Hypercapnic acidosis attenuates endotoxin-induced nuclear factor-[kappa]B activation

Combined angiotensin-converting enzyme and aminopeptidase inhibition for treatment of experimental ventilator-induced lung injury in mice

Introduction: Ventilator-induced lung injury (VILI) may aggravate critical illness. Although angiotensin-converting enzyme (ACE) inhibition has beneficial effects in ventilator-induced lung injury, its clinical application is impeded by concomitant hypotension. We hypothesized that the aminopeptidase inhibitor ALT-00 may oppose the hypotension induced by an angiotensin-converting enzyme inhibitor, and that this combination would activate the alternative renin-angiotensin system (RAS) axis to counteract ventilator-induced lung injury. Methods: In separate experiments, C57BL/6 mice were mechanically ventilated with low (LVT, 6 mL/kg) and high tidal volumes (HVT, 30 mL/kg) for 4 h or remained unventilated (sham). High tidal volume-ventilated mice were treated with lisinopril (0.15 μg/kg/min) ± ALT-00 at 2.7, 10 or 100 μg/kg/min. Blood pressure was recorded at baseline and after 4 h. Lung histology was evaluated for ventilator-induced lung injury and the angiotensin (Ang) metabolite profile in plasma (equilibrium levels of Ang I, Ang II, Ang III, Ang IV, Ang 1-7, and Ang 1-5) was measured with liquid chromatography tandem mass spectrometry at the end of the experiment. Angiotensin concentration-based markers for renin, angiotensin-converting enzyme and alternative renin-angiotensin system activities were calculated. Results: High tidal volume-ventilated mice treated with lisinopril showed a significant drop in the mean arterial pressure at 4 h compared to baseline, which was prevented by adding ALT-00 at 10 and 100 μg/kg/min. Ang I, Ang II and Ang 1-7 plasma equilibrium levels were elevated in the high tidal volumes group versus the sham group. Lisinopril reduced Ang II and slightly increased Ang I and Ang 1-7 levels versus the untreated high tidal volumes group. Adding ALT-00 at 10 and 100 μg/kg/min increased Ang I and Ang 1-7 levels versus the high tidal volume group, and partly prevented the downregulation of Ang II levels caused by lisinopril. The histological lung injury score was higher in the high tidal volume group versus the sham and low tidal volume groups, and was attenuated by lisinopril ± ALT-00 at all dose levels. Conclusion: Combined angiotensin-converting enzyme plus aminopeptidase inhibition prevented systemic hypotension and maintained the protective effect of lisinopril. In this study, a combination of lisinopril and ALT-00 at 10 μg/kg/min appeared to be the optimal approach, which may represent a promising strategy to counteract ventilator-induced lung injury that merits further exploration.

Response of TRPM2 Channel to Hypercapnic Acidosis and Role of Zn, Se, and GSH

Hypercapnia can increase the production of reactive oxygen species (ROS) by inducing oxidative stress in cells. Transient receptor potential melastatin 2 (TRPM2) channel activation that is realized by ROS plays a critical role in the cellular mechanism. It was shown that antioxidants such as zinc (Zn), selenium (Se), and glutathione (GSH) can partake in the structures of enzymes and create a protective effect against oxidative stress. This study revealed the relationship between TRPM2 channel and hypercapnia, and the interaction of zinc, selenium, and glutathione. In our study, normoxia, hypercapnia, hypercapnia + Zn, hypercapnia + Se, and hypercapnia + GSH were created, in transfected HEK293 cells. The cells were exposed to normoxia or hypercapnia gasses in two different times (30 min and 60 min), while Zn, Se, and GSH were applied to the cells in the other groups before being exposed to the gas mixtures. The statistical evaluation showed a significant increase in lipid peroxidation (LPO) level and lactate dehydrogenase (LDH)% in the hypercapnia 30 min and 60 min groups, compared to the normoxia 30 min and 60 min groups, and an increase in LPO level and LDH% in the hypercapnia groups that Zn, Se, and GSH were applied. It was determined that in comparison with the normoxia 30 min and 60 min groups, the amount of inward Ca⁺² current across TRPM2 channels and mean current density increased in the groups that were exposed to hypercapnia for 30 min and 60 min, while the same values significantly decreased in the hypercapnia groups that Zn, Se, and GSH were applied. Also, it was shown that oxidative stress rose as the duration of hypercapnia exposure increased. It was concluded that hypercapnia increased oxidative stress and caused cellular membrane damage, while the addition of Zn, Se, and GSH could protect the cell membrane from these damaging effects.

Ubiquitin is a carbon dioxide-binding protein

The identification of CO₂-binding proteins is crucial to understanding CO₂-regulated molecular processes. CO₂ can form a reversible posttranslational modification through carbamylation of neutral N-terminal α-amino or lysine ε-amino groups. We have previously developed triethyloxonium (TEO) ion as a chemical proteomics tool for covalent trapping of carbamates, and here, we deploy TEO to identify ubiquitin as a mammalian CO₂-binding protein. We use ¹³C-NMR spectroscopy to demonstrate that CO₂ forms carbamates on the ubiquitin N terminus and ε-amino groups of lysines 6, 33, 48, and 63. We demonstrate that biologically relevant pCO₂ levels reduce ubiquitin conjugation at lysine-48 and down-regulate ubiquitin-dependent NF-κB pathway activation. Our results show that ubiquitin is a CO₂-binding protein and demonstrates carbamylation as a viable mechanism by which mammalian cells can respond to fluctuating pCO₂.

Carbon dioxide-dependent signal transduction in mammalian systems

Carbon dioxide (CO2) is a fundamental physiological gas known to profoundly influence the behaviour and health of millions of species within the plant and animal kingdoms in particular. A recent Royal Society meeting on the topic of 'Carbon dioxide detection in biological systems' was extremely revealing in terms of the multitude of roles that different levels of CO2 play in influencing plants and animals alike. While outstanding research has been performed by leading researchers in the area of plant biology, neuronal sensing, cell signalling, gas transport, inflammation, lung function and clinical medicine, there is still much to be learned about CO2-dependent sensing and signalling. Notably, while several key signal transduction pathways and nodes of activity have been identified in plants and animals respectively, the precise wiring and sensitivity of these pathways to CO2 remains to be fully elucidated. In this article, we will give an overview of the literature relating to CO2-dependent signal transduction in mammalian systems. We will highlight the main signal transduction hubs through which CO2-dependent signalling is elicited with a view to better understanding the complex physiological response to CO2 in mammalian systems. The main topics of discussion in this article relate to how changes in CO2 influence cellular function through modulation of signal transduction networks influenced by pH, mitochondrial function, adenylate cyclase, calcium, transcriptional regulators, the adenosine monophosphate-activated protein kinase pathway and direct CO2-dependent protein modifications. While each of these topics will be discussed independently, there is evidence of significant cross-talk between these signal transduction pathways as they respond to changes in CO2. In considering these core hubs of CO2-dependent signal transduction, we hope to delineate common elements and identify areas in which future research could be best directed.

Effects of hypercapnia in sepsis: A scoping review of clinical and pre-clinical data

BACKGROUND

Perform a scoping review of (1) pre-clinical studies testing the physiological effects of higher PaCO2 levels in the setting of sepsis models and (2) clinical investigations testing the effects of hypercapnia on clinical outcomes in mechanically ventilated patients with sepsis.

METHODS

We performed a search of CENTRAL, PUBMED, CINAHL, and EMBASE. Study inclusion criteria for pre-clinical studies were: (1) bacterial sepsis model (2) measurement of PaCO2 , and (3) comparison of outcome measure between different PaCO2 levels. Inclusion criteria for clinical studies were: (1) diagnosis of sepsis, (2) receiving invasive mechanical ventilation, (3) measurement of PaCO2 , and (4) comparison of outcomes between different PaCO2 levels. We performed a qualitative analysis to collate and summarize the physiological and clinical effects of hypercapnia according to the recommended methodology from the Cochrane Handbook.

RESULTS

Fifteen pre-clinical and nine clinical studies were included. Among pre-clinical studies, the individual studies found higher PaCO2 augments tissue blood flow and oxygenation, and attenuates inflammation and lung injury; however, all pre-clinical studies were found to have some degree of risk of bias. Six of the nine clinical studies were deemed to be good quality. Among clinical studies hypercapnia was associated with increased cerebral perfusion and oxygenation; however, there were conflicting results testing the association between hypercapnia and mortality.

CONCLUSION

While individual pre-clinical studies identified potential mechanisms by which changes in PaCO2 levels could affect pathophysiology in sepsis, there is a paucity of clinical data as to the optimal PaCO2 range, demonstrating a need for future research.

REGISTRATION

PROSPERO number CRD42018086703.

