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 pilot study will aim to create a small dataset of local pleural strain values assessed at predetermined pulmonary areas using ultrasound imaging in patients undergoing thoracic surgery requiring one-lung ventilation. This dataset will be used to help plan larger scale studies.

At four different time points during thoracic surgery, images of the pleura of the dependent lung will be made at 2 predetermined areas. The images will be made: after induction (tidal volume of 10 mL/kg), during two-lung ventilation (tidal volume of 10 mL/kg) and during one-lung ventilation (tidal volume of 10 mL/kg and 5 mL/kg). The sites to be studied will be: the 3rd intercostal space at the mid-clavicular line (dependent lung), the 8th intercostal space at the posterior axillary line (dependent lung). Three consecutive respiratory cycles at each site will be recorded for subsequent analysis.

Lung ultrasonography will be performed by the principal investigator and a co-investigator using a Terason (Teratech Corporation, Burlington, MA) device and a 12L5 linear ultrasound probe. For each image, the probe will be oriented perpendicularly to the pleura with the pointer towards the participant's head. A depth of 4 cm will be used and adjusted in order to have the pleural line located between the center and the three-quarts of the screen. The beam's focal zone will be positioned at the level of the pleural line. A 12 MHz frequency will be used.

Using a reference ultrasonographic image, an experienced lung ultrasonographer will segment the pleura. From this image, an algorithm will define a region of interest which will be followed throughout the rest of the images of the video sequence. Thereafter, the algorithm will calculate the various components of pulmonary strain in relation to tidal volume. The principal investigator or a co-investigator will visually validate the speckle-tracking.

Inclusion Criteria:

• Patients undergoing thoracic surgery in lateral position requiring one-lung ventilation

Exclusion Criteria:

• Obesity (Body Mass Index superior to 30 kg/m2)
• Emergency surgery
• Previous thoracic procedure (chest tube, thoracotomy, thoracoscopy)
• Previous participation to the study
• Patient refusal