A New Ultrasonographic Tool to Assess Pulmonary Strain in the ICU

Mechanical ventilation is frequently used in 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 severe 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. Presently 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 in 4 predetermined pulmonary areas using ultrasonographic imaging in mechanically ventilated patients suffering from pulmonary diseases in the intensive care setting. This dataset will be used to help plan larger scale studies.

Methods:

Intensive care patients under mechanical ventilation will undergo imaging of the pleura at 4 predetermined areas. The sites to be studied will be: the 3rd intercostal space at the mid-clavicular line (left and right side), the 8th intercostal space at the posterior axillary line (left and right side). 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. The beam's focal zone will be positioned at the level of the pleural line.

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. An experienced technician will visually validate the algorithm's tracking.

Inclusion criteria

• Adult intensive care patients under mechanical ventilation suffering from a pulmonary disease with a static lung compliance of less than 40 mL/cm H2O.

Exclusion criteria

• Obesity (Body Mass Index superior to 30 kg/m2)
• Previous thoracic procedure (chest tube, thoracotomy, thoracoscopy)
• Intra-abdominal pressure inferior to 15 mm Hg (if available)
• Previous participation to the study