Effect of inspiratory muscle fatigue on inspiratory muscle relaxation rates in healthy subjects.

Simple methods to diagnose inspiratory muscle fatigue in the clinical setting would be of considerable benefit. Inspiratory muscle relaxation rates are known to slow following induction of fatigue. Inspiratory muscle relaxation rates have been measured following a short sharp inspiratory effort against an occluded airway (sniffmouth) or through the unoccluded nostrils (sniffnostrils). Relaxation rates in the absence of fatigue are faster when sniffs are performed through the unoccluded nostrils. While both methods have been shown to be capable of detecting inspiratory muscle fatigue, there may be quantitative or qualitative differences between the two techniques in their ability to detect fatigue similar to the differences observed in the fresh state. Accordingly, we measured relaxation rates with the two sniff techniques in five healthy naive male subjects before and after induction of fatigue. Inspiratory muscle fatigue was induced by threshold loading at 80 percent of Pesmax until the subjects were unable to generate the target pressure. For those trials in which sniffnostrils were performed, the maximum relaxation rate from the esophageal pressure curve (MRRes) was significantly decreased following induction of fatigue in nine of ten trials, while the exponential time constant (taues) was significantly increased in all ten trials. In contrast, for those trials in which sniffmouth were performed, the MRRes was significantly decreased following induction of fatigue in only six of ten trials. Similarly, taues was significantly increased following induction of fatigue in only six of ten trials. In addition, the magnitude of change in the MRR or tau following induction of fatigue was quantitatively greater with sniffnostrils compared with sniffmouth. Similar findings were obtained when relaxation rates were measured from the diaphragmatic pressure tracing. In conclusion, changes in relaxation rate following induction of fatigue were quantitatively greater and more consistently observed when sniffs were performed through the unoccluded nostrils rather than against an occluded airway.