Tidal volume increases do not affect alveolar mechanics in normal lung but cause alveolar overdistension and exacerbate alveolar instability after surfactant deactivation

Nanotechnology-Based Approaches and Investigational Therapeutics against COVID-19

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel coronavirus which is currently responsible for the global pandemic since December 2019. This class of coronavirus has affected 217 countries around the world. Most of the countries have taken some non-remedial preventive actions like country lockdown, work from home, travel bans, and the most significant one is social isolation. Pharmacists, doctors, nurses, technologists, and all other healthcare professionals are playing a pivotal role during this pandemic. Unluckily, there is no specific drug that can treat patients who are confirmed with COVID-19, though favipiravir and remdesivir have appeared as favorable antiviral drugs. Some vaccines have already developed, and vaccination has started worldwide. Different nanotechnologies are in the developing stage in many countries for preventing SARS-COV-2 and treating COVID-19 conditions. In this article, we review the COVID-19 pandemic situation as well as the nanotechnology-based approaches and investigational therapeutics against COVID-19.

Ventilation and Perfusion at the Alveolar Level: Insights From Lung Intravital Microscopy

Intravital microscopy (IVM) offers unique possibilities for the observation of biological processes and disease related mechanisms in vivo. Especially for anatomically complex and dynamic organs such as the lung and its main functional unit, the alveolus, IVM provides exclusive advantages in terms of spatial and temporal resolution. By the use of lung windows, which have advanced and improved over time, direct access to the lung surface is provided. In this review we will discuss two main topics, namely alveolar dynamics and perfusion from the perspective of IVM-based studies. Of special interest are unanswered questions regarding alveolar dynamics such as: What are physiologic alveolar dynamics? How do these dynamics change under pathologic conditions and how do those changes contribute to ventilator-induced lung injury? How can alveolar dynamics be targeted in a beneficial way? With respect to alveolar perfusion IVM has propelled our understanding of the pulmonary microcirculation and its perfusion, as well as pulmonary vasoreactivity, permeability and immunological aspects. Whereas the general mechanism behind these processes are understood, we still lack a proper understanding of the complex, multidimensional interplay between alveolar ventilation and microvascular perfusion, capillary recruitment, or vascular immune responses under physiologic and pathologic conditions. These are only part of the unanswered questions and problems, which we still have to overcome. IVM as the tool of choice might allow us to answer part of these questions within the next years or decades. As every method, IVM has advantages as well as limitations, which have to be taken into account for data analysis and interpretation, which will be addressed in this review.

Detection of optimal PEEP for equal distribution of tidal volume by volumetric capnography and electrical impedance tomography during decreasing levels of PEEP in post cardiac-surgery patients

Background

Homogeneous ventilation is important for prevention of ventilator-induced lung injury. Electrical impedance tomography (EIT) has been used to identify optimal PEEP by detection of homogenous ventilation in non-dependent and dependent lung regions. We aimed to compare the ability of volumetric capnography and EIT in detecting homogenous ventilation between these lung regions.

Methods

Fifteen mechanically-ventilated patients after cardiac surgery were studied. Ventilator settings were adjusted to volume-controlled mode with a fixed tidal volume (Vt) of 6–8 ml kg⁻¹ predicted body weight. Different PEEP levels were applied (14 to 0 cm H₂O, in steps of 2 cm H₂O) and blood gases, Vcap and EIT were measured.

Results

Tidal impedance variation of the non-dependent region was highest at 6 cm H₂O PEEP, and decreased significantly at 14 cm H₂O PEEP indicating decrease in the fraction of Vt in this region. At 12 cm H₂O PEEP, homogenous ventilation was seen between both lung regions. Bohr and Enghoff dead space calculations decreased from a PEEP of 10 cm H₂O. Alveolar dead space divided by alveolar Vt decreased at PEEP levels ≤6 cm H₂O. The normalized slope of phase III significantly changed at PEEP levels ≤4 cm H₂O. Airway dead space was higher at higher PEEP levels and decreased at the lower PEEP levels.

Conclusions

In postoperative cardiac patients, calculated dead space agreed well with EIT to detect the optimal PEEP for an equal distribution of inspired volume, amongst non-dependent and dependent lung regions. Airway dead space reduces at decreasing PEEP levels.

Personalizing mechanical ventilation according to physiologic parameters to stabilize alveoli and minimize ventilator induced lung injury (VILI)

It has been shown that mechanical ventilation in patients with, or at high-risk for, the development of acute respiratory distress syndrome (ARDS) can be a double-edged sword. If the mechanical breath is improperly set, it can amplify the lung injury associated with ARDS, causing a secondary ventilator-induced lung injury (VILI). Conversely, the mechanical breath can be adjusted to minimize VILI, which can reduce ARDS mortality. The current standard of care ventilation strategy to minimize VILI attempts to reduce alveolar over-distension and recruitment-derecruitment (R/D) by lowering tidal volume (Vt) to 6 cc/kg combined with adjusting positive-end expiratory pressure (PEEP) based on a sliding scale directed by changes in oxygenation. Thus, Vt is often but not always set as a "one-size-fits-all" approach and although PEEP is often set arbitrarily at 5 cmH2O, it may be personalized according to changes in a physiologic parameter, most often to oxygenation. However, there is evidence that oxygenation as a method to optimize PEEP is not congruent with the PEEP levels necessary to maintain an open and stable lung. Thus, optimal PEEP might not be personalized to the lung pathology of an individual patient using oxygenation as the physiologic feedback system. Multiple methods of personalizing PEEP have been tested and include dead space, lung compliance, lung stress and strain, ventilation patterns using computed tomography (CT) or electrical impedance tomography (EIT), inflection points on the pressure/volume curve (P/V), and the slope of the expiratory flow curve using airway pressure release ventilation (APRV). Although many studies have shown that personalizing PEEP is possible, there is no consensus as to the optimal technique. This review will assess various methods used to personalize PEEP, directed by physiologic parameters, necessary to adaptively adjust ventilator settings with progressive changes in lung pathophysiology.

