Differential effects of hypercapnia on expiratory phases of respiration inthe piglet

Hypercapnia induces prolongation of expiratory time (TE) during early devel opment. In the present study, we determined the response to steady state hy percapnia of three neural phases of the total respiratory cycle, inspiratio n (Tr), stage 1 or passive expiration, TE-1 and stage 2 or active expiratio n, TE-2. Experiments were performed in decerebrate, vagotomized, spontaneou sly breathing piglets aged 5-10 days. Neural phases of the respiratory cycl e were based on electrical activities of the thyroarytenoid (TA, laryngeal adductor) and triangularis sternii (TS, chest wall expiratory muscle) in re lation to diaphragm (D) activity. We observed that hypercapnia induced prol ongation of both expiratory phases. The greater prolongation of TE-1 was as sociated with an increase in TA activity and an increase in laryngeal resis tance, which peal;ed early in Te-1, and then progressively decreased. These findings demonstrate that, in early postnatal life, a hypercapnia induced increase in respiratory drive is associated with centrally mediated prolong ation of both phases of expiration, a greater prolongation of TE-1, and an increase in laryngeal resistance during post-inspiration. We speculate that the latter serves to optimize gas exchange by reducing large fluctuations in functional residual capacity. (C) 2001 Elsevier Science B.V. All rights reserved.