Early Endothelial Signaling Transduction in Developing Lung Edema.

The lung promptly responds to edemagenic conditions through functional adaptations that contrast the increase in microvascular filtration. This review presents evidence for early signaling transduction by endothelial lung cells in two experimental animal models of edema, hypoxia exposure, and fluid overload (hydraulic edema). The potential role of specialized sites of the plasma membranes considered mobile signaling platforms, referred to as membrane rafts, that include caveolae and lipid rafts, is presented. The hypothesis is put forward that early changes in the lipid composition of the bilayer of the plasma membrane might trigger the signal transduction process when facing changes in the pericellular microenvironment caused by edema. Evidence is provided that for an increase in the extravascular lung water volume not exceeding 10%, changes in the composition of the plasma membrane of endothelial cells are evoked in response to mechanical stimuli from the interstitial compartment as well as chemical stimuli relating with changes in the concentration of the disassembled portions of structural macromolecules. In hypoxia, thinning of endothelial cells, a decrease in caveolae and AQP-1, and an increase in lipid rafts are observed. The interpretation of this response is that it favors oxygen diffusion and hinder trans-cellular water fluxes. In hydraulic edema, which generates greater capillary water leakages, an increase in cell volume and opposite changes in membrane rafts were observed; further, the remarkable increase in caveolae suggests a potential abluminal-luminal vesicular-dependent fluid reabsorption.

The impact of heterogeneity of the air-blood barrier on control of lung extravascular water and alveolar gas exchange.

The architecture of the air-blood barrier is effective in optimizing the gas exchange as long as it retains its specific feature of extreme thinness reflecting, in turn, a strict control on the extravascular water to be kept at minimum. Edemagenic conditions may perturb this equilibrium by increasing microvascular filtration; this characteristically occurs when cardiac output increases to balance the oxygen uptake with the oxygen requirement such as in exercise and hypoxia (either due to low ambient pressure or reflecting a pathological condition). In general, the lung is well equipped to counteract an increase in microvascular filtration rate. The loss of control on fluid balance is the consequence of disruption of the integrity of the macromolecular structure of lung tissue. This review, merging data from experimental approaches and evidence in humans, will explore how the heterogeneity in morphology, mechanical features and perfusion of the terminal respiratory units might impact on lung fluid balance and its control. Evidence is also provided that heterogeneities may be inborn and they could actually get worse as a consequence of a developing pathological process. Further, data are presented how in humans inter-individual heterogeneities in morphology of the terminal respiratory hinder the control of fluid balance and, in turn, hamper the efficiency of the oxygen diffusion-transport function.

A century of exercise physiology: lung fluid balance during and following exercise.

PURPOSE: This review recalls the principles developed over a century to describe trans-capillary fluid exchanges concerning in particular the lung during exercise, a specific condition where dyspnea is a leading symptom, the question being whether this symptom simply relates to fatigue or also implies some degree of lung edema. METHOD: Data from experimental models of lung edema are recalled aiming to: (1) describe how extravascular lung water is strictly controlled by "safety factors" in physiological conditions, (2) consider how waning of "safety factors" inevitably leads to development of lung edema, (3) correlate data from experimental models with data from exercising humans. RESULTS: Exercise is a strong edemagenic condition as the increase in cardiac output leads to lung capillary recruitment, increase in capillary surface for fluid exchange and potential increase in capillary pressure. The physiological low microvascular permeability may be impaired by conditions causing damage to the interstitial matrix macromolecular assembly leading to alveolar edema and haemorrhage. These conditions include hypoxia, cyclic alveolar unfolding/folding during hyperventilation putting a tensile stress on septa, intensity and duration of exercise as well as inter-individual proneness to develop lung edema. CONCLUSION: Data from exercising humans showed inter-individual differences in the dispersion of the lung ventilation/perfusion ratio and increase in oxygen alveolar-capillary gradient. More recent data in humans support the hypothesis that greater vasoconstriction, pulmonary hypertension and slower kinetics of alveolar-capillary O2 equilibration relate with greater proneness to develop lung edema due higher inborn microvascular permeability possibly reflecting the morpho-functional features of the air-blood barrier.

Role of the Air-Blood Barrier Phenotype in Lung Oxygen Uptake and Control of Extravascular Water.