Purinergic signalling links mechanical breath profile and alveolar mechanics with the pro-inflammatory innate immune response causing ventilation-induced lung injury

Severe pulmonary infection or vigorous cyclic deformation of the alveolar epithelial type I (AT I) cells by mechanical ventilation leads to massive extracellular ATP release. High levels of extracellular ATP saturate the ATP hydrolysis enzymes CD39 and CD73 resulting in persistent high ATP levels despite the conversion to adenosine. Above a certain level, extracellular ATP molecules act as danger-associated molecular patterns (DAMPs) and activate the pro-inflammatory response of the innate immunity through purinergic receptors on the surface of the immune cells. This results in lung tissue inflammation, capillary leakage, interstitial and alveolar oedema and lung injury reducing the production of surfactant by the damaged AT II cells and deactivating the surfactant function by the concomitant extravasated serum proteins through capillary leakage followed by a substantial increase in alveolar surface tension and alveolar collapse. The resulting inhomogeneous ventilation of the lungs is an important mechanism in the development of ventilation-induced lung injury. The high levels of extracellular ATP and the upregulation of ecto-enzymes and soluble enzymes that hydrolyse ATP to adenosine (CD39 and CD73) increase the extracellular adenosine levels that inhibit the innate and adaptive immune responses rendering the host susceptible to infection by invading microorganisms. Moreover, high levels of extracellular adenosine increase the expression, the production and the activation of pro-fibrotic proteins (such as TGF-β, α-SMA, etc.) followed by the establishment of lung fibrosis.

Mechanical ventilation in acute respiratory distress syndrome: The open lung revisited

Acute respiratory distress syndrome (ARDS) is still related to high mortality and morbidity rates. Most patients with ARDS will require ventilatory support. This treatment has a direct impact upon patient outcome and is associated to major side effects. In this regard, ventilator-associated lung injury (VALI) is the main concern when this technique is used. The ultimate mechanisms of VALI and its management are under constant evolution. The present review describes the classical mechanisms of VALI and how they have evolved with recent findings from physiopathological and clinical studies, with the aim of analyzing the clinical implications derived from them. Lastly, a series of knowledge-based recommendations are proposed that can be helpful for the ventilator assisted management of ARDS at the patient bedside.

Four-dimensional visualization of subpleural alveolar dynamics in vivo during uninterrupted mechanical ventilation of living swine

Pulmonary alveoli have been studied for many years, yet no unifying hypothesis exists for their dynamic mechanics during respiration due to their miniature size (100-300 μm dimater in humans) and constant motion, which prevent standard imaging techniques from visualizing four-dimensional dynamics of individual alveoli in vivo. Here we report a new platform to image the first layer of air-filled subpleural alveoli through the use of a lightweight optical frequency domain imaging (OFDI) probe that can be placed upon the pleura to move with the lung over the complete range of respiratory motion. This device enables in-vivo acquisition of four-dimensional microscopic images of alveolar airspaces (alveoli and ducts), within the same field of view, during continuous ventilation without restricting the motion or modifying the structure of the alveoli. Results from an exploratory study including three live swine suggest that subpleural alveolar air spaces are best fit with a uniform expansion (r (2) = 0.98) over a recruitment model (r (2) = 0.72). Simultaneously, however, the percentage change in volume shows heterogeneous alveolar expansion within just a 1 mm x 1 mm field of view. These results signify the importance of four-dimensional imaging tools, such as the device presented here. Quantification of the dynamic response of the lung during ventilation may help create more accurate modeling techniques and move toward a more complete understanding of alveolar mechanics.

Higher Frequency Ventilation Attenuates Lung Injury during High-frequency Oscillatory Ventilation in Sheep Models of Acute Respiratory Distress Syndrome

Background:

High-frequency oscillatory ventilation (HFOV) at higher frequencies minimizes the tidal volume. However, whether increased frequencies during HFOV can reduce ventilator-induced lung injury remains unknown.

Methods:

After the induction of acute respiratory distress syndrome in the model by repeated lavages, 24 adult sheep were randomly divided into four groups (n = 6): three HFOV groups (3, 6, and 9 Hz) and one conventional mechanical ventilation (CMV) group. Standard lung recruitments were performed in all groups until optimal alveolar recruitment was reached. After lung recruitment, the optimal mean airway pressure or positive end-expiratory pressure was determined with decremental pressure titration, 2 cm H2O every 10 min. Animals were ventilated for 4 h.

Results:

After lung recruitment, sustained improvements in gas exchange and compliance were observed in all groups. Compared with the HFOV-3 Hz and CMV groups, the transpulmonary pressure and tidal volumes were statistically significantly lower in the HFOV-9 Hz group. The lung injury scores and wet/dry weight ratios were significantly reduced in the HFOV-9 Hz group compared with the HFOV-3 Hz and CMV groups. Expression of interleukin-1β and interleukin-6 in the lung tissue, decreased significantly in the HFOV-9 Hz group compared with the HFOV-3 Hz and CMV groups. Malondialdehyde expression and myeloperoxidase activity in lung tissues in the HFOV-9 Hz group decreased significantly, compared with the HFOV-3 Hz and CMV groups.

Conclusion:

The use of HFOV at 9 Hz minimizes lung stress and tidal volumes, resulting in less lung injury and reduced levels of inflammatory mediators compared with the HFOV-3 Hz and CMV conditions.

Effect of body position on ventilation distribution in ventilated preterm infants

RATIONALE

Positioning is considered vital to the maintenance of good lung ventilation by optimizing oxygen transport and gas exchange in ventilated premature infants. Previous studies suggest that the prone position is advantageous; however, no data exist on regional ventilation distribution for this age group.

OBJECTIVES

To investigate the effect of body position on regional ventilation distribution in ventilated and nonventilated preterm infants using electrical impedance tomography.

DESIGN

Randomized crossover study design.

SETTING

Neonatal ICU.

PATIENTS

A total of 24 ventilated preterm infants were compared with six spontaneously breathing preterm infants.

INTERVENTIONS

Random assignment of the order of the positions supine, prone, and quarter prone.

