A New Ultrasonographic Tool to Assess Regional Pulmonary Strain

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 assess the feasibility of the measurement of local pleural strain in 4 predetermined pulmonary areas using ultrasonographic imaging.

Following the induction of general anesthesia and the patient's intubation, 4 different tidal volumes of 6 ml/kg, 8ml/kg, 10 ml/kg and 12 ml/kg will be studied. For each volume, images of the pleura will be made at 4 predetermined areas. The sites to be studied will be: the 4th intercostal space at the mid-clavicular line (left and right side), the 7th intercostal space at the posterior axillary line (left and right side). For each tidal volume, 3 consecutive respiratory cycles at each site will be recorded for subsequent analysis.

To assess intra- and inter-observer variability, for the 10 ml/kg tidal volume only, the examination will be immediately repeated by a second observer at the 4 predetermined sites. The first observer will also return to repeat the examination at the same sites.

Mechanical ventilation parameters will be standardized throughout the study as follows: volume-controlled ventilation, respiratory rate at 12 breaths per minute adjusted to obtain expired carbon dioxide (CO2) between 30 and 40 mm Hg, initial inspired oxygen fraction of 40% adjusted between 40% and 80% to obtain oxygen saturation ≥ 92% and positive end-expiratory pressure at 6 cm H2O. In the event of a desaturation (saturation inferior to 90%) despite an increase in inspired oxygen fraction, the study will be stopped and adjustments of mechanical ventilation parameters will be left to the attending anesthesiologist's discretion.

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 ribs and pleura with the pointer towards the participant's head. Depth will be adjusted in order to have the pleural line in the center of the screen. The beam's focal zone will be positioned at the level of the pleural line. A 12 megahertz (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. An experienced technician will visually validate the speckle-tracking.