Action and interaction of respiratory muscles in dogs

How the respiratory muscles, the so-called "vital pump", works is relevant to medicine and nevertheless, poorly understood. The present studies attempt to obtain a better insight in the action and interaction of respiratory muscles by measuring the mechanical outcome of individual respiratory muscle contraction with two different techniques in anesthetized dogs. First, we measured the changes in length undergone by respiratory muscles using sonomicrometry. We observed that the costal and crural parts of the diaphragm frequently behaved differently, supporting the concept that these two parts behave as two different muscles. Moreover, crural diaphragmatic length appeared to be linked to abdominal dimensions, the classical used estimate of diaphragmatic length, whereas costal diaphragmatic length was not. The parasternal or the interchondral parts of the internal intercostals invariably shortened with inspiration supporting their role as inspiratory agonists. By way of contrast, the respiratory length changes in the external and interosseus internal intercostal were variable and not consistently in different directions for the two layers. The changes in length, during rotation of the trunk were consistent, larger and systematically in opposite directions for these two layers, suggesting that these muscles predominantly function as rotators of the trunk or at least that a role in rotation of the trunk explains better the need for two layers of intercostal muscles with different fiber orientation than a respiratory role. Second, we measured changes in intramuscular pressure in the costal and crural parts of the diaphragm, and in the parasternal intercostals, using mini-transducers. In the diaphragm intramuscular pressure appeared to be a complex variable determined by the tension developed by the muscle as well as the pleural and abdominal pressures applied to the muscle. As a consequence, intramuscular pressure often decreased during contraction, suggesting that in contrast to other muscles, diaphragmatic contraction may enhance diaphragmatic blood flow. Conversely, in the parasternal intercostals, intramuscular pressure invariably increased during contraction, and a linear relationship between intramuscular pressure and contractile force was present. The forces developed by these muscles during quiet breathing and other respiratory maneuvers were found to be significant, supporting the important inspiratory role of the parasternal intercostals. Intramuscular pressure thus appeared to be a perfect estimate of the force developed by these muscles. Finally, we examined the changes in respiratory muscle interaction with hyperinflation.(ABSTRACT TRUNCATED AT 400 WORDS)