Over several days’ exposure to hypoxia there is a progressive rise in ventilation and increasing arterial oxygenation, often referred to as ventilatory acclimatization to hypoxia, which is well described but poorly understood. Numerous theories have been put forward to explain this phenomenon, including resolution of respiratory alkalosis in blood or brain, central neural excitation, and even progressive improvement in lung mechanics, but none has been clearly established as a likely mechanism. A cardinal feature of acclimatization, the progressive increase in ventilation in the face of a deceasing stimulus, arterial hypoxemia, suggests that the ventilatory sensitivity to hypoxia increases during acclimatization. However, only a few studies have directiy assessed the ventilatory responsiveness to hypoxia during acclimatization and show a substantial progressive increase. This augmentation of hypoxic ventilatory sensitivity could reflect either an augmentation of carotid body sensitivity, enhanced central translation of chemoreceptor input into ventilatory output, or an improvement in efficiency of the peripheral respiratory apparatus. Several lines of evidence suggest that the most important changes are those in the carotid body. First, hypoxemia, limited to the carotid body by isolated perfusion, seems to lead to a progressive rise in ventilation which resembles acclimatization and does so in the absence of changes in systemic or cerebral blood chemistry. Second, the hypoxic responsiveness of the carotid body, assessed from carotid sinus nerve recordings, is substantially increased following acclimatization. Thus, it appears that increasing ventilation during hours to days of exposure to sustained hypoxia may, to a large degree, reflect an enhancement of the relationship linking the response ventilation to its stimulus—hypoxia. This suggests that hypoxia in some way enhances the sensitivity of the carotid body, although nothing is known regarding the cytochemical events responsible for these changes.
Tags: Chronic obstructive pulmonary disease, Hypercapnia, Hypoxia