The air blood barrier phenotype can be reasonably described by the ratio of lung capillary blood volume to the diffusion capacity of the alveolar membrane (Vc/Dm), which can be determined at rest in normoxia. The distribution of the Vc/Dm ratio in the population is normal; Vc/Dm shifts from ∼1, reflecting a higher number of alveoli of smaller radius, providing a high alveolar surface and a limited extension of the capillary network, to just opposite features on increasing Vc/Dm up to ∼6. We studied the kinetics of alveolar-capillary equilibration on exposure to edemagenic conditions (work at ∼60% maximum aerobic power) in hypoxia (HA) (PIO2 90 mmHg), based on an estimate of time constant of equilibration (τ) and blood capillary transit time (Tt). A shunt-like effect was described for subjects having a high Vc/Dm ratio, reflecting a longer τ (>0.5 s) and a shorter Tt (<0.8 s) due to pulmonary vasoconstriction and a larger increase in cardiac output (>3-fold). The tendency to develop lung edema in edemagenic conditions (work in HA) was found to be directly proportional to the value of Vc/Dm as suggested by an estimate of the mechanical properties of the respiratory system with the forced frequency oscillation technique.

Pulmonary Interstitial Matrix and Lung Fluid Balance From Normal to the Acutely Injured Lung.

This review analyses the mechanisms by which lung fluid balance is strictly controlled in the air-blood barrier (ABB). Relatively large trans-endothelial and trans-epithelial Starling pressure gradients result in a minimal flow across the ABB thanks to low microvascular permeability aided by the macromolecular structure of the interstitial matrix. These edema safety factors are lost when the integrity of the interstitial matrix is damaged. The result is that small Starling pressure gradients, acting on a progressively expanding alveolar barrier with high permeability, generate a high transvascular flow that causes alveolar flooding in minutes. We modeled the trans-endothelial and trans-epithelial Starling pressure gradients under control conditions, as well as under increasing alveolar pressure (Palv) conditions of up to 25 cmH2O. We referred to the wet-to-dry weight (W/D) ratio, a specific index of lung water balance, to be correlated with the functional state of the interstitial structure. W/D averages ∼5 in control and might increase by up to ∼9 in severe edema, corresponding to ∼70% loss in the integrity of the native matrix. Factors buffering edemagenic conditions include: (i) an interstitial capacity for fluid accumulation located in the thick portion of ABB, (ii) the increase in interstitial pressure due to water binding by hyaluronan (the "safety factor" opposing the filtration gradient), and (iii) increased lymphatic flow. Inflammatory factors causing lung tissue damage include those of bacterial/viral and those of sterile nature. Production of reactive oxygen species (ROS) during hypoxia or hyperoxia, or excessive parenchymal stress/strain [lung overdistension caused by patient self-induced lung injury (P-SILI)] can all cause excessive inflammation. We discuss the heterogeneity of intrapulmonary distribution of W/D ratios. A W/D ∼6.5 has been identified as being critical for the transition to severe edema formation. Increasing Palv for W/D > 6.5, both trans-endothelial and trans-epithelial gradients favor filtration leading to alveolar flooding. Neither CT scan nor ultrasound can identify this initial level of lung fluid balance perturbation. A suggestion is put forward to identify a non-invasive tool to detect the earliest stages of perturbation of lung fluid balance before the condition becomes life-threatening.

Differences in alveolo-capillary equilibration in healthy subjects on facing O2 demand.

Oxygen diffusion across the air-blood barrier in the lung is commensurate with metabolic needs and ideally allows full equilibration between alveolar and blood partial oxygen pressures. We estimated the alveolo-capillary O2 equilibration in 18 healthy subjects at sea level at rest and after exposure to increased O2 demand, including work at sea level and on hypobaric hypoxia exposure at 3840 m (PA ~ 50 mmHg). For each subject we estimated O2 diffusion capacity (DO2), pulmonary capillary blood volume (Vc) and cardiac output ([Formula: see text]). We derived blood capillary transit time [Formula: see text] and the time constant of the equilibration process ([Formula: see text], ß being the slope of the hemoglobin dissociation curve). O2 equilibration at the arterial end of the pulmonary capillary was defined as [Formula: see text]. Leq greately differed among subjects in the most demanding O2 condition (work in hypoxia): lack of full equilibration was found to range from 5 to 42% of the alveolo-capillary PO2 gradient at the venous end. The present analysis proves to be sensible enough to highlight inter-individual differences in alveolo-capillary equilibration among healthy subjects.

