Parallel baroreceptor control of sinoatrial rate and atrioventricular conduction

The vagal efferent limb of baroreceptor reflexes was studied in 10 anesthetized (alpha-chloralose) dogs to determine its differential regulatory significance on sinoatrial (SA) rate and antegrade atrioventricular (AV) conduction. The arterial pressure stimulus for these reflexes was produced by intra-aortic balloon inflation (increase) and hemorrhage (decrease); the efferent sympathetic reflex was blocked by spinal cord transection. The dromotropic response to arterial pressure changes was studied over a physiological range of arterial pressures (60-150 mmHg) with the chronotropic response intact, with atrial pacing to prevent the chronotropic response, and after atropine or bilateral vagotomy. In all animals, the chronotropic response to arterial pressure changes balanced the dromotropic response to preserve 1:1 AV conduction over most arterial pressures. If the chronotropic response was prevented with atrial pacing, an unbalanced and independent vagal regulation of AV conduction was "unmasked" with resultant high-grade second-degree AV block. The independent regulations of SA rate and AV conduction were further found to be finely matched to produce an equilibrium of chronotropic-dromotropic changes as arterial pressure varied. Clinical conditions arising from disruption of this equilibrium are discussed.