MEASUREMENTS AND MAIN RESULTS

Ventilation distribution was measured with regional impedance amplitudes and global inhomogeneity indices using electrical impedance tomography. In the spontaneously breathing infants, regional impedance amplitudes were increased in the posterior compared with the anterior lung (p < 0.01) and in the right compared with the left lung (p = 0.03). No differences were found in the ventilated infants. Ventilation was more inhomogeneous in the ventilated compared with the healthy infants (p < 0.01). Assessment of temporal regional lung filling showed that the posterior lung filled earlier than the anterior lung in the spontaneously breathing infants (p < 0.02) whereas in the in the ventilated infants the right lung filled before the left lung (p < 0.01).

CONCLUSIONS

In contrast to previous studies showing that ventilation is distributed to the nondependent lung in infants and children, this study shows that gravity has little effect on regional ventilation distribution.

Analysis of regional compliance in a porcine model of acute lung injury

Lung protective ventilation in acute lung injury (ALI) focuses on using low tidal volumes and adequate levels of positive end-expiratory pressure (PEEP). Identifying optimal pressure is difficult because pressure-volume (PV) relations differ regionally. Precise analysis demands local measurements of pressures and related alveolar morphologies. In a porcine model of surfactant depletion (n=24), we combined measuring static pressures with endoscopic microscopy and electrical impedance tomography (EIT) to examine regional PV loops and morphologic heterogeneities between healthy (control group; CON) and ALI lungs ventilated with low (LVT) or high tidal volumes (HVT). Quantification included indices for microscopy (Volume Air Index (VAI), Heterogeneity and Circularity Index), EIT analysis and calculation of regional compliances due to generated PV loops. We found that: (1) VAI decreased in lower lobe after ALI, (2) electrical impedance decreased in dorsal regions and (3) PV loops differed regionally. Further studies should prove the potentials of these techniques on individual respiratory settings and clinical outcome.

Three-dimensional simultaneous optical coherence tomography and confocal fluorescence microscopy for investigation of lung tissue

Although several strategies exist for a minimal-invasive treatment of patients with lung failure, the mortality rate of acute respiratory distress syndrome still reaches 30% at minimum. This striking number indicates the necessity of understanding lung dynamics on an alveolar level. To investigate the dynamical behavior on a microscale, we used three-dimensional geometrical and functional imaging to observe tissue parameters including alveolar size and length of embedded elastic fibers during ventilation. We established a combined optical coherence tomography (OCT) and confocal fluorescence microscopy system that is able to monitor the distension of alveolar tissue and elastin fibers simultaneously within three dimensions. The OCT system can laterally resolve a 4.9 μm line pair feature and has an approximately 11 μm full-width-half-maximum axial resolution in air. confocal fluorescence microscopy visualizes molecular properties of the tissue with a resolution of 0.75 μm (laterally), and 5.9 μm (axially) via fluorescence detection of the dye sulforhodamine B specifically binding to elastin. For system evaluation, we used a mouse model in situ to perform lung distension by application of different constant pressure values within the physiological regime. Our method enables the investigation of alveolar dynamics by helping to reveal basic processes emerging during artificial ventilation and breathing.

Effect of body position on ventilation distribution in preterm infants on continuous positive airway pressure

RATIONALE

Although continuous positive airway pressure is used extensively in neonatal intensive care units, and despite the belief that positioning is considered vital to the maintenance of good lung ventilation, no data exist on regional ventilation distribution in infants on continuous positive airway pressure ventilatory support.

OBJECTIVES

To investigate the effect of body position on regional ventilation in preterm infants on continuous positive airway pressure ventilatory support using electrical impedance tomography.

DESIGN

Randomized crossover study design.

SETTING

Neonatal intensive care unit.

PATIENTS

Twenty-four preterm infants on continuous positive airway pressure were compared to six spontaneously breathing preterm infants.

INTERVENTIONS

Random assignment of the order of the positions supine, prone, and quarter prone.

MEASUREMENTS AND RESULTS

Changes in global and regional lung volume were measured with electrical impedance tomography. Although there were no differences between positions, regional tidal volume was increased in the posterior compared with the anterior lung (p < .01) and in the right compared with the left lung (p < .03) in both the spontaneously breathing infants and in the infants on continuous positive airway pressure. The posterior lung filled earlier than the anterior lung in the spontaneously breathing infants (p < .02), whereas in the infants on continuous positive airway pressure the right lung filled before the left lung (p < .01). There was more ventilation inhomogeneity in the infants on continuous positive airway pressure than in the healthy infants (p < .01).

CONCLUSIONS

This study presents the first results on regional ventilation distribution in preterm infants on continuous positive airway pressure using electrical impedance tomography. Gravity had little impact on regional ventilation distribution in preterm infants on continuous positive airway pressure or in spontaneously breathing infants in the supine or prone position, indicating that ventilation distribution in preterm infants is not gravity-dependent but follows an anatomical pattern. AUSTRALIA NEW ZEALAND CLINICAL TRIALS REGISTRY:: ACTRN12606000210572.

Mechanobiology in lung epithelial cells: measurements, perturbations, and responses

Epithelial cells of the lung are located at the interface between the environment and the organism and serve many important functions including barrier protection, fluid balance, clearance of particulate, initiation of immune responses, mucus and surfactant production, and repair following injury. Because of the complex structure of the lung and its cyclic deformation during the respiratory cycle, epithelial cells are exposed to continuously varying levels of mechanical stresses. While normal lung function is maintained under these conditions, changes in mechanical stresses can have profound effects on the function of epithelial cells and therefore the function of the organ. In this review, we will describe the types of stresses and strains in the lungs, how these are transmitted, and how these may vary in human disease or animal models. Many approaches have been developed to better understand how cells sense and respond to mechanical stresses, and we will discuss these approaches and how they have been used to study lung epithelial cells in culture. Understanding how cells sense and respond to changes in mechanical stresses will contribute to our understanding of the role of lung epithelial cells during normal function and development and how their function may change in diseases such as acute lung injury, asthma, emphysema, and fibrosis.

