Estimation of changes in cyclic lung strain by electrical impedance tomography: Proof-of-concept study

Abstract

Rationale Cyclic strain may be a determinant of ventilator-induced lung injury. The standard for strain assessment is the computed tomography (CT), which does not allow continuous monitoring and exposes to radiation. Electrical impedance tomography (EIT) is able to monitor changes in regional lung ventilation. In addition, there is a correlation between mechanical deformation of materials and detectable changes in its electrical impedance, making EIT a potential surrogate for cyclic lung strain measured by CT (Strain(CT)).

Objectives To compare the global Strain(CT) with the change in electrical impedance (Delta Z).

Methods Acute respiratory distress syndrome patients under mechanical ventilation (V-T 6 mL/kg ideal body weight with positive end-expiratory pressure 5 [PEEP 5] and best PEEP according to EIT) underwent whole-lung CT at end-inspiration and end-expiration. Biomechanical analysis was used to construct 3D maps and determine Strain(CT) at different levels of PEEP. CT and EIT acquisitions were performed simultaneously. Multilevel analysis was employed to determine the causal association between Strain(CT) and Delta Z. Linear regression models were used to predict the change in lung Strain(CT) between different PEEP levels based on the change in Delta Z.

Main results Strain(CT) was positively and independently associated with Delta Z at global level (P < .01). Furthermore, the change in Strain(CT) (between PEEP 5 and Best PEEP) was accurately predicted by the change in Delta Z (R-2 0.855, P < .001 at global level) with a high agreement between predicted and measured Strain(CT).

Conclusions The change in electrical impedance may provide a noninvasive assessment of global cyclic strain, without radiation at bedside.