Hypercapnia selectively modulates LPS-induced changes in innate immune and DNA replication-related gene transcription in the macrophage

Hypercapnia, the elevation of CO₂ in blood and tissues, commonly occurs in severe acute and chronic respiratory diseases and is associated with increased risk of death. Recent studies have shown that hypercapnia inhibits expression of select innate immune genes and suppresses host defence against bacterial and viral pneumonia in mice. In the current study, we evaluated the effect of culture under conditions of hypercapnia (20% CO₂) versus normocapnia (5% CO₂), both with normoxia, on global gene transcription in human THP-1 and mouse RAW 264.7 macrophages stimulated with lipopolysaccharide (LPS). We found that hypercapnia selectively downregulated transcription of LPS-induced genes associated with innate immunity, antiviral response, type I interferon signalling, cytokine signalling and other inflammatory pathways in both human and mouse macrophages. Simultaneously, hypercapnia increased expression of LPS-downregulated genes associated with mitosis, DNA replication and DNA repair. These CO₂-induced changes in macrophage gene expression help explain hypercapnic suppression of antibacterial and antiviral host defence in mice and reveal a mechanism that may underlie, at least in part, the high mortality of patients with severe lung disease and hypercapnia.

Treatment with 7% and 10% CO2 enhanced expression of IL-1β, TNF-α, and IL-6 in hypoxic cultures of human whole blood

Objective

This study investigated whether hypercapnia influenced the inflammatory response of hypoxic blood.

Methods

Human whole blood was cultured with 0.2% oxygen (O₂) and treated with 5%, 7%, or 10% carbon dioxide (CO₂). Interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and IL-6 were evaluated in whole blood cultures. Reactive oxygen species (ROS) production and expression levels of caspase-1 and IL-1β were evaluated in THP-1 monocytic cells.

Results

IL-1β, TNF-α, and IL-6 levels were higher in the hypoxia + 7% CO₂ group than in the hypoxia + 5% CO₂ group. The hypoxia + 10% CO₂ group had the highest IL-1β, TNF-α, and IL-6 levels, compared with the hypoxia + 7% CO₂ and hypoxia + 5% CO₂ groups. Expression levels of IL-1β, TNF-α, and IL-6 were significantly negatively correlated with pH levels in the cell culture medium. Treatment with 7% and 10% CO₂ increased the production of ROS and the expression of caspase-1 and IL-1β in hypoxia-activated THP-1 cells.

Conclusions

High levels of CO₂ treatment increased expression levels of IL-1β, TNF-α, and IL-6 in hypoxic whole blood cultures. High levels of CO₂-induced ROS overproduction and NLRP3 inflammasome activation in monocytes may comprise a target to mitigate the inflammatory response of hypoxic blood.

Carbon dioxide inhibits UVB-induced inflammatory response by activating the proton-sensing receptor, GPR65, in human keratinocytes

Carbon dioxide (CO₂) is the predominant gas molecule emitted during aerobic respiration. Although CO₂ can improve blood circulation in the skin via its vasodilatory effects, its effects on skin inflammation remain unclear. The present study aimed to examine the anti-inflammatory effects of CO₂ in human keratinocytes and skin. Keratinocytes were cultured under 15% CO₂, irradiated with ultraviolet B (UVB), and their inflammatory cytokine production was analyzed. Using multiphoton laser microscopy, the effect of CO₂ on pH was observed by loading a three-dimensional (3D)-cultured epidermis with a high-CO₂ concentration formulation. Finally, the effect of CO₂ on UVB-induced erythema was confirmed. CO₂ suppressed the UVB-induced production of tumor necrosis factor-α (TNFα) and interleukin-6 (IL-6) in keratinocytes and the 3D epidermis. Correcting medium acidification with NaOH inhibited the CO₂-induced suppression of TNFα and IL-6 expression in keratinocytes. Moreover, the knockdown of H⁺-sensing G protein-coupled receptor 65 inhibited the CO₂-induced suppression of inflammatory cytokine expression and NF-κB activation and reduced CO₂-induced cyclic adenosine monophosphate production. Furthermore, the high-CO₂ concentration formulation suppressed UVB-induced erythema in human skin. Hence, CO₂ suppresses skin inflammation and can be employed as a potential therapeutic agent in restoring skin immune homeostasis.

High CO2 Levels Impair Lung Wound Healing

Delayed lung repair leads to alveolopleural fistulae, which are a major cause of morbidity after lung resections. We have reported that intrapleural hypercapnia is associated with delayed lung repair after lung resection. Here, we provide new evidence that hypercapnia delays wound closure of both large airway and alveolar epithelial cell monolayers because of inhibition of epithelial cell migration. Cell migration and airway epithelial wound closure were dependent on Rac1-GTPase activation, which was suppressed by hypercapnia directly through the upregulation of AMP kinase and indirectly through inhibition of injury-induced NF-κB-mediated CXCL12 (pleural CXC motif chemokine 12) release, respectively. Both these pathways were independently suppressed, because dominant negative AMP kinase rescued the effects of hypercapnia on Rac1-GTPase in uninjured resting cells, whereas proteasomal inhibition reversed the NF-κB-mediated CXCL12 release during injury. Constitutive overexpression of Rac1-GTPase rescued the effects of hypercapnia on both pathways as well as on wound healing. Similarly, exogenous recombinant CXCL12 reversed the effects of hypercapnia through Rac1-GTPase activation by its receptor, CXCR4. Moreover, CXCL12 transgenic murine recipients of orthotopic tracheal transplantation were protected from hypercapnia-induced inhibition of tracheal epithelial cell migration and wound repair. In patients undergoing lobectomy, we found inverse correlation between intrapleural carbon dioxide and pleural CXCL12 levels as well as between CXCL12 levels and alveolopleural leak. Accordingly, we provide first evidence that high carbon dioxide levels impair lung repair by inhibiting epithelial cell migration through two distinct pathways, which can be restored by recombinant CXCL12.

Beneficial Effects of Hypercapnic Acidosis on the Inhibition of Transforming Growth Factor β-1-induced Corneal Fibrosis in Vitro

PURPOSE

Corneal scarring is a common poor outcome of corneal trauma. Transforming growth factor β-1 plays a vital role in corneal fibrosis, inducing keratocyte transformation to myofibroblasts. Other than corneal transplantation, no other curative treatment methods for corneal scarring are currently available. Hypercapnic acidosis exerts anti-inflammatory and anti-migratory effects on numerous organs; however, its effect on corneal fibroblasts remains unknown. Hence, this study aimed to evaluate the effect of hypercapnic acidosis on transforming growth factor β-1-induced fibrosis in corneal fibroblasts and to elucidate the underlying mechanisms.

MATERIALS AND METHODS

Corneal fibroblasts were obtained from human limbal tissue and cultured with or without transforming growth factor β-1 under hypercapnic acidosis or no-hypercapnic acidosis conditions, and subjected to scratch wound, cell migration, and collagen matrix contraction assays. Furthermore, immunocytochemistry was performed to evaluate the alpha-smooth muscle actin stress fiber. Finally, western blotting was performed to assess the expression of proteins in the NF-κB and Smad pathways.

RESULTS

Hypercapnic acidosis suppressed collagen gel contraction capacity in transforming growth factor β-1-treated corneal fibroblasts and inhibited transforming growth factor β-1-induced cell migration. Moreover, hypercapnic acidosis downregulated corneal fibrosis marker alpha-smooth muscle actin in transforming growth factor β-1-treated corneal fibroblasts. Furthermore, hypercapnic acidosis suppressed transforming growth factor β-1-induced fibrosis, at least partly, by inhibiting Smad2/3 phosphorylation and down-regulating p-IκB-dependent and RelB signaling transduction.

CONCLUSIONS

Hypercapnic acidosis inhibits transforming growth factor β-1-induced corneal fibroblast migration, collagen gel contraction capacity, and alpha smooth muscle actin expression, potentially through the Smad and NF-κB pathways. Therefore, hypercapnic acidosis may be a potentially useful anti-fibrotic therapy for corneal scarring.

Lung-Protective Mechanical Ventilation Strategies in Pediatric Acute Respiratory Distress Syndrome

OBJECTIVES

Reduced morbidity and mortality associated with lung-protective mechanical ventilation is not proven in pediatric acute respiratory distress syndrome. This study aims to determine if a lung-protective mechanical ventilation protocol in pediatric acute respiratory distress syndrome is associated with improved clinical outcomes.

DESIGN

This pilot study over April 2016 to September 2019 adopts a before-and-after comparison design of a lung-protective mechanical ventilation protocol. All admissions to the PICU were screened daily for fulfillment of the Pediatric Acute Lung Injury Consensus Conference criteria and included.

SETTING

Multidisciplinary PICU.

PATIENTS

Patients with pediatric acute respiratory distress syndrome.

INTERVENTIONS

Lung-protective mechanical ventilation protocol with elements on peak pressures, tidal volumes, end-expiratory pressure to FIO2 combinations, permissive hypercapnia, and permissive hypoxemia.

