A model of transient oscillatory pressure-flow relationships of canine airways

Emergent behavior of regional heterogeneity in the lung and its effects on respiratory impedance

The ability to maintain adequate gas exchange depends on the relatively homogeneous distribution of inhaled gas throughout the lung. Structural alterations associated with many respiratory diseases may significantly depress this function during tidal breathing. These alterations frequently occur in a heterogeneous manner due to complex, emergent interactions among the many constitutive elements of the airways and parenchyma, resulting in unique signature changes in the mechanical impedance spectrum of the lungs and total respiratory system as measured by forced oscillations techniques (FOT). When such impedance spectra are characterized by appropriate inverse models, one may obtain functional insight into derangements in global respiratory mechanics. In this review, we provide an overview of the impact of structural heterogeneity with respect to dynamic lung function. Recent studies linking functional impedance measurements to the structural heterogeneity observed in acute lung injury, asthma, and chronic obstructive pulmonary disease are highlighted, as well as current approaches for the modeling and interpretation of impedance. Finally, we discuss the potential diagnostic role of FOT in the context of therapeutic interventions.

An imaging-based computational approach to model ventilation distribution and soft-tissue deformation in the ovine lung

RATIONALE AND OBJECTIVES

A computational modeling framework of soft-tissue mechanics and air flow has been developed toward the aim of linking computed tomography measures of ventilation distribution to subject-specific predictions in imaging-based geometric models of the lung. The aim of this approach is to enable predictions of the effect of perturbations in geometry or functional parameters on imaged function of the lung.

MATERIALS AND METHODS

Computational techniques that can deal with anatomic detail and spatially distributed nonlinear material properties are used to model parenchymal soft-tissue mechanics in a physically realistic model of the ovine lung. The lung is modeled as a homogeneous, compressible, nonlinearly elastic body. Using equations for large deformation mechanics, change in geometry of the lung is simulated at static inflation pressures from 25 cm H2O to 0 cm H2O. Multidetector row computed tomography imaging defines the model geometry, the movement of the model lung surface during inflation, and displacement of airway bifurcations for comparison with predicted internal displacements of the model.

RESULTS

A novel modeling framework has been formulated that links equations for large deformation of the lung tissue to equations for airway flow and pressure. This preliminary model predicts airway displacements that are in good agreement with imaged displacements (total root mean square [RMS] error < 4 mm from 25 to 0 cm H2O).

CONCLUSIONS

State-of-the-art computed tomography imaging is interpreted using a modeling framework to predict ventilation distribution and changes in the geometry of the lung during increments in inflation pressure. Further development will provide a predictive link between subject-specific anatomy and function.

Inspiratory lung impedance in COPD: effects of PEEP and immediate impact of lung volume reduction surgery

Frequency-dependent characteristics of lung resistance (RL) and elastance (EL) are sensitive to different patterns of airway obstruction. We used an enhanced ventilator waveform (EVW) to measure inspiratory RL and EL spectra in ventilated patients during thoracic surgery. The EVW delivers an inspiratory flow waveform with enhanced spectral excitation from 0.156 to 8.1 Hz. Estimates of the coefficients in a trigonometric approximation of the EVW flow and transpulmonary pressure inspirations yielded inspiratory RL and EL spectra. We applied the EVW in a group with mild obstruction undergoing various thoracoscopic procedures (n = 6), and another group with severe chronic obstructive pulmonary disease undergoing lung volume reduction surgery (n = 8). Measurements were made at positive end-expiratory pressure (PEEP) of 0, 3, and 6 cmH(2)O. Inspiratory RL was similar in both groups despite marked differences in spirometry. The chronic obstructive pulmonary disease patients demonstrated a pronounced frequency-dependent increase in inspiratory EL consistent with severe heterogeneous peripheral airway obstruction. PEEP appears to have beneficial effects by reducing peripheral airway resistance. Lung volume reduction surgery resulted in increased inspiratory RL and EL at all frequencies and PEEPs, possibly due to loss of diseased lung tissue, pulmonary edema, increased mechanical heterogeneity, and/or an improvement in airway tethering.