The ECG of a biventricular pacemaker takes on a "fusion" and more narrow QRS complex (although still somewhat wide) with an upright QRS complex in V1 similar to a right bundle branch block pattern.

Remember to interpret the ECG in the appropriate clinical context and treat a wide QRS complex tachycardia like ventricular tachycardia until proven otherwise. A fusion beat (a.k.a. Dressler beat ...