MEASUREMENTS AND MAIN RESULTS

Ventilator and blood gas data were collected for the first 7 days of pediatric acute respiratory distress syndrome and compared between the protocol (n = 63) and nonprotocol groups (n = 69). After implementation of the protocol, median tidal volume (6.4 mL/kg [5.4-7.8 mL/kg] vs 6.0 mL/kg [4.8-7.3 mL/kg]; p = 0.005), PaO2 (78.1 mm Hg [67.0-94.6 mm Hg] vs 74.5 mm Hg [59.2-91.1 mm Hg]; p = 0.001), and oxygen saturation (97% [95-99%] vs 96% [94-98%]; p = 0.007) were lower, and end-expiratory pressure (8 cm H2O [7-9 cm H2O] vs 8 cm H2O [8-10 cm H2O]; p = 0.002] and PaCO2 (44.9 mm Hg [38.8-53.1 mm Hg] vs 46.4 mm Hg [39.4-56.7 mm Hg]; p = 0.033) were higher, in keeping with lung protective measures. There was no difference in mortality (10/63 [15.9%] vs 18/69 [26.1%]; p = 0.152), ventilator-free days (16.0 [2.0-23.0] vs 19.0 [0.0-23.0]; p = 0.697), and PICU-free days (13.0 [0.0-21.0] vs 16.0 [0.0-22.0]; p = 0.233) between the protocol and nonprotocol groups. After adjusting for severity of illness, organ dysfunction and oxygenation index, the lung-protective mechanical ventilation protocol was associated with decreased mortality (adjusted hazard ratio, 0.37; 95% CI, 0.16-0.88).

CONCLUSIONS

In pediatric acute respiratory distress syndrome, a lung-protective mechanical ventilation protocol improved adherence to lung-protective mechanical ventilation strategies and potentially mortality.

Hypercapnia exacerbates the disruption of the blood‑brain barrier by inducing interleukin‑1β overproduction in the blood of hypoxemic adult rats

Refractory hypoxemia is the main symptom of acute respiratory distress syndrome (ARDS). Low tidal volume ventilation is routinely applied in clinical practice to correct hypoxemia, which aims to prevent ventilator‑induced lung injury. However, this ventilation strategy inevitably leads to hypercapnia. Our previous study demonstrated that hypercapnia aggravated cognitive impairment in hypoxemic rats; however, the underlying mechanism remains unclear. The aim of the present study was to investigate whether hypercapnia exacerbates the blood‑brain barrier (BBB) disruption through inducing interleukin (IL)‑1β overproduction in the blood of hypoxemic rats. The BBB permeability in a rat model of hypercapnia/hypoxemia was evaluated. The levels of IL‑1β in the blood of rats and human whole‑blood cultures were assessed. The expression of IL‑1 receptor 1 (IL‑1R1), phosphorylated IL‑1R1‑associated kinase (p‑IRAK‑1) and tight junctional proteins in cerebral vascular endothelial cells was examined in vitro and in vivo. In addition, IL‑1Ra, an IL‑1 receptor antagonist, was used to determine whether hypercapnia affects tight junctional protein expression in hypoxic cerebral vascular endothelial cells through inducing IL‑1β overproduction. It was observed that hypercapnia alone did not disrupt the BBB, but aggravated the damage to the BBB integrity in hypoxemic rats. Hypercapnia increased IL‑1β expression in the blood of hypoxemic rats as well as in hypoxic human whole‑blood cultures. IL‑1R1 and p‑IRAK‑1 expression was increased, while that of tight junctional proteins was reduced by hypercapnia in hypoxemic cerebral vascular endothelial cells in vitro and in vivo. Additionally, the expression of tight junctional proteins was markedly increased following treatment with IL‑1Ra. These results suggest that hypercapnia‑induced IL‑1β overproduction in the hypoxemic blood may decrease tight junctional protein expression in cerebrovascular endothelial cells via the IL‑1R1/p‑IRAK‑1 pathway, further disrupting BBB integrity, and eventually resulting in increased BBB permeability.

The Organ-Protective Effect of Higher Partial Pressure of Arterial Carbon Dioxide in the Normal Range for Infant Patients Undergoing Ventricular Septal Defect Repair

Hypercapnia has been reported to play an active role in protection against organ injury. The aim of this study was to determine whether a higher level of partial pressure of arterial carbon dioxide (PaCO₂) within the normal range in pediatric patients undergoing cardiac surgery had a similar organ-protective effect. From May 2017 to May 2018, 83 consecutive infant patients undergoing ventricular septal defect (VSD) repair with cardiopulmonary bypass were retrospectively enrolled. We recorded the end-expiratory tidal partial pressure of carbon dioxide (Pet-CO₂) as an indirect and continuous way to reflect the PaCO₂. The patients were divided into a low PaCO₂ group (LPG; 30 mmHg < Pet-CO₂ < 40 mmHg) and a high PaCO₂ group (HPG; 40 mmHg < Pet-CO₂ < 50 mmHg). The regional cerebral oxygen saturation (rScO₂), cerebral blood flow velocity (CBFV), and hemodynamics at five time points throughout the operation, and perioperative data were recorded and analyzed for the two groups. In total, 34 LPG and 49 HPG patients were included. Demographics and perioperative clinical data showed no significant difference between the groups. Compared with LPG, the HPG produced lower postoperative creatine kinase isoenzyme-MB (40.88 versus 50.34 ng/mL, P = 0.038). The postoperative C-reactive protein of HPG trended lower than in LPG (61.09 versus 73.4 mg/L, P = 0.056). The rScO₂ and mean CBFV of HPG were significantly higher compared with LPG (P < 0.05) except at the end of cardiopulmonary bypass. Hemodynamic data showed no significant difference between the groups. As a convenient and safe approach, higher-normal PaCO₂ could attenuate brain injury, heart injury, and inflammatory response in infant patients undergoing VSD repair.

Hypercapnia attenuates ventilator-induced lung injury through vagus nerve activation

Purpose:

To investigate the role of vagus nerve activation in the protective effects of hypercapnia in ventilator-induced lung injury (VILI) rats.

Methods:

Male Sprague-Dawley rats were randomized to either high-tidal volume or low-tidal volume ventilation (control) and monitored for 4h. The high-tidal volume group was further divided into either a vagotomy or sham-operated group and each surgery group was further divided into two subgroups: normocapnia and hypercapnia. Injuries were assessed hourly through hemodynamics, respiratory mechanics and gas exchange. Protein concentration, cell count and cytokines (TNF-α and IL-8) in bronchoalveolar lavage fluid (BALF), lung wet-to-dry weight and pathological changes were examined. Vagus nerve activity was recorded for 1h.

Results:

Compared to the control group, injurious ventilation resulted in a decrease in PaO₂/FiO₂ and greater lung static compliance, MPO activity, enhanced BALF cytokines, protein concentration, cell count, and histology injury score. Conversely, hypercapnia significantly improved VILI by decreasing the above injury parameters. However, vagotomy abolished the protective effect of hypercapnia on VILI. In addition, hypercapnia enhanced efferent vagus nerve activity compared to normocapnia.

Conclusion:

These results indicate that the vagus nerve plays an important role in mediating the anti-inflammatory effect of hypercapnia on VILI.

Mechanisms and Consequences of Oxygen and Carbon Dioxide Sensing in Mammals

Molecular oxygen (O₂) and carbon dioxide (CO₂) are the primary gaseous substrate and product of oxidative phosphorylation in respiring organisms, respectively. Variance in the levels of either of these gasses outside of the physiological range presents a serious threat to cell, tissue, and organism survival. Therefore, it is essential that endogenous levels are monitored and kept at appropriate concentrations to maintain a state of homeostasis. Higher organisms such as mammals have evolved mechanisms to sense O₂ and CO₂ both in the circulation and in individual cells and elicit appropriate corrective responses to promote adaptation to commonly encountered conditions such as hypoxia and hypercapnia. These can be acute and transient nontranscriptional responses, which typically occur at the level of whole animal physiology or more sustained transcriptional responses, which promote chronic adaptation. In this review, we discuss the mechanisms by which mammals sense changes in O₂ and CO₂ and elicit adaptive responses to maintain homeostasis. We also discuss crosstalk between these pathways and how they may represent targets for therapeutic intervention in a range of pathological states.

The role of hypercapnia in acute respiratory failure

The biological effects and physiological consequences of hypercapnia are increasingly understood. The literature on hypercapnia is confusing, and at times contradictory. On the one hand, it may have protective effects through attenuation of pulmonary inflammation and oxidative stress. On the other hand, it may also have deleterious effects through inhibition of alveolar wound repair, reabsorption of alveolar fluid, and alveolar cell proliferation. Besides, hypercapnia has meaningful effects on lung physiology such as airway resistance, lung oxygenation, diaphragm function, and pulmonary vascular tree.

In acute respiratory distress syndrome, lung-protective ventilation strategies using low tidal volume and low airway pressure are strongly advocated as these have strong potential to improve outcome. These strategies may come at a price of hypercapnia and hypercapnic acidosis. One approach is to accept it (permissive hypercapnia); another approach is to treat it through extracorporeal means. At present, it remains uncertain what the best approach is.

