In an effort to better understand the basic physiology concerning heart/lung interactions, and thus define the mechanical determinants of decreased cardiac output during clinical management involving assisted ventilation, we propose to obtain detailed high temporal resolution, three-dimensional cardiac and pulmonary morphologic and physiologic data in intact experimental animals under various experimental conditions (including stepwise changes in airway pressure, changes in body position, increased pericardial pressure, and increased pleural pressure with constant lung volume) using the DSR, a unique multiple x-ray source transaxial reconstructor. For the first time we will have simultaneous, accurate information on a beat-to-beat basis regarding cardiac chamber shapes and dimension, regional cardiac wall dynamics, distribution of flowing pulmonary blood volume, regional lung expansion, vena caval blood flow, pleural and pericardial pressures, directly sampled left atrial pressures, and lung and chest wall shapes and dimensions. It is hoped that this approach will yield new insights regarding a three decade old problem: specifically, the determinants of decreased cardiac output during positive end-expiratory pressure (PEEP).