EFFECTS OF POSTURAL CHANGES VERSUS EXERCISE ON THE ELECTROPHYSIOLOGIC PARAMETERS OF THE ACCESSORY PATHWAY

The electrophysiologic profile of the accessory pathway was studied in 17 patients (mean age +/- standard deviation 32 +/- 14 years) with Wolff-Parkinson-White (WPW) syndrome who were either lying down in a supine position, standing, or undergoing isometric or dynamic treadmill exercise. There were significant decreases in the PP interval after isometric exercise, standing and dynamic exercise (supine 764 +/- 224, standing 638 +/- 146, isometric 605 +/- 170, treadmill 455 +/- 86 ms; p < 0.05). Both anterograde and retrograde accessory pathway refractory periods were measured after a constant drive of 400 ms during lying down supine, standing and isometric and treadmill exercise (Bruce protocol stage II). There was no significant decrease in the anterograde accessory pathway refractory period during isometric exercise (lying down 265 +/- 22 to isometric exercise 256 +/- 13 ms, p < 0.05), but there were significant decreases (p < 0.05) during standing (246 +/- 24 ms) and treadmill exercise (235 +/- 17 ms). The retrograde accessory pathway refractory period also showed a significant decrease (supine 272 +/- 16, isometric 267 +/- 23, standing 249 +/- 15, treadmill 237 +/- 17 ms; p < 0.05). The relative change in refractory periods was greater when patients changed from lying down to standing than when they changed from standing to treadmill exercise, despite obvious higher adrenergic neuronal activities during exercise. These findings suggest that testing the accessory pathway during free standing can give a reliable indication of the sensitivity of the accessory pathway to sympathetic stimulation. Further increase in sympathetic stimulation, as judged by increases in the sinus rate, does not cause proportional reduction in the refractoriness of the accessory pathways.