Maximal airway response in mature and immature rabbits during tidal ventilation. Tepper, R. S., X. Shen, E. Bakan, S. J. Gunst. Departments of Pediatrics and Physiology, and Biophysics, Indiana University School of Medicine, Indianapolis, Indiana 46223
APStracts 2:0231A, 1995.
Airway closure during maximal methacholine (Mch) challenge was evaluated using alveolar capsules in 8 immature and 8 mature anesthetized rabbits in vivo during imposed tidal ventilation. Airway opening and alveolar pressures(DPao, DPalv) and pulmonary resistance(Rpulm) were measured during Mch challenge at PEEP = 5 cm H2O. In immature rabbits, DPalv remained > 3 cm H2O in all animals indicating no detectable airway closure. This contrasts to our previous study of isolated immature rabbit lungs under static conditions in which DPalv was < 0.1 cm during maximal Mch challenge; findings consistent with airway closure. Airway closure also did not occur in mature animals during tidal ventilation in vivo; however, the frequency of closure in isolated lungs under static conditions was very low. With increasing Mch, end-expiratory Palv increased in immature but not in mature rabbits. Rpulm did not reach a plateau in immature rabbits during Mch whereas a plateau was reached in mature rabbits. Immature rabbits also had greater increases in Rpulm. These results suggest that tidal ventilation can limit bronchoconstriction in immature rabbits and prevent airway closure during maximal Mch challenge. Tidal ventilation may limit bronchoconstriction by inhibitory effects of stretch on airway smooth muscle contraction and also by causing hyper-inflation and thereby increasing transpulmonary pressure. 

Received 13 February 1995; accepted in final form 12 May 1995.
APS Manuscript Number A165-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 May 1995.