Mechanical advantage of the canine triangularis sterni.

Recent studies on the canine parasternal intercostal, sternomastoid, and scalene muscles have shown that the maximal changes in airway opening pressure (delta Pao) obtained per unit muscle mass (delta Pao/m) during isolated contraction are closely related to the fractional changes in muscle length per unit volume increase of the relaxed chest wall. In the present study, we have examined the validity of this relationship for the triangularis sterni, an important expiratory muscle of the rib cage in dogs. Passive inflation above functional residual capacity (FRC) induced a virtually linear increase in muscle length, such that, with a 1.0-liter inflation, the muscle lengthened by 17.9 +/- 1.6 (SE) % of its FRC length. When the muscle in one interspace was maximally stimulated at FRC, Pao increased by 0.84 +/- 0.11 cmH2O. However, in agreement with the length-tension characteristics of the muscle, when lung volume was increased by 1.0 liter before stimulation, the rise in Pao amounted to 1.75 +/- 0.12 cmH2O. At the higher volume, delta Pao/m therefore averaged +.053 +/- 0.05 cmH2O/g, such that the coefficient of proportionality between the change in triangularis sterni length during passive inflation and delta Pao/m was the same as that previously obtained for the parasternal intercostal and neck inspiratory muscles. These observations, therefore, confirm that there is a unique relationship between the fractional changes in length of the respiratory muscles, both inspiratory and expiratory, during passive inflation and their delta Pao/m. Consequently, the maximal effect of a particular muscle on the lung can be predicted on the basis of its change in length during passive inflation and its mass. A geometric analysis of the rib cage also established that the lengthening of the canine triangularis sterni during passive inflation is much greater than the shortening of the parasternal intercostals because, in dogs, the costal cartilages slope downward from the sternum.