Acute lung injury: effects of prone positioning on cephalocaudal distribution of lung inflation--CT assessment in dogs

Abstract

PURPOSE: To quantify cephalocaudal gradient of lung inflation in acute lung injury in a dog model in prone versus supine position. MATERIALS AND METHODS: Experiments were performed in accordance with Guide for the Care and Use of Laboratory Animals, as approved by National Research Council (National Institutes of Health), and were approved by committee on care and use of animals in research at Seoul National University Hospital. After induction of acute lung injury with intravenous injection of oleic acid, dogs were randomized to be ventilated in either prone (n = 6) or supine (n = 6) position. Spiral computed tomography (CT) and hemodynamic measurement were performed sequentially on an hourly basis. Volume and mean attenuation of lung were measured quantitatively by using software to evaluate each CT section. Cephalocaudal gradient of mean lung attenuation, distribution of gas and tissue, and alveolar expansion were assessed. Functional residual capacity and net alveolar expansion of entire lung were measured. Statistical analysis was performed with Friedman, sign, and Mann-Whitney tests. RESULTS: Mean lung attenuation increased gradually from apex to base of lung in supine position. Thus, inflation gradient along cephalocaudal axis was found. Gas was located predominantly in upper lung, whereas tissue was dominant in lower lung in supine position. In supine group, cephalocaudal inflation gradient showed no significant change from baseline up to 4 hours. After prone positioning, cephalocaudal inflation gradient was reduced, and gas and tissue proportions became more uniform along cephalocaudal axis. In prone group, absolute values of cephalocaudal inflation gradient at time points of prone positioning for 1, 2, and 3 hours were significantly lower than baseline values (P < .05) and those in supine group (P < .05). Alveolar expansion occurred in caudal regions, and alveolar contraction occurred in cephalic regions; accordingly, net alveolar volume of entire lung was not altered significantly. Functional residual capacity was unchanged by prone positioning. CONCLUSION: In acute lung injury, prone positioning induced more uniform distribution of gas and tissue along cephalocaudal axis by reducing cephalocaudal inflation gradient.