic rhythm, which can falsely narrow the QRS and simulate appropriate capture 3

Left Bundle Branch Area Pacing (LBBAP)

• LBBAP produces distinct V1 morphologies: Qr pattern (60.7%), qR (19.6%), rSR' (7.1%), or QS (12.5%) 8
• The terminal R' wave duration in V1 is significantly shorter during LBBAP (51 ± 12 ms) compared to native RBBB (85 ± 19 ms), which helps differentiate these patterns 8
• V6 R-wave peak time remains short and constant (approximately 65 ms) during left bundle capture, indicating physiological left ventricular activation 7, 8

Pacemaker Stimulus Characteristics

Visible Pacing Spikes

• A sharp vertical deflection (pacing spike) precedes the QRS complex, marking the electrical stimulus delivery 3
• The spike should be immediately followed by ventricular depolarization (captured beat) in properly functioning systems 3

Latency Considerations

• Prolonged latency is more common with LV than RV pacing due to indirect stimulation through coronary veins and higher incidence of LV pathology including scars 3
• Marked latency may require V-V interval programming to optimize BiV pacing patterns 3

Differential Diagnosis: Paced Rhythm vs. Ventricular Tachycardia

Key Distinguishing Features

• Paced rhythms show regular pacing spikes before each QRS complex, while VT does not 6
• During wide complex tachycardia, AV dissociation with visible P waves (marked by arrows on ECG) indicates VT rather than paced rhythm 6
• If the diagnosis cannot be proven easily, treat as VT until proven otherwise, as medications for SVT (particularly verapamil or diltiazem) may precipitate hemodynamic collapse in VT 6

LBBB Morphology Tachycardia Criteria

• R wave >30 ms duration in V1 or V2 suggests VT over paced rhythm 5
• Any Q wave in V6 favors VT 5
• Duration >60 ms from QRS onset to S wave nadir in V1 or V2 indicates VT 5
• Notching on the downstroke of S wave in V1 or V2 suggests VT 5

Clinical Context and Reversible Causes

Before Attributing Findings to Pacing

• Exclude reversible causes of conduction abnormalities: electrolyte abnormalities, medications, hypothermia, or inflammation near the AV junction 6, 1
• Verify proper lead placement on chest X-ray, as incorrect positioning can create abnormal ECG patterns 1, 2

When to Suspect Malfunction

• RBBB pattern during RV pacing requires immediate investigation 2
• Loss of capture (pacing spikes without subsequent QRS) indicates lead or generator malfunction 3
• Unexpected QRS morphology changes warrant lead position verification and threshold testing 3