Intratidal Overdistention and Derecruitment in the Injured Lung: A Simulation Study

Abstract

Goal: Ventilated patients with the acute respiratory distress syndrome (ARDS) are predisposed to cyclic parenchymal overdistention and derecruitment, which may worsen existing injury. We hypothesized that intratidal variations in global mechanics, as assessed at the airway opening, would reflect such distributed processes.

Methods: We developed a computational lung model for determining local instantaneous pressure distributions and mechanical impedances continuously during a breath. Based on these distributions and previous literature, we simulated the within-breath variability of airway segment dimensions, parenchymal viscoelasticity, and acinar recruitment in an injured canine lung for tidal volumes( V_T ) of 10, 15, and 20 mL·kg^-1 and positive end-expiratory pressures (PEEP) of 5, 10, and 15 cm H₂O. Acini were allowed to transition between recruited and derecruited states when exposed to stochastically determined critical opening and closing pressures, respectively.

Results: For conditions of low V_T and low PEEP, we observed small intratidal variations in global resistance and elastance, with a small number of cyclically recruited acini. However, with higher V_T and PEEP, larger variations in resistance and elastance were observed, and the majority of acini remained open throughout the breath. Changes in intratidal resistance, elastance, and impedance followed well-defined parabolic trajectories with tracheal pressure, achieving minima near 12 to 16 cm H₂O.

Conclusion: Intratidal variations in lung mechanics may allow for optimization of ventilator settings in patients with ARDS, by balancing lung recruitment against parenchymal overdistention.

Significance: Titration of airway pressures based on variations in intratidal mechanics may mitigate processes associated with injurious ventilation.

Fig. 1

(a) Percentage of recruited lung vs. mean tracheal pressure. Symbols are computed from the experimental data of Kaczka et al. [13], as the ratio healthy to injured dynamic elastance multiplied by 100%. Solid line is the linear regression for the four data points, with horizontal dashed lines denoting the imposed upper and lower limits of recruitment. Thus the model assumes that 7.6% of the lung is recruited at a mean tracheal pressure of 0 cm H₂O, while 100% of the lung is recruited at about 28 cm H₂O. (b) Distribution of acinar critical opening / closing pressures (P_crit,n), expressed as the fraction of total acini in the model. This distribution is computed as the slope (i.e., derivative) of the function in (a).

Fig. 2

Algorithm used to simulate time-domain variations in flow and pressure based on nonlinear variations in impedance. At time t = 0, the model is initiated by setting a PEEP and sinusoidal tracheal flow magnitude |V̇_tr |. The local impedances throughout the tree (Z_k) are then computed at this PEEP. Regional pressure magnitudes (P_k) and phases (ϕ_k) throughout the tree are determined in frequency-domain, and then converted to time-domain values according to (4). The time index is then incremented as t = t + dt.

Fig. 3

Acinar flow patterns in the model are divided into three distinct subgroups: (a) consistently recruited; (b) transitional; and (c) consistently derecruited.

Fig. 4

Percentages of consistently recruited, transitional, and consistently derecruited acini in the model for 5, 10, and 15 cm H₂O PEEP and 10, 15, and 20 mL kg⁻¹V_T.

Fig. 5

Histograms of the acinar recruitment duty cycle (RDC_n), defined as the fraction of time that acinus stays opened throughout a breath. Histograms are shown for PEEPs of (a) 5, (b) 10, and (c) 15 cm H₂O, and tidal volumes of 10, 15, and 20 mL kg⁻¹.

Fig. 6

Tracheal pressure variations and within breath variations of lung resistance, elastance, impedance, and acinar recruitment for 5, 10, and 15 cm H₂O PEEP and 10, 15, and 20 mL kg⁻¹V_T.

Fig. 7

(a) Total lung resistance (R_L), (b) elastances (E_L), and (c) impedance magnitude (|Z_L|) as functions of tracheal pressure. Trajectories are color coded (shaded) to denote the percentage of recruited lung as a function of tracheal pressure. Vertical dashed /dotted lines denote ‘optimal’ tracheal pressures, according to the pressure for which R_L, E_L, or |Z_L| is minimized.

Fig. 8

Simulated dynamic pressure-volume loops for single breaths at (a) 10, (b) 15, and (c) 20 mL kg⁻¹ and 5, 10, and 15 cm H₂O PEEP (left to right within each panel). Loops are color coded (shaded) to denote the percentage of recruited lung during the course of a breath. Solid lines denote inspiration, while dashed lines denote expiration. Vertical lines denote ‘optimal’ tracheal pressure, as defined according to the minimum E_L or |Z_L| (11.6 cm H₂O, dotted line) or R_L minimum (15.9 cm H₂O, dashed line) criteria (Fig. 7).