Modeling airflow-related shear stress during heterogeneous constriction and mechanical ventilation

Interplay between environmental yielding and dynamic forcing modulates bacterial growth

Many bacterial habitats-ranging from gels and tissues in the body to cell-secreted exopolysaccharides in biofilms-are rheologically complex, undergo dynamic external forcing, and have unevenly distributed nutrients. How do these features jointly influence how the resident cells grow and proliferate? Here, we address this question by studying the growth of Escherichia coli dispersed in granular hydrogel matrices with defined and highly tunable structural and rheological properties, under different amounts of external forcing imposed by mechanical shaking, and in both aerobic and anaerobic conditions. Our experiments establish a general principle: that the balance between the yield stress of the environment that the cells inhabit, σy, and the external stress imposed on the environment, σ, modulates bacterial growth by altering transport of essential nutrients to the cells. In particular, when σy<σ, the environment is easily fluidized and mixed over large scales, providing nutrients to the cells and sustaining complete cellular growth. By contrast, when σy>σ, the elasticity of the environment suppresses large-scale fluid mixing, limiting nutrient availability and arresting cellular growth. Our work thus reveals a new mechanism, beyond effects that change cellular behavior via local forcing, by which the rheology of the environment may modulate microbial physiology in diverse natural and industrial settings.

Examination of changes in the breathing intolerance index to determine the respiratory support needs of very low birth weight infants

BACKGROUND

The breathing intolerance index (BITI) is used to evaluate respiratory muscle tolerance. The higher the ratio of the inspiratory time to the total breathing time and the ratio of the tidal volume to the vital capacity, the more easily the respiratory muscles become fatigued. The BITI is high with chronic respiratory failure, and values of 0.15 or more indicate the need for assisted ventilation.

OBJECTIVE

This study aimed to examine the changes in the BITI of very low birth weight infants (VLBWIs) and determine whether it is possible to use the BITI as an objective indicator of the timing of noninvasive respiratory support (NRS) withdrawal.

METHODS

VLBWIs admitted to our neonatal intensive care unit between July 2020 and July 2022 under NRS at 36 weeks' postmenstrual age (PMA). If the BITI was less than 0.15, then we attempted to determine whether NRS could be withdrawn.

RESULTS

Sixteen infants with a median gestational age and birth weight of 30.9 weeks and 1249 g, respectively, were eligible. The median PMA at the time of the first examination was 36.6 weeks. The BITI of two VLBWIs was less than 0.15. For 11 of the other VLBWIs, the BITI decreased over time to less than 0.15 at 39 weeks' PMA. After confirming that the BITI was less than 0.15, weaning from NRS was attempted. Weaning from NRS was possible except one VLBWI.

CONCLUSION

The BITI can help evaluate respiratory tolerance and could be an objective indicator of the timing of NRS withdrawal.

Challenges in ARDS Definition, Management, and Identification of Effective Personalized Therapies

Over the last decade, the management of acute respiratory distress syndrome (ARDS) has made considerable progress both regarding supportive and pharmacologic therapies. Lung protective mechanical ventilation is the cornerstone of ARDS management. Current recommendations on mechanical ventilation in ARDS include the use of low tidal volume (VT) 4-6 mL/kg of predicted body weight, plateau pressure (PPLAT) < 30 cmH2O, and driving pressure (∆P) < 14 cmH2O. Moreover, positive end-expiratory pressure should be individualized. Recently, variables such as mechanical power and transpulmonary pressure seem promising for limiting ventilator-induced lung injury and optimizing ventilator settings. Rescue therapies such as recruitment maneuvers, vasodilators, prone positioning, extracorporeal membrane oxygenation, and extracorporeal carbon dioxide removal have been considered for patients with severe ARDS. Regarding pharmacotherapies, despite more than 50 years of research, no effective treatment has yet been found. However, the identification of ARDS sub-phenotypes has revealed that some pharmacologic therapies that have failed to provide benefits when considering all patients with ARDS can show beneficial effects when these patients were stratified into specific sub-populations; for example, those with hyperinflammation/hypoinflammation. The aim of this narrative review is to provide an overview on current advances in the management of ARDS from mechanical ventilation to pharmacological treatments, including personalized therapy.

Flow-controlled expiration reduces positive end-expiratory pressure requirement in dorsally recumbent, anesthetized horses

Introduction

Equine peri-anesthetic mortality is higher than that for other commonly anesthetized veterinary species. Unique equine pulmonary pathophysiologic aspects are believed to contribute to this mortality due to impairment of gas exchange and subsequent hypoxemia. No consistently reliable solution for the treatment of peri-anesthetic gas exchange impairment is available. Flow-controlled expiration (FLEX) is a ventilatory mode that linearizes gas flow throughout the expiratory phase, reducing the rate of lung emptying and alveolar collapse. FLEX has been shown to improve gas exchange and pulmonary mechanics in anesthetized horses. This study further evaluated FLEX ventilation in anesthetized horses positioned in dorsal recumbency, hypothesizing that after alveolar recruitment, horses ventilated using FLEX would require a lower positive end-expiratory pressure (PEEP) to prevent alveolar closure than horses conventionally ventilated.

Methods

Twelve adult horses were used in this prospective, randomized study. Horses were assigned either to conventional volume-controlled ventilation (VCV) or to FLEX. Following induction of general anesthesia, horses were placed in dorsal recumbency mechanically ventilated for a total of approximately 6.5 hours. Thirty-minutes after starting ventilation with VCV or FLEX, a PEEP-titration alveolar recruitment maneuver was performed at the end of which the PEEP was reduced in decrements of 3 cmH₂O until the alveolar closure pressure was determined. The PEEP was then increased to the previous level and maintained for additional three hours. During this time, the mean arterial blood pressure, pulmonary arterial pressure, central venous blood pressure, cardiac output (CO), dynamic respiratory system compliance and arterial blood gas values were measured.

Results

The alveolar closure pressure was significantly lower (6.5 ± 1.2 vs 11.0 ± 1.5 cmH₂O) and significantly less PEEP was required to prevent alveolar closure (9.5 ± 1.2 vs 14.0 ± 1.5 cmH₂O) for horses ventilated using FLEX compared with VCV. The CO was significantly higher in the horses ventilated with FLEX (37.5 ± 4 vs 30 ± 6 l/min).

Discussion

We concluded that FLEX ventilation was associated with a lower PEEP requirement due to a more homogenous distribution of ventilation in the lungs during expiration. This lower PEEP requirement led to more stable and improved cardiovascular conditions in horses ventilated with FLEX.

Time dependency and unique etiology of barotrauma in COVID-19: A retrospective cohort study with landmark analysis and pathological approach

BACKGROUND

Barotrauma frequently occurs in coronavirus disease 2019. Previous studies have reported barotrauma to be a mortality-risk factor; however, its time-dependent nature and pathophysiology are not elucidated. To investigate the time-dependent characteristics and the etiology of coronavirus disease 2019-related-barotrauma.

