THE INFLUENCE OF PACING RATE AND AUTONOMIC BLOCKADE ON HUMAN PRIMARY AND SECONDARY ATRIAL PACEMAKERS

The morphology of the first spontaneous post-pacing P wave was assessed in 106 patients who underwent electrophysiological study for various arrhythmias. An overall number of 589 atrial pacing sessions, from 60-200 b/min, were analyzed (mean 5.55 +/- 1.3 session per patient). After cessation of 138 (23.42%) of them, spontaneous post-pacing P wave arose from extrasinus foci. Nonsinus spontaneous post-pacing P waves were recorded after 1 or more pacing rates (from 1 to 7) in 58 patients (54.7%, Group 1). In the remaining 48 patients the spontaneous post-pacing P waves were sinus after all pacing sessions (45.3%, Group 2). Sinus node disease (SND) was present in 32.7% of Group 1 patients and in 2% of Group 2 (p = 0.002). Spontaneous post-pacing P wave of extrasinus origin was recorded in 19/20 of SND patients (95%) compared to 39/86 of patients with normal sinus function (45.34%, p < 0.001). SND patients had a greater number of pacing sessions resulting in nonsinus spontaneous post-pacing P waves. Pacing rate associated with nonsinus spontaneous post-pacing P waves was lower in SND patients compared to patients with normal sinus node function. In the SND group, nonsinus spontaneous post-pacing P waves were recorded beyond the first spontaneous one in 9 patients compared to only 1 in patients with normal sinus node function (p < 0.005). Extrasinusal first spontaneous post-pacing P waves at a pacing rate of 140 b/min had the highest sensitivity (75%). Increasing of pacing rate from 60 to 140 b/min was associated with progressive arousal of secondary foci with highest value of 36.8% in pacing rate of 140 b/min. Further increase of pacing rate resulted in a slight reduction of spontaneous post-pacing firing from secondary foci. Atropine was administered in 12 Group 1 patients and abolished all measurable morphological changes of spontaneous post-pacing P waves compared to pre-pacing. Autonomic blockade was performed in 14 Group 1 patients and 16 Group 2 patients. In 35.7% of Group 1 patients autonomic blockade abolished the appearance of nonsinus spontaneous post-pacing P waves, while in the remaining patients increased the lowest pacing rate resulting in nonsinus spontaneous post-pacing P waves. In 12.5% Group 2 patients autonomic blockade resulted in nonsinus spontaneous post-pacing P waves after cessation of atrial pacing. In conclusion, the extrasinus spontaneous rise of atrial impulse is a very frequent phenomenon after atrial pacing especially in patients with SND. Pacing rate has different quantitative effects on sinus and secondary atrial pacemakers. Secondary pacemakers are less under autonomic control compared to sinus node.