Rodolfo Margaria and the First Walk on the Moon.

INTRODUCTION: During the Cold War years, the Space Race was largely supported by the efforts of many engineers and scientists, in particular human physiologists. Rodolfo Margaria (1901-1983), director of the Institute of Human Physiology at the University of Milan, was one of the most eminent and focused his studies on the mechanics of human locomotion in subgravity, in particular on the Moon's surface. Long before the real Moon landing, Margaria was able to correctly theorize how astronauts would walk on lunar soil, what would be the optimal pattern of progression, as well as determine the optimum and maximum speed at one-sixth of the Earth's gravity. On 21st July 1969 at 02:56 UTC, great excitement was aroused by the television images of Neil Armstrong's first steps on the Moon. Instead of walking, he moved around making small leaps, as expected from Margaria and colleagues.Grasso GS, Beretta EP, Miserocchi GA, Riva MA. Rodolfo Margaria and the first walk on the Moon. Aerosp Med Hum Perform. 2019; 90(11):982-985.

Understanding Vasomotion of Lung Microcirculation by In Vivo Imaging.

The balance of lung extravascular water depends upon the control of blood flow in the alveolar distribution vessels that feed downstream two districts placed in parallel, the corner vessels and the alveolar septal network. The occurrence of an edemagenic condition appears critical as an increase in extravascular water endangers the thinness of the air-blood barrier, thus negatively affecting the diffusive capacity of the lung. We exposed anesthetized rabbits to an edemagenic factor (12% hypoxia) for 120 min and followed by in vivo imaging the micro-vascular morphology through a "pleural window" using a stereo microscope at a magnification of 15× (resolution of 7.2 µm). We measured the change in diameter of distribution vessels (50-200 µm) and corner vessels (<50 µm). On average, hypoxia caused a significant decrease in diameter of both smaller distribution vessels (about ~50%) and corner vessels (about ~25%) at 30 min. After 120 min, reperfusion occurred. Regional differences in perivascular interstitial volume were observed and could be correlated with differences in blood flow control. To understand such difference, we modelled imaged alveolar capillary units, obtained by Voronoi method, integrating microvascular pressure parameters with capillary filtration. Results of the analysis suggested that at 120 min, alveolar blood flow was diverted to the corner vessels in larger alveoli, which were found also to undergo a greater filtration indicating greater proneness to develop lung edema.

Tissue specificity of mitochondrial adaptations in rats after 4 weeks of normobaric hypoxia.

PURPOSE: Exposure to hypoxia has been suggested to activate multiple adaptive pathways so that muscles are better able to maintain cellular energy homeostasis. However, there is limited research regarding the tissue specificity of this response. The aim of this study was to investigate the influence of tissue specificity on mitochondrial adaptations of rat skeletal and heart muscles after 4 weeks of normobaric hypoxia (FiO2: 0.10). METHODS: Twenty male Wistar rats were randomly assigned to either normobaric hypoxia or normoxia. Mitochondrial respiration was determined in permeabilised muscle fibres from left and right ventricles, soleus and extensorum digitorum longus (EDL). Citrate synthase activity and the relative abundance of proteins associated with mitochondrial biogenesis were also analysed. RESULTS: After hypoxia exposure, only the soleus and left ventricle (both predominantly oxidative) presented a greater maximal mass-specific respiration (+48 and +25%, p < 0.05) and mitochondrial-specific respiration (+75 and +28%, p < 0.05). Citrate synthase activity was higher in the EDL (0.63 ± 0.08 vs 0.41 ± 0.10 µmol min- 1 µg- 1) and lower in the soleus (0.65 ± 0.17 vs 0.87 ± 0.20 µmol min- 1 µg- 1) in hypoxia with respect to normoxia. There was a lower relative protein abundance of PGC-1α (-25%, p < 0.05) in the right ventricle and a higher relative protein abundance of PGC-1ß (+43%, p < 0.05) in the left ventricle of rats exposed to hypoxia, with few differences for protein abundance in the other muscles. CONCLUSION: Our results show a muscle-specific response to 4 weeks of normobaric hypoxia. Depending on fibre type, and the presence of ventricular hypertrophy, muscles respond differently to the same degree of environmental hypoxia.

