A 46-year-old man with a nonischemic dilated cardiomyopathy most likely secondary to a viral myocarditis from which he suffered 8 years earlier whom at the time of his myocarditis had a pacemaker implanted because of second degree AV block is presenting since his left ventricular failure has progresses to class III NYHA. LV ejection fraction 32% at rest and 19% during mild-to-moderate exercise:

• Sinus rhythm, 60/min, with DDD pacing (atrial sensed, ventricular paced rhythm)
  
• Left axis deviation – QRS duration 160/min.
• ECG pattern typical of Right Ventricular apical pacing

Because of progressive advanced LV failure despite optimal medical therapy the patient is referred to the Electrophysiology Service and a therapeutic procedure is carried out:

• Sinus rhythm, 63/min
• DDD pacing (atrial sensed, ventricular paced rhythm)
• One premature ventricular complex
• Compared with ECG 1, there is now right superior QRS axis (240 º)
• The ECG findings are suggestive of biventricular pacing

Comments: The patient underwent an upgrade from RV apical pacemaker to biventricular pacemaker. The atrial lead was positioned in the right atrial appendix, the LV lead in a lateral coronary vein branch. The right ventricular lead remained in the RV apex. The two ECG tracings demonstrate the typical  QRS patterns of RV apical pacing (left axis deviation, negative QRS  in V1 to V6) and biventricular pacing (right superior axis deviation).