Effect of lung volume on the respiratory action of the canine sternomastoid.

We tested the hypothesis that because the resting length of the canine sternomastoid (SM) muscles is relatively insensitive to lung volume change, the SM may maintain its inspiratory force generation regardless of lung volume. The relationships between SM pre- and postcontraction in situ fiber lengths and SM-produced inspiratory pressure generation [i.e., esophageal (Pes)] and rib cage displacements were examined in adult supine anesthetized dogs at residual volume (RV), functional residual capacity, and total lung capacity. SM muscle contraction was produced by isolated bilateral supramaximal electrical stimulation during hyperventilation-induced apnea. In all animals, SM contraction produced negative change in Pes (i.e., an inspiratory action). Passively increasing lung volume from RV to total lung capacity decreased (P < or = 0.01) the SM-produced Pes by -66 +/- 4% but had a relatively small effect on SM in situ pre- and postcontraction fiber length (< 3%). Whereas SM contraction at RV produced a cranial displacement of the sternum and increased the upper rib cage cross-sectional area, passively elevating lung volume diminished the SM-produced expansion of the upper rib cage. Hyperinflation did not increase the impedance of the sternum to cranial displacement during SM contraction, suggesting that hyperinflation caused a dissociation between the mechanical action of the sternum and the upper rib cage. These results suggest that mechanical dissociation of the ribs and sternum may diminish the contribution of the SM to inspiratory volume generation when breathing is done from elevated end-expiratory lung volumes.