Can V-Paced Rhythm Have Nonspecific Intraventricular Block?
No, a ventricular-paced rhythm cannot technically exhibit nonspecific intraventricular conduction delay (NICD) because the paced QRS morphology itself represents an iatrogenic conduction pattern that supersedes the native conduction system. 1
Understanding the Terminology Conflict
Nonspecific intraventricular conduction delay is defined as QRS duration >110 ms where morphology criteria for right bundle branch block (RBBB) or left bundle branch block (LBBB) are not present during native conduction. 1 This definition inherently requires assessment of the intrinsic conduction system, which is bypassed during ventricular pacing.
Why V-Paced Rhythms Cannot Show NICD
Ventricular pacing produces a QRS morphology similar to LBBB pattern (when pacing from the right ventricle), with broad, notched complexes and markedly deviated ST-T waves. 1
The paced QRS represents artificial depolarization originating from the pacing lead tip, not conduction through the His-Purkinje system where bundle branch blocks and intraventricular conduction delays occur. 1
During 100% ventricular pacing, you are observing pacemaker-induced ventricular activation, which creates its own characteristic wide QRS pattern that cannot be classified using native conduction terminology. 1
What You Actually See in V-Paced Rhythm
Right Ventricular Pacing Morphology
Right ventricular apical pacing typically produces a LBBB-like pattern with QS or rS complexes in V1 and broad R waves in lateral leads (I, aVL, V5-V6). 1, 2
Occasionally, right ventricular pacing can produce a RBBB-like pattern (seen in approximately 8% of cases), which may suggest alternative lead positions or unusual conduction patterns, but this still represents paced morphology, not native RBBB. 2
The QRS duration during ventricular pacing is typically ≥120 ms and often exceeds 150 ms, reflecting the slow myocyte-to-myocyte conduction from the pacing site rather than rapid His-Purkinje conduction. 3
Clinical Implications and Common Pitfalls
Why This Distinction Matters
ST-segment monitoring is unreliable in ventricular-paced rhythms because the markedly deviated ST-T waves (similar to LBBB) cause frequent false alarms and obscure ischemia detection. 1
Patients who fluctuate between spontaneous rhythm and pacing rhythm are especially prone to false ST alarms due to the dramatic morphology changes between native and paced complexes. 1
Ventricular pacing induces mechanical dyssynchrony, particularly in patients whose native rhythm shows normal conduction or LBBB, which can worsen heart failure over time. 3
Assessing Native Conduction in Pacemaker Patients
If you need to determine whether a patient has underlying NICD, LBBB, or RBBB, you must:
Reduce the pacing rate below the patient's intrinsic rate (if safe and the patient has adequate native rhythm) to observe native QRS morphology. 1
Review pre-implantation ECGs to document the baseline conduction pattern before pacing was initiated. 1
Perform ambulatory monitoring to capture any periods of native conduction if the patient is not pacemaker-dependent. 1
Special Consideration: Fusion and Pseudofusion Beats
Fusion beats (simultaneous activation from both the pacemaker and native conduction) can produce intermediate QRS morphologies that may appear to show features of both paced and native conduction, but these are transient phenomena, not stable conduction patterns. 1
These fusion complexes should not be classified as NICD, LBBB, or RBBB because they represent hybrid activation patterns. 1
Bottom Line for Clinical Practice
When you see a wide QRS complex during ventricular pacing, document it as "ventricular-paced rhythm" rather than attempting to classify it using bundle branch block or intraventricular conduction delay terminology. 1 The paced morphology tells you about lead position and capture, not about the integrity of the native conduction system. If assessment of native conduction is clinically necessary (for example, to determine CRT candidacy or evaluate for progressive conduction disease), you must obtain an ECG during intrinsic rhythm. 1, 4