It is well established that chronic right ventricular apical pacing has an adverse effect on left ventricular systolic function. An alternative is pacing the left ventricular apex, which has shown more favourable results in terms of left ventricular pump function than the RVA in small series. The left ventricular apex may be paced via the coronary sinus tributary (e.g. in the anterior cardiac vein). Our study will compare effects of these two pacing sites on left ventricular ejection fraction measured by 3D-echocardiography.