MECHANISM OF GRAVITY-DEPENDENT ATELECTASIS - ANALYSIS BY NONRADIOACTIVE XENON-ENHANCED DYNAMIC COMPUTED-TOMOGRAPHY

RATIONALE AND OBJECTIVES. The physiologic mechanism of gravity-depende nt atelectasis (GDA), a common finding identified during anesthesia, i s not well understood. The purpose of this study was to determine whet her an inherent reduction in alveolar volume or a reduction in alveola r ventilation is the more important causative factor for the developme nt of GDA in an experimental animal model. METHODS. After uniform redu ction of lung volume in ten rabbits by artificially induced pneumoperi toneum, dynamic inhalation computed tomography (CT) was performed usin g 50% nonradioactive xenon in oxygen. Time-CT attenuation value curves were fitted to an exponential function, CT value = a - b x e(-kt), an d K value, which is proportional to the alveolar ventilation/alveolar volume ratio, was calculated by regression analysis. RESULTS. Gravity- dependent atelectasis occurred only in 5 of 10 rabbits. In this group, K values in the dorsal regions increased before the appearance of GDA . No significant change in K values in the ventral regions was observe d. CONCLUSION. One mechanism of GDA may be a preferential reduction in alveolar volume without small airway collapse rather than alveolar vo lume loss secondary to decreased ventilation.