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

Abstract

The maximum atrial paced rate with 1:1 atrioventricular conduction (R max) 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. R max determined (1) under quietly resting, control conditions, (2) following β-adrenergic blockade, (3) following muscarinic blockade, and (4) following combined β-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, R max under control conditions averaged 136 ± b4 beats per minute (bpm). Following β-blockade, R max was 126 ± 5 bpm; while with muscarinic blockade, R max averaged 373 ± 4 bpm ( P < 0.001, with control). With combined β- and muscarinic blockade, R max was 300 ± 14. After AVNR parasympathectomy, although the resting heart rate was unchanged, the R max under control conditions was 342 ± 10 bpm. β-Blockade reduced this significantly ( P < 0.001) to 278 ± 15 bpm. With muscarinic blockade, R max averaged 346 ± 11 bpm, which was not different from the control R max after AVNR parasympathectomy. Combined β- and muscarinic blockade produced an R max of 280 ± 14 bpm. These experiments indicate a selective and differential parasympathetic regulation of the A-V region in conscious, resting dogs. Selective parasympathectomy of the atrioventricular nodal region results in conduction of impulses at very rapid rates to the ventricles.