METHODS AND FINDINGS

We retrospectively reviewed intubated patients with coronavirus disease 2019 from March 2020 to May 2021. We compared the 90-day survival between the barotrauma and non-barotrauma groups and performed landmark analyses on days 7, 14, 21, and 28. Barotrauma within seven days before the landmark was defined as the exposure. Additionally, we evaluated surgically treated cases of coronavirus disease 2019-related pneumothorax. We included 192 patients. Barotrauma developed in 44 patients (22.9%). The barotrauma group's 90-day survival rate was significantly worse (47.7% vs. 82.4%, p < 0.001). In the 7-day landmark analysis, there was no significant difference (75.0% vs. 75.7%, p = 0.79). Contrastingly, in the 14-, 21-, and 28-day landmark analyses, the barotrauma group's survival rates were significantly worse (14-day: 41.7% vs. 69.1%, p = 0.044; 21-day: 16.7% vs. 62.5%, p = 0.014; 28-day: 20.0% vs. 66.7%, p = 0.018). Pathological examination revealed a subpleural hematoma and pulmonary cyst with heterogenous lung inflammation.

CONCLUSIONS

Barotrauma was a poor prognostic factor for coronavirus disease 2019, especially in the late phase. Heterogenous inflammation may be a key finding in its mechanism. Barotrauma is a potentially important sign of lung destruction.

Understanding the effects of aerodynamic and hydrodynamic shear forces on Pseudomonas aeruginosa biofilm growth

Biofilms are communities of bacterial cells encased in a self-produced polymeric matrix and exhibit high tolerance towards environmental stress. Despite the plethora of research on biofilms, most biofilm models are produced using mono-interface culture in static flow conditions, and knowledge of the effects of interfaces and mechanical forces on biofilm development remains fragmentary. This study elucidated the effects of air-liquid (ALI) or liquid-liquid (LLI) interfaces and mechanical shear forces induced by airflow and hydrodynamic flow on biofilm growing using a custom-designed dual-channel microfluidic platform. Results from this study showed that comparing biofilms developed under continuous nutrient supply and shear stresses free condition to those developed with limited nutrient supply, ALI biofilms were four times thicker, 60% less permeable, and 100 times more resistant to antibiotics, while LLI biofilms were two times thicker, 20% less permeable, and 100 times more resistant to antibiotics. Subjecting the biofilms to mechanical shear stresses affected the biofilm structure across the biofilm thickness significantly, resulting in generally thinner and denser biofilm compared to their controlled biofilm cultured in the absence of shear stresses, and the ALI and LLI biofilm's morphology was vastly different. Biofilms developed under hydrodynamic shear stress also showed increased antibiotic resistance. These findings highlight the importance of investigating biofilm growth and its mechanisms in realistic environmental conditions and demonstrate a feasible approach to undertake this work using a novel platform. This article is protected by copyright. All rights reserved.

Ventilation‐induced epithelial injury drives biological onset of lung trauma in vitro and is mitigated with prophylactic anti‐inflammatory therapeutics

Mortality rates among patients suffering from acute respiratory failure remain perplexingly high despite the maintenance of blood oxygen homeostasis during ventilatory support. The biotrauma hypothesis advocates that mechanical forces from invasive ventilation trigger immunological mediators that spread systemically. Yet, how these forces elicit an immune response remains unclear. Here, a biomimetic in vitro three‐dimensional (3D) upper airways model allows to recapitulate lung injury and immune responses induced during invasive mechanical ventilation in neonates. Under such ventilatory support, flow‐induced stresses injure the bronchial epithelium of the intubated airways model and directly modulate epithelial cell inflammatory cytokine secretion associated with pulmonary injury. Fluorescence microscopy and biochemical analyses reveal site‐specific susceptibility to epithelial erosion in airways from jet‐flow impaction and are linked to increases in cell apoptosis and modulated secretions of cytokines IL‐6, ‐8, and ‐10. In an effort to mitigate the onset of biotrauma, prophylactic pharmacological treatment with Montelukast, a leukotriene receptor antagonist, reduces apoptosis and pro‐inflammatory signaling during invasive ventilation of the in vitro model. This 3D airway platform points to a previously overlooked origin of lung injury and showcases translational opportunities in preclinical pulmonary research toward protective therapies and improved protocols for patient care.

In Vitro Measurements of Cellular Forces and their Importance in the Lung-From the Sub- to the Multicellular Scale

Throughout life, the body is subjected to various mechanical forces on the organ, tissue, and cellular level. Mechanical stimuli are essential for organ development and function. One organ whose function depends on the tightly connected interplay between mechanical cell properties, biochemical signaling, and external forces is the lung. However, altered mechanical properties or excessive mechanical forces can also drive the onset and progression of severe pulmonary diseases. Characterizing the mechanical properties and forces that affect cell and tissue function is therefore necessary for understanding physiological and pathophysiological mechanisms. In recent years, multiple methods have been developed for cellular force measurements at multiple length scales, from subcellular forces to measuring the collective behavior of heterogeneous cellular networks. In this short review, we give a brief overview of the mechanical forces at play on the cellular level in the lung. We then focus on the technological aspects of measuring cellular forces at many length scales. We describe tools with a subcellular resolution and elaborate measurement techniques for collective multicellular units. Many of the technologies described are by no means restricted to lung research and have already been applied successfully to cells from various other tissues. However, integrating the knowledge gained from these multi-scale measurements in a unifying framework is still a major future challenge.

Pharmacokinetic Analysis of Epithelial/Endothelial Cell Barriers in Microfluidic Bilayer Devices with an Air-Liquid Interface

As the range of applications of organs-on-chips is broadening, the evaluation of aerosol-based therapies using a lung-on-a-chip model has become an attractive approach. Inhalation therapies are not only minimally invasive but also provide optimal pharmacokinetic conditions for drug absorption. As drug development evolves, it is likely that better screening through use of organs-on-chips can significantly save time and cost. In this work, bio-aerosols of various compounds including insulin were generated using a jet nebulizer. The aerosol flows were driven through microfluidic bilayer devices establishing an air–liquid interface to mimic the blood–air barrier in human small airways. The aerosol flow in the microfluidic devices has been characterized and adjusted to closely match physiological values. The permeability of several compounds, including paracellular and transcellular biomarkers, across epithelial/endothelial cell barriers was measured. Concentration–time plots were established in microfluidic devices with and without cells; the curves were then utilized to extract standard pharmacokinetic parameters such as the area under the curve, maximum concentration, and time to maximum concentration. The cell barrier significantly affected the measured pharmacokinetic parameters, as compound absorption through the barrier decreases with its increasing molecular size. Aerosolizing insulin can lead to the formation of fibrils, prior to its entry to the microfluidic device, with a substantially larger apparent molecular size effectively blocking its paracellular transport. The results demonstrate the advantage of using lung-on-a-chip for drug discovery with applications such as development of novel inhaled therapies.

Airborne transmission of severe acute respiratory syndrome coronavirus-2 to healthcare workers: a narrative review

Healthcare workers are at risk of infection during the severe acute respiratory syndrome coronavirus‐2 pandemic. International guidance suggests direct droplet transmission is likely and airborne transmission occurs only with aerosol‐generating procedures. Recommendations determining infection control measures to ensure healthcare worker safety follow these presumptions. Three mechanisms have been described for the production of smaller sized respiratory particles (‘aerosols’) that, if inhaled, can deposit in the distal airways. These include: laryngeal activity such as talking and coughing; high velocity gas flow; and cyclical opening and closure of terminal airways. Sneezing and coughing are effective aerosol generators, but all forms of expiration produce particles across a range of sizes. The 5‐μm diameter threshold used to differentiate droplet from airborne is an over‐simplification of multiple complex, poorly understood biological and physical variables. The evidence defining aerosol‐generating procedures comes largely from low‐quality case and cohort studies where the exact mode of transmission is unknown as aerosol production was never quantified. We propose that transmission is associated with time in proximity to severe acute respiratory syndrome coronavirus‐1 patients with respiratory symptoms, rather than the procedures per se. There is no proven relation between any aerosol‐generating procedure with airborne viral content with the exception of bronchoscopy and suctioning. The mechanism for severe acute respiratory syndrome coronavirus‐2 transmission is unknown but the evidence suggestive of airborne spread is growing. We speculate that infected patients who cough, have high work of breathing, increased closing capacity and altered respiratory tract lining fluid will be significant producers of pathogenic aerosols. We suggest several aerosol‐generating procedures may in fact result in less pathogen aerosolisation than a dyspnoeic and coughing patient. Healthcare workers should appraise the current evidence regarding transmission and apply this to the local infection prevalence. Measures to mitigate airborne transmission should be employed at times of risk. However, the mechanisms and risk factors for transmission are largely unconfirmed. Whilst awaiting robust evidence, a precautionary approach should be considered to assure healthcare worker safety.

