Preventing mediastinal shift after pneumonectomy does not abolish physiological compensation

To determine the role of mediastinal shift after pneumonectomy (PNX) on com pensatory responses, we performed right PNX in adult dogs and replaced the resected lung with a custom-shaped inflatable silicone prosthesis. Prosthes is was inflated (Inf) to prevent mediastinal shift, or deflated (Def), allo wing mediastinal shift to occur. Thoracic, lung air, and tissue volumes wer e measured by computerized tomography scan. Lung diffusing capacities for c arbon monoxide (DLCO) and its components, membrane diffusing capacity for c arbon monoxide (Dm(CO)) and capillary blood volume (Vc), were measured at r est and during exercise by a rebreathing technique. In the Inf group, lung air volume was significantly smaller than in Def group; however, the lung b ecame elongated and expanded by 20% via caudal displacement of the left hem idiaphragn. Consequently, rib cage volume was similar, but total thoracic v olume was higher in the Inf group. Extravascular septal tissue volume was n ot different between groups. At a given pulmonary blood flow, DLCO and Dm(C O) were significantly lower in the Inf group, but Ve was similar. In one do g, delayed mediastinal shift occurred 9 mo after PNX; both lung volume and DLCO progressively increased over the subsequent 3 mo. We conclude that pre venting mediastinal shift after PNX impairs recruitment of diffusing capaci ty but does not abolish expansion of the remaining lung or the compensatory increase in extravascular septal tissue volume.