Hypercapnic acidosis induces mitochondrial dysfunction and impairs the ability of mesenchymal stem cells to promote distal lung epithelial repair

Acute respiratory distress syndrome (ARDS) is a devastating disorder characterized by diffuse inflammation and edema formation. The main management strategy, low tidal volume ventilation, can be associated with the development of hypercapnic acidosis (HCA). Mesenchymal stem cells (MSCs) are a promising therapeutic candidate currently in early-phase clinical trials. The effects of HCA on the alveolar epithelium and capillary endothelium are not well established. The therapeutic efficacy of MSCs has never been reported in HCA. In the present study, we evaluated the effects of HCA on inflammatory response and reparative potential of the primary human small airway epithelial and lung microvasculature endothelial cells as well as on the capacity of bone marrow-derived MSCs to promote wound healing in vitro. We demonstrate that HCA attenuates the inflammatory response and reparative potential of primary human small airway epithelium and capillary endothelium and induces mitochondrial dysfunction. It was found that MSCs promote lung epithelial wound repair via the transfer of functional mitochondria; however, this proreparative effect of MSCs was lost in the setting of HCA. Therefore, HCA may adversely impact recovery from ARDS at the cellular level, whereas MSCs may not be therapeutically beneficial in patients with ARDS who develop HCA.-Fergie, N., Todd, N., McClements, L., McAuley, D., O'Kane, C., Krasnodembskaya, A. Hypercapnic acidosis induces mitochondrial dysfunction and impairs the ability of mesenchymal stem cells to promote distal lung epithelial repair.

Protective effects of hypercapnic acidosis on Ischemia-reperfusion-induced retinal injury

Ischemia–reperfusion (I/R) injury is associated with numerous retinal diseases, such as diabetic retinopathy, acute glaucoma, and other vascular retinopathies. Hypercapnic acidosis (HCA) has a protective effect on lung, myocardial, and central nervous system ischemic injury models. However, no study has evaluated its protective effects in an experimental retinal I/R injury model. In this study, retinal I/R injury was induced in Sprague Dawley rats by elevating the intraocular pressure to 110 mmHg for 60 minutes. HCA was induced before and after the injury. After 24 hours, the terminal dUTP nick end labeling assay was performed. Moreover, the ratios of cleaved caspase-3/total caspase-3, phosphorylated IκB/IκB, and phosphorylated p38 were measured through Western blotting. After 7 days, the rats’ aqueous humor was analyzed. In addition, electroretinography and retinal thickness measurement were performed in the rats. Moreover, the retinal neural cell line RGC-5 was exposed to 500 μM H₂O₂ for 24 hours to induce a sustained oxidative stress in vitro. The effects of HCA were evaluated by comparing oxidative stress, MAPK signals, NF-κB signals, survival rates, and apoptosis rates in the RGC-5 cells before and after H₂O₂ exposure. We further investigated whether the potent I/R-protective heat shock protein (HSP) 32 contribute to protective effects of HCA. Our results indicated that HCA has protective effects against retinal I/R injury both in vivo and in vitro, at multiple levels, including antiapoptotic, anti-inflammatory, antioxidative, and functional retinal cell protection. Further research clarifying the role of HCA in retinal I/R injury prevention and treatment is warranted.

The HMGB1-RAGE/TLR-TNF-α signaling pathway may contribute to kidney injury induced by hypoxia

The hypoxia-reoxygenation process of obstructive sleep apnea (OSA) may cause oxidative stress injury of the kidney, but the molecular mechanisms are not clear. The present study aimed to investigate whether high mobility group box 1 protein (HMGB1) and its subsequent inflammatory pathway served a role in kidney injury. Adult Sprague Dawley rats were used to establish hypoxia models: Continuous hypoxia, intermittent hypoxia and intermittent hypoxia with hypercapnia. Rat kidney tissues and peripheral blood samples were obtained. Histopathological and ultrastructural changes were observed by light and electron microscopy. Immunohistochemical (IHC) staining was used to detect the distribution of HMGB1. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was used to detect the expression of HMGB1, receptor for advanced glycosylation end products (RAGE), toll-like receptor 4 (TLR4), nuclear factor kappa-light-chain-enhancer of active B cells (NF-κB) p65 subunit, tumor necrosis factor-α (TNF-α), interleukin (IL)-6, NAD-dependent protein deacetylase sirtuin-1 (SIRT1), peroxisome proliferator-activated receptor (PPAR) mRNA in renal tissues. An ELISA was used to detect the expression of soluble TLR2, TLR4, PPAR-γ, TNF-α, IL-6 in peripheral blood. Hematoxylin & eosin staining demonstrated that there was no serious injury to the kidneys due to hypoxia, with the exception of a certain degree of renal tubular epithelial cell vacuolation. By contrast, ultrastructural changes by electron microscopy were more significant in the hypoxia groups compared with the control, including foot process fusion in the glomerulus and degeneration of mitochondria in the proximal convoluted tubules. IHC also indicated increased expression of HMGB1 and nuclear translocation in the hypoxia groups. The results of the RT-qPCR demonstrated that hypoxia stimulation increased the expression of HMGB1, PPAR, RAGE and TNF-α mRNA, and decreased the expression of SIRT1 mRNA in kidney tissues (P<0.05). The results of the ELISA suggested that hypoxia stimulation increased the expression of soluble TLR4, TNF-α and IL-6 in the peripheral blood, and decreased the expression of soluble TLR2 and PPAR-γ. In summary, hypoxia stimulation may cause early renal injury at the subcellular level and increase the expression and translocation of HMGB1. Hypoxia also upregulated the mRNA expression of the HMGB1-RAGE-TNF-α pathway in kidney tissue and increased the expression of soluble TLR4, TNF-α and IL-6 in the peripheral blood. This suggested that the HMGB1-RAGE/TLR-TNF-α pathway may contribute to the molecular mechanisms of early renal injury induced by hypoxia. The pathway may contain potential markers for OSA-associated early renal injury and drug intervention targets in the future.

Hypercapnia and hypercapnic acidosis in sepsis: harmful, beneficial or unclear?

Mortality related to sepsis among critically ill patients remains high. Recent literature suggests that hypercapnia may affect the pathophysiology of sepsis. The effects of hypercapnia on sepsis are largely related to the direct effect of hypercapnic acidosis on immune function and, as a consequence, of increased cardiac output that subsequently leads to improved tissue oxygenation. Appropriate management of hypercapnia may aid in improving the outcomes of sepsis. Our aim was to review the effects of compensated hypercapnia and hypercapnic acidosis on sepsis, with a specific focus on critically ill patients. Hypercapnic acidosis has been extensively studied in various in vivo animal models of sepsis and ex vivo studies. Published data from animal experimental studies suggest that the effects of hypercapnic acidosis are variable, with benefit shown in some settings of sepsis and harm in others. The effects may also vary at different time points during the course of sepsis. There are very few clinical studies investigating the effects of hypercapnia in prevention of sepsis and in established sepsis. It appears from these very limited clinical data that hypercapnia may be associated with adverse outcomes. There are no clinical studies investigating clinical outcomes of hypercapnic acidosis or compensated hypercapnia in sepsis and septic shock in critical care settings, thus extrapolation of the experimental results to guide critical care practice is difficult. Clinical studies are needed, especially in critically ill patients, to define the effects of compensated hypercapnia and hypercapnic acidosis that may aid clinicians to improve the outcomes in sepsis.

Association Between Partial Pressure of Arterial Carbon Dioxide and Survival to Hospital Discharge Among Patients Diagnosed With Sepsis in the Emergency Department

OBJECTIVE

The objective of this study was to test the association between the partial pressure of arterial carbon dioxide and survival to hospital discharge among mechanically ventilated patients diagnosed with sepsis in the emergency department.

DESIGN

Retrospective cohort study of a single center trial registry.

SETTING

Academic medical center.

PATIENTS

Mechanically ventilated emergency department patients.

INCLUSION CRITERIA

age 18 years and older, diagnosed with sepsis in the emergency department, and mechanical ventilation initiated in the emergency department.

INTERVENTIONS

Arterial blood gases obtained after initiation of mechanical ventilation were analyzed. The primary outcome was survival to hospital discharge. We tested the association between partial pressure of arterial carbon dioxide and survival using multivariable logistic regression adjusting for potential confounders. Sensitivity analyses, including propensity score matching were also performed.

MEASUREMENTS AND MAIN RESULTS

Six hundred subjects were included, and 429 (72%) survived to hospital discharge. The median (interquartile range) partial pressure of arterial carbon dioxide was 42 (34-53) mm Hg for the entire cohort and 44 (35-57) and 39 (31-45) mm Hg among survivors and nonsurvivors, respectively (p < 0.0001 Wilcox rank-sum test). On multivariable analysis, a 1 mm Hg rise in partial pressure of arterial carbon dioxide was associated with a 3% increase in odds of survival (adjusted odds ratio, 1.03; 95% CI, 1.01-1.04) after adjusting for tidal volume and other potential confounders. These results remained significant on all sensitivity analyses.