Ventilation-induced jet suggests biotrauma in reconstructed airways of the intubated neonate

We investigate respiratory flow phenomena in a reconstructed upper airway model of an intubated neonate undergoing invasive mechanical ventilation, spanning conventional to high-frequency ventilation (HFV) modes. Using high-speed tomographic particle image velocimetry, we resolve transient, three-dimensional flow fields and observe a persistent jet flow exiting the endotracheal tube whose strength is directly modulated according to the ventilation protocol. We identify this synthetic jet as the dominating signature of convective flow under intubated ventilation. Concurrently, our in silico wall shear stress analysis reveals a hitherto overlooked source of ventilator-induced lung injury as a result of jet impingement on the tracheal carina, suggesting damage to the bronchial epithelium; this type of injury is known as biotrauma. We find HFV advantageous in mitigating the intensity of such impingement, which may contribute to its role as a lung protective method. Our findings may encourage the adoption of less invasive ventilation procedures currently used in neonatal intensive care units.

Expiratory flow-limitation in mechanically ventilated patients: A risk for ventilator-induced lung injury?

Expiratory flow limitation (EFL), that is the inability of expiratory flow to increase in spite of an increase of the driving pressure, is a common and unrecognized occurrence during mechanical ventilation in a variety of intensive care unit conditions. Recent evidence suggests that the presence of EFL is associated with an increase in mortality, at least in acute respiratory distress syndrome (ARDS) patients, and in pulmonary complications in patients undergoing surgery. EFL is a major cause of intrinsic positive end-expiratory pressure (PEEPi), which in ARDS patients is heterogeneously distributed, with a consequent increase of ventilation/perfusion mismatch and reduction of arterial oxygenation. Airway collapse is frequently concomitant to the presence of EFL. When airways close and reopen during tidal ventilation, abnormally high stresses are generated that can damage the bronchiolar epithelium and uncouple small airways from the alveolar septa, possibly generating the small airways abnormalities detected at autopsy in ARDS. Finally, the high stresses and airway distortion generated downstream the choke points may contribute to parenchymal injury, but this possibility is still unproven. PEEP application can abolish EFL, decrease PEEPi heterogeneity, and limit recruitment/derecruitment. Whether increasing PEEP up to EFL disappearance is a useful criterion for PEEP titration can only be determined by future studies.

A lubricant boundary condition for a biological body lined by a thin heterogeneous biofilm

We study the asymptotic behavior of an incompressible viscous fluid flow in a biological body lined by a thin biological film with a cellular microstructure, varying thickness, and a heterogeneous viscosity regulated by a time random process. Letting the thickness of the film tend to zero, we derive an effective biological slip boundary condition on the boundary of the body. This law relates the tangential fluxes to the tangential velocities via a proportional coefficient corresponding to the energy of some local problem. This law describes the ability of the biological film to function as a lubricant reducing friction at the wall of the body. The tangential velocities are functions of the random trajectories of a finely concentrated biological particle.

Effect of frequency on pressure cost of ventilation and gas exchange in newborns receiving high-frequency oscillatory ventilation

BackgroundWe hypothesized that ventilating at the resonant frequency of the respiratory system optimizes gas exchange while limiting the mechanical stress to the lung in newborns receiving high-frequency oscillatory ventilation (HFOV). We characterized the frequency dependence of oscillatory mechanics, gas exchange, and pressure transmission during HFOV.MethodsWe studied 13 newborn infants with a median (interquartile range) gestational age of 29.3 (26.4-30.4) weeks and body weight of 1.00 (0.84-1.43) kg. Different frequencies (5, 8, 10, 12, and 15 Hz) were tested, keeping carbon dioxide diffusion coefficient (DCO₂) constant. Oscillatory mechanics and transcutaneous blood gas were measured at each frequency. The attenuation of pressure swings (ΔP) from the airways opening to the distal end of the tracheal tube (TT) and to the alveolar compartment was mathematically estimated.ResultsBlood gases were unaffected by frequency. The mean (SD) resonant frequency was 16.6 (3.5) Hz. Damping of ΔP increased with frequency and with lung compliance. ΔP at the distal end of the TT was insensitive to frequency, whereas ΔP at the peripheral level decreased with frequency.ConclusionThere is no optimal frequency for gas exchange when DCO₂ is held constant. Greater attenuation of oscillatory pressure at higher frequencies offers more protection from barotrauma, especially in patients with poor compliance.

Decreased Pseudomonas aeruginosa biofilm formation on nanomodified endotracheal tubes: a dynamic lung model

Ventilator-associated pneumonia (VAP) is a serious complication of mechanical ventilation that has been shown to be associated with increased mortality rates and medical costs in the pediatric intensive care unit. Currently, there is no cost-effective solution to the problems posed by VAP. Endotracheal tubes (ETTs) that are resistant to bacterial colonization and that inhibit biofilm formation could provide a novel solution to the problems posed by VAP. The objective of this in vitro study was to evaluate differences in the growth of Pseudomonas aeruginosa on unmodified polyvinyl chloride (PVC) ETTs and on ETTs etched with a fungal lipase, Rhizopus arrhizus, to create nanoscale surface features. These differences were evaluated using an in vitro model of the pediatric airway to simulate a ventilated patient in the pediatric intensive care unit. Each experiment was run for 24 hours and was supported by computational models of the ETT. Dynamic conditions within the ETT had an impact on the location of bacterial growth within the tube. These conditions also quantitatively affected bacterial growth especially within the areas of tube curvature. Most importantly, experiments in the in vitro model revealed a 2.7 log reduction in the number (colony forming units/mL) of P. aeruginosa on the nanoroughened ETTs compared to the untreated PVC ETTs after 24 hours. This reduction in total colony forming units/mL along the x-axis of the tube was similar to previous studies completed for Staphylococcus aureus. Thus, this dynamic study showed that lipase etching can create surface features of nanoscale roughness on PVC ETTs that decrease bacterial attachment of P. aeruginosa without the use of antibiotics and may provide clinicians with an effective and inexpensive tool to combat VAP.

Biofilm Cohesive Strength as a Basis for Biofilm Recalcitrance: Are Bacterial Biofilms Overdesigned?

Bacterial biofilms are highly resistant to common antibacterial treatments, and several physiological explanations have been offered to explain the recalcitrant nature of bacterial biofilms. Herein, a biophysical aspect of biofilm recalcitrance is being reported on. While engineering structures are often overdesigned with a factor of safety (FOS) usually under 10, experimental measurements of biofilm cohesive strength suggest that the FOS is on the order of thousands. In other words, bacterial biofilms appear to be designed to withstand extreme forces rather than typical or average loads. In scenarios requiring the removal or control of unwanted biofilms, this emphasizes the importance of considering strategies for structurally weakening the biofilms in conjunction with bacterial inactivation.

