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\(\left[ {\Delta \dot V_{\left( {\text{s}} \right)} } \right]\) were obtained as the difference in ventilation between hypoxia (mean values ± S.D. of\(P_{{\text{ET}}_{{\text{O}}_{\text{2}} } }\)=7.36±0.20 kPa or of\(P_{{\text{a}}_{{\text{O}}_{\text{2}} } }\) 7.10 ±0.41 kPa) and hyperoxia (\(P_{{\text{ET}}_{{\text{O}}_{\text{2}} } }\)>26.7 kPa) with the same degree of hypercapnia (\(P_{{\text{ET}}_{{\text{CO}}_{\text{2}} } }\) 6.12±0.22 kPa). On the other band, withdrawal responses\(\left[ {\Delta \dot V_{\left( {\text{w}} \right)} } \right]\) were obtained as the magnitude of depression in ventilation caused by two bicaths of O₂ from the above mentioned hypoxic hypercapnia. Averaged\(\Delta \dot V_{\left( {\text{s}} \right)}\) and\(\Delta \dot V_{\left( {\text{w}} \right)}\) were 9.57±5.45 and 6.45 ±4.90l/min, respectively, the difference being statistically significant (P<0.01). Furthermore, if we assume the presence of ventilatory depression to be due to tissue\({\text{P}}_{{\text{CO}}_{\text{2}} }\) 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\(\Delta \dot V_{\left( {\text{w}} \right)}\).