Respiratory variation in the atrioventricular conduction time on dogs.

Respiratory variation in the intraatrial and atrioventricular conduction time and its mechanism were studied on 14 dogs under the following 3 conditions: (1) unanesthetized, (2) anesthetized with alpha-chloralose, and (3) anesthetized with pentobarbital sodium. Beautiful respiratory variation in the atrioventricular conduction time (shortening during inspiration and prolongation during expiration) was observed on both conditions (1) and (2), but nothing was observed in (3). No significant variation in the intraand interatrial conduction time was observed under any condition. This "respiratory-conduction time response" (RCR) was almost in phase with the variation in the cardiac interval. Amplitude of the variation in RCR was 14.7•}3.9msec. which corresponded to 3.8% of that of the variation in the cardiac interval. Augmentation of the RCR was observed on right atrial pacing with constant stimulus interval. It is concluded that RCR may result mainly from the waxing and waning of the efferent vagal activity to the atrioventricular node and in part from factor dependent upon heart rate change itself i.e. chronotropic dromotropism. It is supposed that the afferent pathway from the lung is not so important in the physiological state as the central origin in this phenomenon.