Analysis of the method for ventilation heterogeneity assessment using the Otis model and forced oscillations

Increased heterogeneity of the lung disturbs pulmonary gas exchange. During bronchoconstriction, inflammation of lung parenchyma or acute respiratory distress syndrome, inhomogeneous lung ventilation can become bimodal and increase the risk of ventilator-induced lung injury during mechanical ventilation. A simple index sensitive to ventilation heterogeneity would be very useful in clinical practice. In the case of bimodal ventilation, the index (H) can be defined as the ratio between the longer and shorter time constant characterising regions of contrary mechanical properties. These time constants can be derived from the Otis model fitted to input impedance (Zin) measured using forced oscillations. In this paper we systematically investigated properties of the aforementioned approach. The research included both numerical simulations and real experiments with a dual-lung simulator. Firstly, a computational model mimicking the physical simulator was derived and then used as a forward model to generate synthetic flow and pressure signals. These data were used to calculate the input impedance and then the Otis inverse model was fitted to Zin by means of the Levenberg-Marquardt (LM) algorithm. Finally, the obtained estimates of model parameters were used to compute H. The analysis of the above procedure was performed in the frame of Monte Carlo simulations. For each selected value of H, forward simulations with randomly chosen lung parameters were repeated 1000 times. Resulting signals were superimposed by additive Gaussian noise. The estimated values of H properly indicated the increasing level of simulated inhomogeneity, however with underestimation and variation increasing with H. The main factor responsible for the growing estimation bias was the fixed starting vector required by the LM algorithm. Introduction of a correction formula perfectly reduced this systematic error. The experimental results with the dual-lung simulator confirmed potential of the proposed procedure to properly deduce the lung heterogeneity level. We conclude that the heterogeneity index H can be used to assess bimodal ventilation imbalances in cases when this phenomenon dominates lung properties, however future analyses, including the impact of lung tissue viscoelasticity and distributed airway or tissue inhomogeneity on H estimates, as well as studies in the time domain, are advisable.

Mechanosensitive activation of CFTR by increased cell volume and hydrostatic pressure but not shear stress

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl(-) channel that is essential for electrolyte and fluid homeostasis. Preliminary evidence indicates that CFTR is a mechanosensitive channel. In lung epithelia, CFTR is exposed to different mechanical forces such as shear stress (Ss) and membrane distention. The present study questioned whether Ss and/or stretch influence CFTR activity (wild type, ∆F508, G551D). Human CFTR (hCFTR) was heterologously expressed in Xenopus oocytes and the response to the mechanical stimulus and forskolin/IBMX (FI) was measured by two-electrode voltage-clamp experiments. Ss had no influence on hCFTR activity. Injection of an intracellular analogous solution to increase cell volume alone did not affect hCFTR activity. However, hCFTR activity was augmented by injection after pre-stimulation with FI. The response to injection was similar in channels carrying the common mutations ∆F508 and G551D compared to wild type hCFTR. Stretch-induced CFTR activation was further assessed in Ussing chamber measurements using Xenopus lung preparations. Under control conditions increased hydrostatic pressure (HP) decreased the measured ion current including activation of a Cl(-) secretion that was unmasked by the CFTR inhibitor GlyH-101. These data demonstrate activation of CFTR in vitro and in a native pulmonary epithelium in response to mechanical stress. Mechanosensitive regulation of CFTR is highly relevant for pulmonary physiology that relies on ion transport processes facilitated by pulmonary epithelial cells.

A biomechanical model of pendelluft induced lung injury

Lung ventilation using high frequency oscillatory techniques have been documented to attain adequate gas exchange through various gas transport mechanisms. Among them, the pendelluft flow is considered one of the most crucial mechanisms. In this work, we computationally investigate the induction of abnormal mechanical stresses and a regionally trapped volume of gas due to pendelluft flow. Large eddy simulation was used to model the turbulence in an upper tracheobronchial lung geometry that was derived from CT scans. The pendelluft flow was captured by modeling physiological boundary conditions at the truncated level of the lung model that is sensitive to the coupled resistance and compliance of individual patients. The flow-volume and volume-pressure loops are characterized by irregular shapes and suggest abnormal regional lung ventilation. Incomplete loops were observed indicating gas trapping in these regions signifying a potential for local injury due to incomplete ventilation from a residual volume build-up at the end of the expiration phase. In addition, the gas exchange between units was observed to create a velocity gradient causing a region of high wall shear stress surrounding the carina ridges. The recurrence of the pendelluft flow could cause a rupture to the lung epithelium layer. The trapped gas and wall shear stress were observed to amplify with increasing compliance asymmetry and ventilator operating frequency. In general, despite the significant contribution of the pendelluft flow to the gas exchange augmentation there exists significant risks of localized lung injury, phenomena we describe as pendelluft induced lung injury or PILI.

Assessment of acute bronchodilator effects from specific airway resistance changes in stable COPD patients

BACKGROUND

In COPD patients, reversibility is currently evaluated from the changes of forced expiratory volume at 1s (ΔFEV1) and forced vital capacity (ΔFVC). By lowering peripheral airway smooth muscle tone, bronchodilators should decrease dynamic hyperinflation, gas trapping, and possibly dyspnea at rest. Hence, we hypothesize that specific airway resistance changes (ΔsRAW) should better characterize the acute response to bronchodilators.

METHODS

On two days, 60 COPD patients underwent dyspnea evaluation (VAS score) and pulmonary function testing at baseline and one hour after placebo or 300μg indacaterol administration.

RESULTS

Spirographic and ΔsRAW-based criteria identified as responders 24 and 45 patients, respectively. ΔsRAW correlated with changes of intrathoracic gas volume (ΔITGV) (r=0.61; p<0.001), residual volume (ΔRV) (r=0.60; p<0.001), ΔFVC (r=0.44; p=0.001), and ΔVAS (r=0.73; p<0.001), while ΔFEV1 correlated only with ΔFVC (r=0.34; p=0.008). Significant differences in terms of ΔITGV (p=0.002), ΔRV (p=0.023), and ΔVAS (p<0.001) occurred only if patients were stratified according to ΔsRAW.

CONCLUSIONS

In assessing the acute functional effect of bronchodilators, ΔsRAW-based criterion is preferable to FEV1-FVC-based criteria, being more closely related to bronchodilator-induced improvements of lung mechanics and dyspnea at rest.

Wall shear stress distributions in a model of normal and constricted small airways

Previous studies have highlighted flow shear stress as a possible damage mechanism for small airways, in particular those liable to constriction through disease or injury due to mechanical ventilation. Flow experiments in vitro have implicated shear stress as a relevant factor for mechanotransduction pathways with respect to airway epithelial cell function. Using computational fluid dynamics analysis, this study reports velocity profiles and calculations for wall shear stress distributions in a three-generation, asymmetric section of the small airways subjected to a steady, inspiratory flow. The results show distal variation of wall shear stress distributions due to velocity gradients on the carina side of each daughter airway branch. The maximum wall shear stresses in both normal and constricted small airways are shown to exceed those calculated using data from previous simpler one-dimensional experimental analyses. These findings have implications for lung cell flow experiments involving shear stress in the consideration of both normal airway function and pathology due to mechanotransduction mechanisms.

