Single-Lead DDD System: A Comparative Evaluation of Unipolar, Bipolar and Overlapping Biphasic Stimulation

Single-lead DDD pacing using unipolar or bipolar stimulation has been limited by high atrial thresholds. Overlapping biphasic (OLBI) waveform stimulation via atrial floating ring electrodes may preferentially enhance atrial stimulation and avoid diaphragmatic pacing. Single-lead DDD pacing using the OLBI pacing configuration was studied in 12 patienta (6 male, 6 female, mean age 74±7 years) with complete heart block. At implantation, atrial rings (area 27 mm2, separation 10 mm) were positioned at radiological defined high, mid and low right atrium (RA). The P-wave amplitude, atrial and diaphragmatic pacing thresholds were determined at each position using unipolar, bipolar and OLBI stimulation configurations in randomized order. Although statistically insignificant, the P-wave amplitude tended to be lower in the low RA position. Independent of the stimulation modes, minimum atrial pacing threshold occured in the mid RA. In the mid RA position , the atrial pacing threshold was significantly lower with the OLBI mode compared to either unipolar or bipolar mode (3,9±2,2V vs. 6,7±3,5V vs. 6,9±3,5V, p < 0,05). Althrough the diaphragmatic thresholds were similar, OLBI pacing modes in mid RA and final location sigificantly improved the safety margin for preventing diaphragmatic pacing compared to unipolar mode. By the time of pre-discharge testing, one patient developed atrial fibrillation, the remaining 11 patients had satisfactory atrial capture and stable atrial pacing threshold (day 0: 2,6±1,1V vs. day 2: 3,2±1,3V, p = ns). However, diaphragmatic pacing occured in 4/11 (36%) of patients especially during upright postures (sitting and standing). Preliminary clinical results suggest that OLBI pacing via atrial floating ring electrodes can reduce atrial pacing treshold compared to unipolar and bipolar configurations. To optimize atrial pacing and sensing, the bipolar electrodes should be located at the mid RA position first, althrough the right RA may be alternative. Despite significant improvement of safety marginfor diaphragmatic pacing with OLBI pacing mode, diaphragmatic stimulation remains a clinical problem.