CONCLUSION

In this sample of mechanically ventilated sepsis patients, we found an association between increasing levels of partial pressure of arterial carbon dioxide and survival to hospital discharge. These findings justify future studies to determine the optimal target partial pressure of arterial carbon dioxide range for mechanically ventilated sepsis patients.

Pediatric acute respiratory distress syndrome - current views

Acute respiratory distress syndrome (ARDS) mainly involves acute respiratory failure. In addition to this affected patients feel progressive arterial hypoxemia, dyspnea, and a marked increase in the work of breathing. The only clinical solution for the above pathological state is ventilation. Mechanical ventilation is necessary to support life in ARDs but it itself worsen lung injury and the term is known clinically as ‘ventilation induced lung injury’ (VILI). At the cellular level, respiratory epithelial cells are subjected to cyclic stretch, i.e. repeated cycles of positive and negative strain, during normal tidal ventilation. In aerated areas of diseased lungs, or even normal lungs subjected to injurious positive pressure mechanical ventilation, the cells are at risk of being over distended, and worsening injury by disrupting the alveolar epithelial barrier. Further, hypercapnic acidosis (HCA) in itself confers protection from stretch injury, potentially via a mechanisms involving inhibition of nuclear factor κB (NF-κB), a transcription factor central to inflammation, injury and repair. Mesenchymal stem cells are the latest in the field and are being investigated as a possible therapy for ARDS.

Re-examining Permissive Hypercapnia in ARDS: A Narrative Review

Lung-protective ventilation (LPV) has become the cornerstone of management in patients with ARDS. A subset of patients is unable to tolerate LPV without significant CO₂ elevation. In these patients, permissive hypercapnia is used. Although thought to be benign, it is becoming increasingly evident that elevated CO₂ levels have significant physiological effects. In this narrative review, we highlight clinically relevant end-organ effects in both animal models and clinical studies. We also explore the association between elevated CO₂, acute cor pulmonale, and ICU mortality. We conclude with a brief review of alternative therapies for CO₂ management currently under investigation in patients with moderate to severe ARDS.

Partial pressure of arterial carbon dioxide and survival to hospital discharge among patients requiring acute mechanical ventilation: A cohort study

PURPOSE

To describe the prevalence of hypocapnia and hypercapnia during the earliest period of mechanical ventilation, and determine the association between PaCO2 and mortality.

MATERIALS AND METHODS

A cohort study using an emergency department registry of mechanically ventilated patients. PaCO2 was categorized: hypocapnia (<35mmHg), normocapnia (35-45mmHg), and hypercapnia (>45mmHg). The primary outcome was survival to hospital discharge.

RESULTS

A total of 1,491 patients were included. Hypocapnia occurred in 375 (25%) patients and hypercapnia in 569 (38%). Hypercapnia (85%) had higher survival rate compared to normocapnia (74%) and hypocapnia (66%), P<0.001. PaCO2 was an independent predictor of survival to hospital discharge [hypocapnia (aOR 0.65 (95% confidence interval [CI] 0.48-0.89), normocapnia (reference category), hypercapnia (aOR 1.83 (95% CI 1.32-2.54)]. Over ascending ranges of PaCO2, there was a linear trend of increasing survival up to a PaCO2 range of 66-75mmHg, which had the strongest survival association, aOR 3.18 (95% CI 1.35-7.50).

CONCLUSIONS

Hypocapnia and hypercapnia occurred frequently after initiation of mechanical ventilation. Higher PaCO2 levels were associated with increased survival. These data provide rationale for a trial examining the optimal PaCO2 in the critically ill.

Effects of Hypercapnia and Hypercapnic Acidosis on Hospital Mortality in Mechanically Ventilated Patients

OBJECTIVES

Lung-protective ventilation is used to prevent further lung injury in patients on invasive mechanical ventilation. However, lung-protective ventilation can cause hypercapnia and hypercapnic acidosis. There are no large clinical studies evaluating the effects of hypercapnia and hypercapnic acidosis in patients requiring mechanical ventilation.

DESIGN

Multicenter, binational, retrospective study aimed to assess the impact of compensated hypercapnia and hypercapnic acidosis in patients receiving mechanical ventilation.

SETTINGS

Data were extracted from the Australian and New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation Adult Patient Database over a 14-year period where 171 ICUs contributed deidentified data.

PATIENTS

Patients were classified into three groups based on a combination of pH and carbon dioxide levels (normocapnia and normal pH, compensated hypercapnia [normal pH with elevated carbon dioxide], and hypercapnic acidosis) during the first 24 hours of ICU stay. Logistic regression analysis was used to identify the independent association of hypercapnia and hypercapnic acidosis with hospital mortality.

INTERVENTIONS

Nil.

MEASUREMENTS AND MAIN RESULTS

A total of 252,812 patients (normocapnia and normal pH, 110,104; compensated hypercapnia, 20,463; and hypercapnic acidosis, 122,245) were included in analysis. Patients with compensated hypercapnia and hypercapnic acidosis had higher Acute Physiology and Chronic Health Evaluation III scores (49.2 vs 53.2 vs 68.6; p < 0.01). The mortality was higher in hypercapnic acidosis patients when compared with other groups, with the lowest mortality in patients with normocapnia and normal pH. After adjusting for severity of illness, the adjusted odds ratio for hospital mortality was higher in hypercapnic acidosis patients (odds ratio, 1.74; 95% CI, 1.62-1.88) and compensated hypercapnia (odds ratio, 1.18; 95% CI, 1.10-1.26) when compared with patients with normocapnia and normal pH (p < 0.001). In patients with hypercapnic acidosis, the mortality increased with increasing PCO2 until 65 mm Hg after which the mortality plateaued.

CONCLUSIONS

Hypercapnic acidosis during the first 24 hours of intensive care admission is more strongly associated with increased hospital mortality than compensated hypercapnia or normocapnia.

Carbon dioxide-dependent regulation of NF-κB family members RelB and p100 gives molecular insight into CO2-dependent immune regulation

CO₂ is a physiological gas normally produced in the body during aerobic respiration. Hypercapnia (elevated blood pCO₂ >≈50 mm Hg) is a feature of several lung pathologies, e.g. chronic obstructive pulmonary disease. Hypercapnia is associated with increased susceptibility to bacterial infections and suppression of inflammatory signaling. The NF-κB pathway has been implicated in these effects; however, the molecular mechanisms underpinning cellular sensitivity of the NF-κB pathway to CO₂ are not fully elucidated. Here, we identify several novel CO₂-dependent changes in the NF-κB pathway. NF-κB family members p100 and RelB translocate to the nucleus in response to CO₂ A cohort of RelB protein-protein interactions (e.g. with Raf-1 and IκBα) are altered by CO₂ exposure, although others are maintained (e.g. with p100). RelB is processed by CO₂ in a manner dependent on a key C-terminal domain located in its transactivation domain. Loss of the RelB transactivation domain alters NF-κB-dependent transcriptional activity, and loss of p100 alters sensitivity of RelB to CO₂ Thus, we provide molecular insight into the CO₂ sensitivity of the NF-κB pathway and implicate altered RelB/p100-dependent signaling in the CO₂-dependent regulation of inflammatory signaling.

Effect of low-level CO2 on innate inflammatory protein response to organic dust from swine confinement barns

BACKGROUND

Organic hog barn dust (HDE) exposure induces lung inflammation and long-term decreases in lung function in agricultural workers. While concentrations of common gasses in confined animal facilities are well characterized, few studies have been done addressing if exposure to elevated barn gasses impacts the lung immune response to organic dusts. Given the well documented effects of hypercapnia at much higher levels we hypothesized that CO2 at 8 h exposure limit levels (5000 ppm) could alter innate immune responses to HDE.

METHODS

Using a mouse model, C57BL/6 mice were nasally instilled with defined barn dust extracts and then housed in an exposure box maintained at one of several CO2 levels for six hours. Bronchiolar lavage (BAL) was tested for several cytokines while lung tissue was saved for mRNA purification and immunohistochemistry.

RESULTS

Exposure to elevated CO2 significantly increased the expression of pro-inflammatory markers, IL-6 and KC, in BAL fluid as compared to dust exposure alone. Expression of other pro-inflammatory markers, such as ICAM-1 and matrix metalloproteinase-9 (MMP-9), were also tested and showed similar increased expression upon HDE + CO2 exposure. A chemokine array analysis of BAL fluid revealed that MIP-1γ (CCL9) shows a similar increased response to HDE + CO2. Further testing showed CCL9 was significantly elevated by barn dust and further enhanced by CO2 co-exposure in a dose-dependent manner that was noticeable at the protein and mRNA levels. In all cases, except for ICAM-1, increases in tested markers in the presence of elevated CO2 were only significant in the presence of HDE as well.

