Ventilatory responses to hypercapnia and hypoxia following chronic hypercapnia in the rat

This study investigated the effects of an 18 week exposure to 10% CO2 in air on minute ventilation ((V) over dot(E)), breathing pattern and the chemoresponiveness of rats to hypoxic and hyperoxic stimuli. We found that (V) over dot(E), remained elevated over the 18 weeks. Nonetheless, the breathing pattern changed significantly. Tidal volume increased and the durations of inspiration and the total cycle decreased. After the sustained hypercapnia the mean Pa-CO2 was 72.0 + 5.1 (S.D.) mmHg. Every 6 weeks the chemoresponiveness of the CO2-exposed rats was tested by an acute exposure sequentially to room air, then a 6% O-2, 10% CO2 and 84% N-2 gas mixture, and finally a 90% O-2 in 10% CO2 mixture. On either room air or the hyperoxic-hypercapnic mixture (V) over dot(E) fell to its pre-hypercapnic level. On the hypoxic-hypercapnic mixture ir, increased significantly. These results demonstrate that the initial stimulating effect of 10% CO2 on (V) over dot(E) persisted for the entire 18 weeks without altering hypoxic or hyperoxic ventilatory responses. (C) 2000 Elsevier Science B.V. All rights reserved.