This ECG shows ventricular pacing. Note the pacer spike prior to each QRS complex, most apparent in leads V5 and V6. The P waves appear sinus in origin and have no association with the QRS complexes indicating 3rd degree heart block (complete heart block). Note that when a pacemaker lead is inserted into the right ventricle (the standard position), the QRS morphology takes on a left bundle branch block pattern. This occurs since electrical activation will start with the right ventricle first, then slowly spread to the left ventricle, similar to what occurs in the setting of a left bundle branch block. This right ventricular pacing can induce "ventricular dyssynchrony" which can cause problems at times. Therefore, a "biventricular pacemaker" has been developed placing a second lead into the coronary sinus around the back of the heart which can pace the left ventricle as well. 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.
