Ventilatory response to high inspired carbon dioxide concentrations in anesthetized dogs

Background: The ventilation (V-I.) response to inspired CO2 has been extensively studied, but rarely with concentrations >10%. Aims: These experiments were performed to determine whether V-I. would increase correspondingly to higher concentrations and according to conventional chemoreceptor time delays. Materials and Methods: We exposed anesthetized dogs acutely, with and without vagotomy and electrical stimulation of the right vagus, to 20-100% CO2-balance O-2. and to 0 and 10% O-2-balance N-2. Results: The V-I. 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 V-I. at 100% CO2, with the response to 0% O-2 being similar to 3 s slower. Right vagotomy significantly reduced baseline breathing frequency (fR), increased tidal volume (VT) and increased the time delay by similar to 3 s. Bilateral vagotomy further reduced baseline fR and V-I., and reduced the response to CO2 and increased the time delay by similar to 12 s. Electro-stimulation of the peripheral right vagus while inspiring CO2 caused a 13 s asystole and further reduced and delayed the V-I. response, especially after bilateral vagotomy, shifting the mode from VT to fR. Conclusions: 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 CO2 concentrations by receptors upstream from the aortic bodies.