Mechanical ventilation is frequently used in the operating room and the intensive care settings. Although essential in many cases, mechanical ventilation can be responsible for ventilator-induced lung injury (VILI). The relationship between mechanical ventilation and VILI has been clearly demonstrated in animals and is highly suspected in humans. The putative mechanism responsible for VILI is excessive pulmonary strain or overdistension. Frequently observed in mechanically ventilated patients, the presence of a severe pre-existing pulmonary disease can increase the risk of overdistension. The development of a tool allowing early detection of pulmonary overdistension would represent a great asset in the prevention of VILI by allowing safer adjustments of mechanical ventilation parameters. Ultrasonographic imaging is a non-radiant, non-invasive technique already available in the intensive care setting. Already used for cardiac strain measurements, ultrasonography is a promising avenue to assess pulmonary strain.This study will aim to establish normal pleural strain values using ultrasonography in healthy volunteers.

Purpose: The primary objective is to calculate 95% confidence intervals in pleural strain for a set of 8 inspired volumes at 15 predetermined lung areas in healthy volunteers.

The secondary objectives of the study are:

• to modelize the relation between pleural strain and inspired volume
• to modelize the relation between pleural strain and global pulmonary volumetric strain
• to modelize the relation between pleural strain and maximal echo intensity change
• to compare the regional distribution pattern of pleural strain in healthy volunteers in dependent versus non-dependent areas.

Hypothesis: Elastography using the Lagrangian speckle model estimator based on optical flow allows the determination of normal mean values and 95% confidence intervals of pleural strain (average Von Mises coefficient) in 15 predetermined lung areas for a set of 8 inspired volumes in healthy volunteers.

Participants will have their functional residual capacity (FRC) measured by a nitrogen dilution technique. Subsequently, they will be instructed to breathe 8 different inspired volumes (5 to 15 ml/kg predicted body weight). Tidal volumes will be measured using a spirometer. For each inspired volume, 3 respiratory cycles will be recorded at 15 predetermined anatomic sites: 1st and 3rd left and right intercostal spaces at the mid-clavicular line, 5th right intercostal space at the mid-clavicular line, 2nd, 4th and 6th left and right intercostal spaces at the anterior axillary line, and 5th and 7th left intercostal spaces and right to the posterior axillary line. For each image, the probe will be oriented perpendicularly to the ribs. A research, non-commercially available, non-invasive vascular elastography platform will be used to calculate the various strain parameters for all of the recorded cineloops. For each recorded clip, an experienced lung ultrasonographer will segment the pleura on a single reference image. From this image, an algorithm will define a region of interest which will be tracked throughout the rest of the images of the video sequence. Finally, the algorithm will calculate the various components of pleural strain.

Inclusion Criteria:

• Healthy volunteers (20 men/20 women)
• 18 years old and over
• Functional capacity superior to 4 METs (metabolic equivalent of task)

Exclusion Criteria:

• Previous thoracic procedure (chest tube, thoracotomy, thoracoscopy)
• Pre-existing pulmonary disease (asthma, chronic obstructive pulmonary disease, lung fibrosis)
• Active or previous history of smoking
• Obesity (Body Mass Index superior to 30 kg/m2)

An equal number of men and women will be recruited.