Study design and participants
This study was part of a larger, prospective observational cohort study conducted in an 18-bed intensive care unit (ICU) between January 2014 and December 2014 . The study was approved by our Institutional Review Board (Comité de Protection des Personnes Nord-Ouest 3); NCT02363231 www.clinicaltrials.gov. In conformity with the Declaration of Helsinki, all patients participated voluntarily.
Patients were included if they were over 18 years of age and had undergone a minimum of 24 h of MV. They were not included if they had chronic loss of autonomy (a KATZ score below 6/6 , a degenerative neurological pathology with disabling muscle weakness, were agitated prior to the evaluation (Ramsay score of 1 or Richmond Agitation-Sedation Scale (RASS) greater than 1) or a decision to withhold life sustaining treatment had been made. Patients who were included but had to be re-intubated during the first 24 h of extubation were excluded from the analysis.
In our ICU, patients are assessed daily (without sedation) to determine whether they are ready to wean from MV. If a patient fulfils extubation criteria and level of cooperation is satisfactory, a weaning trial is carried out under pressure support (inspiratory positive airway pressure of 7 cmH2O with no expiratory positive airway pressure for 30–120 min) . For the purpose of the study, if the trial was successful and extubation was planned, the patient underwent maximum inspiratory and expiratory pressure measurements (MIPs and MEPs) (intubation condition). Twenty-four hours following extubation, MIPs and MEPs were re-measured, this time using a mouthpiece (mouthpiece condition).
Demographic data, reasons for admission to ICU and comorbidities were collected at the time of inclusion, prior to carrying out the MIP and MEP measurements under MV.
In both conditions, the MIP and MEP measurements were carried out with the patient lying in bed with the backrest inclined to 45°. Respiratory physiotherapy was carried out first to ensure that secretions were evacuated, and endotracheal aspiration was carried out for intubated patients.
An electronic manometer, micro-RPM® (Eolys, PAYS), with a unidirectional valve was used to measure respiratory pressures. In both conditions, MIP was measured at the residual volume and patients were instructed accordingly.
In the intubation condition, the manometer was connected to the endotracheal tube using a catheter mount. The patient was disconnected from the ventilator for a minimum of 20 s .
In the mouthpiece condition, it was not possible to leave the manometer in position for 20 s. MIP was measured after a maximal exhalation (at the residual volume).
MEP was measured after a maximal inspiration in both conditions. Three MIP and three MEP measurements were carried out for each patient, and the best result was used for the analysis.
Descriptive statistics are reported as counts and percentages for categorical data, and means and standard deviations or medians and 25th–75th percentiles for continuous variables, depending on the normality of the distribution. Differences between values were evaluated using a Wilcoxon matched-pairs signed rank test. Univariate linear regression analysis was performed using the least squares method. The Bland–Altman limits of agreement method was used to calculate bias and precision.
Statistical analyses were performed using GraphPad Prism 5. A two-tailed p value of 0.05 was considered significant for all analyses.