Respiratory control in humans after 8 h of lowered arterial Po-2, hemodilution, or carboxyhemoglobinemia

In humans exposed to 8 h of isocapnic hypoxia, there is a progressive incre ase in ventilation that is associated with an increase in the ventilatory s ensitivity to acute hypoxia. To determine the relative roles of lowered art erial Po-2 and oxygen content in generating these changes, the acute hypoxi c ventilatory response was determined in 11 subjects after four 8-h exposur es: 1) protocol IH (isocapnic hypoxia), in which end-tidal Po-2 was held at 55 Torr and end-tidal Pco(2) was maintained at the preexposure value; 2) p rotocol PB (phlebotomy), in which 500 ml of venous blood were withdrawn; 3) protocol CO, in which carboxyhemoglobin was maintained at 10% by controlle d carbon monoxide inhalation; and 4) protocol C as a control. Both hypoxic sensitivity and ventilation in the absence of hypoxia increased significant ly after protocol IH (P < 0.001 and P < 0.005, respectively, ANOVA) but not after the other three protocols. This indicates that it is the reduction i n arterial Po-2 that is primarily important in generating the increase in t he acute hypoxic ventilatory response in prolonged hypoxia. The associated reduction in arterial oxygen content is unlikely to play an important role.