Similarities and differences between electrocardiogram signs of left bundle-branch block and left-ventricular...

A left bundle-branch block (LBBB) electrocardiogram (ECG) type may be caused by either a block in the left branch of the ventricular conduction system or by uncoupling in the working myocardium. We used a realistic large-scale computer model to evaluate the effects of uncoupling with and without left-sided block and in combination with biventricular pacing.

Action potential propagation was simulated using a reaction-diffusion model of the human ventricles. Electrocardiograms and cardiac electrograms were computed from the simulated action potentials by solving the bidomain equations. In all situations, diffuse uncoupling reduced QRS amplitude, prolonged QRS duration, and rotated the QRS axis leftward. Uncoupling by 50% increased QRS duration from 90 to 120 ms with a normal conduction system and from 140 to 190 ms when the left bundle branch was blocked. Biventricular pacing did not change QRS duration but reduced total ventricular activation time.

Uncoupling in the working myocardium can mimic left-sided block in the ventricular conduction system and can explain an LBBB ECG pattern with relatively low amplitude. Biventricular pacing improves ventricular activation in true LBBB with or without uncoupling but not in case of 50% uncoupling alone.