Ventilation, EELV and diaphragmatic activity in rats during chronic normobaric hypoxia.

We determined the effects of chronic hypoxia on end-expiratory lung volume (EELV), end-expiratory diaphragmatic activity (DE) and ventilation (V̇E) in 27 intact (awake and anesthetized) and six carotid body-denervated (CBD; anesthetized) rats. Twenty-nine control animals were also studied. Recordings were made during hypoxia and normoxia before and after 2 or 3 weeks of hypoxia (+3 days of recovery from chronic hypoxia). In awake rats, 2 weeks of chronic hypoxia increased only normoxic V̇E, while 3 weeks of chronic hypoxia did not change V̇E or DE. In anesthetized intact rats, after both exposures, hypoxic and normoxic V̇E tended to decrease, DE did not change and hypoxic and normoxic EELV were enlarged. In CBD animals, 2 weeks of chronic hypoxia did not affect hypoxic V̇E but decreased normoxic ventilation and enlarged EELV similar to the intact animals. After 3 days of recovery in normoxia, all parameters except EELV were restored to prehypoxic values. Also, transition from hypoxia to normoxia induced parallel changes in EELV and DE while chronic hypoxia increased only EELV. Therefore, chronic normobaric hypoxia induced, (1) an increase in normoxic ventilation reflecting a process of acclimatization; (2) an enlargement of EELV that did not depend on changes in DE and carotid chemoreceptors.