BackgroundDiaphragm dysfunction (DD) is a major concern in intensive care unit (ICU) patients. The gold standard method to diagnose DD is the measure of intra-thoracic pressure in response to a bilateral stimulation of the phrenic nerve (Ptr,stim). This technique is costly, requires expertise and is not widely available. Diaphragm ultrasound (US) could be an interesting alternative.ObjectivesTo assess the time course of diaphragm function simultaneously with Ptr,stim and US, and to compare US to Ptr,stim to detect DD.MethodsMonocentric prospective study in a medical ICU. DD was detected at three key time points: 1) during the first 24h of controlled ventilation (CV), 2) as soon as patients could tolerate pressure support ventilation (PSV) and 3) before the first spontaneous breathing trial (SBT). Ptr,stimwas measured using bilateral anterior magnetic stimulation and the thickening fraction (TF) of the diaphragm was obtained from M-mode US. A Ptr,stim< -11 cmH₂O defined DD.ResultsForty-three critically ill patients were investigated (77 measurements). DD was present in 79%, 85% and 58% of patients during CV, PSV and SBT respectively. The respective mean Ptr,stim were 8.1±4, 7.5±4 and 11.8±6 cm H₂O (p=0.02 for CV vs. SBT). There was a significant correlation between Ptr,stim and TF during PSV and SBT (R2=0.88, p<0.001) but not during CV. During PSV and SBT, a TF < 30% had a sensitivity and specificity of 90 and 100% respectively to detect DD (area under the receiver operating characteristics curve 0.98).ConclusionDD is frequent on admission but can improve during ICU stay. Diaphragm US seems a reliable method to detect and monitor DD in patients with spontaneous breathing.