Objective :
The main objective is to correlate ultrasound indices (thickness, thickening, rigidity) to the measurement of transdiaphragmatic pressure in the evaluation of the diaphragmatic function of patients under mechanical ventilation.
Secondary objectives are: 1. Correlate thickening of the diaphragm with Pdi, 2. Correlate diaphragmatic thickening with diaphragmatic stiffness 3. Describe these correlations under different levels of pressure ventilatory support, 4. Describe the evolution of diaphragmatic stiffness, thickening and Pdi pressure during a ventilator liberating trial.
The primary endpoint is the measurement of Pdi. The secondary endpoints are the diaphragm thickening fraction and diaphragm stiffness.
Methods :
The Pdi will be obtained by measuring the esophageal and gastric pressures through nasogastric tube equipped with balloons. The position of each of the balloons will be checked by displaying the esophageal and gastric pressure lines. The intragastric position will be confirmed by mild abdominal compression. The oesophageal position is based on visualization of cardiac artifacts on the pressure pattern and signal deflations related to inspiratory movements. The signals of the esophageal and gastric pressures will be displayed continuously and their resultant - transdiapragmatic pressure - will be automatically calculated in real time by the software. It will also record pressure in the airways at the endotracheal tube.
Patients will be placed in half-sitting position to allow better recognition of the diaphragm. Using the ultrasound system Aixplorer® (Aix en Provence, France V9), equipped with a high resolution probe (6 MHz). The probe will be positioned at the intercostal space above the tenth rib on the right axillary line and directed perpendicular to the diaphragm (apposition zone). The measurements will be performed "offline" and the calculation of the thickening fraction will be performed in blind clinical conditions.
The diaphragmatic elastography will be determined using the same ultrasound system (Aixplorer®, Aix en Provence, France V9, provided with a high resolution probe (SL10-2, central frequency, 6 MHz)). The anatomical approach is the same as the intercostal approach described previously. After finding the diaphragm in B mode, a transition to SWE (elastography) mode will be performed. Three successive measurements will be made on the same region of interest, the average of the three measurements will be retained. The variation of the shear modulus during the inspiratory time will also be calculated post hoc. As for standard ultrasound, records analysis will be performed in blind conditions.
Pdi, ultrasound derived indices and shear wave modulus will be measured under several conditions.
1. At baseline: initial ventilator settings set by the physician in charge of patient
2. 25%-increase in pressure support and initial PEEP during 10 minutes
3. 25%-decrease in pressure support and initial PEEP during 10 minutes
4. initial level of pressure support without PEEP (PEEP = 0) during 10 minutes
5. Measurement of the maximal inspiratory pressure during an inspiratory occlusion maneuver lasting 20 seconds.
6. A 30 minutes Ventilator liberating trial without PEEP nor Pressure Support (CPAP=0)
