Does this variable chemosensitivity influence the nature and quality of adaptation to hypoxia at high altitude and in disease? In the case of high altitude, several studies suggest such an association. For example, individuals with the ability to climb to very high altitudes seem to have especially high hypoxic ventilatory responses, compared with climbers unable to reach such high altitudes, which contrasts with the low hypoxic responses associated with another kind of athletic prowess—endurance running. Further, individuals with a high pre-ascent hypoxic response manifest higher ventilation and arterial oxygen saturation during hypoxic exercise. Finally, individuals with high hypoxic response manifest fewer symptoms of acute mountain sickness than individuals with low response. Thus, a high hypoxic response seems to be associated with ability to climb to higher altitudes, higher ventilation and oxygenation during hypoxic exercise, and fewer symptoms of hypoxic maladap-tation. buy prednisone
Although pre-ascent ventilatory sensitivity to hypoxia may be a determinant of function and adaptation to the hypoxia of high altitude, important changes in ventilatory sensitivity to hypoxia seem to be acquired during, and as a consequence of, hypoxic exposure. It has long been known that severe hypoxia (Pa02 around 20 mm Hg) has profound ventilatory depressant effects. Whether this is true of more moderate hypoxia of the sort encountered commonly at high altitude or in clinical hypoxemia has been less certain. An early indication that moderate hypoxia might act to reduce ventilation was the finding that, in subjects at simulated altitude where carbon dioxide had been added to the ambient air to minimize the development of hypocapnia, ventilation was found to be substantially below values measured in those same individuals during brief isocapnic hypoxia. This suggested that the ventilatory response to sustained hypoxia may be less than that observed with brief exposure. Subsequent studies by several investigators have demonstrated that indeed in moderate hypoxia (Pa02 in the range of 40-50 mm Hg) ventilation rises to an initial peak value within the first few minutes and after 5-10 minutes declines to a new lower plateau value 20-30% below the peak response but well above the prehypoxic baseline.
Tags: Chronic obstructive pulmonary disease, Hypercapnia, Hypoxia