Rib cage muscle interaction in airway pressure generation

We have previously demonstrated in dogs that the change in airway opening pressure (DeltaPao) produced by isolated maximum activation of the parasternal intercostal or triangularis sterni muscle in a single interspace, 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 the DeltaPao obtained during simultaneous activation of a pair of muscles (measured DeltaPao) to the sum of the DeltaPao values obtained during their separate activation (predicted DeltaPao). Measured and predicted DeltaPao values were compared for the following pairs of muscles: the parasternal intercostals in two interspaces, the parasternal intercostals in one interspace and either the sternomastoids or the scalenes, two segments of the triangularis sterni, and the interosseous intercostals in two contiguous interspaces. For all these pairs, the measured DeltaPao was within approximately 10% of the predicted value. We therefore conclude that 1) the pressure changes generated by the rib cage muscles are essentially additive; and 2) measurements of the mass of a particular muscle and of its fractional change in length during passive inflation can be used to estimate the potential pressure-generating ability of the muscle during coordinated activity as well as during isolated activation.