In situ methods for assessing alveolar mechanics

Lung mechanics are an important determinant of physiological and pathophysiological lung function. Recent light microscopy studies of the intact lung have furthered the understanding of lung mechanics but used methodologies that may have introduced artifacts. To address this concern, we employed a short working distance water immersion objective to capture confocal images of a fluorescently labeled alveolar field on the costal surface of the isolated, perfused rat lung. Surface tension held a saline drop between the objective tip and the lung surface, such that the lung surface was unconstrained. For comparison, we also imaged with O-ring and coverslip; with O-ring, coverslip, and vacuum pressure; and without perfusion. Under each condition, we ventilated the lung and imaged the same region at the endpoints of ventilation. We found use of a coverslip caused a minimal enlargement of the alveolar field; additional use of vacuum pressure caused no further dimensional change; and absence of perfusion did not affect alveolar field dimension. Inflation-induced expansion was unaltered by methodology. In response to inflation, percent expansion was the same as recorded by all four alternative methods.

Quantitative imaging of alveolar recruitment with hyperpolarized gas MRI during mechanical ventilation

The aim of this study was to assess the utility of (3)He MRI to noninvasively probe the effects of positive end-expiratory pressure (PEEP) maneuvers on alveolar recruitment and atelectasis buildup in mechanically ventilated animals. Sprague-Dawley rats (n = 13) were anesthetized, intubated, and ventilated in the supine position ((4)He-to-O(2) ratio: 4:1; tidal volume: 10 ml/kg, 60 breaths/min, and inspiration-to-expiration ratio: 1:2). Recruitment maneuvers consisted of either a stepwise increase of PEEP to 9 cmH(2)O and back to zero end-expiratory pressure or alternating between these two PEEP levels. Diffusion MRI was performed to image (3)He apparent diffusion coefficient (ADC) maps in the middle coronal slices of lungs (n = 10). ADC was measured immediately before and after two recruitment maneuvers, which were separated from each other with a wait period (8-44 min). We detected a statistically significant decrease in mean ADC after each recruitment maneuver. The relative ADC change was -21.2 ± 4.1 % after the first maneuver and -9.7 ± 5.8 % after the second maneuver. A significant relative increase in mean ADC was observed over the wait period between the two recruitment maneuvers. The extent of this ADC buildup was time dependent, as it was significantly related to the duration of the wait period. The two postrecruitment ADC measurements were similar, suggesting that the lungs returned to the same state after the recruitment maneuvers were applied. No significant intrasubject differences in ADC were observed between the corresponding PEEP levels in two rats that underwent three repeat maneuvers. Airway pressure tracings were recorded in separate rats undergoing one PEEP maneuver (n = 3) and showed a significant relative difference in peak inspiratory pressure between pre- and poststates. These observations support the hypothesis of redistribution of alveolar gas due to recruitment of collapsed alveoli in presence of atelectasis, which was also supported by the decrease in peak inspiratory pressure after recruitment maneuvers.

Comparison of "open lung" modes with low tidal volumes in a porcine lung injury model

BACKGROUND

Ventilator strategies that maintain an "open lung" have shown promise in treating hypoxemic patients. We compared three "open lung" strategies with standard of care low tidal volume ventilation and hypothesized that each would diminish physiologic and histopathologic evidence of ventilator induced lung injury (VILI).

MATERIALS AND METHODS

Acute lung injury (ALI) was induced in 22 pigs via 5% Tween and 30-min of injurious ventilation. Animals were separated into four groups: (1) low tidal volume ventilation (LowVt -6 mL/kg); (2) high-frequency oscillatory ventilation (HFOV); (3) airway pressure release ventilation (APRV); or (4) recruitment and decremental positive-end expiratory pressure (PEEP) titration (RM+OP) and followed for 6 h. Lung and hemodynamic function was assessed on the half-hour. Bronchoalveolar lavage fluid (BALF) was analyzed for cytokines. Lung tissue was harvested for histologic analysis.

RESULTS

APRV and HFOV increased PaO(2)/FiO(2) ratio and improved ventilation. APRV reduced BALF TNF-α and IL-8. HFOV caused an increase in airway hemorrhage. RM+OP decreased SvO(2), increased PaCO(2), with increased inflammation of lung tissue.

CONCLUSION

None of the "open lung" techniques were definitively superior to LowVt with respect to VILI; however, APRV oxygenated and ventilated more effectively and reduced cytokine concentration compared with LowVt with nearly indistinguishable histopathology. These data suggest that APRV may be of potential benefit to critically ill patients but other "open lung" strategies may exacerbate injury.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

Gravity-dependent ventilation distribution in rats measured with electrical impedance tomography

Ventilation in larger animals and humans is gravity dependent and mainly distributed to the dependent lung. Little is known of the effect of gravity on ventilation distribution in small animals such as rodents. The aim of this study was to investigate gravity-dependent ventilation distribution and regional filling characteristics in rats. Ventilation distribution and regional lung filling were measured in six rats using electrical impedance tomography (EIT). Measurements were performed in four body positions (supine, prone, left and right lateral), and all animals were ventilated with increasing tidal volumes from 3 to 8 mL kg(-1). The effect of gravity on regional ventilation distribution was assessed with profiles of relative impedance change and calculation of the geometric centre. Regional filling was measured by calculating the slope of the plot of regional versus global relative impedance change on a breath-by-breath basis. Ventilation was significantly distributed to the non-dependent lung regardless of body position and tidal volume used. The geometric centre was located in the dependent lung in all but prone position. The regional filling characteristics followed an anatomical pattern with the posterior and the right lung generally filling faster. Gravity had little impact on regional filling. Ventilation distribution in rats is gravity dependent, whereas regional filling characteristics are dependent on anatomy.

Regional ventilation distribution in non-sedated spontaneously breathing newborns and adults is not different

BACKGROUND

In adults, ventilation is preferentially distributed towards the dependent lung. A reversal of the adult pattern has been observed in infants using radionuclide ventilation scanning. But these results have been obtained in infants and children with lung disease. In this study we investigate whether healthy infants have a similar reverse pattern of ventilation distribution.

STUDY DESIGN

Measurement of regional ventilation distribution in healthy newborn infants during non-REM sleep in comparison to adults.

METHODS

Twenty-four healthy newborns and 13 adults were investigated with electrical impedance tomography (EIT) in supine and prone position. Regional ventilation distribution was assessed with profiles of relative impedance change. The phase lag between dependent and non-dependent ventilation was calculated as a measure of asynchronous ventilation.