CONCLUSIONS

We show that even at mandated safe exposure limits, CO2 is capable of enhancing multiple markers of inflammation in response to HDE.

The Right Ventricle in ARDS

ARDS is associated with poor clinical outcomes, with a pooled mortality rate of approximately 40% despite best standards of care. Current therapeutic strategies are based on improving oxygenation and pulmonary compliance while minimizing ventilator-induced lung injury. It has been demonstrated that relative hypoxemia can be well tolerated, and improvements in oxygenation do not necessarily translate into survival benefit. Cardiac failure, in particular right ventricular dysfunction (RVD), is commonly encountered in moderate to severe ARDS and is reported to be one of the major determinants of mortality. The prevalence rate of echocardiographically evident RVD in ARDS varies across studies, ranging from 22% to 50%. Although there is no definitive causal relationship between RVD and mortality, severe RVD is associated with increased mortality. Factors that can adversely affect RV function include hypoxic pulmonary vasoconstriction, hypercapnia, and invasive ventilation with high driving pressure. It might be expected that early diagnosis of RVD would be of benefit; however, echocardiographic markers (qualitative and quantitative) used to prospectively evaluate the right ventricle in ARDS have not been tested in adequately powered studies. In this review, we examine the prognostic implications and pathophysiology of RVD in ARDS and discuss available diagnostic modalities and treatment options. We aim to identify gaps in knowledge and directions for future research that could potentially improve clinical outcomes in this patient population.

α-Tocopherol transfer protein mediates protective hypercapnia in murine ventilator-induced lung injury

RATIONALE

Hypercapnia is common in mechanically ventilated patients. Experimentally, 'therapeutic hypercapnia' can protect, but it can also cause harm, depending on the mechanism of injury. Hypercapnia suppresses multiple signalling pathways. Previous investigations have examined mechanisms that were known a priori, but only a limited number of pathways, each suppressed by CO₂, have been reported.

OBJECTIVE

Because of the complexity and interdependence of processes in acute lung injury, this study sought to fill in knowledge gaps using an unbiased screen, aiming to identify a specifically upregulated pathway.

METHODS AND RESULTS

Using genome-wide gene expression analysis in a mouse model of ventilator-induced lung injury, we discovered a previously unsuspected mechanism by which CO₂ can protect against injury: induction of the transporter protein for α-tocopherol, α-tocopherol transfer protein (αTTP). Pulmonary αTTP was induced by inspired CO₂ in two in vivo murine models of ventilator-induced lung injury; the level of αTTP expression correlated with degree of lung protection; and, absence of the αTTP gene significantly reduced the protective effects of CO₂. α-Tocopherol is a potent antioxidant and hypercapnia increased lung α-tocopherol in wild-type mice, but this did not alter superoxide generation or expression of NRF2-dependent antioxidant response genes in wild-type or in αTTP^-/- mice. In concordance with a regulatory role for α-tocopherol in lipid mediator synthesis, hypercapnia attenuated 5-lipoxygenase activity and this was dependent on the presence of αTTP.

CONCLUSIONS

Inspired CO₂ upregulates αTTP which increases lung α-tocopherol levels and inhibits synthesis of a pathogenic chemoattractant.

Protective Ventilation Improves Gas Exchange, Reduces Incidence of Atelectases, and Affects Metabolic Response in Major Pancreatoduodenal Surgery

Background

Protective perioperative ventilation has been shown to improve outcomes and reduce the incidence of postoperative pulmonary complications. The goal of this study was to assess the effects of ventilation with low tidal volume (V_T) either alone or in a combination with moderate permissive hypercapnia in major pancreatoduodenal interventions.

Materials and methods

Sixty adult patients scheduled for elective pancreatoduodenal surgery with duration >2 h were enrolled into a prospective single-center study. All patients were randomized to three groups receiving high V_T [10 mL/kg of predicted body weight (PBW), the HVT group, n = 20], low V_T (6 mL/kg PBW, the LVT group, n = 20), and low V_T combined with a moderate hypercapnia and hypercapnic acidosis (6 mL/kg PBW, PaCO₂ 45–60 mm Hg, the LVT + HC group, n = 20). Cardiopulmonary parameters and the incidence of complications were registered during surgery and postoperatively.

Results and discussion

The values of V_T were 610 (563–712), 370 (321–400), and 340 (312–430) mL/kg for the HVT, the LVT, and the LVT + HC groups, respectively (p < 0.001). Compared to the HVT group, PaO₂/FiO₂ ratio was increased in the LVT group by 15%: 333 (301–381) vs. 382 (349–423) mm Hg at 24 h postoperatively (p < 0.05). The HVT group had significantly higher incidence of atelectases (n = 6), despite lower incidence of smoking compared with the LVT (n = 1) group (p = 0.017) and demonstrated longer length of hospital stay. The patients of the LVT + HC group had lower arterial lactate and bicarbonate excess values by the end of surgery.

Conclusion

In major pancreatoduodenal interventions, preventively protective V_T improves postoperative oxygenation, reduces the incidence of atelectases, and shortens length of hospital stay. The combination of low V_T and permissive hypercapnia results in hypercapnic acidosis decreasing the lactate concentration but adding no additional benefits and warrants further investigations.

Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is a serious inflammatory disorder with high mortality. Its main pathologic mechanism seems to result from increased alveolar permeability. Its definition has also changed since first being described according to the Berlin definition, which now classifies ARDS on a severity scale based on PaO2 (partial pressure of oxygen, arterial)/FIO2 (fraction of inspired oxygen) ratio. The cornerstone of therapy was found to be a low tidal volume strategy featuring volumes of 6 to 8 mL per kg of ideal body weight that has been shown to have decreased mortality as proven by the ARDSnet trials. There are other areas of treatment right now that include extracorporeal membrane oxygenation, as well for severe refractory hypoxemia. Other methods that include prone positioning for ventilation have also shown improvements in oxygenation. Positive end-expiratory pressure with lung recruitment maneuvers has also been found to be helpful. Other therapies that include vasodilators and neuromuscular agents are still being explored and need further studies to define their role in ARDS.

Ventilator-induced Lung Injury

Prevention of ventilator-induced lung injury (VILI) can attenuate multiorgan failure and improve survival in at-risk patients. Clinically significant VILI occurs from volutrauma, barotrauma, atelectrauma, biotrauma, and shear strain. Differences in regional mechanics are important in VILI pathogenesis. Several interventions are available to protect against VILI. However, most patients at risk of lung injury do not develop VILI. VILI occurs most readily in patients with concomitant physiologic insults. VILI prevention strategies must balance risk of lung injury with untoward side effects from the preventive effort, and may be most effective when targeted to subsets of patients at increased risk.

Hypercapnia slows down proliferation and apoptosis of human bone marrow promyeloblasts

Stem cells are being applied in increasingly diverse fields of research and therapy; as such, growing and culturing them in scalable quantities would be a huge advantage for all concerned. Gas mixtures containing 5 % CO2 are a typical concentration for the in vitro culturing of cells. The effect of varying the CO2 concentration on promyeloblast KG-1a cells was investigated in this paper. KG-1a cells are characterized by high expression of CD34 surface antigen, which is an important clinical surface marker for human hematopoietic stem cells (HSCs) transplantation. KG-1a cells were cultured in three CO2 concentrations (1, 5 and 15 %). Cells were batch-cultured and analyzed daily for viability, size, morphology, proliferation, and apoptosis using flow cytometry. No considerable differences were noted in KG-1a cell morphological properties at all three CO2 levels as they retained their myeloblast appearance. Calculated population doubling time increased with an increase in CO2 concentration. Enhanced cell proliferation was seen in cells cultured in hypercapnic conditions, in contrast to significantly decreased proliferation in hypocapnic populations. Flow cytometry analysis revealed that apoptosis was significantly (p = 0.0032) delayed in hypercapnic cultures, in parallel to accelerated apoptosis in hypocapnic ones. These results, which to the best of our knowledge are novel, suggest that elevated levels of CO2 are favored for the enhanced proliferation of bone marrow (BM) progenitor cells such as HSCs.

Hypercapnic acidosis attenuates pulmonary epithelial stretch-induced injury via inhibition of the canonical NF-κB pathway

BACKGROUND

Hypercapnia, with its associated acidosis (HCA), is a consequence of respiratory failure and is also seen in critically ill patients managed with conventional "protective" ventilation strategies. Nuclear factor kappa-B (NF-κB), a pivotal transcription factor, is activated in the setting of injury and repair and is central to innate immunity. We have previously established that HCA protects against ventilation-induced lung injury in vivo, potentially via a mechanism involving inhibition of NF-κB signaling. We wished to further elucidate the role and mechanism of HCA-mediated inhibition of the NF-κB pathway in attenuating stretch-induced injury in vitro.

