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A 76 year old patient with paroxysmal atrial fibrillation and recurrent syncope caused by sinus arrest was admitted for permanent pacemaker implantation. This was uncomplicated with satisfactory pacing parameters (ventricular lead: threshold 0.7 V at 0.5 ms, R wave 18.1 mV). A routine pacing check the following day revealed complete loss of ventricular pacing and abdominal twitching during unipolar ventricular pacing. The sensed unipolar ventricular electrogram demonstrated a dominant, upright R wave (panel A) and a postero-anterior chest x ray showed the lead tip pointing superiorly, outside the right ventricular boundaries. An epicardial position was suspected and screening in the left anterior oblique projection during subsequent repositioning showed the lead overlying the apical left ventricular free wall (panel B). Repositioning was performed without further complication and a post-procedure unipolar electrogram was obtained (panel C).
Ventricular depolarisation normally occurs from endocardium to epicardium. Endocardial lead positions therefore produce a predominantly negative unipolar electrogram (panel C) and epicardial sites result in a predominantly positive electrogram (panel A). The endocardial electrogram at the repositioned site (panel C) shows a small initial R wave, which is normal for an apical endocardial right ventricular lead position. This is because during normal intrinsic conduction the earliest site of ventricular activation is the septal aspect of the left ventricular endocardium and septal activation then occurs left to right.
Perforation of the right ventricular free wall during pacemaker implantation is often clinically silent. Findings may include chest pain, a pericardial rub, and a right bundle branch block (RBBB) pacing pattern. If hypotension occurs cardiac tamponade must be excluded.