Abstract
To determine whether chronic exposure to hypoxia during adulthood produces alterations in the control of ventilation, measurements of the resting ventilatory response to hypoxia and hypercapnia, as well as ventilatory response to hypoxia during exercise, were carried out in a group of 10 long-term (3-39 yr) non-native residents of Leadville, Colo. (elevation 3100 m). A group of 8 subjects native to Leadville was also studied and 10 low altitude subjects of Denver, Colo. (elevation 1600 m) served as controls. Hypoxic ventilatory drive was measured as the shape parameter A of isocapnic VE-PA(o2) curves. In the non-native high altitude resident this parameter averaged 43% of the value for low altitude controls (P<0.05) denoting a diminished ventilatory response to hypoxia. The degree of attenuation was related to the length of time spent at high altitude. In the high altitude natives the parameter A averaged 9.6% of control (P<0.01). Similarly hypercapnic ventilatory drive as measured by the slope of the isoxic VE-PA(co2) lines was reduced in the non-native residents to 65% of control (P<0.05) and in the natives averaged 54% of control (P<0.01). In contrast with these findings at rest induction of hypoxia during exercise produced an increase in ventilation comparable to that in the controls in both groups of highlanders. Hence chronic exposure to hypoxia during adulthood in man results in marked attenuation of the ventilatory response to hypoxia at rest and this is a function of the length of exposure to hypoxia. This attenuation of the ventilatory response to hypoxia was associated with a decrease in hypercapnic ventilatory drive. The fact that hypoxic ventilatory drive was almost completely absent while hypercapnic drive was only partially reduced parallels closely the more important role of the peripheral chemoreceptors in mediating ventilatory responses to hypoxia than to hypercapnia. This suggests that the alterations in ventilatory control at altitude are due to failure of peripheral chemoreceptor function.