Predicting ventilator-induced lung injury using a lung injury cost function

Abstract

Managing patients with acute respiratory distress syndrome (ARDS) requires mechanical ventilation that balances the competing goals of sustaining life while avoiding ventilator-induced lung injury (VILI). In particular, it is reasonable to suppose that for any given ARDS patient, there must exist an optimum pair of values for tidal volume (VT) and positive end-expiratory pressure (PEEP) that together minimize the risk for VILI. To find these optimum values, and thus develop a personalized approach to mechanical ventilation in ARDS, we need to be able to predict how injurious a given ventilation regimen will be in any given patient so that the minimally injurious regimen for that patient can be determined. Our goal in the present study was therefore to develop a simple computational model of the mechanical behavior of the injured lung in order to calculate potential injury cost functions to serve as predictors of VILI. We set the model parameters to represent normal, mildly injured, and severely injured lungs and estimated the amount of volutrauma and atelectrauma caused by ventilating these lungs with a range of VT and PEEP. We estimated total VILI in two ways: 1) as the sum of the contributions from volutrauma and atelectrauma and 2) as the product of their contributions. We found the product provided estimates of VILI that are more in line with our previous experimental findings. This model may thus serve as the basis for the objective choice of mechanical ventilation parameters for the injured lung.

Keywords: acute respiratory distress syndrome; atelectrauma; mechanical ventilation; volutrauma.

Fig. 1.

Schematic representation of the computational model. The variable x represents tissue distension, with the threshold value x_inj demarcating the level above which volutrauma starts to occur. The variable 0 < y ≤ 1 represents lung recruitment. E_x is the stiffness of the spring that controls tissue distension. P_A, alveolar pressure; V_A, alveolar compartment volume; R_aw, airway resistance; V̇, tracheal flow, V, time-integral of V̇; P, airway pressure.

Fig. 2.

A: pressure-volume relationship of the lung assuming fully recruited tissue. TLC, total lung capacity; FRC, functional residual capacity. B–D: relationship between the open fraction of the lung as it recruits during inspiration (solid lines) and derecruits during expiration (dashed lines) vs. alveolar pressure for a healthy lung (B), moderately injured lung (C), and severely injured lung (D).

Fig. 3.

Normalized lung injury prediction surfaces for range of positive-end expiratory pressure (PEEP) and tidal volume (V_T) values. Atelectrauma, Ω_atl, is shown for a healthy (A), moderately injured (B), and severely injured (C) lung, and volutrauma, Ω_vol, is shown for a healthy (D), moderately injured (E), and severely injured (F) lung. The surfaces are normalized to the maximum value indicated in each plot.

Fig. 4.

Normalized lung injury prediction surfaces for range of positive-end expiratory pressure (PEEP) and tidal volume (V_T) values. VILI calculated as the sum (Ω_sum) and the product (Ω_prod) of atelectrauma and volutrauma is shown for a healthy (A and D), moderately injured (B and E), and severely injured (C and F) lung. The surfaces are normalized to the maximum value indicated in each plot. The points indicate V_T-PEEP combinations from VILI experiments that were noninjurious (white circles) and injurious (black Xs) in healthy mice and injurious in mice with preexisting injury (gray triangles). The white outlined regions show the reduction in the range of minimally injurious V_T-PEEP combinations in progressively injured lungs.