Mechanical advantage of the canine diaphragm

The mechanical advantage (mu) of a respiratory muscle is defined as the res piratory pressure generated per unit muscle mass and per unit active stress . The value of mu can be obtained by measuring the change in the length of the muscle during inflation of the passive lung and chest wall. We report v alues of mu for the muscles of the canine diaphragm that were obtained by m easuring the lengths of the muscles during a passive quasistatic vital capa city maneuver. Radiopaque markers were attached along six muscle bundles of the costal and two muscle bundles of the crural left hemidiaphragms of fou r bred-for-research beagle dogs. The three-dimensional locations of the mar kers were obtained from biplane videofluoroscopic images taken at four volu mes during a passive relaxation maneuver from total lung capacity to functi onal residual capacity in the prone and supine postures. Muscle lengths wer e determined as a function of lung volume, and from these data, values of m u were obtained. Values of mu are fairly uniform around the ventral midcost al and crural diaphragm but significantly lower at the dorsal end of the co stal diaphragm. The average values of mu are -0.35 +/- 0.18 and -0.27 +/- 0 .16 cmH(2)O . g(-1) . kg(-1) . cm(-2) in the prone and supine dog, respecti vely. These values are 1.5-2 times larger than the largest values of mu of the intercostal muscles in the supine dog. From these data we estimate that during spontaneous breathing the diaphragm contributes similar to 40% of i nspiratory pressure in the prone posture and similar to 30% in the supine p osture. Passive shortening, and hence mu, in the upper one-third of inspira tory capacity is less than one-half of that at lower lung volume. The lower mu is attributed primarily to a lower abdominal compliance at high lung vo lume.