Effects of acute hyperinflation on the mechanical effectiveness of the parasternal intercostals

We studied the mechanical effectiveness of the parasternal intercostals at FRC and near TLC in 14 supine, vagotomized, and anesthetized dogs. First, we determined the relationship between parasternal intramuscular pressure (Pps), measured with Gaeltec 12 CT-mini-transducers, and parasternal EMG activity (Eps) during breathing at FRC and near TLC. Second, we examined the changes in Pps and the changes in parasternal force (Fps) generated during bilateral parasternal stimulation at FRC and near TLC with a given supramaximal stimulus. Before phrenicotomy, the inspiratory increases in Pps remained relatively constant near TLC (FRC, 50.4 +/- 16.5 versus TLC, 48.7 +/- 13.3 cm H2O, NS), whereas the Eps clearly decreased (82.9 +/- 5.5% FRC, p less than 0.01). This indicates that the gain converting electrical activity into pressure for the parasternals is greater near TLC than at FRC. A similar pattern of changes in Pps and in Eps was observed during quiet inspiration at FRC and near TLC after phrenicotomy. During bilateral parasternal stimulation the increases in Pps near TLC tended to be greater than those at FRC (140.7 +/- 28.6 versus 100 +/- 28.3 cm H2O, NS), whereas the increases in Fps were significantly greater near TLC than at FRC (277.4 +/- 60.6 versus 214.2 +/- 47.1 g, p less than 0.05). Therefore, we conclude that the mechanical effectiveness of the parasternal contraction near TLC remains relatively unchanged and is even greater in relation to that at FRC.