Ventilation, EELV and diaphragmatic activity in rats during the early phase of normobaric hypoxia

We tested whether the enhancement of end-expiratory activity of the diaphra gm (DE) induced by acute hypoxia persists during long-lasting hypoxia and p articipates in the enlargement of end-expiratory lung volume (EELV). We thu s measured these two parameters together with ventilation ((V)over dot E) i n 30 rats, either awake or anesthetized, exposed to (1) poikilocapnic hypox ia sustained for 2 or 3 h, or (2) chronic normobaric hypoxia for 7 days int errupted by short episodes of normoxia. Twelve control animals were also st udied. (1) Sustained hypoxia induced a stable increase in DE VE and EELV. ( 2) In awake rats. chronic hypoxia induced a transient increase in VE after 1 day of hypoxia. and an increase persisting during acute normoxia througho ut the exposure, DE followed the same, although less pronounced. Course as VE. In anesthetized animals, only EELV was increased in both chronic hypoxi a and acute normoxia, but its enlargement in normoxia was not associated wi th a concomitant increase in DE. The transition front hypoxia to normoxia a lways induced a decrease in DE and EELV. Therefore, (1) during hypoxia sust ained for 2 or 3 h, the ventilatory and diaphragmatic responses were stable ; (2) during chronic hypoxia lasting 1 week. a ventilatory acclimatization was expressed by a transient increase in hypoxic VE and a hyperventilation continuing during acute normoxia (3) EELV enlargement in chronic hypoxia wa s partly related to changes in DE and partly due to another mechanism possi bly involving morphological adaptations. (C) 2001 Elsevier Science B.V. All rights reserved.