Ultrasound assessment of the diaphragm in healthy volunteers

After meeting all inclusion and no exclusion criteria, healthy volunteers are asked to undergo to ultrasound assessment of the diaphragm. While normally breathing, at rest, the investigator checks if none of both emi-diaphragm is affected by palsy or by some paradoxical motion. Therefore, two types of measurement are record: 1) the cranio-caudal displacement and 2) the thickening fraction. The cranio-caudal displacement is performed using a 3.5–5 MHz probe. The probe is placed immediately below the right or left costal margin in the midclavicular line, or in the right or left anterior axillary line and is directed medially, cephalad and dorsally, so that the ultrasound beam reaches perpendicularly the posterior third of the corresponding hemidiaphragm. The two-dimensional (2D) mode is initially used to obtain the best approach and select the exploration line; the M-mode is then used to display the motion of the anatomical structures along the selected line. Patients are scanned along the long axis of the intercostal spaces, with the liver serving as an acoustic window to the right, and the spleen to the left. Normal inspiratory diaphragmatic movement is caudal, since the diaphragm moves toward the probe; normal expiratory trace is cranial, as the diaphragm moves away from the probe. In the M mode, the diaphragmatic excursion (displacement, cm), the speed of diaphragmatic contraction (slope, cm/s), the inspiratory time (Tinsp, s) and the duration of the cycle (Ttot, s) can be measured (Matamis et al. Intensive Care Med (2013) 39:801–810). The thickening fraction is evaluated in the zone of apposition of the diaphragm to the rib cage. The zone of apposition is the area of the chest wall where the abdominal contents reach the lower rib cage. In this area, the diaphragm is observed as a structure made of three distinct layers: a non-echogenic central layer bordered by two echogenic layers, the peritoneum and the diaphragmatic pleurae. To obtain adequate images of diaphragmatic thickness in M mode and 2D mode, a linear high-frequency probe (C10 MHz) is necessary. The diaphragmatic thickness can be measured during quiet spontaneous breathing and during a maximal inspiratory and expiratory effort. An index of diaphragmatic thickening, the thickening fraction (TF) can be calculated using the M mode (TF = thickness at endinspiration - thickness at end-expiration/thickness at end-expiration) (Matamis et al. Intensive Care Med (2013) 39:801–810).