Intermittent left bundle branch block
An ECG That Changes From Beat to Beat
An 82-year-old woman is referred for an abnormal ECG. She reports occasional exertional breathlessness.
The quiz uses the 12 leads without the annotated rhythm strip. The library retains the published figure.
Look for
- Changing QRS width and morphology.
- Some complexes have LBBB morphology.
Where to look. Compare complexes in V1 and I, aVL, V5–V6, then measure their duration.
When left bundle conduction is impaired, the left ventricle activates later. Intermittency is recognized by comparing complexes; it does not establish the precise mechanism.
The question concerns intermittent conduction without requiring a phase 3 or phase 4 block mechanism.
References & provenance
The ECG Made Easy, 9e, ch. 3, PDF 77–83; The ECG Made Practical, 7e, ch. 1, p. 27; Lilly, 6e, ch. 4, p. 92–94.
A Case of Intermittent Left Bundle Branch Block
Syed M Saad; Faran S Polani; Paul LeLorier; Alexander Muacevic; John R Adler. A Case of Intermittent Left Bundle Branch Block. 2021. doi:10.7759/cureus.15851. Figure 1. CC BY 4.0. CC BY 4.0
Adaptation: cropping and removal of headers/annotations outside the tracing where applicable; metadata removed. Waveforms have not been redrawn.
Compared with the published source; no independent clinical validation is claimed.
Original annotations & limitations
Bloc de branche gauche intermittent
Femme de 82 ans adressée pour ECG anormal et dyspnée d’effort occasionnelle; bilan ischémique non contributif; BBG intermittent.
Question avancée centrée sur les QRS alternants; pas sur la théorie des blocs de phase 3/4.