Effects of resting vagal tone on accessory atrioventricular connections

The purpose of this study was to determine the effects of resting vagal tone on accessory atrioventricular (AV) connections. Atropine (0.04 mg/kg) was administered to 13 patients with the Wolff-Parkinson-White syndrome and was found to have the following effects on the accessory AV connection: the anterograde block cycle length shortened from 305 +/- 51 to 279 +/- 54 msec (mean +/- SD; p less than 0.001); the retrograde block cycle length shortened from 288 +/- 57 to 251 +/- 50 msec (p less than 0.001); and the effective refractory period measured at a basic drive cycle length of 400 msec shortened from 295 +/- 45 to 265 +/- 47 msec in the anterograde direction (p less than 0.001) and from 283 +/- 18 to 261 +/- 12 msec in the retrograde direction (p less than 0.01). During atrial fibrillation, the mean ventricular cycle length decreased from 434 +/- 88 to 352 +/- 56 msec (p less than 0.001), and the shortest preexcited RR interval decreased from 302 +/- 56 to 256 +/- 43 msec (p less than 0.01). In another seven patients, propranolol (0.2 mg/kg) was administered before atropine, and atropine lengthened the anterograde block cycle length and the effective refractory period of the accessory AV connection; the magnitude of these effects was similar to that in the patients who did not receive propranolol. In conclusion, these data demonstrate that resting vagal tone exerts a direct depressant effect on accessory AV connections that does not require background sympathetic activity to be manifest.