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Background: Diaphragm dysfunction is highly prevalent in mechanically ventilated patients. Recent work showed that changes in diaphragm shear modulus (ΔSMdi) assessed using ultrasound shear wave elastography (SWE) are strongly related to changes in Pdi (ΔPdi) in healthy subjects. The aims of this study were to investigate the relationship between ΔSMdi and ΔPdi in mechanically ventilated patients, and whether ΔSMdi is responsive to change in respiratory load when varying the ventilator settings., Methods: A prospective, monocentric study was conducted in a 15-bed ICU. Patients were included if they met the readiness-to-wean criteria. Pdi was continuously monitored using a double-balloon feeding catheter orally introduced. The zone of apposition of the right hemidiaphragm was imaged using a linear transducer (SL10-2, Aixplorer, Supersonic Imagine, France). Ultrasound recordings were performed under various pressure support settings and during a spontaneous breathing trial (SBT). A breath-by-breath analysis was performed, allowing the direct comparison between ΔPdi and ΔSMdi. Pearson's correlation coefficients (r) were used to investigate within-individual relationships between variables, and repeated measure correlations (R) were used for determining overall relationships between variables. Linear mixed models were used to compare breathing indices across the conditions of ventilation., Results: Thirty patients were included and 930 respiratory cycles were analyzed. Twenty-five were considered for the analysis. A significant correlation was found between ΔPdi and ΔSMdi (R = 0.45, 95% CIs [0.35 0.54], p < 0.001). Individual correlation displays a significant correlation in 8 patients out of 25 (r = 0.55-0.86, all p < 0.05, versus r = - 0.43-0.52, all p > 0.06). Changing the condition of ventilation similarly affected ΔPdi and ΔSMdi. Patients in which ΔPdi-ΔSMdi correlation was non-significant had a faster respiratory rate as compared to that of patient with a significant ΔPdi-ΔSMdi relationship (median (Q1-Q3), 25 (18-33) vs. 21 (15-26) breaths.min -1 , respectively)., Conclusions: We demonstrate that ultrasound SWE may be a promising surrogate to Pdi in mechanically ventilated patients. Respiratory rate appears to negatively impact SMdi measurement. Technological developments are needed to generalize this method in tachypneic patients., Trial Registration: NCT03832231 .

Background: Greater diaphragm fatigue has been reported after hypoxic versus normoxic exercise, but whether this is due to increased ventilation and therefore work of breathing or reduced blood oxygenation per se remains unclear. Hence, we assessed the effect of different blood oxygenation level on isolated hyperpnoea-induced inspiratory and expiratory muscle fatigue., Methods: Twelve healthy males performed three 15-min isocapnic hyperpnoea tests (85% of maximum voluntary ventilation with controlled breathing pattern) in normoxic, hypoxic (SpO2 = 80%) and hyperoxic (FiO2 = 0.60) conditions, in a random order. Before, immediately after and 30 min after hyperpnoea, transdiaphragmatic pressure (P(di,tw)) was measured during cervical magnetic stimulation to assess diaphragm contractility, and gastric pressure (P(ga,tw)) was measured during thoracic magnetic stimulation to assess abdominal muscle contractility. Two-way analysis of variance (time x condition) was used to compare hyperpnoea-induced respiratory muscle fatigue between conditions., Results: Hypoxia enhanced hyperpnoea-induced P(di,tw) and P(ga,tw) reductions both immediately after hyperpnoea (P(di,tw) : normoxia -22 +/- 7% vs hypoxia -34 +/- 8% vs hyperoxia -21 +/- 8%; P(ga,tw) : normoxia -17 +/- 7% vs hypoxia -26 +/- 10% vs hyperoxia -16 +/- 11%; all P < 0.05) and after 30 min of recovery (P(di,tw) : normoxia -10 +/- 7% vs hypoxia -16 +/- 8% vs hyperoxia -8 +/- 7%; P(ga,tw) : normoxia -13 +/- 6% vs hypoxia -21 +/- 9% vs hyperoxia -12 +/- 12%; all P < 0.05). No significant difference in (di,tw) or P(ga,tw) reductions was observed between normoxic and hyperoxic conditions. Also, heart rate and blood lactate concentration during hyperpnoea were higher in hypoxia compared to normoxia and hyperoxia., Conclusions: These results demonstrate that hypoxia exacerbates both diaphragm and abdominal muscle fatigability. These results emphasize the potential role of respiratory muscle fatigue in exercise performance limitation under conditions coupling increased work of breathing and reduced O2 transport as during exercise in altitude or in hypoxemic patients.