Airway gas flow

Local characteristics of airflow and its global distribution in the lung are determined by interaction between resistance to flow through the airways and the compliance of the tissue, with tissue compliance dominating flow distribution in the healthy lung. Current understanding is that conceptualizing the airways of the lung as a system of smooth adjoined cylinders through which air traverses laminarly is insufficient for understanding flow and energy dissipation and is particularly poor for predicting physiologically realistic transport of particles by the airflow. With rapid advances in medical imaging, computer technologies, and computational techniques, computational fluid dynamics is now becoming a viable tool for providing detailed information on the mechanics of airflow in the human respiratory tract. Studies using such techniques have shown that the upper airway (specifically its development of a turbulent laryngeal jet in the trachea), airway geometry, branching and rotation angle, and the pattern of joining of successive bifurcations are important in determining airflow structures. It is now possible to compute airflow in physical domains that are anatomically accurate and subject specific, enabling comparisons among intersubjects, that among subjects of different ages, and that among different species.

Recruitment maneuvers modulate epithelial and endothelial cell response according to acute lung injury etiology

OBJECTIVE

To investigate the effects of the rate of increase in airway pressure and duration of lung recruitment maneuvers in experimental pulmonary and extrapulmonary acute lung injury.

DESIGN

Prospective, randomized, controlled experimental study.

SETTINGS

University research laboratory.

SUBJECTS

Fifty adult male Wistar rats.

INTERVENTIONS

Acute lung injury was induced by Escherichia coli lipopolysaccharide either intratracheally (pulmonary acute lung injury) or intraperitoneally (extrapulmonary acute lung injury). After 24 hours, animals were assigned to one of three different recruitment maneuvers, targeted to maximal airway pressure of 30 cm H2O: 1) continuous positive airway pressure for 30 seconds (CPAP-30); 2) stepwise airway pressure increase (5 cm H2O/step, 8.5 s at each step) over 51 seconds (STEP-51) to achieve a pressure-time product similar to that of CPAP-30; and 3) stepwise airway pressure increase (5 cm H2O/step, 5 s at each step) over 30 seconds with maximum pressure sustained for a further 30 seconds (STEP-30/30).

MEASUREMENTS AND MAIN RESULTS

All recruitment maneuvers reduced static lung elastance independent of acute lung injury etiology. In pulmonary acute lung injury, CPAP-30 yielded lower surfactant protein-B and higher type III procollagen expressions compared with STEP-30/30. In extrapulmonary acute lung injury, CPAP-30 and STEP-30/30 increased vascular cell adhesion molecule-1 expression, but the type of recruitment maneuver did not influence messenger ribonucleic acid expression of receptor for advanced glycation end products, surfactant protein-B, type III procollagen, and pro-caspase 3.

CONCLUSIONS

CPAP-30 worsened markers of potential epithelial cell damage in pulmonary acute lung injury, whereas both CPAP-30 and STEP-30/30 yielded endothelial injury in extrapulmonary acute lung injury. In both acute lung injury groups, recruitment maneuvers improved respiratory mechanics, but stepwise recruitment maneuver without sustained airway pressure appeared to associate with less biological impact on lungs.

Airway dysfunction in elite athletes--an occupational lung disease?

Airway dysfunction is prevalent in elite endurance athletes and when left untreated may impact upon both health and performance. There is now concern that the intensity of hyperpnoea necessitated by exercise at an elite level may be detrimental for an athlete's respiratory health. This article addresses the evidence of causality in this context with the aim of specifically addressing whether airway dysfunction in elite athletes should be classified as an occupational lung disease. The approach used highlights a number of concerns and facilitates recommendations to ensure airway health is maintained and optimized in this population. We conclude that elite athletes should receive the same considerations for their airway health as others with potential and relevant occupational exposures.

Urinary CC16 after challenge with dry air hyperpnoea and mannitol in recreational summer athletes

Airway epithelial injury is regarded as a key contributing factor to the pathogenesis of exercise-induced bronchoconstriction (EIB) in athletes. The concentration of the pneumoprotein club cell (Clara cell) CC16 in urine has been found to be a non-invasive marker for hyperpnoea-induced airway epithelial perturbation. Exercise-hyperpnoea induces mechanical, thermal and osmotic stress to the airways. We investigated whether osmotic stress alone causes airway epithelial perturbation in athletes with suspected EIB. Twenty-four recreational summer sports athletes who reported respiratory symptoms on exertion performed a standard eucapnic voluntary hyperpnoea test with dry air and a mannitol test (osmotic challenge) on separate days. Median urinary CC16 increased from 120 to 310 ρg μmol creatinine(-1) after dry air hyperpnoea (P = 0.002) and from 90 to 191 ρg μmol creatinine(-1) after mannitol (P = 0.021). There was no difference in urinary CC16 concentration between athletes who did or did not bronchoconstrict after dry air hyperpnoea or mannitol. We conclude that, in recreational summer sports athletes with respiratory symptoms, osmotic stress per se to the airway epithelium induces a rise in urinary excretion of CC16. This suggests that hyperosmolarity of the airway surface lining perturbs the airway epithelium in symptomatic athletes.

Surface mapping for visualization of wall stresses during inhalation in a human nasal cavity

Airflow analysis can assist in better understanding the physiology however the human nasal cavity is an extremely complicated geometry that is difficult to visualize in 3D space, let alone in 2D space. In this paper, an anatomically accurate 3D surface of the nasal passages derived from CT data was unwrapped and transformed into a 2D space, into a UV-domain (where u and v are the coordinates) to allow a complete view of the entire wrapped surface. This visualization technique allows surface flow parameters to be analyzed with greater precision. A UV-unwrapping tool is developed and a strategy is presented to allow deeper analysis to be performed. This includes (i) the ability to present instant comparisons of geometry and flow variables between any number of different nasal cavity models through normalization of the 2D unwrapped surface; (ii) visualization of an entire surface in one view and; (iii) a planar surface that allows direct 1D and 2D analytical solutions of diffusion of inhaled vapors and particles through the nasal walls. This work lays a foundation for future investigations that correlates adverse and therapeutic health responses to local inhalation of gases and particles.

Effect of terbutaline on hyperpnoea-induced bronchoconstriction and urinary club cell protein 16 in athletes

Repeated injury of the airway epithelium caused by hyperpnoea of poorly conditioned air has been proposed as a key factor in the pathogenesis of exercise-induced bronchoconstriction (EIB) in athletes. In animals, the short-acting β2-agonist terbutaline has been shown to reduce dry airflow-induced bronchoconstriction and the associated shedding of airway epithelial cells. Our aim was to test the efficacy of inhaled terbutaline in attenuating hyperpnoea-induced bronchoconstriction and airway epithelial injury in athletes. Twenty-seven athletes with EIB participated in a randomized, double-blind, placebo-controlled, crossover study. Athletes completed an 8-min eucapnic voluntary hyperpnoea (EVH) test with dry air on two separate days 15 min after inhaling 0.5 mg terbutaline or a matching placebo. Forced expiratory volume in 1 s (FEV1) and urinary concentration of the club cell (Clara cell) protein 16 (CC16, a marker of airway epithelial perturbation) were measured before and up to 60 min after EVH. The maximum fall in FEV1 of 17 ± 8% (SD) on placebo was reduced to 8 ± 5% following terbutaline (P < 0.001). Terbutaline gave bronchoprotection (i.e., post-EVH FEV1 fall <10%) to 22 (81%) athletes. EVH caused an increase in urinary excretion of CC16 in both conditions (P < 0.001), and terbutaline significantly reduced this rise (pre- to postchallenge CC16 increase 416 ± 495 pg/μmol creatinine after placebo vs. 315 ± 523 pg/μmol creatinine after terbutaline, P = 0.016). These results suggest that the inhalation of a single therapeutic dose of terbutaline offers significant protection against hyperpnoea-induced bronchoconstriction and attenuates acute airway epithelial perturbation in athletes.

