Sternomastoid, rib cage, and expiratory muscle activity during weaning failure

We hypothesized that patients who fail weaning from mechanical ventilation recruit their inspiratory rib cage muscles sooner than they recruit their expiratory muscles, and that rib cage muscle recruitment is accompanied by recruitment of stemomastoid muscles. Accordingly, we measured stemomastoid electrical activity and changes in esophageal (Delta P-es) and gastric pressure (Delta P-ga) in 11 weaning-failure and 8 weaning-success patients. At the start of trial, failure patients exhibited a higher Delta p(ga)-to-Delta P-es ratio than did success patients (P = 0.05), whereas expiratory rise in P-ga was equivalent in the two groups. Between the start and end of the trial, failure patients developed additional increases in Delta P-ga-to-Delta P-es ratio (P < 0.0014) and the expiratory rise in P-ga also increased (P < 0.004). At the start of trial, stemomastoid activity was present in 8 of 11 failure patients contrasted with I of 8 success patients. Over the course of the trial, sternomastoid activity increased by 53.0 +/- 9.3% in the failure patients (P = 0.0005), whereas it did not change in the success patients. Failure patients recruited their respiratory muscles in a sequential manner. The sequence began with activity of diaphragm and greater-than-normal activity of inspiratory rib cage muscles; recruitment of sternomastoids and rib cage muscles approached near maximum within 4 min of trial commencement; expiratory muscles were recruited slowest of all. In conclusion, not only is activity of the inspiratory rib cage muscles increased during a failed weaning trial, but respiratory centers also recruit sternomastoid and expiratory muscles. Extradiaphragmatic muscle recruitment may be a mechanism for offsetting the effects of increased load on a weak diaphragm.