METHODS

Initial experiments examined the effect of HCA on cyclic stretch-induced inflammation and injury in human bronchial and alveolar epithelial cells. Subsequent experiments examined the role of the canonical NF-κB pathway in mediating stretch-induced injury and the mechanism of action of HCA. The contribution of pH versus CO2 in mediating this effect of HCA was also examined.

RESULTS

Pulmonary epithelial high cyclic stretch (22 % equibiaxial strain) activated NF-κB, enhanced interleukin-8 (IL-8) production, caused cell injury, and reduced cell survival. In contrast, physiologic stretch (10 % strain) did not activate inflammation or cause cell injury. HCA reduced cyclic mechanical stretch-induced NF-κB activation, attenuated IL-8 production, reduced injury, and enhanced survival, in bronchial and alveolar epithelial cells, following shorter (24 h) and longer (120 h) cyclic mechanical stretch. Pre-conditioning with HCA was less effective than when HCA was applied after commencement of cell stretch. HCA prevented the stretch-induced breakdown of the NF-κB cytosolic inhibitor IκBα, while IκBα overexpression "occluded" the effect of HCA. These effects were mediated by a pH-dependent mechanism rather than via CO2 per se.

CONCLUSIONS

HCA attenuates adverse mechanical stretch-induced epithelial injury and death, via a pH-dependent mechanism that inhibits the canonical NF-κB activation by preventing IκBα breakdown.

The Effects of Lung Protective Ventilation or Hypercapnic Acidosis on Gas Exchange and Lung Injury in Surfactant Deficient Rabbits

Background

Permissive hypercapnia has been shown to reduce lung injury in subjects with surfactant deficiency. Experimental studies suggest that hypercapnic acidosis by itself rather than decreased tidal volume may be a key protective factor.

Objectives

To study the differential effects of a lung protective ventilatory strategy or hypercapnic acidosis on gas exchange, hemodynamics and lung injury in an animal model of surfactant deficiency.

Methods

30 anesthetized, surfactant-depleted rabbits were mechanically ventilated (FiO₂ = 0.8, PEEP = 7cmH₂O) and randomized into three groups: Normoventilation-Normocapnia (NN)-group: tidal volume (Vt) = 7.5 ml/kg, target PaCO₂ = 40 mmHg; Normoventilation-Hypercapnia (NH)-group: Vt = 7.5 ml/kg, target PaCO₂ = 80 mmHg by increasing FiCO₂; and a Hypoventilation-Hypercapnia (HH)-group: Vt = 4.5 ml/kg, target PaCO₂ = 80 mmHg. Plasma lactate and interleukin (IL)-8 were measured every 2 h. Animals were sacrificed after 6 h to perform bronchoalveolar lavage (BAL), to measure lung wet-to-dry weight, lung tissue IL-8, and to obtain lung histology.

Results

PaO₂ was significantly higher in the HH-group compared to the NN-group (p<0.05), with values of the NH-group between the HH- and NN-groups. Other markers of lung injury (wet-dry-weight, BAL-Protein, histology-score, plasma-IL-8 and lung tissue IL-8) resulted in significantly lower values for the HH-group compared to the NN-group and trends for the NH-group towards lower values compared to the NN-group. Lactate was significantly lower in both hypercapnia groups compared to the NN-group.

Conclusion

Whereas hypercapnic acidosis may have some beneficial effects, a significant effect on lung injury and systemic inflammatory response is dependent upon a lower tidal volume rather than resultant arterial CO₂ tensions and pH alone.

Metabolic acidosis aggravates experimental acute kidney injury

AIMS

Ischemia/reperfusion (I/R) injury and metabolic acidosis (MA) are two critical conditions that may simultaneously occur in clinical practice. The result of this combination can be harmful to the kidneys, but this issue has not been thoroughly investigated. The present study evaluated the influence of low systemic pH on various parameters of kidney function in rats that were subjected to an experimental model of renal I/R injury.

MAIN METHODS

Metabolic acidosis was induced in male Wistar rats by ingesting ammonium chloride (NH4Cl) in tap water, beginning 2 days before ischemic insult and maintained during the entire study. Ischemia/reperfusion was induced by clamping both renal arteries for 45 min, followed by 48 h of reperfusion. Four groups were studied: control (subjected to sham surgery, n=8), I/R (n=8), metabolic acidosis (MA; 0.28 M NH4Cl solution and sham surgery, n=6), and MA+I/R (0.28 M NH4Cl solution plus I/R, n=9).

KEY FINDINGS

Compared with I/R rats, MA+I/R rats exhibited higher mortality (50 vs. 11%, p=0.03), significant reductions of blood pH, plasma bicarbonate (pBic), and standard base excess (SBE), with a severe decline in the glomerular filtration rate and tubular function. Microscopic tubular injury signals were detected. Immunofluorescence revealed that the combination of MA and I/R markedly increased nuclear factor κB (NF-κB) and heme-oxygenase 1 (HO-1), but it did not interfere with the decrease in endothelial nitric oxide synthase (eNOS) expression that was caused by I/R injury.

SIGNIFICANCE

Acute ischemic kidney injury is exacerbated by acidic conditions.

Focused Screening Identifies Evoxine as a Small Molecule That Counteracts CO2-Induced Immune Suppression

Patients with severe lung disease may develop hypercapnia, elevation of the levels of CO2 in the lungs and blood, which is associated with increased risk of death, often from infection. To identify compounds that ameliorate the adverse effects of hypercapnia, we performed a focused screen of 8832 compounds using a CO2-responsive luciferase reporter in Drosophila S2* cells. We found that evoxine, a plant alkaloid, counteracts the CO2-induced transcriptional suppression of antimicrobial peptides in S2* cells. Strikingly, evoxine also inhibits hypercapnic suppression of interleukin-6 and the chemokine CCL2 expression in human THP-1 macrophages. Evoxine's effects are selective, since it does not prevent hypercapnic inhibition of phagocytosis by THP-1 cells or CO2-induced activation of AMPK in rat ATII pulmonary epithelial cells. The results suggest that hypercapnia suppresses innate immune gene expression by definable pathways that are evolutionarily conserved and demonstrate for the first time that specific CO2 effects can be targeted pharmacologically.

Lung protective ventilation in patients undergoing major surgery: a systematic review incorporating a Bayesian approach

OBJECTIVE

Protective ventilation (PV) has been validated in patients with acute respiratory distress syndrome. However, the effect of PV in patients undergoing major surgery is controversial. The study aimed to explore the beneficial effect of PV on patients undergoing a major operation by systematic review and meta-analysis.

SETTING

Various levels of medical centres.

PARTICIPANTS

Patients undergoing general anaesthesia.

INTERVENTIONS

PV with low tidal volume.

PRIMARY AND SECONDARY OUTCOME MEASURES

Study end points included acute lung injury (ALI), pneumonia, atelectasis, mortality, length of stay (LOS) in intensive care unit (ICU) and hospital.

METHODS

Databases including PubMed, Scopus, EBSCO and EMBASE were searched from inception to May 2015. Search strategies consisted of terms related to PV and anaesthesia. We reported OR for binary outcomes including ALI, mortality, pneumonia, atelectasis and other adverse outcomes. Weighted mean difference was reported for continuous outcomes such as LOS in the ICU and hospital, pH value, partial pressure of carbon dioxide, oxygenation and duration of mechanical ventilation (MV).

MAIN RESULTS

A total of 22 citations were included in the systematic review and meta-analysis. PV had protective effect against the development of ALI as compared with the control group, with an OR of 0.41 (95% CI 0.19 to 0.87). PV tended to be beneficial with regard to the development of pneumonia (OR 0.46, 95% CI 0.16 to 1.28) and atelectasis (OR 0.68, 95% CI 0.46 to 1.01), but statistical significance was not reached. Other adverse outcomes such as new onset arrhythmia were significantly reduced with the use of PV (OR 0.47, 95% CI 0.48 to 0.93).

CONCLUSIONS

The study demonstrates that PV can reduce the risk of ALI in patients undergoing major surgery. However, there is insufficient evidence that such a beneficial effect can be translated to more clinically relevant outcomes such as mortality or duration of MV.

TRIAL REGISTRATION NUMBER

The study was registered in PROSPERO (http://www.crd.york.ac.uk/PROSPERO/) under registration number CRD42013006416.

Effect of elevated carbon dioxide on bronchial epithelial innate immune receptor response to organic dust from swine confinement barns

Hypercapnia is known to have immunoregulatory effects within the lung. Cell culture systems demonstrate this in both macrophages and alveolar cell lines, suggesting that the alveoli are affected by changes in CO2 levels. We hypothesized that hypercapnia would also modulate human bronchial epithelial cell immune responses. Innate immune responses to Pam3CSK4 (TLR2 ligand), LPS (TLR4 ligand) and a complex innate immune stimulus, an extract from the organic dust of swine confinement barns (barn dust extract or BDE), were tested in a human bronchial epithelial cell line, BEAS-2B. Both TLR ligands showed a decrease in IL-6 and IL-8 production, and an increase in MCP-1 in response to elevated CO2 indicating an enhancement in cytokine production to hypercapnia. This change was not reflected in expression levels of TLR receptor RNA which remained unchanged in response to elevated CO2. Interestingly, barn dust showed an increase in IL-6, IL-8 and MCP-1 response at 9% CO2, suggesting that elevated CO2 exerts different effects on different stimuli. Our results show that airway epithelial cell immune responses to barn dust respond differently to hypercapnic conditions than individual TLR ligands.