RESULTS

In newborns and adults the geometric center of ventilation was centrally located in the lung at 52.2 +/- 6.2% from anterior to posterior and at 50.5 +/- 14.7%, respectively. Using impedance profiles, ventilation was equally distributed to the dependent and non-dependent lung regions in newborns. Ventilation distribution in adults was similar. Phase lag characteristics of the impedance signal showed that infants had slower emptying of the dependent lung than adults.

CONCLUSION

The speculated reverse pattern of regional ventilation distribution in healthy infants compared to adults could not be demonstrated. Gravity had little effect on ventilation distribution in both infants and adults measured in supine and prone position.

Mechanical stretch decreases migration of alveolar epithelial cells through mechanisms involving Rac1 and Tiam1

Mechanical ventilation can overdistend the lungs or generate shear forces in them during repetitive opening/closing, contributing to lung injury and inflammation in patients with acute respiratory distress syndrome (ARDS). Repair of the injured lung epithelium is important for restoring normal barrier and lung function. In the current study, we investigated the effects of cyclic mechanical strain (CS), constant distention strain (CD), and simulated positive end-expiratory pressure (PEEP) on activation of Rac1 and wound closure of rat primary alveolar type 2 (AT2) cells. Cyclic stretch inhibited the migration of wounded AT2 cells in a dose-dependent manner with no inhibition occurring with 5% CS, but significant inhibition with 10% and 15% CS. PEEP conditions were investigated by stretching AT2 cells to 15% maximum strain (at a frequency of 10 cycles/min) with relaxation to 10% strain. AT2 cells were also exposed to 20% CD. All three types of mechanical strain inhibited wound closure of AT2 cells compared with static controls. Since lamellipodial extensions in migrating cells at the wound edge were significantly smaller in stretched cells, we measured Rac1 activity and found it to be decreased in stretched cells. We also demonstrate that Tiam1, a Rac1-specific guanine nucleotide exchange factor, was expressed mainly in the cytosol of AT2 cells exposed to mechanical strain compared with membrane localization in static cells. Downregulation of Tiam1 with 100 microM NSC-23766 inhibited activation of Rac1 and migration of AT2 cells, suggesting its involvement in repair mechanisms of AT2 cells subjected to mechanical strain.

Positive end-expiratory pressure in acute respiratory distress syndrome: should the 'open lung strategy' be replaced by a 'protective lung strategy'?

In patients with acute respiratory distress syndrome, positive end-expiratory pressure is associated with alveolar recruitment and lung hyperinflation despite the administration of a low tidal volume. The best positive end-expiratory pressure should correspond to the best compromise between recruitment and distension, a condition that coincides with the best respiratory elastance.

Alveolar dynamics during respiration: are the pores of Kohn a pathway to recruitment?

The change in alveolar size and number during the full breathing cycle in mammals remains unanswered, yet these descriptors are fundamental for understanding alveolar-based diseases and for improving ventilator management. Genetic and environmental mouse models are used increasingly to evaluate the evolution of disease in the peripheral lung; however, little is known regarding alveolar structure and function in the fresh, intact lung. Therefore, we have developed an optical confocal process to evaluate alveolar dynamics in the fresh intact mouse lung and as an initial experiment, have evaluated mouse alveolar dynamics during a single respiratory cycle immediately after passive lung deflation. We observe that alveoli become smaller and more numerous at the end of inspiration, and propose that this is direct evidence for alveolar recruitment in the mouse lung. The findings reported support a new hypothesis that requires recruitable secondary (daughter) alveoli to inflate via primary (mother) alveoli rather than from a conducting airway.

High-frequency percussive ventilation attenuates lung injury in a rabbit model of gastric juice aspiration

OBJECTIVE

To test the effects of high-frequency percussive ventilation (HFPV) compared with high-frequency oscillatory ventilation (HFOV) and low-volume conventional mechanical ventilation (LVCMV), on lung injury course in a gastric juice aspiration model.

DESIGN

Prospective, randomized, controlled, in-vivo animal study.

SETTING

University animal research laboratory.

SUBJECTS

Forty-three New Zealand rabbits.

INTERVENTIONS

Lung injury was induced by intratracheal instillation of human gastric juice in order to achieve profound hypoxaemia (PaO2/FIO2< or =50). Animals were ventilated for 4h after randomization in one of the following four groups: HFPV (median pressure 15cmH2O); LVCMV (VT 6mlkg(-1) and PEEP set to reach 15cmH2O plateau pressure); HFOV (mean pressure 15cmH2O); and a high-volume control group HVCMV (VT 12ml kg(-1) and ZEEP).

MEASUREMENTS AND RESULTS

Static respiratory compliance increased after the ventilation period in the HFPV, LVMCV and HFOV groups, in contrast with the HVCMV group. PaO2/FIO2 improved similarly in the HFPV, LVCMV and HFOV groups, and remained lower in the HVCMV group than in the three others. Lung oedema, myeloperoxidase and histological lung injury score were higher in the HVCMV group, but not different among all others. Arterial lactate markedly increased after 4h of ventilation in the HVCMV group, while lower but similar levels were observed in the three other groups.

CONCLUSION

HFPV, like HFOV and protective CMV, improves respiratory mechanics and oxygenation, and attenuates lung damage. The HFPV provides attractive lung protection, but further studies should confirm these results before introducing HFPV into the clinical arena.

Correlation between alveolar recruitment /derecruitment and inflection points on the pressure-volume curve

OBJECTIVE

To determine whether individual alveolar recruitment/derecruitment (R/D) is correlated with the lower and upper inflections points on the inflation and deflation limb of the whole-lung pressure-volume (P-V) curve.

DESIGN AND SETTING

Prospective experimental study in an animal research laboratory.

SUBJECTS

Five anesthetized rats subjected to saline-lavage lung injury.

INTERVENTIONS

Subpleural alveoli were filmed continuously using an in vivo microscope during the generation of a whole-lung P-V curve using the super syringe technique. Alveolar R/D was correlated to the calculated inflection points on both limbs of the P-V curve.

