Effect of inhaled gas density on the pendelluft-induced lung injury

Abstract

Helium, sulfur hexafluoride–oxygen, and air were modeled to examine the role of the gas density on the pendelluft-induced lung injury (PILI) under high frequency oscillatory ventilation (HFOV). Large eddy simulation coupled with physiological resistance–compliance boundary conditions was applied to capture pendelluft-induced gas entrapment and mechanical stresses in an image-based human lung model. The flow characteristics were strongly dependent on the inspired gas density. The flow partitioning, globally between the left and right lung and locally between adjacent units branches, was significantly affected by the density of inhaled gas and was more balanced when inspiring lighter gas. The incomplete loops of flow–volume and volume–pressure curves were significantly influenced by the variations of the flow redistribution, resistance, and turbulence associated with the pendelluft mechanism. Inhaling light gas reduced the entrapped gas volume and mechanical stress surrounding carina ridges signifying the important role of inhaled gas properties on PILI. In general, lung ventilation by HFOV with a gas mixture of large amounts of Helium is thought to mitigate ventilator complications.

Introduction

Pendelluft flow has long been documented as one of the crucial mechanisms for gas exchange augmentation during high frequency oscillatory ventilation (HFOV) (Alzahrany et al., 2014a, Chang, 1984, Ultman et al., 1988). The asymmetric resistance (R) and compliance (C) of the adjacent lung units causes a variation in time constant ($\tau =RC$) (Otis et al., 1956) that causes a phase-shift in the ventilation cycle. As a result, an interregional gas exchange known as pendelluft occurs between neighboring lung units that are out-of-phase at the end of inspiration/expiration cycles (Chang, 1984). Several studies investigated pendelluft characteristics under different frequencies and tidal volume during HFOV (Alzahrany et al., 2014b, Elad et al., 1998, High et al., 1991). The majority of the earlier studies were simple in geometrical and mathematical representations. Otis et al. (1956) applied a theoretical analysis of lung ventilation under variation of the time constant in a single idealized bifurcation. A linear model consisting of airways’ resistance and compliance was proposed to capture pendelluft in small airways for nonuniform daughter branches resistances. Later, Ultman et al. (1988) extended Otis׳s theoretical model to include a non-uniform compliance and inertance (I) that accounts for the gas acceleration in large airways. The importance of the new inclusions was affirmed by the captured pendelluft in larger airways. A similar approach was later adopted (High et al., 1991) to explore optimum physiological transport under HFOV therapy. Lee et al. (2006) generated a pendelluft experimentally in a micro channel; and later Hirahara et al. (2011) implemented lumped-parameter and direct resistance and compliance in small airways model. Hitherto, the geometry models that have been used were either single or double simplified bifurcations. Recently, Alzahrany et al. (2014b) have utilized instantaneous pressure outlet boundary conditions based on the airways resistance–compliance (R–C) that was applied at outlets of a 3D Image-Based large-scale tracheobronchial model. The pendelluft flow was successfully captured at different regions in the whole geometry; these were in agreement with the in vitro physiological data obtained by (Andrey and Murphy, 2012).

HFOV is meant to replace conventional mechanical ventilation (CMV) to reduce the risk of lung injury associated with CMV while at the same time preserving adequate gas exchange. However, recent observations have suggested a potential of lung injury due to pendelluft (Alzahrany and Banerjee, 2015b, Hupp et al., 2015, Stawicki et al., 2009, Yoshida et al., 2013). For instance, Yoshida et al. (2013) evaluated in vitro pendelluft flow in anesthetized pigs and reported that lung injury was detected due to pendelluft during protective-lung ventilation with small volume. This conclusion was recently confirmed using a biomechanical model by the authors (Alzahrany and Banerjee, 2015b) by utilizing R–C boundary conditions (Alzahrany et al., 2014b) where a potential for lung injury was identified as a consequence of the gas entrapment and shear stress surrounding the carinas added by the existence of pendelluft. Physicians typically use cocktails of various gas mixtures during ventilation therapy. These gases have a wide range of density and dynamic viscosity, which alter the flow dynamics and consequently affect the ventilation process. In the present work, we have investigated the effect of changing the gas cocktail on lung injury mechanics related to PILI. Two non-reactive gases were utilized Helium (He) and sulfur hexafluoride–oxygen mixture (SF₆–O₂); the results were compared to our previous work that used air as a ventilation medium (Alzahrany and Banerjee, 2015a). Large eddy simulation (LES) along with R–C boundary conditions were implemented to model the gas transport and pendelluft in a patient-specific lung model. The current study aims to improve our understanding of HFOV mechanics and lead to a pathway towards improving lung-protective ventilation therapy.