Physiological measurement of the small airways

Noninvasive physiological measurements are reviewed that have been reported in the literature with the specific aim being to study the small airways in lung disease. This has mostly involved at-the-mouth noninvasive measurement of flow, pressure or inert gas concentration, with the intent of deriving one or more indices that are representative of small airway structure and function. While these measurements have remained relatively low-tech, the effort and sophistication increasingly reside with the interpretation of such indices. When aspiring to derive information at the mouth about structural and mechanical processes occurring several airway generations away in a complex cyclically changing cul-de-sac structure, conceptual or semi-quantitative lung models can be valuable. Two assumptions that are central to small airway structure-function measurement are that of an average airway change at a given peripheral lung generation and of a parallel heterogeneity in airway changes. While these are complementary pieces of information, they can affect certain small airways tests in confounding ways. We critically analyzed the various small airway tests under review, while contending that negative outcomes of these tests are probably a true reflection of the fact that no change occurred in the small airways. Utmost care has been taken to not favor one technique over another, given that most current small airways tests still have room for improvement in terms of rendering their content more specific to the small airways. One way to achieve this could consist of the coupling of signals collected at the mouth to spatial information gathered from imaging in the same patient.

Decreased Staphylococcus aureus biofilm formation on nanomodified endotracheal tubes: a dynamic airway model

Ventilator-associated pneumonia (VAP) is a serious and costly clinical problem. Specifically, receiving mechanical ventilation for over 24 hours increases the risk of VAP and is associated with high morbidity, mortality, and medical costs. Cost-effective endotracheal tubes (ETTs) that are resistant to bacterial infections could help prevent this problem. The objective of this study was to determine differences in the growth of Staphylococcus aureus on nanomodified and unmodified polyvinyl chloride (PVC) ETTs under dynamic airway conditions simulating a ventilated patient. PVC ETTs were modified to have nanometer surface features by soaking them in Rhizopus arrhisus, a fungal lipase. Twenty-four-hour experiments (supported by computational models) showed that airflow conditions within the ETT influenced both the location and the concentration of bacterial growth on the ETTs, especially within areas of tube curvature. More importantly, experiments revealed a 1.5 log reduction in the total number of S. aureus on the novel nanomodified ETTs compared with the conventional ETTs after 24 hours of airflow. This dynamic study showed that lipase etching can create nanorough surface features on PVC ETTs that suppress S. aureus growth, and thus may provide clinicians with an effective and inexpensive tool to combat VAP.

Effect of inspired air conditions on exercise-induced bronchoconstriction and urinary CC16 levels in athletes

Injury to the airway epithelium has been proposed as a key susceptibility factor for exercise-induced bronchoconstriction (EIB). Our goals were to establish whether airway epithelial cell injury occurs during EIB in athletes and whether inhalation of warm humid air inhibits this injury. Twenty-one young male athletes (10 with a history of EIB) performed two 8-min exercise tests near maximal aerobic capacity in cold dry (4°C, 37% relative humidity) and warm humid (25°C, 94% relative humidity) air on separate days. Postexercise changes in urinary CC16 were used as a biomarker of airway epithelial cell perturbation and injury. Bronchoconstriction occurred in eight athletes in the cold dry environment and was completely blocked by inhalation of warm humid air [maximal fall in forced expiratory volume in 1 s = 18.1 ± 2.1% (SD) in cold dry air and 1.7 ± 0.8% in warm humid air, P < 0.01]. Exercise caused an increase in urinary excretion of CC16 in all subjects ( P < 0.001), but this rise in CC16 was blunted following inhalation of warm humid air [median CC16 increase pre- to postchallenge = 1.91 and 0.35 ng/μmol in cold dry and warm humid air, respectively, in athletes with EIB ( P = 0.017) and 1.68 and 0.48 ng/μmol in cold dry and warm humid air, respectively, in athletes without EIB ( P = 0.002)]. The results indicate that exercise hyperpnea transiently disrupts the airway epithelium of all athletes (not only in those with EIB) and that inhalation of warm moist air limits airway epithelial cell perturbation and injury.

Simulating ventilation distribution in heterogenous lung injury using a binary tree data structure

To determine the impact of mechanical heterogeneity on the distribution of regional flows and pressures in the injured lung, we developed an anatomic model of the canine lung comprised of an asymmetric branching airway network, which can be stored as binary tree data structure. The entire tree can be traversed using a recursive flow divider algorithm, allowing for efficient computation of acinar flow and pressure distributions in a mechanically heterogeneous lung. These distributions were found to be highly dependent on ventilation frequency and the heterogeneity of tissue elastances, reflecting the preferential distribution of ventilation to areas of lower regional impedance.

Impact of pressure profile and duration of recruitment maneuvers on morphofunctional and biochemical variables in experimental lung injury

OBJECTIVE

To investigate the effects of the rate of airway pressure increase and duration of recruitment maneuvers on lung function and activation of inflammation, fibrogenesis, and apoptosis in experimental acute lung injury.

DESIGN

Prospective, randomized, controlled experimental study.

SETTING

University research laboratory.

SUBJECTS

Thirty-five Wistar rats submitted to acute lung injury induced by cecal ligation and puncture.

INTERVENTIONS

After 48 hrs, animals were randomly distributed into five groups (seven animals each): 1) nonrecruited (NR); 2) recruitment maneuvers (RMs) with continuous positive airway pressure (CPAP) for 15 secs (CPAP15); 3) RMs with CPAP for 30 secs (CPAP30); 4) RMs with stepwise increase in airway pressure (STEP) to targeted maximum within 15 secs (STEP15); and 5) RMs with STEP within 30 secs (STEP30). To perform STEP RMs, the ventilator was switched to a CPAP mode and positive end-expiratory pressure level was increased stepwise. At each step, airway pressure was held constant. RMs were targeted to 30 cm H2O. Animals were then ventilated for 1 hr with tidal volume of 6 mL/kg and positive end-expiratory pressure of 5 cm H2O.

MEASUREMENTS AND MAIN RESULTS

Blood gases, lung mechanics, histology (light and electronic microscopy), interleukin-6, caspase 3, and type 3 procollagen mRNA expressions in lung tissue. All RMs improved oxygenation and lung static elastance and reduced alveolar collapse compared to NR. STEP30 resulted in optimal performance, with: 1) improved lung static elastance vs. NR, CPAP15, and STEP15; 2) reduced alveolar-capillary membrane detachment and type 2 epithelial and endothelial cell injury scores vs. CPAP15 (p < .05); and 3) reduced gene expression of interleukin-6, type 3 procollagen, and caspase 3 in lung tissue vs. other RMs.

CONCLUSIONS

Longer-duration RMs with slower airway pressure increase efficiently improved lung function, while minimizing the biological impact on lungs.

