Respiratory muscle response to flail chest.

We have previously shown that flail chest in the dog causes an inspiratory inward displacement of the ribs and an increased inspiratory activity in the external intercostal muscles, and we have speculated that this increased activity is due to an increased spindle afferent activity. The present studies were designed to test this hypothesis. Twenty-nine supine anesthetized dogs were studied, and flail was produced surgically by fracturing ventrally and dorsally two to four contiguous ribs on the right side of the chest. Although flail elicited an increased inspiratory activity in the external intercostal and levator costae muscles in the disconnected segment of the rib cage, it did not alter the inspiratory activity in the diaphragm and parasternal intercostals. Expiratory activity in the triangularis sterni, internal intercostals, and transversus abdominis remained unchanged also, as did the inspiratory activity in the external intercostals on the left side of the chest. After flail, the normal inspiratory shortening of the external intercostal muscles in the disconnected segment was also reversed into an inspiratory muscle lengthening. However, when the fractured ribs were connected to the adjacent ribs so that the external intercostals were prevented from lengthening during inspiration, external intercostal and levator costae inspiratory activity was unaltered. These observations support the hypothesis that the increased external intercostal muscle activity seen in flail chest results primarily from an increased activation of the muscle spindles.