Lung Ultrasound and Electrical Impedance Tomography During Ventilator-Induced Lung Injury

OBJECTIVES: Lung damage during mechanical ventilation involves lung volume and alveolar water content, and lung ultrasound (LUS) and electrical impedance tomography changes are related to these variables. We investigated whether these techniques may detect any signal modification during the development of ventilator-induced lung injury (VILI). DESIGN: Experimental animal study. SETTING: Experimental Department of a University Hospital. Subjects: Forty-two female pigs (24.2 +/- 2.0 kg). INTERVENTIONS: The animals were randomized into three groups (n = 14): high tidal volume (TV) (mean TV, 803.0 +/- 121.7 mL), high respiratory rate (RR) (mean RR, 40.3 +/- 1.1 beats/min), and high positive-end-expiratory pressure (PEEP) (mean PEEP, 24.0 +/- 1.1 cm H2O). The study lasted 48 hours. At baseline and at 30 minutes, and subsequently every 6 hours, we recorded extravascular lung water, end-expiratory lung volume, lung strain, respiratory mechanics, hemodynamics, and gas exchange. At the same time-point, end-expiratory impedance was recorded relatively to the baseline. LUS was assessed every 12 hours in 12 fields, each scoring from 0 (presence of A-lines) to 3 (consolidation). MEASUREMENTS AND MAIN RESULTS: In a multiple regression model, the ratio between extravascular lung water and end-expiratory lung volume was significantly associated with the LUS total score (p < 0.002; adjusted R-2, 0.21). The variables independently associated with the end-expiratory difference in lung impedance were lung strain (p < 0.001; adjusted R-2, 0.18) and extravascular lung water (p < 0.001; adjusted R-2, 0.11). CONCLUSIONS: Data suggest as follows. First, what determines the LUS score is the ratio between water and gas and not water alone. Therefore, caution is needed when an improvement of LUS score follows a variation of the lung gas content, as after a PEEP increase. Second, what determines the end-expiratory difference in lung impedance is the strain level that may disrupt the intercellular junction, therefore altering lung impedance. In addition, the increase in extravascular lung water during VILI development contributed to the observed decrease in impedance.