Contribution of various inspiratory muscles to ventilation and the immediate and distant effect of diaphragmatic paralysis.

The contribution of the diaphragm and that of the other inspiratory muscles (scaleni and all the other extradiaphragmatic muscles) at different levels of tidal volume (VT) and during static inspiratory efforts of various strengths has been studied in supine anaesthetized rabbits by blocking phrenic conduction with an electrotonic current. Rabbits spinalized at T1 were used to measure the separate contribution of the scaleni. When the vagi are left intact there is an hyperactivity of the extradiaphragmatic muscles during the phrenic block which has been ascribed to Hering-Breuer reflexes. The relative contribution of the diaphragm to tidal volume, during quiet breathing, appears to approach 9O percent and is reduced to 75 percent during the maximum tidal volume attained. The scaleni appear to account for about 1/3 of the tidal volume contributed by all the other inspiratory extradiaphragmatic muscles during quiet breathing and their share goes up to 1/2 at the highest ventilation. The contribution of the scaleni starts at the same level of inspiratory output found for all extradiaphragmatic muscles and therefore these muscles should not be considered "accessory" Within the range of VT considered the sternomastoids did not appear to contribute, as shown by the fact that their disinsertion did not change the results obtained in spinalized rabbits. Immediately after block of the phrenic nerves there is a marked decrease of VT which then increases within 10-15 breaths to a steady value, three to four times that of the first breath after the paralysis, which is maintained thereafter. The respiratory frequency decreases immediately to a slightly lower value after phrenic block In control animals and does not change when the vagi are cut. In any case a fairly steady value is maintained thereafter even when various types of stimulation are applied. Only changes in body temperature could change respiratory frequency. Chronically phrenicectomized rabbits show some compensatory phenomena either functionally or morphologically. They become able to change their ventilation when chemical drive is increased. In the external intercostal muscles the myoglobin concentration increases and ultrastructural modifications become apparent.