Mediolateral gradient of mechanical advantage in the canine parasternal intercostals

Previous theoretical studies have postulated that the potential effect of a given respiratory muscle on lung volume or pleural pressure (i.e., its respiratory effect) is proportional to the change in length of the muscle during inflation of the passive chest wall (T. A. Wilson and A. De Troyer J. Appl. Physiol. 73: 2283-2288, 1992). To test this prediction, we have studied the parasternal intercostals in 18 interspaces in 8 supine anesthetized dogs. In each interspace, we have measured the changes in length of the medial and lateral portions of the parasternal during passive inflation and we have also assessed the changes in airway opening pressure (delta Pao) generated by these portions during isolated bilateral stimulation of the internal intercostal nerve. The results showed that 1) the medial fibers shorten more than the lateral fibers during passive inflation (P < 0.001); 2) when stimulated, the medial portion generated a larger fall in Pao than the lateral portion (P < 0.001); and 3) delta Pao was closely related to change in length (r = 0.81; P < 0.001). These observations thus imply that the medial portion of the parasternal intercostals contributes much more to lung expansion during breathing than the lateral portion. These observations also suggest, in agreement with the theoretical prediction, that measurements of the changes in length of the different respiratory muscles during passive inflation can be used to predict the potential respiratory effect of these muscles and to compare their mechanical advantages.