The Elusive Path of Brain Tissue Oxygenation and Cerebral Perfusion in Harness Hang Syncope in Mountain Climbers.

Lanfranconi, Francesca, Luca Pollastri, Giovanni Corna, Manuela Bartesaghi, Massimiliano Novarina, Alessandra Ferri, and Giuseppe Andrea Miserocchi. The elusive path of brain tissue oxygenation and cerebral perfusion in harness hang syncope in mountain climbers. High Alt Med Biol. 18:363-371, 2017. AIM: Harness hang syncope (HHS) is a risk that specifically affects wide ranges of situations requiring safety harnesses in mountains. An irreversible orthostatic stasis could lead to death if a prompt rescue is not performed. We aimed at evaluating the risk of developing HHS and at identifying the characteristics related to the pathogenesis of HHS. RESULTS: Forty adults (aged 39.1 [8.2] years) were enrolled in a suspension test lasting about 28.7 (11.4) minutes. We measured cardiovascular parameters, and near infrared spectroscopy (NIRS) was used to assess cerebral hypoxia by changes in the concentration of oxyhemoglobin (Δ[HbO2]) and de-oxyhemoglobin (Δ[HHb]). In the four participants who developed HHS: (1) systolic and diastolic blood pressure showed ample oscillations with a final abrupt drop (∼30 mmHg); (2) Δ[HbO2] increased after 8-12 minutes of suspension and reached a plateau before HHS; and (3) Δ[HHb] decreased with a final abrupt increase before syncope. CONCLUSIONS: Participants who developed HHS failed to activate cardiovascular reflexes that usually safeguard O2 availability to match the metabolic needs of the brain tissue. Since cerebral hypoxia was detected as an early phenomenon by Δ[HbO2] and Δ[HHb] changes, NIRS measurement appears to be the most important parameter to monitor the onset of HHS.

Air blood barrier phenotype correlates with alveolo-capillary O2 equilibration in hypobaric hypoxia.

The O2 diffusion limitation across the air blood barrier (DO2 and subcomponents Dm and Vc) was evaluated in 17 healthy participants exposed to hypobaric hypoxia (HA, 3840m, PIO2 ∼90mmHg). A 10% decrease in alveolar volume (VA) in all participants suggested the development of sub-clinical interstitial lung edema. In >80% of participants DO2/VA increased, reflecting an individual strategy to cope with the hypoxia stimulus by remodulating Vc or Dm. Opposite changes in Dm/Vc ratio were observed and participants decreasing Vc showed reduced alveolar blood capillary transit time. The interplay between diffusion and perfusion (cardiac output) was estimated in order to investigate the individual adaptive response to hypoxia. It appears remarkable that despite individual differences in the adaptive response to HA, diffusion limitation did not exceed ∼11% of the alveolar-venous PO2 gradient, revealing an admirable functional design of the air-blood barrier to defend the O2 diffusion/perfusion function when facing hypobaric hypoxia corresponding to 50mmHg decreased PAO2.

Reappraisal of DLCO adjustment to interpret the adaptive response of the air-blood barrier to hypoxia.

DLCO measured in hypoxia must be corrected due to the higher affinity (increase in coefficient θ) of CO with Hb. We propose an adjustment accounting for individual changes in the equation relating DLCO to subcomponents Dm (membrane diffusive capacity) and Vc (lung capillary volume): 1/DLCO=1/Dm+1/θVc. We adjusted the individual DLCO measured in hypoxia (HA, 3269m) by interpolating the 1/DLCO to the sea level (SL) 1/θ value. Nineteen healthy subjects were studied at SL and HA. Based on the proposed adjustment, DLCO increased in HA in 53% of subjects, reflecting the increase in Dm that largely overruled the decrease in Vc. We hypothesize that a decrease in Vc (buffering microvascular filtration) and the increase in Dm (possibly resulting from a decrease in thickness of the air-blood barrier) represent the anti-edemagenic adaptation of the lung to hypoxia exposure. The efficiency of this adaptation varied among subjects as DLCO did not change in 31% of subjects and decreased in 16%.

Computational micro-scale model of control of extravascular water and capillary perfusion in the air blood barrier.