MEASUREMENTS AND RESULTS

There was continual alveolar recruitment along the entire inflation limb in all animals. There was some correlation (R2=0.898) between the pressure below which microscopic derecruitment was observed and the upper inflection point on the deflation limb. No correlation was observed between this pressure and the lower inflection point on the inflation limb.

CONCLUSIONS

In this physiological experiment in lungs with pure surfactant deactivation we found that individual alveolar recruitment measured by direct visualization was not correlated with the lower inflection point on inflation whereas alveolar derecruitment was correlated with alveolar derecruitment on deflation. These data suggest that inflection points on the P-V curve do not always represent a change in alveolar number.

Lung Protective Ventilation Strategies: Have We Applied Them in Trauma Patients at Risk for Acute Lung Injury and Acute Respiratory Distress Syndrome?

Lung protective ventilation strategies for patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are well documented, and many medical centers fail to apply these strategies in ALI/ARDS. The objective of this study was to determine if we apply these strategies in trauma patients at risk for ALI/ARDS. We undertook a retrospective review of trauma patients mechanically ventilated for ≥4 days with an ICD-9 for traumatic pneumothorax, hemothorax, lung contusion, and/or fractured ribs admitted from May 1, 1999 through April 30, 2000 (Group 1), the pre-ARDS Network study, and from May 1, 2003 through April 30, 2004 (Group 2), the post-ARDS Network study. Tidal volume (VT)/kg admission body weight, VT/kg ideal body weight (IBW), and plateau and peak pressures were analyzed with respect to mortality. VT/Kg admission body weight and IBW were significantly reduced when comparing Group 1 with Group 2 (9.27 to 8.03 and 11.67 to 10.04, respectively). VT/kg IBW was greater ( P < 0.01) for patients who died in Group 1 (13.81) compared with patients who lived (10.29) or died (9.89) in Group 2. Peak and plateau pressures were greater ( P < 0.01) in patients who died in Group 1 than patients who lived or died in Group 2. A strict ARDS Network ventilation strategy (VT < 6 mL/kg) is not followed, rather a low plateau/peak pressure strategy is used, which is a form of lung protective ventilation.

Injurious mechanical ventilation in the normal lung causes a progressive pathologic change in dynamic alveolar mechanics

INTRODUCTION

Acute respiratory distress syndrome causes a heterogeneous lung injury, and without protective mechanical ventilation a secondary ventilator-induced lung injury can occur. To ventilate noncompliant lung regions, high inflation pressures are required to 'pop open' the injured alveoli. The temporal impact, however, of these elevated pressures on normal alveolar mechanics (that is, the dynamic change in alveolar size and shape during ventilation) is unknown. In the present study we found that ventilating the normal lung with high peak pressure (45 cmH(2)0) and low positive end-expiratory pressure (PEEP of 3 cmH(2)O) did not initially result in altered alveolar mechanics, but alveolar instability developed over time.

METHODS

Anesthetized rats underwent tracheostomy, were placed on pressure control ventilation, and underwent sternotomy. Rats were then assigned to one of three ventilation strategies: control group (n = 3, P control = 14 cmH(2)O, PEEP = 3 cmH(2)O), high pressure/low PEEP group (n = 6, P control = 45 cmH(2)O, PEEP = 3 cmH(2)O), and high pressure/high PEEP group (n = 5, P control = 45 cmH(2)O, PEEP = 10 cmH(2)O). In vivo microscopic footage of subpleural alveolar stability (that is, recruitment/derecruitment) was taken at baseline and than every 15 minutes for 90 minutes following ventilator adjustments. Alveolar recruitment/derecruitment was determined by measuring the area of individual alveoli at peak inspiration (I) and end expiration (E) by computer image analysis. Alveolar recruitment/derecruitment was quantified by the percentage change in alveolar area during tidal ventilation (%I - E Delta).

RESULTS

Alveoli were stable in the control group for the entire experiment (low %I - E Delta). Alveoli in the high pressure/low PEEP group were initially stable (low %I - E Delta), but with time alveolar recruitment/derecruitment developed. The development of alveolar instability in the high pressure/low PEEP group was associated with histologic lung injury.

CONCLUSION

A large change in lung volume with each breath will, in time, lead to unstable alveoli and pulmonary damage. Reducing the change in lung volume by increasing the PEEP, even with high inflation pressure, prevents alveolar instability and reduces injury. We speculate that ventilation with large changes in lung volume over time results in surfactant deactivation, which leads to alveolar instability.

Effect of positive end-expiratory pressure and tidal volume on lung injury induced by alveolar instability

INTRODUCTION

One potential mechanism of ventilator-induced lung injury (VILI) is due to shear stresses associated with alveolar instability (recruitment/derecruitment). It has been postulated that the optimal combination of tidal volume (Vt) and positive end-expiratory pressure (PEEP) stabilizes alveoli, thus diminishing recruitment/derecruitment and reducing VILI. In this study we directly visualized the effect of Vt and PEEP on alveolar mechanics and correlated alveolar stability with lung injury.

METHODS

In vivo microscopy was utilized in a surfactant deactivation porcine ARDS model to observe the effects of Vt and PEEP on alveolar mechanics. In phase I (n = 3), nine combinations of Vt and PEEP were evaluated to determine which combination resulted in the most and least alveolar instability. In phase II (n = 6), data from phase I were utilized to separate animals into two groups based on the combination of Vt and PEEP that caused the most alveolar stability (high Vt [15 cc/kg] plus low PEEP [5 cmH2O]) and least alveolar stability (low Vt [6 cc/kg] and plus PEEP [20 cmH2O]). The animals were ventilated for three hours following lung injury, with in vivo alveolar stability measured and VILI assessed by lung function, blood gases, morphometrically, and by changes in inflammatory mediators.

RESULTS

High Vt/low PEEP resulted in the most alveolar instability and lung injury, as indicated by lung function and morphometric analysis of lung tissue. Low Vt/high PEEP stabilized alveoli, improved oxygenation, and reduced lung injury. There were no significant differences between groups in plasma or bronchoalveolar lavage cytokines or proteases.

