Hypercapnia impairs lung neutrophil function and increases mortality in murine pseudomonas pneumonia

Hypercapnia Increases Influenza A Virus Infection of Bronchial Epithelial Cells by Augmenting Cellular Cholesterol via mTOR and Akt

Hypercapnia, the elevation of CO2 in blood and tissue, is a risk factor for mortality in patients with severe lung disease and pulmonary infections. We previously showed that hypercapnia increases viral replication and mortality in mice infected with influenza A virus (IAV). Elevated CO2 also augmented cholesterol content and pseudo-SARS-CoV-2 entry in bronchial epithelial cells. Interestingly, cellular cholesterol facilitates IAV uptake, replication, assembly, and egress from cells. Here, we report that hypercapnia increases viral protein expression in airway epithelium of mice infected with IAV. Elevated CO2 also enhanced IAV adhesion and internalization, viral protein expression, and viral replication in bronchial epithelial cells. Hypercapnia increased the expression and activation of the transcription factor sterol-regulatory element binding protein 2 (SREBP2), resulting in elevated expression of cholesterol synthesis enzymes, decreased expression of a cholesterol efflux transporter, and augmented cellular cholesterol. Moreover, reducing cellular cholesterol with an SREBP2 inhibitor or statins blocked hypercapnia-induced increases in viral adhesion and internalization, viral protein expression, and IAV replication. Inhibitors of mTOR and Akt also blocked the effect of hypercapnia on viral growth. Our findings suggest that targeting cholesterol synthesis and/or mTOR/Akt signaling may hold promise for reducing susceptibility to influenza infection in patients with advanced lung disease and hypercapnia.

Exploring Alternate Targets for Respiratory Resuscitation in Patients With Sepsis and Septic Shock

Despite limited evidence to support it, resuscitation in sepsis has primarily targeted aggressive fluid administration and liberal administration of oxygen. In 2024, new thought paradigms emerged to suggest that dysregulation of aerobic metabolism are essential underpinnings of sepsis, and that in fact, aggressive resuscitation with fluids liberal oxygen could potentially aggravate oxidative stress and organ failure in sepsis. As sepsis continues to be shaped and molded by the latest research; therapies targeting sepsis and septic shock management warrant similar scrutiny.

METHODS

We searched literature pertaining to what is known about metabolic dysregulation in sepsis, to consider approaches to identifying new targets for resuscitation and management in sepsis.

RESULTS

Therapeutic hypoxemic targets of 88-92% have been shown to have some benefit in sepsis resuscitation in a limited number of studies. The benefit is believed to result from protection from excessive accumulation of harmful reactive oxygen species.

CONCLUSION

Limited supporting evidence exists in the literature to recommend targeted hypoxemia or hypercapnia in patients with sepsis. Mixed results have been observed in the literature, including minimal benefit to mortality. New research designs with consideration to the dysregulated metabolic sequelae in sepsis could improve the meaningfulness of these therapies in sepsis.

Identifying Daytime Hypercapnia Using Transcutaneous Carbon Dioxide Monitoring in Patients With Amyotrophic Lateral Sclerosis

INTRODUCTION/AIMS

Respiratory failure from hypoventilation is the most common cause of death in amyotrophic lateral sclerosis (ALS). However, ALS care rarely assesses hypercapnia, a physiologic measure of hypoventilation. We investigated the prevalence and clinical significance of daytime hypercapnia measured by transcutaneous carbon dioxide (tcCO2) monitoring in patients with ALS.

METHODS

This retrospective study included patients seen at two ALS clinics in the United States between 2012 and 2024 who had tcCO2 measured concurrently with pulmonary function tests (PFTs), which included forced vital capacity (FVC) and, at one site, maximum inspiratory pressure (MIP). We assessed the prevalence of hypercapnia (tcCO2 > 45 mmHg), the sensitivity and specificity of patient symptoms and PFTs for hypercapnia, and the relationship between hypercapnia and survival.