Dynamics of tidal volume and ventilation heterogeneity under pressure-controlled ventilation during bronchoconstriction: a simulation study

The difference in effectiveness between volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV) on mechanically ventilated patients during bronchoconstriction is not totally clear. PCV is thought to deliver a more uniform distribution of ventilation than VCV, but the delivered tidal volume could be unstable and affected by changes in the degree of constriction. To explore the magnitude of these effects, we ran numerical simulations with both modes of ventilation in a network model of the lung in which we incorporated not only the pressure and flow dynamics along the airways but also the effect of cycling pressures and tissue tethering forces during breathing on the dynamic equilibrium of the airway smooth muscle (ASM) (Venegas et al., Nature 434: 777-782). These simulations provided an illustration of changes in airway radii, the total delivered tidal volume stability, and distribution of ventilation following a transition from VCV to PCV and during progressively increasing ASM activation level. These simulations yielded three major results. First, the ventilation heterogeneity and patchiness in ventilation during steady-state VCV were substantially reduced after the transition to PCV. Second, airway radius, tidal volume, and the distribution of ventilation under severe bronchoconstriction were highly sensitive to the setting of inspiratory pressure selected for PCV and to the degree of activation of the ASM. Third, the dynamic equilibrium of active ASM exposed to cycling forces is the major contributor to these effects. These insights may provide a theoretical framework to guide the selection of ventilation mode, the adjustment of ventilator settings, and the interpretation of clinical observations in mechanically ventilated asthmatic patients.

Bronchial epithelial damage after a half-marathon in nonasthmatic amateur runners

High neutrophil counts in induced sputum have been found in nonasthmatic amateur runners at rest and after a marathon, but the pathogenesis of airway neutrophilia in athletes is still poorly understood. Bronchial epithelial damage may occur during intense exercise, as suggested by investigations conducted in endurance-trained mice and competitive human athletes studied under resting conditions. To gain further information on airway changes acutely induced by exercise, airway cell composition, apoptosis, IL-8 concentration in induced sputum, and serum CC-16 level were measured in 15 male amateur runners at rest (baseline) and shortly after a half-marathon. Different from results obtained after a marathon, neutrophil absolute counts were unchanged, whereas bronchial epithelial cell absolute counts and their apoptosis increased significantly ( P < 0.01). IL-8 in induced sputum supernatants almost doubled postrace compared with baseline ( P < 0.01) and correlated positively with bronchial epithelial cell absolute counts ( R2 = 0.373, P < 0.01). Serum CC-16 significantly increased after all races ( P < 0.01). These data show mild bronchial epithelial cell injury acutely induced by intense endurance exercise in humans, extending to large airways the data obtained in peripheral airways of endurance-trained mice. Therefore, neutrophil influx into the airways of athletes may be secondary to bronchial epithelial damage associated with intense exercise.

Airway wall stiffening increases peak wall shear stress: a fluid-structure interaction study in rigid and compliant airways

The airflow characteristics in a computed tomography (CT) based human airway bifurcation model with rigid and compliant walls are investigated numerically. An in-house three-dimensional (3D) fluid-structure interaction (FSI) method is applied to simulate the flow at different Reynolds numbers and airway wall stiffness. As the Reynolds number increases, the airway wall deformation increases and the secondary flow becomes more prominent. It is found that the peak wall shear stress on the rigid airway wall can be five times stronger than that on the compliant airway wall. When adding tethering forces to the model, we find that these forces, which produce larger airway deformation than without tethering, lead to more skewed velocity profiles in the lower branches and further reduced wall shear stresses via a larger airway lumen. This implies that pathologic changes in the lung such as fibrosis or remodeling of the airway wall-both of which can serve to restrain airway wall motion-have the potential to increase wall shear stress and thus can form a positive feed-back loop for the development of altered flow profiles and airway remodeling. These observations are particularly interesting as we try to understand flow and structural changes seen in, for instance, asthma, emphysema, cystic fibrosis, and interstitial lung disease.

Peripheral airways injury in acute lung injury/acute respiratory distress syndrome

PURPOSE OF REVIEW

Peripheral airways are less than 2 mm in diameter and comprise a relatively large cross-sectional area, which allows for slower, laminar airflow. They include both membranous bronchioles and gas exchange ducts, and have been referred to in the past as the 'quiet zone', partly because these structures were felt to contribute little to lung mechanics, and partly because they are difficult to study directly.

RECENT FINDINGS

Recent studies suggest that peripheral airway dysfunction plays a significant role in acute respiratory distress syndrome, which may be exacerbated by injurious mechanical ventilation strategies. The presence of elevated airways resistance, intrinsic positive end-expiratory pressure or a lower inflection point on a pressure-volume curve of the respiratory system may indicate presence of impaired peripheral airway function. In-vitro animal and human studies have begun to elucidate the signaling mechanisms responsible for stretch and shear mediated cellular injury.

SUMMARY

Understanding the pathophysiology of peripheral airway dysfunction in acute respiratory distress syndrome and mechanical ventilation continues to evolve. Greater insight into the signaling mechanisms involved in cellular injury and repair will lead to further alterations in mechanical ventilation strategies, and may lead to specific treatment options.

Numerical simulations for detailed airflow dynamics in a human nasal cavity

Nasal physiology is dependent on the physical structure of the nose. Individual aspects of the nasal cavity such as the geometry and flow rate collectively affect nasal function such as the filtration of foreign particles by bringing inspired air into contact with mucous-coated walls, humidifying and warming the air before it enters the lungs and the sense of smell. To better understand the physiology of the nose, this study makes use of CFD methods and post-processing techniques to present flow patterns between the left and right nasal cavities and compared the results with experimental and numerical data that are available in literature. The CFD simulation adopted a laminar steady flow for flow rates of 7.5 L/min and 15 L/min. General agreement of gross flow features were found that included high velocities in the constrictive nasal valve area region, high flow close to the septum walls, and vortex formations posterior to the nasal valve and olfactory regions. The differences in the left and right cavities were explored and the effects it had on the flow field were discussed especially in the nasal valve and middle turbinate regions. Geometrical differences were also compared with available models.

Pulmonary morphofunctional effects of mechanical ventilation with high inspiratory air flow

OBJECTIVE

Uncertainties about the numerous degrees of freedom in ventilator settings leave many unanswered questions about the biophysical determinants of lung injury. We investigated whether mechanical ventilation with high air flow could yield lung mechanical stress even in normal animals.

DESIGN

Prospective, randomized, controlled experimental study.

SETTING

University research laboratory.

SUBJECTS

Thirty normal male Wistar rats (180-230 g).

INTERVENTIONS

Rats were ventilated for 2 hrs with tidal volume of 10 mL/kg and either with normal inspiratory air flow (V') of 10 mL/s (F10) or high V' of 30 mL/s (F30). In the control group, animals did not undergo mechanical ventilation. Because high flow led to elevated respiratory rate (200 breaths/min) and airway peak inspiratory pressure (PIP,aw = 17 cm H2O), two additional groups were established to rule out the potential contribution of these variables: a) normal respiratory rate = 100 breaths/min and V' = 30 mL/sec; and b) PIP,aw = 17 cm H2O and V' = 10 mL/sec.

MEASUREMENTS AND MAIN RESULTS

Lung mechanics and histology (light and electron microscopy), arterial blood gas analysis, and type III procollagen messenger RNA expression in lung tissue were analyzed. Ultrastructural microscopy was similar in control and F10 groups. High air flow led to increased lung plateau and peak pressures, hypoxemia, alveolar hyperinflation and collapse, pulmonary neutrophilic infiltration, and augmented type III procollagen messenger RNA expression compared with control rats. The reduction of respiratory rate did not modify the morphofunctional behavior observed in the presence of increased air flow. Even though the increase in peak pressure yielded mechanical and histologic changes, type III procollagen messenger RNA expression remained unaltered.