CONCLUSION

A ventilatory strategy employing high Vt and low PEEP causes alveolar instability, and to our knowledge this is the first study to confirm this finding by direct visualization. These studies demonstrate that low Vt and high PEEP work synergistically to stabilize alveoli, although increased PEEP is more effective at stabilizing alveoli than reduced Vt. In this animal model of ARDS, alveolar instability results in lung injury (VILI) with minimal changes in plasma and bronchoalveolar lavage cytokines and proteases. This suggests that the mechanism of lung injury in the high Vt/low PEEP group was mechanical, not inflammatory in nature.

Lung-protective ventilation in neurosurgical patients

PURPOSE OF REVIEW

Concepts of ventilator-induced lung injury have revolutionized our approach to the ventilatory management of patients with acute lung injury and acute respiratory distress syndrome over the past 10 years. The extension of these principles to patients with brain injuries is challenging, as many of them are out of keeping with usual brain-protective management.

RECENT FINDINGS

Many patients with acute lung injury or acute respiratory distress syndrome and an acute brain injury may in fact be managed safely within the confines of a lung-protective strategy. Elevated levels of positive end-expiratory pressure in head-injured patients with acute lung injury or acute respiratory distress syndrome also appear to be safe, particularly when the level is set below that of the intracranial pressure, when patients have a low respiratory system compliance, or when positive end-expiratory pressure results in significant lung volume recruitment. Several novel therapies to minimize ventilator-induced lung injury are currently in the early stages of investigation in neurosurgical patients.

SUMMARY

In many patients with brain injuries and acute lung injury the goals of lung protection can be achieved without threatening cerebral perfusion. In patients with more refractory raised intracranial pressure the optimal balance between brain and lung is not well established. Further research is needed on lung-protective strategies in this vulnerable population.

Physiological consequences of early-life insult

The most commonly observed severe lung injuries in early life are the respiratory distress syndrome in premature infants and the acute respiratory distress syndrome in children. Both diseases are characterised by alveolar instability, fluid filled airspace and some degree of airway obstruction. In the acute phase, collapsed alveoli can be reopened with positive end-expiratory pressure and lung recruitment. New insight into the physiology of lung recruitment suggests that the shape of the pressure-volume curve is defined by the change in rate of alveolar opening and closing. Reduced lung volumes and severe ventilation maldistribution are found in the acute phase but may persist during childhood. Any severe lung injury in this early phase of life can cause significant structural and functional damage to the developing lung. Follow-up studies of children with chronic lung disease have shown that the functional abnormalities will improve but may still be present in later childhood.

Pulmonary impedance and alveolar instability during injurious ventilation in rats

The mechanical derangements in the acutely injured lung have long been ascribed, in large part, to altered mechanical function at the alveolar level. This has not been directly demonstrated, however, so we investigated the issue in a rat model of overinflation injury. After thoracotomy, rats were mechanically ventilated with either 1) high tidal volume (Vt) or 2) low Vt with periodic deep inflations (DIs). Forced oscillations were used to measure pulmonary impedance every minute, from which elastance ( H) and hysteresivity (η) were derived. Subpleural alveoli were imaged every 15 min using in vivo video microscopy. Cross-sectional areas of individual alveoli were measured at peak inspiration and end exhalation, and the percent change was used as an index of alveolar instability (%I-EΔ). Low Vt never led to an increase in %I-EΔ but did result in progressive atelectasis that coincided with an increase in H but not η. DI reversed atelectasis due to low Vt, returning H to baseline. %I-EΔ, H, and η all began to rise by 30 min of high Vt and were not reduced by DI. We conclude that simultaneous increases in both H and η are reflective of lung injury in the form of alveolar instability, whereas an isolated and reversible increase in H during low Vt reflects merely derecruitment of alveoli.

Cellular stress failure in ventilator-injured lungs

The clinical and experimental literature has unequivocally established that mechanical ventilation with large tidal volumes is injurious to the lung. However, uncertainty about the micromechanics of injured lungs and the numerous degrees of freedom in ventilator settings leave many unanswered questions about the biophysical determinants of lung injury. In this review we focus on experimental evidence for lung cells as injury targets and the relevance of these studies for human ventilator-associated lung injury. In vitro, the stress-induced mechanical interactions between matrix and adherent cells are important for cellular remodeling as a means for preventing compromise of cell structure and ultimately cell injury or death. In vivo, these same principles apply. Large tidal volume mechanical ventilation results in physical breaks in alveolar epithelial and endothelial plasma membrane integrity and subsequent triggering of proinflammatory signaling cascades resulting in the cytokine milieu and pathologic and physiologic findings of ventilator-associated lung injury. Importantly, though, alveolar cells possess cellular repair and remodeling mechanisms that in addition to protecting the stressed cell provide potential molecular targets for the prevention and treatment of ventilator-associated lung injury in the future.

Laminin-6 assembles into multimolecular fibrillar complexes with perlecan and participates in mechanical-signal transduction via a dystroglycan-dependent, integrin-independent mechanism

Mechanical ventilation is a valuable treatment regimen for respiratory failure. However, mechanical ventilation (especially with high tidal volumes) is implicated in the initiation and/or exacerbation of lung injury. Hence, it is important to understand how the cells that line the inner surface of the lung [alveolar epithelial cells (AECs)] sense cyclic stretching. Here, we tested the hypothesis that matrix molecules, via their interaction with surface receptors, transduce mechanical signals in AECs. We first determined that rat AECs secrete an extracellular matrix (ECM) rich in anastamosing fibers composed of the alpha3 laminin subunit, complexed with beta1 and gamma1 laminin subunits (i.e. laminin-6), and perlecan by a combination of immunofluorescence microscopy and immunoblotting analyses. The fibrous network exhibits isotropic expansion when exposed to cyclic stretching (30 cycles per minute, 10% strain). Moreover, this same stretching regimen activates mitogen-activated-protein kinase (MAPK) in AECs. Stretch-induced MAPK activation is not inhibited in AECs treated with antagonists to alpha3 or beta1 integrin. However, MAPK activation is significantly reduced in cells treated with function-inhibiting antibodies against the alpha3 laminin subunit and dystroglycan, and when dystroglycan is knocked down in AECs using short hairpin RNA. In summary, our results support a novel mechanism by which laminin-6, via interaction with dystroglycan, transduces a mechanical signal initiated by stretching that subsequently activates the MAPK pathway in rat AECs. These results are the first to indicate a function for laminin-6. They also provide novel insight into the role of the pericellular environment in dictating the response of epithelial cells to mechanical stimulation and have broad implications for the pathophysiology of lung injury.

