Diaphragmatic fatigue during inspiratory muscle loading in normoxia and hypoxia

Abstract Introduction Diaphragmatic fatigue (DF) occurs during strenuous loading of respiratory muscles (e.g., heavy-intensity whole-body exercise, normocapnic hyperpnea, inspiratory resistive breathing). DF develops early on during normoxia, without further decline toward task failure; however, its progression during inspiratory muscle loading in during hypoxia remains unclear. Therefore, the present study used volume-corrected transdiaphragmatic pressures during supramaximal magnetic phrenic nerve stimulation (Pdi,twc) to investigate the effect of hypoxia on the progression of diaphragmatic fatigue during inspiratory muscle loading. Methods Seventeen subjects completed two standardized rounds of inspiratory muscle loading (blinded, randomized) under the following conditions: (i) normoxia, and (ii) normobaric hypoxia (SpO 2 80%), with Pdi,twc assessment every 45s. Results In fatiguers (i.e., Pdi,twc reduction >10%, n  = 10), biometric approximation during normoxia is best represented by Pdi,tw c  = 4.06 + 0.83 exp(−0.19 ×  x ), in contrast to Pdi,tw c  = 4.38 − (0.05 ×  x ) during hypoxia. Conclusion Progression of diaphragmatic fatigue during inspiratory muscle loading assessed by Pdi,tw differs between normoxia and normobaric hypoxia: in the former, Pdi,tw follows an exponential decay, whereas during hypoxia, Pdi,tw follows a linear decline.