Ventilatory response to high inspired carbon dioxide concentrations in anesthetized dogs

The ventilation ( ) response to inspired CO(2) has been extensively studied, but rarely with concentrations >10%.

These experiments were performed to determine whether would increase correspondingly to higher concentrations and according to conventional chemoreceptor time delays.

We exposed anesthetized dogs acutely, with and without vagotomy and electrical stimulation of the right vagus, to 20-100% CO(2)-balance O(2) and to 0 and 10% O(2)-balance N(2).

The time delays decreased and response magnitude increased with increasing concentrations (p<0.01), but at higher concentrations the time delays were shorter than expected, i.e., 0.5 s to double at 100% CO(2), with the response to 0% O(2) being ~3 s slower. Right vagotomy significantly reduced baseline breathing frequency (fR), increased tidal volume (VT) and increased the time delay by ~3 s. Bilateral vagotomy further reduced baseline fR and , and reduced the response to CO(2) and increased the time delay by ~12 s. Electro-stimulation of the peripheral right vagus while inspiring CO(2) caused a 13 s asystole and further reduced and delayed the response, especially after bilateral vagotomy, shifting the mode from VT to fR.

Results indicate that airway or lung receptors responded to the rapid increase in lung H(+) and that vagal afferents and unimpaired circulation seem necessary for the initial rapid response to high CO(2) concentrations by receptors upstream from the aortic bodies.