Electrical and mechanical output of the inspiratory muscles in anesthetized dogs

In anesthetized dogs with progressive hypercapnia, the relationships between tidal volume (VT) or occluded inspiratory esophageal pressure swings (Pes) and diaphragm (Edi) or thoracic inspiratory muscle (Etm) peak electrical activity were determined after either cordotomy at C7-T1 or bilateral phrenicotomy, and used together with the Etm vs Edi relationships established in intact and vagotomized dogs to estimate the contribution of each muscle group to VT and Pes observed under the latter conditions. Etm was obtained as the mean of scalene, 2nd and 5th parasternal peak activity, the relationships between these activities being the same under all conditions. In supine and head-up phrenicotomized dogs, VT and Pes increased linearly with Etm and were unaffected by body position. After cordotomy, VT and Pes increased progressively less with increasing Edi and at any Edi were smaller in the head-up posture. Diaphragm relative contribution to VT and Pes was greater when supine than head-up and greater after than before vagotomy, but in all cases it decreased with increasing chemical drive. That of the thoracic inspiratory muscles increased with chemical drive and eventually became equal to (supine) or larger than (head-up) diaphragm contribution.