The effect of lobar atelectasis on pleural fluid distribution in dogs.

Fluid was introduced into the pleural space in upright anesthetized, ventilated and/or spontaneously breathing dogs, and its distribution was studied radiographically and confirmed on frozen slices. Various lobes were collapsed to determine the resulting effect on fluid distribution and thereby gain an understanding of so-called "atypical" effusions in a clinical setting. It was found that fluid collects in the dependent portion of the thorax and its distribution is affected by deformation of the chest wall and lung. Airway obstruction causes lobar collapse, which creates negative local pressures secondary to distortion of the lung and chest wall. The effusion moves to the area of distortion to diminish these presures. To the extent that these mechanisms operate in man, it appears that pleural effusions remain subpulmonary until atelecasis of the adjacent lung or distortion of the chest wall causes fluid redistribution.