Lung Development and Susceptibility to Ventilator-induced Lung Injury

RATIONALE

Ventilator-induced lung injury has been predominantly studied in adults.

OBJECTIVES

To explore the effects of age and lung development on susceptibility to such injury.

METHODS

Ex vivo isolated nonperfused rat lungs (infant, juvenile, and adult) were mechanically ventilated where VT was based on milliliters per kilogram of body weight or as a percentage of the measured total lung capacity (TLC). In vivo anesthetized rats (infant, adult) were mechanically ventilated with pressure-limited VTs. Allocation to ventilation strategy was randomized.

MEASUREMENTS

Ex vivo injury was assessed by pressure-volume analysis, reduction in TLC, and histology, and in vivo injury by lung compliance, cytokine production, and wet- to dry-weight ratio.

MAIN RESULTS

Ex vivo ventilation (VT 30 ml.kg(-1)) resulted in a significant reduction (36.0 +/- 10.1%, p < 0.05) in TLC in adult but not in infant lungs. Ex vivo ventilation (VT 50% TLC) resulted in a significant reduction in TLC in both adult (27.8 +/- 2.8%) and infant (10.6 +/- 7.0%) lungs, but more so in the adult lungs (p < 0.05); these changes were paralleled by histology and pressure-volume characteristics. After high stretch in vivo ventilation, adult but not infant rats developed lung injury (total lung compliance, wet/dry ratio, tumor necrosis factor alpha). Surface video microscopy demonstrated greater heterogeneity of alveolar distension in ex vivo adult versus infant lungs.

CONCLUSION

These data provide ex vivo and in vivo evidence that comparable ventilator settings are significantly more injurious in the adult than infant rat lung, probably reflecting differences in intrinsic susceptibility or inflation pattern.

Dynamic alveolar mechanics and ventilator-induced lung injury

OBJECTIVES

To review the mechanism of dynamic alveolar mechanics (i.e., the dynamic change in alveolar size and shape during ventilation) in both the normal and acutely injured lung; to investigate the alteration in alveolar mechanics secondary to acute lung injury as a mechanism of ventilator-induced lung injury (VILI); and to examine the hypothesis that the reduced morbidity and mortality associated with protective strategies of mechanical ventilation is related to the normalization of alveolar mechanics.

DATA EXTRACTION AND SYNTHESIS

This review is based on original published articles and review papers dealing with the mechanism of lung volume change at the alveolar level and the role of altered alveolar mechanics as a mechanism of VILI. In addition, data from our laboratory directly visualizing dynamic alveolar mechanics is reviewed and related to the literature.

CONCLUSIONS

The mechanism of alveolar inflation in normal lungs is unclear. Nonetheless, normal alveoli are very stable and change size very little with ventilation. Acute lung injury causes marked destabilization of individual alveoli. Alveolar instability causes pulmonary damage and is believed to be a major component in the mechanism of VILI. Ventilator strategies that reduce alveolar instability may potentially reduce the morbidity and mortality associated with VILI.

Lung volumes and alveolar expansion pattern in immature rabbits treated with serum-diluted surfactant

In acute respiratory distress syndrome, mechanical ventilation often induces alveolar overdistension aggravating the primary insult. To examine the mechanism of overdistension, surfactant-deficient immature rabbits were anesthetized with pentobarbital sodium, and their lungs were treated with serum-diluted modified natural surfactant (porcine lung extract; 2 mg/ml, 10 ml/kg). By mechanical ventilation with a peak inspiration pressure of 22.5 cm H2O, the animals had a tidal volume of 14.7 ml/kg (mean), when 2.5 cm H2O positive end-expiratory pressure was added. This volume was similar to that in animals treated with nondiluted modified natural surfactant (24 mg/ml in Ringer solution, 10 ml/kg). However, the lungs fixed at 10 cm H2O on the deflation limbs of the pressure-volume curve had the largest alveolar/alveolar duct profiles (> or =48,000 microm2), accounting for 38% of the terminal air spaces, and the smallest (<6,000 microm2), accounting for 31%. These values were higher than those in animals treated with nondiluted modified natural surfactant (P <0.05). We conclude that administration of serum-diluted surfactant to immature neonatal lungs leads to patchy overdistension of terminal air spaces, similar to the expansion pattern that may be seen after dilution of endogenous surfactant with proteinaceous edema fluid in acute respiratory distress syndrome.

Alveolar instability causes early ventilator-induced lung injury independent of neutrophils

Intratracheal instillation of Tween causes a heterogeneous surfactant deactivation in the lung, with areas of unstable alveoli directly adjacent to normal stable alveoli. We employed in vivo video microscopy to directly assess alveolar stability in normal and surfactant-deactivated lung and tested our hypothesis that alveolar instability causes a mechanical injury, initiating an inflammatory response that results in a secondary neutrophil-mediated proteolytic injury. Pigs were mechanically ventilated (VT 10 cc/kg, positive end-expiratory pressure [PEEP] 3 cm H2O), randomized to into three groups, and followed for 4 hours: Control group (n = 3) surgery only; Tween group (n = 4) subjected to intratracheal Tween (surfactant deactivator causing alveolar instability); and Tween + PEEP group (n = 4) subjected to Tween with increased PEEP (15 cm H2O) to stabilize alveoli. The magnitude of alveolar instability was quantified by computer image analysis. Surfactant-deactivated lungs developed significant histopathology only in lung areas with unstable alveoli without an increase in neutrophil-derived proteases. PEEP stabilized alveoli and significantly reduced histologic evidence of lung injury. Thus, in this model, alveolar instability can independently cause ventilator-induced lung injury. To our knowledge, this is the first study to directly confirm that unstable alveoli are subjected to ventilator-induced lung injury whereas stable alveoli are not.