Since the development of the first VDD pacemakers which are dual chamber pacemakers that employ a single pacing lead for both atrial sensing and ventricular sensing and pacing, numerous studies have been done regarding the efficacy and sensitivity of the VDD pacemaker in comparison with the more common type of dual chamber pacemaker which is the DDD pacemaker. Each of these dual chamber pacemakers offers several advantages and disadvantages and in spite of that, DDD pacemakers have traditionally been preferred to VDD pacemakers for a variety of reasons. It is the purpose of this study to evaluate the efficacy and sensitivity of two different types of dual chamber pacemakers: (VDD and DDD pacemakers) by various electrophysiological and surgical parameters in an attempt to determine whether VDD pacemakers are a viable alternative to DDD pacemakers in the treatment of patients with 2nd or 3rd degree heart block with normal sinus node function. In addition, this study also aims to assess different sites for positioning the atrial sensing electrodes in VDD pacemakers using certain measurements and electrophysiological parameters in order to achieve a long-term stable atrial sensing and physiological atrial synchronous pacing comparable to that of DDD pacemakers. The study was conducted on 62 patients who were divided into 2 groups:DDD Group (Control Group): consisted of 31 patients suffering from 2nd degree or 3rd degree heart block. VDD Group (Patients Group): consisted of 31 patients suffering from 2nd degree or 3rd degree heart block. The VDD and DDD pacemakers were implanted in the patients and after implantation X-ray images were taken by Fluoroscopy and the images were used (via calibration and distance measuring technique) to measure the distance between the atrial sensing electrodes of the single lead of the VDD pacemaker and the junction between the Superior Vena Cava and the Right Atrium and this technique was used to determine whether the position of the atrial sensing electrodes is in the high, middle, or lower Right Atrium, (thereby subdividing the VDD group into 3 subgroups) and it was also used to measure the distance between the atrial lead of the DDD pacemaker and the junction between the Superior Vena Cava and the Right Atrium. In addition, tests of efficacy and sensitivity were done at implantation and in the followup period for both the control group and the patients group and these tests were: Atrial sensitivity, atrial lead impedance, duration of implantation, duration of fluoroscopy, P-wave amplitude range, event histogram (% of AV synchronous pacing), and atrial sensing threshold test. Comparing the parameters of efficacy and sensitivity between the control group and patients group at implant, the control group (DDD group) showed significantly higher efficacy and sensitivity than the patients group (VDD group). In addition, during the 4 month follow-up duration, the control group (DDD group) also showed significantly higher efficacy and sensitivity than the patients group (VDD group). As for comparing duration of implantation and duration of fluoroscopy, the patients group (VDD group) showed significantly shorter duration of implantation and duration of fluoroscopy than the control group (DDD group). With regards to comparing the parameters of efficacy and sensitivity between the 1st, 2nd, and 3rd subgroups of the patients group (VDD group) representing atrial sensing electrodes in upper, middle and lower RA respectively, the 1st VDD subgroup showed significantly the highest efficacy and sensitivity followed by the 2nd VDD subgroup and both of them showed significantly higher efficacy and sensitivity than the 3rd VDD subgroup. Furthermore, the 1st and 2nd VDD subgroups showed nearly equivalent efficacy and sensitivity to that of the control group (DDD group) unlike the 3rd VDD subgroup which showed much lower efficacy and sensitivity to that of the control group. There was a strong negative correlation between each of the mean P-wave amplitude, % of AV synchronous pacing, atrial sensing threshold and the mean distance of the atrial sensing electrodes from the junction of the Superior Vena Cava and the Right Atrium. In conclusion, mean P-wave amplitude, % of AV synchronous pacing, and atrial sensing threshold are reliable indicators of long term efficacy and sensitivity of both VDD and DDD pacemakers. On one hand, dual lead DDD pacing is superior to single lead VDD pacing for long term maintenance of AV synchronous pacing in patients with second or third degree AV block and preserved SA node function when no consideration is taken for the position of the atrial sensing electrodes of the VDD pacemaker lead within the Right Atrium. On the other hand, single lead VDD pacing is equivalent to dual lead DDD pacing for long term maintenance of AV synchronous pacing in patients with second or third degree AV block and preserved SA node function when the position of the atrial sensing electrodes of the VDD pacemaker lead within the Right Atrium is taken into account, namely, when the atrial sensing electrodes are positioned in the upper or middle Right Atrium. Finally, the high efficacy and sensitivity, shorter duration of implantation, shorter duration of fluoroscopy, and simpler implant procedure all suggest that single lead VDD pacing is a viable alternative to dual lead DDD pacing in patients with second or third degree AV block and preserved SA node function.