Hypercapnia attenuates ventilator-induced lung injury via a disintegrin and metalloprotease-17

Hypercapnic acidosis, common in mechanically ventilated patients, has been reported to exert both beneficial and harmful effects in models of lung injury. Understanding its effects at the molecular level may provide insight into mechanisms of injury and protection. The aim of this study was to establish the effects of hypercapnic acidosis on mitogen‐activated protein kinase (MAPK) activation, and determine the relevant signalling pathways. p44/42 MAPK activation in a murine model of ventilator‐induced lung injury (VILI) correlated with injury and was reduced in hypercapnia. When cultured rat alveolar epithelial cells were subjected to cyclic stretch, activation of p44/42 MAPK was dependent on epidermal growth factor receptor (EGFR) activity and on shedding of EGFR ligands; exposure to 12% CO2 without additional buffering blocked ligand shedding, as well as EGFR and p44/42 MAPK activation. The EGFR ligands are known substrates of the matrix metalloprotease ADAM17, suggesting stretch activates and hypercapnic acidosis blocks stretch‐mediated activation of ADAM17. This was corroborated in the isolated perfused mouse lung, where elevated CO2 also inhibited stretch‐activated shedding of the ADAM17 substrate TNFR1 from airway epithelial cells. Finally, in vivo confirmation was obtained in a two‐hit murine model of VILI where pharmacological inhibition of ADAM17 reduced both injury and p44/42 MAPK activation. Thus, ADAM17 is an important proximal mediator of VILI; its inhibition is one mechanism of hypercapnic protection and may be a target for clinical therapy.

Therapeutic hypercapnia prevents inhaled nitric oxide-induced right-ventricular systolic dysfunction in juvenile rats

Chronic pulmonary hypertension in the neonate and infant frequently presents with right-ventricular (RV) failure. Current clinical management may include protracted treatment with inhaled nitric oxide (iNO), with the goal of reducing RV afterload. We have previously reported that prolonged exposure to iNO causes RV systolic dysfunction in the chronic hypoxia-exposed juvenile rat, which was prevented by a peroxynitrite decomposition catalyst. Given that inhalation of CO2 (therapeutic hypercapnia) may limit oxidative stress and upregulated cytokine expression in the lung and other organs, we hypothesized that therapeutic hypercapnia would attenuate cytokine-mediated nitric oxide synthase (NOS) upregulation, thus limiting peroxynitrite generation. Sprague-Dawley rat pups were exposed to chronic hypoxia (13% O2) from postnatal day 1 to 21, while receiving iNO (20 ppm) from day 14 to 21, with or without therapeutic hypercapnia (10% CO2). Therapeutic hypercapnia completely normalized RV systolic function, RV hypertrophy, and remodeling of pulmonary resistance arteries in animals exposed to iNO. Inhaled nitric oxide-mediated increases in RV peroxynitrite, apoptosis, and contents of tumor necrosis factor (TNF)-α, interleukin (IL)-1α, and NOS-2 were all attenuated by therapeutic hypercapnia. Inhibition of NOS-2 activity with 1400 W (1 mg/kg/day) prevented iNO-mediated upregulation of peroxynitrite and led to improved RV systolic function. Blockade of IL-1 receptor signaling with anakinra (500 mg/kg/day) decreased NOS-2 content and had similar effects compared to NOS-2 inhibition on iNO-mediated effects, whereas blockade of TNF-α signaling with etanercept (0.4 mg/kg on alternate days) had no effects on these parameters. We conclude that therapeutic hypercapnia prevents the adverse effects of sustained exposure to iNO on RV systolic function by limiting IL-1-mediated NOS-2 upregulation and consequent nitration. Therapeutic hypercapnia also acts synergistically with iNO in normalizing RV hypertrophy, vascular remodeling, and raised pulmonary vascular resistance secondary to chronic hypoxia.

Hypercapnic acidosis confers antioxidant and anti-apoptosis effects against ventilator-induced lung injury

Hypercapnic acidosis may attenuate ventilator-induced lung oxidative stress injury and alveolar cell apoptosis, but the underlying mechanisms are poorly understood. We examined the effects of hypercapnic acidosis on the role of apoptosis signal-regulating kinase 1 (ASK1), which activates the c-Jun N-terminal kinase (JNK) and p38 cascade in both apoptosis and oxidative reactions, in high-pressure ventilation stimulated rat lungs. Rats were ventilated with a peak inspiratory pressure (PIP) of 30 cmH2O for 4 h and randomly given FiCO2 to achieve normocapnia (PaCO2 at 35-45 mm Hg) or hypercapnia (PaCO2 at 80-100 mm Hg); normally ventilated rats with PIP of 15 cmH2O were used as controls. Lung injury was quantified by gas exchange, microvascular leaks, histology, levels of inflammatory cytokines, and pulmonary oxidative reactions. Apoptosis through the ASK1-JNK/p38 mitogen-activated protein kinase (MAPK) cascade in type II alveolar epithelial cells (AECIIs) were evaluated by examination of caspase-3 activation. The results showed that injurious ventilation caused significant lung injury, including deteriorative oxygenation, changes of histology, and the release of inflammatory cytokines. In addition, the high-pressure mechanical stretch also induced apoptosis and caspase-3 activation in the AECIIs. Hypercapnia attenuated these responses, suppressing the ASK1 signal pathways with its downstream kinase phosphorylation of p38 MAPK and JNK, and caspase-3 activation. Thus, hypercapnia can attenuate cell apoptosis and oxidative stress damage in rat lungs during injurious ventilation, at least in part, due to the suppression of the ASK1-JNK/p38 MAPK pathways.

Carbon dioxide-sensing in organisms and its implications for human disease

The capacity of organisms to sense changes in the levels of internal and external gases and to respond accordingly is central to a range of physiologic and pathophysiologic processes. Carbon dioxide, a primary product of oxidative metabolism is one such gas that can be sensed by both prokaryotic and eukaryotic cells and in response to altered levels, elicit the activation of multiple adaptive pathways. The outcomes of activating CO2-sensitive pathways in various species include increased virulence of fungal and bacterial pathogens, prey-seeking behavior in insects as well as taste perception, lung function, and the control of immunity in mammals. In this review, we discuss what is known about the mechanisms underpinning CO2 sensing across a range of species and consider the implications of this for physiology, disease progression, and the possibility of developing new therapeutics for inflammatory and infectious disease.

Acidosis activation of the proton-sensing GPR4 receptor stimulates vascular endothelial cell inflammatory responses revealed by transcriptome analysis

Acidic tissue microenvironment commonly exists in inflammatory diseases, tumors, ischemic organs, sickle cell disease, and many other pathological conditions due to hypoxia, glycolytic cell metabolism and deficient blood perfusion. However, the molecular mechanisms by which cells sense and respond to the acidic microenvironment are not well understood. GPR4 is a proton-sensing receptor expressed in endothelial cells and other cell types. The receptor is fully activated by acidic extracellular pH but exhibits lesser activity at the physiological pH 7.4 and minimal activity at more alkaline pH. To delineate the function and signaling pathways of GPR4 activation by acidosis in endothelial cells, we compared the global gene expression of the acidosis response in primary human umbilical vein endothelial cells (HUVEC) with varying level of GPR4. The results demonstrated that acidosis activation of GPR4 in HUVEC substantially increased the expression of a number of inflammatory genes such as chemokines, cytokines, adhesion molecules, NF-κB pathway genes, and prostaglandin-endoperoxidase synthase 2 (PTGS2 or COX-2) and stress response genes such as ATF3 and DDIT3 (CHOP). Similar GPR4-mediated acidosis induction of the inflammatory genes was also noted in other types of endothelial cells including human lung microvascular endothelial cells and pulmonary artery endothelial cells. Further analyses indicated that the NF-κB pathway was important for the acidosis/GPR4-induced inflammatory gene expression. Moreover, acidosis activation of GPR4 increased the adhesion of HUVEC to U937 monocytic cells under a flow condition. Importantly, treatment with a recently identified GPR4 antagonist significantly reduced the acidosis/GPR4-mediated endothelial cell inflammatory response. Taken together, these results show that activation of GPR4 by acidosis stimulates the expression of a wide range of inflammatory genes in endothelial cells. Such inflammatory response can be suppressed by GPR4 small molecule inhibitors and hold potential therapeutic value.