Central hypoxic-hypercapnic interaction in mild hypoxia in man.

Hypoxic-hypercapnic interaction in mild hypoxia was studied in 12 healthy males. Steady state ventilatory responses to hypercapnic-hypoxia [delta V(s)] were obtained as the difference in ventilation between hypoxia (mean values +/- S.D. of PETO2 = 7.36 +/- 0.20 kPa or of PaO2 7.10 +/- 0.41 kPa) and hyperoxia (PETO2 greater than 26.7 kPa) with the same degree of hypercapnia (PETCO2 6.12 +/- 0.22 kPa). On the other hand, withdrawal responses [delta V(w)] were obtained as the magnitude of depression in ventilation caused by two breaths of O2 from the above mentioned hypoxic hypercapnia. Averaged delta V(s) and delta V(w) were 9.57 +/- 5.45 and 6.45 +/- 4.90 l/min, respectively, the difference being statistically significant (P less than 0.01). Furthermore, if we assume the presence of ventilatory depression to be due to tissue PCO2 fall resulting from an increase in cerebral blood flow caused by hypoxia, the magnitude of central hypoxic-hypercapnic interaction was estimated to be as great as the value of delta V(w).