The effect of vagal blockade on the variability of ventilation in the awake dog.

The present study examined the variability of breathing in five (5) awake tracheostomized dogs with the vagi intact and during complete vagal blockade produced by cooling exteriorized cervical vagal loops (VC). Breath by breath variations in both respiratory timing (assessed from the airflow signal) and the drive to the respiratory muscles (as assessed from the rate of inspiratory airflow (VI/TI) and occlusive pressure (P100) were examined. The degree of variability in the parameters characterizing breathing was evaluated from frequency distribution histograms and by calculation of the standard deviation. VC increased the mean values of VT, TI, TE, TI/TTOT, and decreased VT/TI and occlusion pressure, but had no consistent effect on the mean value of VE. The variability of VE, PACO2, VT, TI, TE, TI/TTOT was greater during VC in 4 of the 5 dogs. The increased variability of VE and PACO2 during VC appeared to be due to a poorer correlation between TI and TE. The present study suggests that vagal mechanoreceptors, presumably pulmonary stretch receptors, minimize breath by breath fluctuations in both the level and pattern of ventilation by controlling respiratory timing. An explanation, based on the model of inspiratory off-switching proposed by Beadley et al. (1975) is invoked.