CONCLUSIONS

Ventilation with high inspiratory air flow may lead to high tensile and shear stresses resulting in lung functional and morphologic compromise and elevation of type III procollagen messenger RNA expression.

Reliability of Estimating Stochastic Lung Tissue Heterogeneity from Pulmonary Impedance Spectra: A Forward-Inverse Modeling Study

Heterogeneity of regional lung mechanics is an important determinant of the work of breathing and may be a risk factor for ventilator associated lung injury. The ability to accurately assess heterogeneity may have important implications for monitoring disease progression and optimizing ventilator settings. Inverse modeling approaches, when applied to dynamic pulmonary impedance data (Z(L)), are thought to be sensitive to the detection of mechanical heterogeneity with the ability to characterize global lung function using a minimal number of free parameters. However, the reliability and bias associated with such model-based estimates of heterogeneity are unknown. We simulated Z(L) spectra from healthy, emphysematous, and acutely injured lungs using a computer-generated anatomic canine structure with asymmetric Horsfield branching and various predefined distributions of stochastic lung tissue heterogeneity. Various inverse models with distinct topologies incorporating linear resistive and inertial airways with parallel tissue viscoelasticity were then fitted to these Z(L) spectra and evaluated in terms of their quality of fit as well as the accuracy and reliability of their respective model parameters. While all model topologies detected appropriate changes in tissue heterogeneity, only a topology consisting of lumped airway properties with distributed tissue properties yielded accurate estimates of both mean lung tissue stiffness and the spread of regional elastances. These data demonstrate that inverse modeling approaches applied to noninvasive measures of Z(L) may provide reliable and accurate assessments of lung tissue heterogeneity as well as insight into distributed lung mechanical properties.

Bench-to-bedside review: distal airways in acute respiratory distress syndrome

Distal airways are less than 2 mm in diameter, comprising a relatively large cross-sectional area that allows for slower, laminar airflow. The airways include both membranous bronchioles and gas exchange ducts, and have been referred to in the past as the 'quiet zone', in part because these structures were felt to contribute little to lung mechanics and in part because they were difficult to study directly. More recent data suggest that distal airway dysfunction plays a significant role in acute respiratory distress syndrome. In addition, injurious mechanical ventilation strategies may contribute to distal airway dysfunction. The presence of elevated airway resistance, intrinsic positive end-expiratory pressure or a lower inflection point on a pressure–volume curve of the respiratory system may indicate the presence of impaired distal airway function. There are no proven specific treatments for distal airway dysfunction, and protective ventilation strategies to minimize distal airway injury may be the best therapeutic approach at this time.

Endurance Training Damages Small Airway Epithelium in Mice

RATIONALE

In athletes, airway inflammatory cells were found to be increased in induced sputum or bronchial biopsies. Most data were obtained after exposure to cold and dry air at rest or during exercise. Whether training affects epithelial and inflammatory cells in small airways is unknown.

OBJECTIVES

To test whether endurance training under standard environmental conditions causes epithelial damage and inflammation in the small airways of mice.

METHODS AND MEASUREMENTS

Formalin-fixed, paraffin-embedded lung sections were obtained in sedentary (n = 14) and endurance-trained (n = 16) Swiss mice at baseline and after 15, 30, and 45 days of training. The following variables were assessed (morphometry and immunohistochemistry) in small airways (basement membrane length < 1 mm): (1) integrity, proliferation, and apoptosis of bronchiolar epithelium; and (2) infiltration, activation, and apoptosis of inflammatory cells.

MAIN RESULTS

Compared with sedentary mice, bronchiolar epithelium of trained mice showed progressive loss of ciliated cells, slightly increased thickness, unchanged goblet cell number and appearance, and increased apoptosis and proliferation (proliferating cell nuclear antigen) (p < 0.001 for all variables). Leukocytes (CD45(+) cells) infiltrated airway walls (p < 0.0001) and accumulated within the lumen (p < 0.001); however, apoptosis of CD45(+) cells did not differ between trained and sedentary mice. Nuclear factor-kappaB translocation and inhibitor-alpha of NF-kappaB (IkappaBalpha) phosphorylation were not increased in trained compared with sedentary mice.

CONCLUSIONS

Bronchiolar epithelium showed damage and repair associated with endurance training. Training increased inflammatory cells in small airways, but inflammatory activation was not increased. These changes may represent an adaptive response to increased ventilation during exercise.

Characteristics of the turbulent laryngeal jet and its effect on airflow in the human intra-thoracic airways

A computational fluid dynamics technique is applied to understand the relative importance of the upper and intra-thoracic airways and their role in determining central airflow patterns with particular attention paid to the importance of turbulence. The geometry of the human upper respiratory tract is derived from volumetric scans of a volunteer imaged via multidetector-row computed tomography. Geometry 1 consists of a mouthpiece, the mouth, the oropharynx, the larynx, and the intra-thoracic airways of up to six generations. Geometry 2 comprises only the intra-thoracic airways. The results show that a curved sheet-like turbulent laryngeal jet is observed only in geometry 1 with turbulence intensity in the trachea varying from 10% to 20%, whereas the turbulence in geometry 2 is negligible. The presence of turbulence is found to increase the maximum localised wall shear stress by three-folds. The proper orthogonal decomposition analysis reveals that the regions of high turbulence intensity are associated with Taylor-Görtler-like vortices. We conclude that turbulence induced by the laryngeal jet could significantly affect airway flow patterns as well as tracheal wall shear stress. Thus, airflow modeling, particularly subject specific evaluations, should consider upper as well as intra-thoracic airway geometry.

Closing volume: a reappraisal (1967-2007)

Measurement of closing volume (CV) allows detection of presence or absence of tidal airway closure, i.e. cyclic opening and closure of peripheral airways with concurrent (1) inhomogeneity of distribution of ventilation and impaired gas exchange; and (2) risk of peripheral airway injury. Tidal airway closure, which can occur when the CV exceeds the end-expiratory lung volume (EELV), is commonly observed in diseases characterised by increased CV (e.g. chronic obstructive pulmonary disease, asthma) and/or decreased EELV (e.g. obesity, chronic heart failure). Risk of tidal airway closure is enhanced by ageing. In patients with tidal airway closure (CV > EELV) there is not only impairment of pulmonary gas exchange, but also peripheral airway disease due to injury of the peripheral airways. In view of this, the causes and consequences of tidal airway closure are reviewed, and further studies are suggested. In addition, assessment of the "open volume", as opposed to the "closing volume", is proposed because it is easier to perform and it requires less equipment.

Does mechanical injury of the peripheral airways play a role in the genesis of COPD in smokers?

In the present account it is proposed that in smokers the transition from peripheral airway disease to COPD is characterized by three sequential stages: Stage I, during which the closing volume eventually exceeds the functional residual capacity; Stage II, during which tidal expiratory flow limitation (EFL) is eventually exhibited; and Stage III, during which dynamic hyperinflation progressively increases leading to dyspnea and exercise limitation, which may be considered as markers of overt disease. Presence of airway closure (Stage I) and EFL (Stage II) in the tidal volume range may promote peripheral airway injury and accelerate the abnormalities of lung function. It is such injury that may determine which smoker is destined to develop COPD.