RESULTS

Daytime hypercapnia was present in 33/328 (10%) patients at baseline. Hypercapnia was associated with an increased rate of death (aHR 2.1, 95% CI 1.4-3.3). Orthopnea or dyspnea was 70% sensitive for hypercapnia (95% CI 51%-84%). Absolute value of MIP (|MIP|) < 60 cmH2O was 95% sensitive (95% CI 74%-100%) and 22% specific (95% CI 16%-30%), FVC < 50% predicted was 33% sensitive (95% CI 18%-52%) and 82% specific (95% CI 78%-87%), and FVC < 80% predicted was 85% sensitive (95% CI 68%-95%) and 31% specific (95% CI 26%-36%) for hypercapnia.

DISCUSSION

TcCO2 monitoring identified strengths and weaknesses of PFTs in identifying hypercapnia in ALS. We found high sensitivity of |MIP| < 60 cmH2O and FVC < 80% predicted and high specificity of FVC < 50% predicted. Prospective studies should investigate the optimal clinical role for tcCO2.

Initiation of home noninvasive ventilation in hypercapnic chronic obstructive pulmonary disease: when, where, and how?

PURPOSE OF REVIEW

This review aims to highlight the importance of timely initiation of home noninvasive ventilation (homeNIV) for patients with chronic hypercapnic respiratory failure (CHRF) due to chronic obstructive pulmonary disease (COPD). As emerging evidence continues to show substantial benefits in reducing mortality and hospitalizations, it's crucial to identify which patients will benefit most and to provide clear guidance on implementing homeNIV effectively.

RECENT FINDINGS

Recent research supports the use of high intensity homeNIV for CHRF secondary to COPD, showing marked reductions in hospitalizations and mortality. However, despite its proven benefits, homeNIV is underutilized, often due to significant barriers related to payor policies and gaps in knowledge by those most likely to be evaluating and managing patients with advanced COPD. The literature also reveals ongoing debate about the optimal timing and setting for starting homeNIV, whether in outpatient clinics or directly after hospital discharge.

SUMMARY

The evidence suggests that homeNIV should be more widely used, with a focus on early initiation and careful titration to normalize PaCO 2 over time. By addressing the barriers to its broader use, we can improve outcomes for patients with CHRF due to COPD.

Rapamycin improves satellite cells' autophagy and muscle regeneration during hypercapnia

Both CO2 retention, or hypercapnia, and skeletal muscle dysfunction predict higher mortality in critically ill patients. Mechanistically, muscle injury and reduced myogenesis contribute to critical illness myopathy, and while hypercapnia causes muscle wasting, no research has been conducted on hypercapnia-driven dysfunctional myogenesis in vivo. Autophagy flux regulates myogenesis by supporting skeletal muscle stem cell - satellite cell - activation, and previous data suggest that hypercapnia inhibits autophagy. We tested whether hypercapnia worsens satellite cell autophagy flux and myogenic potential and if autophagy induction reverses these deficits. Satellite cell transplantation and lineage-tracing experiments showed that hypercapnia undermined satellite cells' activation, replication, and myogenic capacity. Bulk and single-cell sequencing analyses indicated that hypercapnia disrupts autophagy, senescence, and other satellite cell programs. Autophagy activation was reduced in hypercapnic cultured myoblasts, and autophagy genetic knockdown phenocopied these changes in vitro. Rapamycin stimulation led to AMPK activation and downregulation of the mTOR pathway, which are both associated with accelerated autophagy flux and cell replication. Moreover, hypercapnic mice receiving rapamycin showed improved satellite cell autophagy flux, activation, replication rate, and posttransplantation myogenic capacity. In conclusion, we have shown that hypercapnia interferes with satellite cell activation, autophagy flux, and myogenesis, and systemic rapamycin administration improves these outcomes.

Hypercapnia outcome in COVID-19 acute respiratory distress syndrome patients on mechanical ventilator: A retrospective observational cohort

Introduction

Acute respiratory distress syndrome (ARDS) is characterized by progressive lung inflammation which leads to increased dead space that can cause hypercapnia and can increase the risk of patient morbidity and mortality. In an attempt to improve ARDS patient outcomes provision of protective lung ventilation has been shown to improve patient mortality but increases the incidence of hypercapnia. Therefore, the role of carbon dioxide in ARDS remains contradicted by conflicted evidence. This study aims to examine this conflicting relationship between hyper-capnia and mortality in mechanically ventilated COVID-19 ARDS patients.

