This variation in hypoxic response at high altitude, which proved in part to be traceable to an identifiable factor—the duration of hypoxic exposure —raised the question of whether there might be variation in such responses at low altitude traceable to other definable factors. Indeed, as the number of normal subjects in whom this response was measured grew, it became apparent that ventilatory responses to both hypoxia and hypercapnia spanned a broad range (Fig 3). This persisted with repeated testing—that is, individuals in whom low responses are measured on one occasion tend to have a low response on another. That this variation was nonrandom was suggested by subsequent studies that revealed familial clusters of low hypoxic ventilatory responses among the immediate relatives of patients with unexplained hypoventilation and low hypoxic ventilatory response. It was also found that athletes, particularly those who excel in endurance running, have relatively low hypoxic and hypercapnic ventilatory responses, N which are also seen in their nonathletic parents and siblings. Subsequent studies in twins to determine the relative importance of genetic and environmental contributions to these familial clusters demonstrated substantially greater concordance for hypoxic response among identical than fraternal twins, suggesting that genetic factors may be important determinants of the strength of this response. ventolin inhaler
Figure 3. Histogram describing the distribution of hypoxic and hypercapnic ventilatory responses in 44 normal low-altitude subjects. The distribution of responses is broad and possibly bimodal for both, suggesting substantial variation.
Tags: Chronic obstructive pulmonary disease, Hypercapnia, Hypoxia