Respiratory mechanical effects of abdominal distension

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Abstract
A theory was developed to predict the partitioning of a change in volume of the abdominal contents into the end-expiratory volume change of the lung, rib cage and anterior abdominal wall. The distribution of such a volume change was developed using the relative compliances of the 3 compartments. The inspiratory influence of abdominal pressure on the rib cage, and its effect on the distribution of this volume change, was calculated. The theory was tested by inducing gastric distension in 3 experienced laboratory personnel. From a gastric balloon, 1 l of water was instilled and withdrawn, and the effects of this change in gastric volume on the relaxation characteristics of the respiratory system were examined. The distribution of the volume change that would be expected from the observed relative compliances of the 3 compartments would be .apprx. 65% into change in lung volume, 25% into change in rib cage volume, and 9% into change in abdominal volume. Instead, .apprx. 33% went into decrease in lung volume, 40% into increase in rib cage volume and 26% into increase in abdominal volume. The interactions among the rib cage, abdomen and diaphragm may be such as to defend against large changes in end-expiratory lung volume in the face of abdominal distension.