Influence of the depth of anaesthesia on the peripheral and central ventilatory CO2 sensitivity during hyperoxia

The influence of the depth of anaesthesia on the contribution of central and peripheral chemoreceptors to the slope of the ventilatory response to CO2 during hyperoxia was studied in 12 cats anaesthetized with chloralose-urethane or pentobarbital. By artificial perfusion of the pontomedullary region of the brainstem it was possible to assess the peripheral (Sp) and central (Sc) ventilatory sensitivity to CO2, as well as to vary selectively the level of anaesthesia at the ponto-medullary region (central level) or the overall level. In each cat Sp and Sc were assessed at the initial and at one or two deeper anaesthetic levels. Going from the initial to deeper anaesthetic levels both Sp and Sc decreased. However, Sp/Sc did not change significantly, whether the overall or the central level of anaesthesia was increased. Also the B-value, i.e. the PaCO2 of the extrapolated ventilation-PaCO2 curve at zero ventilation, did not change significantly when going to deeper anaesthetic levels. It is concluded that the respiratory depression caused by the anaesthetics used is due to an influence on the respiratory integrating centres, central chemoreceptors, or both.