Airway strain during mechanical ventilation in an intact animal model.
BACKGROUND
Mechanical ventilation with large tidal volumes causes ventilator-induced lung injury in animal models. Little direct evidence exists regarding the deformation of airways in vivo during mechanical ventilation, or in the presence of positive end-expiratory pressure (PEEP).


OBJECTIVE
To measure airway strain and to estimate airway wall tension during mechanical ventilation in an intact animal model.


METHODS
Sprague-Dawley rats were anesthetized and mechanically ventilated with tidal volumes of 6, 12, and 25 cm(3)/kg with and without 10-cm H(2)O PEEP. Real-time tantalum bronchograms were obtained for each condition, using microfocal X-ray imaging. Images were used to calculate circumferential and longitudinal airway strains, and on the basis of a simplified mathematical model we estimated airway wall tensions.


RESULTS
Circumferential and longitudinal airway strains increased with increasing tidal volume. Levels of mechanical strain were heterogeneous throughout the bronchial tree. Circumferential strains were higher in smaller airways (less than 800 mum). Airway size did not influence longitudinal strain. When PEEP was applied, wall tensions increased more rapidly than did strain levels, suggesting that a "strain limit" had been reached. Airway collapse was not observed under any experimental condition.


CONCLUSIONS
Mechanical ventilation results in significant airway mechanical strain that is heterogeneously distributed in the uninjured lung. The magnitude of circumferential but not axial strain varies with airway diameter. Airways exhibit a "strain limit" above which an abrupt dramatic rise in wall tension is observed.
Sinclair
Scott E
SE
Department of Physiology, University of Tennessee Health Science Center, 894 Union Avenue, Nash 426, Memphis, TN 38163, USA.
Molthen
Robert C
RC
Haworth
Steve T
ST
Dawson
Christopher A
CA
Waters
Christopher M
CM
D000403:Airway Resistance / Q000502:physiology*; D000818:Animals; D001995:Bronchography; D023421:Models, Animal; D008954:Models, Biological; D051381:Rats; D017207:Rats, Sprague-Dawley; D012121:Respiration, Artificial; D013314:Stress, Mechanical; D013990:Tidal Volume / Q000502:physiology