VENTILATORY RESPONSES TO CO2 - CONSTANT FRACTION VS CONSTANT INFLOW ADMINISTRATION

The purpose of this study was to determine whether the transfer functi on characteristics of the respiratory CO2 control system differs accor ding to whether the CO2 administration method is constant fraction (CF ) or constant inflow (CFlow). Ventilatory responses to CO2 changes wer e measured in seven healthy subjects during random PET(CO2) perturbati on by the CF and CFlow administration methods in normoxia and hyperoxi a. The transfer function from P-CO2 to VE was estimated in the frequen cy domain from 0.002 to 0.02 Hz. The transfer function characteristics showed a low-pass filter character in both of CFlow and CE The impuls e responses to both the methods persisted for greater than or equal to 60 sec, while the maximum amplitude (h(max)) of the CFlow response wa s statistically greater than that of the CF response in normoxic condi tion. The time required until the peak (t(max)) of the CFlow impulse r esponse was shorter than that of CF in normoxia. Hyperoxia retarded th e t(max) and reduced h(max) in both CFlow and CF, with the result that significant differences in the normoxic impulse responses were not ob served between CFlow and CF in hyperoxia. To characterize the CO2 cont rol system quantitatively, we determined the static transfer gain, osc illatory frequency, damping factor, and pure time delay by applying a second-order delay model to the observed transfer function. The static gain was not significantly different between CFlow and CF responses i n both normoxia and hyperoxia. The pure time delay and damping factor were significantly decreased for CFlow only in normoxia. We suggest th at inhalation of CO2 by CFlow modifies ventilatory response, probably mediating through the peripheral chemoreceptor activity.