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Pulmonary impedance and alveolar instability during injurious ventilation in rats

¹Department of Medicine, Vermont Lung Center, University of Vermont, Burlington; ²Fletcher Allen Health Care, Burlington, Vermont; and ³Department of Surgery, State University of New York Upstate Medical University, Syracuse, New York

Submitted 1 December 2004 ; accepted in final form 11 April 2005

The mechanical derangements in the acutely injured lung have long been ascribed, in large part, to altered mechanical function at the alveolar level. This has not been directly demonstrated, however, so we investigated the issue in a rat model of overinflation injury. After thoracotomy, rats were mechanically ventilated with either 1) high tidal volume (VT) or 2) low VT with periodic deep inflations (DIs). Forced oscillations were used to measure pulmonary impedance every minute, from which elastance (H) and hysteresivity () were derived. Subpleural alveoli were imaged every 15 min using in vivo video microscopy. Cross-sectional areas of individual alveoli were measured at peak inspiration and end exhalation, and the percent change was used as an index of alveolar instability (%I-E). Low VT never led to an increase in %I-E but did result in progressive atelectasis that coincided with an increase in H but not . DI reversed atelectasis due to low VT, returning H to baseline. %I-E, H, and all began to rise by 30 min of high VT and were not reduced by DI. We conclude that simultaneous increases in both H and are reflective of lung injury in the form of alveolar instability, whereas an isolated and reversible increase in H during low VT reflects merely derecruitment of alveoli.

ventilator-induced lung injury; acute respiratory distress syndrome; in vivo microscopy; alveolar recruitment; respiratory mechanics

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