Diaphragmatic contraction and the gradient of alveolar expansion in the lateral posture

Using 133Xe we measured the vertical distribution of regional volume in four subjects in the lateral decubitus posture at 20, 40, 60, and 80% of vital capacity (VC). To study the influence of diaphragmatic tone, all measurements were performed either when transdiaphragmatic pressure (Pdi) was low, i.e., diaphragm was "relaxed" (RD) or during voluntary diaphragmatic contraction (VDC). The latter was achieved by tensing the abdominal muscles while keeping the glottis open. Under both conditions the gradient of alveolar expansion tended to be curvilinear, with a discontinuity at the level of the mediastinum. At all lung volumes the difference in regional volume between dependent and nondependent lung regions was less during VDC than during RD. At 70% total lung capacity (TLC) this difference, expressed as percent of regional TLC (%TLCr), decreased from 19.7 +/- 1.7 (mean +/- 1 SE) %TLCr during RD to 3.9 +/- 1.5% TLCr during VDC. It is likely that diaphragmatic tension influences the pleural pressure gradient and regional volume distribution 1) by modifying the transmission of the abdominal hydrostatic pressure gradient to the thorax, and 2) by an upward displacement of the mediastinum.