Influence of selective parasympathectomy of the A-V nodal region on atrioventricular conduction in conscious dogs.

The maximum atrial paced rate with 1:1 atrioventricular conduction (Rmax) was compared before and after selective parasympathectomy of the atrioventricular nodal region (AVNR). Each animal was instrumented with right atrial and right ventricular bipolar electrodes. Rmax was determined (1) under quietly resting, control conditions, (2) following beta-adrenergic blockade, (3) following muscarinic blockade, and (4) following combined beta-adrenergic and muscarinic blockade. During a second surgical procedure approximately two weeks later, parasympathectomy was achieved by dissection and topical application of phenol to the fat pad and underlying epicardium at the inferior left atrial junction with the inferior vena cava; completeness of AVNR parasympathectomy was tested at surgery by supramaximal stimulation of right and left cervical vagi, with and without rapid atrial pacing. AVNR sympathetic innervation remained intact. All studies were conducted while the animals were conscious and quietly resting. Before parasympathectomy, Rmax under control conditions averaged 136 +/- b4 beats per minute (bpm). Following beta-blockade, Rmax was 126 +/- 5 bpm; while with muscarinic blockade, Rmax averaged 373 +/- 4 bpm (P less than 0.001, with control). With combined beta- and muscarinic blockade, Rmax was 300 +/- 14. After AVNR parasympathectomy, although the resting heart rate was unchanged, the Rmax under control conditions was 342 +/- 10 bpm. beta-Blockade reduced this significantly (P less than 0.001) to 278 +/- 15 bpm. With muscarinic blockade, Rmax averaged 346 +/- 11 bpm, which was not different from the control Rmax after AVNR parasympathectomy. Combined beta- and muscarinic blockade produced an Rmax of 280 +/- 14 bpm.(ABSTRACT TRUNCATED AT 250 WORDS)