A computational model of a morphologically-based alveolar capillary unit (ACU) in the rabbit is developed to relate lung fluid balance to mechanical forces between capillary surface and interstitium during development of interstitial edema. We hypothesize that positive values of interstitial liquid pressure Pliq impact on capillary transmural pressure and on blood flow. ACU blood flow, capillary recruitment and filtration are computed by modulating vascular and interstitial pressures. Model results are compared with experimental data of Pliq increasing from ~-10 (control) up to ~4cmH2O in two conditions, hypoxia and collagenase injection. For hypoxia exposure, fitting data requires a linear increase in hydraulic conductivity Lp and capillary pressure PC, that fulfils the need of increase in oxygen delivery. For severe fragmentation of capillary endothelial barrier (collagenase injection), fitting requires a rapid increase in both hydraulic and protein permeability, causing ACU de-recruitment, followed by an increase in PC as a late response to restore blood flow. In conclusion, the model allows to describe the lung adaptive response to edemagenic perturbations; the increase in Pliq, related to the low interstitial compliance, provides an efficient control of extravascular water, by limiting microvascular filtration.

Air leak after lung resection: pathophysiology and patients' implications.

Protocols for the management of air leaks are critical aspects in the postoperative course of patients following lung resections. Many investigations in the last decade are focusing on the chest tube modalities or preventative measures, however, little is known about the pathophysiology of air leak and the patient perception of this common complication. This review concentrates on understanding the reasons why a pulmonary parenchyma may start to leak or an air leak may be longer than others. Experimental works support the notion that lung overdistension may favor air leak. These studies may represent the basis of future investigations. Furthermore, the standardization of nomenclature in the field of pleural space management and the creation of novel air leak scoring systems have contributed to improve the knowledge among thoracic surgeons and facilitate the organization of trials on this matter. We tried to summarize available evidences about the patient perception of a prolonged air leak and about what would be useful for them in order to prevent worsening of their quality of life. Future investigations are warranted to better understand the pathophysiologic mechanisms responsible of prolonged air leak in order to define tailored treatments and protocols. Improving the care at home with web-based telemonitoring or real time connected chest drainage may in a future improve the quality of life of the patients experience this complication and also enhance hospital finances.

Mild training program in metabolic syndrome improves the efficiency of the oxygen pathway.

In sedentary patients suffering of metabolic syndrome, we evaluated the effects of mild exercise program (EP) on the efficiency of the oxygen delivery system. The prescription of exercise (40 min/session, 3 times/week) was tailored at workload corresponding to ∼90% individual anaerobic threshold (AT). EP improved significantly by ∼10% peak values of oxygen consumption (VO2) and heart rate (HR). Furthermore, in response to steady state workload at 90% AT, EP shortened the time constant of VO2, HR and the ratio VO2/HR (reflecting arterio-venous O2 concentration difference) by ∼6s. EP also decreased the elastic respiratory work due to a change in breathing pattern implying a larger contribution of respiratory rate, at the expense of tidal volume during exercise hyperventilation. In all subjects the perceived fatigue (Borg) decreased after training. This study supports a positive effect of a mild EP for the adaptive response of the oxygen chain to face metabolic needs compatible with daily life in patients affected by metabolic syndrome.

The effect of exogenous surfactant on alveolar interdependence.

To investigate the nature of alveolar mechanical interdependence, we purposefully disturbed the equilibrium condition by administering exogenous surfactant in physiological non-surfactant deprived conditions. Changes in alveolar morphology induced by intra-tracheal delivery of CUROSURF were evaluated after opening a pleural window allowing in-vivo microscopic imaging of sub-pleural alveoli in 6 male anesthetized, tracheotomized and mechanically ventilated rabbits. Surfactant instillation increased the surface area of alveoli smaller than 20,000 µm(2) up to ∼ 50% at 15 min after instillation, reflecting a lowering of surface tension due to local surfactant enrichment. Conversely, for alveoli greater than 20,000 µm(2), surface area decreased by ∼ 5%. Opposite changes in alveolar surface are interpreted as reflecting a new inter-alveolar mechanical equilibrium modified by local surfactant distribution and by a decrease in lung distending pressure. We propose that smaller alveoli, representing the majority of alveolar population, might mostly contribute to improve the oxygenation index following surfactant replacement therapy in case of surfactant deficiency.

