Mechanism of gravity-depend atelectasis : Analysis by nouradioactive xenon-euhance dynamic CT

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Reduction, in lung volume and shallow breathing produce progressive atelectasis in the dorsal lung(gravity-dependent atelectasis : GDA). We have developed an animal model of GDA(Invest Radiol 24 : 522, 1'989). The purpose of this study is to determine whether airway occlusion is a key factor for GDA.Methods : Dynamic inhalation CT was performed in ten rabbits, using 50%'iionradioactive xenon in oxygen on high frequency oscillatory ventilation(HFOV). Time-CT value curves were. -assumed to be described as exponential function, C=C_o(1-e^<-Kt>)(Kety's model). K value, which is equal to alveolar ventilation /alveolar volume, was calculated with re. gres. sion analysis. I. n experimental group(n=5)lung volume was reduced with artificial pneumoperitoneus of 10 cmH_2, while control group(n=5)did not have pneuxoperitoneum. Both groups were placed on HFOV for six hours and K values were calculated every one hour.Results : In all of the experimental group GDA was formed. In the dorial regions. wash-in of xenon. was observed and' K values increased just before GDA appeared'' Meanwhile K values in the ventral regions did not change significantly. The control group did not reveal-GDA'and K values did not change in either region.Conclusions : GDA is. related to the increase in alveolar ventilation /alveolar volume ratio. This result suggests that GDA is not caused. by airway occlusion. By means of this procedure-we can. diagnose GDA before"' the decrease in arterial oxygen pressure or. detection on CT images.