Effects of body position and lung volume on in situ operating length of canine diaphragm

The performance of the diaphragm is influenced by its in situ length relative to its optimal force-generating length (L(o)). Lead markers were sutured to the abdominal surface of the diaphragm along bundles of the left ventral, middle, and dorsal regions of the costal diaphragm and the left crural diaphragm of six beagle dogs. After 2-3 wk postoperative recovery, the dogs were anesthetized, paralyzed, and scanned prone and supine in the Dynamic Spatial Reconstructor (DSR) at a total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV). The location of each marker was digitized from the reconstructed DSR images, and in situ lengths were determined. After an overdose of anesthetic had been administered to the dogs, each marked diaphragm bundle was removed, mounted in a 37°C in vitro chamber, and adjusted to L(o) (maximum tetanic force). The operating length of the diaphragm, or in situ length expressed as percent L(o), varied from region to region at the lung volumes studied; variability was least at RV and increased with increasing lung volume. At FRC, all regions of the diaphragm were shorter in the prone posture compared with the supine, but there was no clear gravity-dependent vertical gradient of in situ length in either posture. Because in vitro length-tension characteristics were similar for all diaphragm regions, regional in vivo length differences indicate that the diaphragm's potential to generate maximal force is nonuniform.