Alveolar mechanics studied by in vivo microscopy imaging through intact pleural space.

In six male anesthetized, tracheotomized, and mechanically ventilated rabbits we derived indications on alveolar mechanics from in vivo imaging, using a "pleural window" technique (pleural space intact) that allows unrestrained movement of the same subpleural alveoli (N=60) on increasing alveolar pressure from 4 to 8 cmH2O. Absolute compliance (C(abs), ratio of change in alveolar surface area to the change in alveolar pressure) was significantly lower in smaller compared to larger alveoli. Specific compliance, C(sp), obtained by normalizing C(abs) to alveolar surface area, was essentially independent of alveolar size. Both C(abs) and C(sp) were affected by large variability likely reflecting the complex matching between elastic and surface forces. We hypothesize that the relative constancy of C(sp) might contribute to reduce interregional differences in parenchymal and surface forces in the lung tissue by contributing to assure a uniform stretching in a model of mechanically inter-dependent alveoli.

Near infrared spectroscopy (NIRS) as a new non-invasive tool to detect oxidative skeletal muscle impairment in children survived to acute lymphoblastic leukaemia.

BACKGROUND: Separating out the effects of cancer and treatment between central and peripheral components of the O2 delivery chain should be of interest to clinicians for longitudinal evaluation of potential functional impairment in order to set appropriate individually tailored training/rehabilitation programmes. We propose a non-invasive method (NIRS, near infrared spectroscopy) to be used in routine clinical practice to evaluate a potential impairment of skeletal muscle oxidative capacity during exercise in children previously diagnosed with acute lymphoblastic leukaemia (ALL). The purpose of this study was to evaluate the capacity of skeletal muscle to extract O2 in 10 children diagnosed with ALL, 1 year after the end of malignancy treatment, compared to a control group matched for gender and age (mean±SD = 7.8±1.5 and 7.3±1.4 years, respectively). METHODS AND FINDINGS: Participants underwent an incremental exercise test on a treadmill until exhaustion. Oxygen uptake ([Formula: see text]), heart rate (HR), and tissue oxygenation status (Δ[HHb]) of the vastus lateralis muscle evaluated by NIRS, were measured. The results showed that, in children with ALL, a significant linear regression was found by plotting [Formula: see text] vs Δ[HHb] both measured at peak of exercise. In children with ALL, the slope of the HR vs [Formula: see text] linear response (during sub-maximal and peak work rates) was negatively correlated with the peak value of Δ[HHb]. CONCLUSIONS: The present study proves that the NIRS technique allows us to identify large inter-individual differences in levels of impairment in muscle O2 extraction in children with ALL. The outcome of these findings is variable and may reflect either muscle atrophy due to lack of use or, in the most severe cases, an undiagnosed myopathy.

From morphological heterogeneity at alveolar level to the overall mechanical lung behavior: an in vivo microscopic imaging study.

In six male anesthetized, tracheotomized, and mechanically ventilated rabbits, we imaged subpleural alveoli under microscopic view (60×) through a "pleural window" obtained by stripping the endothoracic fascia and leaving the parietal pleura intact. Three different imaging scale levels were identified for the analysis on increasing stepwise local distending pressure (P ld) up to 16.5 cmH2O: alveoli, alveolar cluster, and whole image field. Alveolar profiles were manually traced, clusters of alveoli of similar size were identified through a contiguity-constrained hierarchical agglomerative clustering analysis and alveolar surface density (ASD) was estimated as the percentage of air on the whole image field. Alveolar area distributions were remarkably right-skewed and showed an increase in median value with a large topology-independent heterogeneity on increasing P ld. Modeling of alveolar area distributions on increasing P ld led to hypothesize that absolute alveolar compliance (change in surface area over change in P ld) increases fairly linearly with increasing initial alveolar size, the corollary of this assumption being a constant specific compliance. Clusters were reciprocally interweaved due to their highly variable complex shapes. ASD was found to increase with a small coefficient of variation (CV <25%) with increasing P ld. The CV of lung volume at each transpulmonary pressure was further decreased (about 6%). The results of the study suggest that the considerable heterogeneity of alveolar size and of the corresponding alveolar mechanical behavior are homogenously distributed, resulting in a substantially homogenous mechanical behavior of lung units and whole organ.