Effect of body position on regional diaphragm function in dogs

The in situ lengths of muscle bundles of the crural and three regions of the costal diaphragm between origin and insertion were determined with a video roentgenographic technique in dogs. At total lung capacity (TLC) in both the prone and supine positions, the length of the diaphragm is not significantly different from the unstressed excised length, suggesting that the diaphragm is not under tension at TLC and that there is a hydrostatic gradient of pleural pressure on the diaphragmatic surface. Except for the ventral region of the costal diaphragm, which does not change length at lung volumes >70% TLC, all other regions are stretched during passive deflations from TLC. Therefore below TLC the diaphragm is under passive tension and supports a transdiaphragmatic pressure (Pdi). The length of the diaphragm relative to its unstressed length is not uniform at functional residual capacity (FRC) and does not follow a strict vertical gradient that reverses when the animal is changed from the supine to the prone position. By inference, the length of muscle bundles is determined by factors other than the vertical gradient of Pdi. During mechanical ventilation, regional shortening is identical to the passive deflation length-volume relationship near FRC. Prone and supine FRC is the same, but the diaphragm is slightly shorter in the prone position. In both positions, during spontaneous ventilation there are no consistent differences in regional fractional shortening, despite regional differences in initial length relative to unstressed length.