Intraventricular conduction delay’s need to be differentiated from other conditions resulting in wide QRS complex such as LVH, pacemaker rhythms and accessory pathway arrythmias.

• Left ventricular hypertrophy : Dilatation and thickening of the LV wall produce a QRS very similar to LBBB. Wide QRS and secondary T wave discordance can be seen, but the prolonged intrinsicoid deflection (time from the QRS onset to the peak of the R wave in leads V5-V6) to over 60 msec (1.5 small boxes) characteristically seen in LBBB is not seen. Often, LVH progresses to incomplete and then to complete LBBB.
• Pacemaker rhythms : During right ventricular pacing, the ventricles are activated from the electrode positioned in the right ventricular apex and can produce an ECG pattern similar to LBBB. In, biventricular pacing, left ventricular lead activating the heart from back to front (toward lead V1) may produce ECG pattern resembling RBBB.
• Wolff-Parkinson-White syndrome : Left ventricular preexcitation (WPW type A) produces a positive R wave in lead V1 similar to RBBB, whereas right ventricular preexcitation (WPW type B) creates deep S waves in lead V1 similar to that seen in LBBB because the right ventricle is activated first. Short PR interval and delta wave of the QRS, characteristic of WPW differentiates it from bundle branch blocks.