If the ventricular pacing is originating in the right ventricle like a normal pacemaker, a left bundle branch morphology will be present. This occurs since the right ventricle is activated first and the activation of the left ventricle occurs slowly from myocyte to myocyte. This is the same phenomenon that occurs in the presence of a left bundle branch block. This ECG has ventricular pacing as seen by pacemaker spikes prior to each QRS complex, however has a more right bundle branch appearance. This is a biventricular pacemaker which has one lead in right ventricle and a second lead in the coronary sinus which wraps around the posterior heart to pace the lateral left ventricle. These two leads pace simultaneously to be sure that both ventricles contract at the same time, resulting in a fusion of a left and right bundle QRS morphology. Biventricular pacemakers are sometimes referred to as "Cardiac Resynchronization Therapy" or CRT.
