Identification of fast and slow ventilatory responses to carbon dioxide under hypoxic and hyperoxic conditions in humans

1. Under conditions of both euoxia and hypoxia, it is generally accepted th at the ventilatory response to CO2 has both rapid (peripheral chemoreflex) and slow (central chemoreflex) components. However, under conditions of hyp eroxia, it is unclear in humans whether the fast component is completely ab olished or merely attenuated in magnitude. 2. The present study develops a technique to determine whether or not a two -compartment model fits the ventilatory response to CO2 significantly bette r than a one-compartment model. Data were collected under both hypoxic (end -tidal P-O2 = 50 Torr) conditions, when two components would be expected, a nd under hyperoxic (end-tidal P-O2 = 200 Torr) conditions, when the presenc e of the fast compartment is under question. 3. Ten subjects were recruited, of whom nine completed the study. The end-t idal P-CO2 of each subject was varied according to a multi-frequency binary sequence that involved 13 steps into and 13 steps out of hypercapnia lasti ng altogether 1408 s. 4. In four out of nine subjects in hypoxia, and six out of nine subjects in hyperoxia, the two-compartment model fitted the data significantly better than the one-compartment model (F ratio test on residuals). This improvemen t in fit was significant for the pooled data in both hypoxia (P < 0.05) and hyperoxia (P < 0.005). Mean ventilatory sensitivities for the central chem oreflex were (mean +/- S.E.M.) 1.69 +/- 0.39 l min(-1) Torr(-1) in hypoxia and 2.00 +/- 0.32 l min(-1) Torr(-1) in hyperoxia. Mean ventilatory sensiti vities for the peripheral chemoreflex were 2.42 +/- 0.36 l min(-1) Torr(-1) in hypoxia and 0.75 +/- 0.16 l min(-1) Torr(-1) in hyperoxia. 5. It is concluded that the rapid and slow components of the ventilatory re sponse to CO2 can be separately identified, and that a rapid component pers ists under conditions of hyperoxia.