[The effect of PaO2 on apneic threshold PCO2 (PATCO2)]

Recovery from posthyperventilation apnea accompanies a rise in PaCO2, and ventilation starts only when the PaCO2 attains the apneic threshold PCO2 (PATCO2). In the present study, we evaluated the effect of decreasing PaO2 on PATCO2 under light enflurane anesthesia in 17 female patients undergoing elective surgery. The apnea was produced by hyperventilating the lungs under two different conditions [with 1.3% (end-tidal) enflurane in 100% O2 or with 1.3% enflurane in 30% O2]. The initiation of ventilation after apnea was identified by the first expiratory act observed in capnogram, at which time an arterial blood sample was taken to determine PATCO2. The values of PaO2 at the initiation of ventilation with 1.3% enflurane in 100% O2 and 1.3% enflurane in 30% O2 were 394.7 +/- 65.0 mmHg (mean +/- SD), and 51.9 +/- 18.2 mmHg, respectively. There was a significant difference between the two values (P < 0.001). The values of PATCO2 with 1.3% enflurane in 100% O2 and 1.3% enflurane in 30% O2 were 48.3 +/- 4.9 mmHg and 47.2 +/- 3.9 mmHg, respectively, and no significant difference was observed between the two values. Our results indicate that a decrease in PaO2 exerts no effect on PATCO2 at 0.7 MAC of enflurane anesthesia. This suggests that hypoxic stimulation does not play an important role in initiation of ventilation following posthyperventilation apnea at a light level of anesthesia.