Mechanical coupling between the hemithoraces in humans

Unilateral airway occlusion permits measurement of single lung function. Since the results suggest that the occluded lung influences the mobility of the contralateral hemithorax, we wanted to test this hypothesis. In eight healthy subjects, we measured, using computed tomography, lung parenchymal area and inner rib cage parasagittal and transverse diameters at three different levels and at maximal inspiration and expiration. These measurements were made without and during balloon occlusion of one mainstem bronchus at residual volume (RV) and at total lung capacity (TLC). Unilateral occlusion at RV reduced the ipsilateral diameters in maximal inspiration, but the increase during inspiration was still 39-50% of that without occlusion. The inspiratory increase in contralateral diameters was reduced to 64-80% of the increase without occlusion. Occlusion at TLC reduced the expiratory decrease in ipsilateral diameters to 37-57% of that without occlusion. The expiratory decrease on the contralateral side was reduced to 56-70% of that without occlusion. Due to accompanying mediastinal shifts the parenchymal areas of the occluded lung barely changed. In contrast, the contralateral area was 86-97% of that without occlusion. We conclude that the movement of the two hemithoraces are, at least partially, interdependent. The occluded hemithorax prevents full expansion/compression of the nonoccluded contralateral side, whilst its own mobility appears to be increased by the presence of this nonoccluded side. Potential negative effects are outweighed by the physiological benefit of the coupling, as this mechanism could secure ventilation to a chest half without own movement.