The Effects of Vagotomy on Ventilation and Blood Gas Composition in Dog, Sheep and Rabbit

Abstract

The influence of vagotomy on resting arterial blood gases (PaCO2 and PaO2) and on the ventilatory responses to hypercapnia and hypoxia was studied in three different species of anaesthetized animals. After bilateral cervical vagotomy resting PaCO2 generally declined. In the dog, where this was studied more fully, resting PaCO2 declined significantly (8.13+O067 mm Hg) over several hours, but PaO2 changes were not significant.After vagotomy tidal volume (VT), but not frequency (f), increased with increasing chemical drive. It is concluded that the increase in frequency in response to increased CO2 or decreased 02 is dependent on the vagal mechanisms. [1880] suggested that stimulation of the vagus was ineffective in animals made apnoeic by hyperventilation. Rice [1938] reported that hypercapnia produced by increased respiratory dead space caused a decrease in the effectiveness of the vagal reflexes while Woldring [1965] showed that the sensitivity of the inspiratory muscles to a given change in lung volume in anaesthetized cats varied with the level of arterial PaCO2.Although it is well known that the vagus nerves are the afferent pathway for a number of respiratory reflexes affecting rate and depth of breathing and there is some information on the minute volume/end tidal PaCO2 relationship after vagotomy, it is not clear what influence the vagi exert on maintenance of a normal arterial CO2 tension. The present study endeavours to establish a more quantitative relationship between the effects of vagotomy on arterial blood gas tension in the resting state as well as during hypercapnia and hypoxia in three different species. METHODSExperiments were carried out in thirteen rabbits (body weight 1-9-3-7 kg), twenty dogs (7419-2 kg) and twelve sheep (22-0-43-0 kg). Food was withheld for 12 hr before the experiment but the animals were allowed free access to water.

Ventilatory response to high inspired carbon dioxide concentrations in anesthetized dogs

“…1 and Table 1) was not far removed from the 10 s time delay reported in humans and dogs and occurred at estimated PAo 2 values close to those reported for steady state breathing[4], (c) the /B response leveled off with 100% CO 2 after vagotomy (Fig. 3C), as reported in dogs[5] and (d) the response was greatest and time delay shortest with 100% CO 2 when PAo 2 fell most rapidly (Fig. 2D), demonstrating the well-known enhanced ventilatory sensitivity to CO 2 when combined with hypoxia[6].…”

Breathing movements in fetal lambs and the effect of hypercapnia.

“…The rate is raised only in so far as the amount of low frequency breathing is reduced. The effects are similar to those in adult sheep, where hypercapnia is accompanied mainly by an increase in tidal volume with little rise in rate (Kashani & Haigh, 1976). The breathing during hypercapnia is indistinguishable from much of the regular, vigorous breathing that occurs intermittently in normocapnic fetuses.…”

Encoding of the cough reflex in anesthetized guinea pigs

“…With each breath in animals as different as sheep, dogs, guinea pigs, pigeons, and lizards, lung stretch receptors regulate respiratory rate by signaling the brain stem information about lung volume and stretch (1,9,26,48). Cutting the vagi in most animals results in an immediate and precipitous fall in respiratory rate coupled with a marked increase in tidal volume.…”