Transient Mechanical Response of Lung Airway Tissue during Mechanical Ventilation

Abstract

Patients with acute lung injury, airway and other pulmonary diseases often require Mechanical Ventilation (MV). Knowledge of the stress/strain environment in lung airway tissues is very important in order to avoid lung injuries for patients undergoing MV. Airway tissue strains responsible for stressing the lung's fiber network and rupturing the lung due to compliant airways are very difficult to measure experimentally. Multi-level modeling is adopted to investigate the transient mechanical response of the tissue under MV. First, airflow through a lung airway bifurcation (Generation 4-6) is modeled using Computational Fluid Dynamics (CFD) to obtain air pressure during 2 seconds of MV breathing. Next, the transient air pressure was used in structural analysis to obtain mechanical strain experienced by the airway tissue wall. Structural analysis showed that airway tissue from Generation 5 in one bifurcation can stretch eight times that of airway tissue of the same generation number but with different bifurcation. The results suggest sensitivity of load to geometrical features. Furthermore, the results of strain levels obtained from the tissue analysis are very important because these strains at the cellular-level can create inflammatory responses, thus damaging the airway tissues.

Keywords: finite element analysis; lung airway; mechanical strains; mechanical ventilation.

Figure 1

Multi-level analysis adopted for transient mechanical response of airway lung tissue. Top: model and boundary condition used for fluid flow analysis. Bottom: model for mechanical analysis.

Figure 2

Velocity contour on bifurcation of Generation 5 lung airway, showing tendency to form horseshoe pattern.

Figure 3

Average transient pressure for MV breathing in Section 1, Section 2 and Section 3, obtained from airflow simulation.

Figure 4

Pressure distribution in lung airway at several locations along airway at t = 0.3 s (inspiration) on four section, including junction. Color legend shows pressure value range in Pascal.

Figure 5

Pressure distribution in lung airway at several locations along airway at t = 0.5 s (peak expiration) on four section, including junction. Color legend shows pressure value range in Pascal.

Figure 6

Mesh convergence test for tissue domain. Von Mises strain at two time values, t, was taken as criteria. (a) Maximum strain; (b) minimum strain.

Figure 7

Maximum Von Mises strains in (a) Layer 1; (b) Layer 2; and (c) Layer 3. Right column shows strain progression over time, left column captures the Von Mises strain at a specific time for a clearer comparison.

Figure 8

Maximum radial strains occurring in (a) Layer 1; (b) Layer 2; and (c) Layer 3. Right column shows strain progression over time, left column captures the Von Mises strain at specific time for a clearer comparison.

Figure 8

Maximum radial strains occurring in (a) Layer 1; (b) Layer 2; and (c) Layer 3. Right column shows strain progression over time, left column captures the Von Mises strain at specific time for a clearer comparison.

Figure 9

Maximum Von Mises strains occurring in (a) Section 1; (b) Section 2; and (c) Section 3. Right column shows strain progression over time, left column captures the Von Mises strain at specific time for a clearer comparison.

Figure 9

Maximum Von Mises strains occurring in (a) Section 1; (b) Section 2; and (c) Section 3. Right column shows strain progression over time, left column captures the Von Mises strain at specific time for a clearer comparison.

Figure 10

Maximum radial strains occurring in (a) Section 1; (b) Section 2; and (c) Section 3. Right column shows strain progression over time, left column captures the Von Mises strain at specific time for a clearer comparison.

Figure 10

Maximum radial strains occurring in (a) Section 1; (b) Section 2; and (c) Section 3. Right column shows strain progression over time, left column captures the Von Mises strain at specific time for a clearer comparison.

Figure 11

Maximum strains at peak expiration (0.5 s). Von Mises strain in Section 3 is 4.89 times and 2.9 times higher than in Section 2 and Section 1, respectively. Radial strain in Section 3 is 8.15 times and 4.83 times higher than in Section 2 and Section 1, respectively. Axial strain in Section 3 is 6.04 times and 3.6 times higher than in Section 2 and Section 1, respectively.

Figure 12

Comparison of maximum strains with different material model at peak expiration (t = 0.5 s). The plot is log₁₀ plot to exaggerate the proportion of strains from Neo-Hookean material model.