Effect of unilateral and bilateral vagotomy on the mechanical stability of ventilation in anesthetized cats

The effect of the resistive loading of inspiration and expiration on pulmonary stretch receptor discharge

Anesthetized, spontaneously breathing cats were used to examine the hypothesized role of slowly adapting pulmonary stretch receptors (PSR) in the control of breath duration. Initially, graded inspiratory and expiratory resistive loads were added to elucidate the inspiratory and expiratory volume-time relationship with both vagi intact. Unilateral vagotomy increased the slope of the VI--TI relationship indicating a reduction of the volume related modulation of TI. PSR frequency (fPSR) at end-inspiration also progressively decreased resulting in a fPSR--TI relationship qualitatively similar to the VI--TI curve. Expiratory resistive loading also produced an increased slope for the VE--TE relationship when the right vagus nerve was severed. The prolongation of TE was associated with a progressive increase in the number of PSR discharges during the loaded expiration. These results support the hypothesized role of PSR in the vagally mediated prolongation of TI and TE during resistive loading. In a subsequent series of experiments, the changes in fPSR were correlated with the tidal volume and transpulmonary pressure (PTP) changes. The fPSR was linearly related to PTP during both eupnic and loaded breathing. When fPSR was plotted against volume, a clockwise hysteresis was observed. These results suggest that in the spontaneously breathing cat, intrathoracic PSR frequency varies as a function of the transmural pressure across the airways.

Tonic vagal influences on inspiratory duration

The tidal volume (VT), inspiratory (Ti) and expiratory (Te) durations were determined in anesthetized rabbits, cats and dogs under normal conditions, during continuous positive or negative pressure breathing. Elastic loads up to complete occlusion of the airways were applied in order to determine the VT-Ti and Te-Ti relationships. The larger the end-expiratory lung volume the steeper the VT-Ti relationship, the smaller the increase of Ti and the greater the increase of Te produced by the load. Hence, the larger the end-expiratory lung volume the steeper the Te-Ti relationship. In vagotomized animals airways occlusion did not affect Ti and Te. Ti of occluded breaths in vagotomized animals was longer than that of occluded breaths in intact animals, whereas Te was similar at the resting volume of the respiratory system (FRC), shorter above FRC and longer below FRC. These results show that both phasic and tonic vagal discharges determine Ti and Te.