RIB CAGE MUSCLE INTERACTION IN AIRWAY PRESSURE GENERATION

We have previously demonstrated in dogs that the change in airway open ing pressure (Delta Pao) produced by isolated maximum activation of th e parasternal intercostal or triangularis sterni muscle in a single in terspace, the sternomastoids, and the scalenes is proportional to the product of muscle mass and the fractional change in muscle length per unit volume increase of the relaxed chest wall. In the present study, we have assessed the interactions between these muscles by comparing t he Delta Pao obtained during simultaneous activation of a pair of musc les (measured Delta Pao) to the sum of the Delta Pao values obtained d uring their separate activation (predicted Delta Pao). Measured and pr edicted Delta Pao values were compared for the following pairs of musc les: the parasternal intercostals in two interspaces, the parasternal intercostals in one interspace and either the sternomastoids or the sc alenes, two segments of the triangularis sterni, and the interosseous intercostals in two contiguous interspaces. For all these pairs, the m easured Delta Pao was within similar to 10% of the predicted value. We therefore conclude that I)the pressure changes generated by the rib c age muscles are essentially additive; and 2) measurements of the mass of a particular muscle and of its fractional change in length during p assive inflation can be used to estimate the potential pressure-genera ting ability of the muscle during coordinated activity as well as duri ng isolated activation.