AV blocking due to asynchronous vagal stimulation in rats

The parasympathetic nervous system innervates the heart through two cervical vagal branches. The right vagal branch mainly influences the heart rate by the modulation of the rhythmogenesis of the sinoatrial node. The left branch predominantly influences the conduction properties of the atrioventricular (AV) node. We investigated the effect of asynchronous stimulation by the vagal nerves on the occurrence of irregularities in heart rate. In rats, the vagal nerves were isolated and cut. Different vagal stimulation patterns (continuous, pulsed) were applied. The heart was beating spontaneously under continuous vagal stimulation. In case of pulsed vagal stimulation, the atria were paced at different rates. Asynchronicity was induced by delaying the right stimulus with respect to the left stimulus (early right) or the left stimulus with respect to the right stimulus (early left). The value of the fraction of deviated R-R or P-Q intervals in the distribution in the histogram was used to characterize irregularities during a stimulation protocol (duration in case of continuous stimulation: 20 s; pulsed stimulation: 120 s). Under both stimulation patterns (continuous or pulsed), we found that early left vagal stimulation introduced a much larger fraction of deviated intervals in the R-R or P-Q histogram (in R-R: 29.1 +/- 4.9%; in P-Q: 12.90 +/- 1.95%) than early right vagal stimulation (in R-R: 7.4 +/- 2.0%; in P-Q: 1. 05 +/- 0.50%) or synchronous stimulation (in R-R: 8.2 +/- 3.6%; in P-Q: 2.15 +/- 0.75%). We conclude that early stimulation by the left vagal nerve can introduce irregularities in heart rate, mainly due to different degrees of AV nodal blockade.