Estimation of Deflation Parameters as a Surrogate for Transpulmonary Pressure from Regional Pressure-Flow-Curves derived from Electrical Impedance Tomography in a porcine model of the Acute Respiratory Distress Syndrome

OBJECTIVE: Adjustment of positive end-expiratory pressure is part of lung-protective ventilation in patients with acute respiratory distress syndrome and can be performed invasively using an esophageal catheter (Talmor & Sarge et al. 2008). The objective of this proof-of-concept study was to investigate whether the transpulmonary pressure of 0 mbar, at which alveolar collapse occurs, can be estimated non-invasively using electrical impedance tomography. Approach: Acute respiratory distress syndrome was induced in 14 pigs by repeated lung lavage and a 2-hour period of ventilator-induced lung injury ventilation. The transpulmonary pressure was calculated as the difference between end-expiratory airway pressure and the corresponding esophageal pressure. During a descending pressure-controlled ramp maneuver from 50 to 0 mbar, pressure-flow curves at the pixel levels of the candidate dependent lung region were generated and regional deflation parameters calculated. Main results: During the descending pressure ramp the beginning of alveolar collapse could be determined at approximately 25 mbar in the dependent region of the electrical impedance tomography images based on regional characteristic points in both lung conditions. A strong correlation was found (r = 0.929, B = 1.005, R2 = 0.862, p = 0.007) in healthy lungs and an intermediate correlation (r = 0.817, B = 0.894, R2 = 0.668, p = 0.025) in acute respiratory distress syndrome between the second characteristic point and the positive end-expiratory pressure according to Talmor, corresponding to a transpulmonary pressure of 0. Significance: The findings of the study have demonstrated that electrical impedance tomography can identify regional characteristic points that correlate with regional transpulmonary pressure changes in the dependent lung region. This knowledge can be used to optimize a lung protective ventilation strategy by titrating transpulmonary pressure guided positive end-expiratory pressure non-invasively.