Methods

We conducted a retrospective cohort study. The data was collected from the medical records of the patients admitted with COVID-19 ARDS in Sindh Infectious Disease Hospital & Research Centre (SIDH & RC) from August 2020 to August 2022 and who received mechanical ventilation for more than 48 hours. The patients were grouped into severe and no severe hypercapnia groups based on their arterial blood carbon dioxide levels (PaCO2). To understand the effect of hypercapnia on mortality we performed multivariable logistic regression, and inverse probability-weighted regression to adjust for time-varying confounders.

Results

We included 288 patients to detect at least 3% of the effect on mortality. Our analysis revealed an association of severe hypercapnia with severe lung injury, low PaO2/FiO2, high dead space, and poor compliance. In univariate analysis severe hypercapnia showed higher mortality: OR=3.50, 95% CI (1.46-8.43). However, after, adjusting for disease severity hypercapnia is not found to be associated with mortality: OR=1.08, 95% CI (0.32-3.64). The sensitive analysis with weighted regression also shows no significant effect on mortality: OR=1.04, 95% CI (0.95-1.14).

Conclusion

This study showed that hypercapnia is not associated with mortality in COVID-19 ARDS patients.

Extracorporeal Support for Acute Respiratory Distress Syndrome

Extracorporeal life support (ECLS) has a long history in the management of the acute respiratory distress syndrome (ARDS). The objectives of this review are to summarize the rationale and evidence for ECLS in ARDS including its role in reducing ventilator-induced lung injury (VILI), suggest best practice management strategies during ECLS, and identify areas that require additional research to better inform patient care.

Development and validation of a nomogram for predicting pulmonary complications in elderly patients undergoing thoracic surgery

BACKGROUND

Postoperative pulmonary complications (PPCs) remain a prevalent concern among elderly patients undergoing surgery, with a notably higher incidence observed in elderly patients undergoing thoracic surgery. This study aimed to develop a nomogram to predict the risk of PPCs in this population.

METHODS

A total of 2963 elderly patients who underwent thoracic surgery were enrolled and randomly divided into a training cohort (80%, n = 2369) or a validation cohort (20%, n = 593). Univariate and multivariate logistic regression analyses were conducted to identify risk factors for PPCs, and a nomogram was developed based on the findings from the training cohort. The validation cohort was used to validate the model. The predictive accuracy of the model was evaluated by receiver operating characteristic (ROC) curve, area under ROC (AUC), calibration curve, and decision curve analysis (DCA).

RESULTS

A total of 918 (31.0%) patients reported PPCs. Nine independent risk factors for PPCs were identified: preoperative presence of chronic obstructive pulmonary disease (COPD), elevated leukocyte count, higher partial pressure of arterial carbon dioxide (PaCO2) level, surgical site, thoracotomy, intraoperative hypotension, blood loss > 100 mL, surgery duration > 180 min, and malignant tumor. The AUC value for the training cohort was 0.739 (95% CI: 0.719-0.762), and it was 0.703 for the validation cohort (95% CI: 0.657-0.749). The P-values for the Hosmer-Lemeshow test were 0.633 and 0.144 for the training and validation cohorts, respectively, indicating a notable calibration curve fit. The DCA curve indicated that the nomogram could be applied clinically if the risk threshold was between 12% and 84%, which was found to be between 8% and 82% in the validation cohort.

CONCLUSION

This study highlighted the pressing need for early detection of PPCs in elderly patients undergoing thoracic surgery. The nomogram exhibited promising predictive efficacy for PPCs in elderly patients undergoing thoracic surgery, enabling the identification of high-risk patients and consequently aiding in the implementation of preventive interventions.

Obstructive Sleep Apnea and Acute Lower Respiratory Tract Infections: A Narrative Literature Review

Both obstructive sleep apnea (OSA) and acute lower respiratory tract infections (LRTIs) are important global health issues. The pathophysiological links between OSA and LRTIs include altered immune responses due to chronic intermittent hypoxia and sleep fragmentation, increased aspiration risk, and a high burden of comorbidities. In this narrative review, we evaluated the current evidence on the association between OSA and the incidence and outcomes of acute LRTIs in adults, specifically community-acquired pneumonia and viral pneumonia caused by influenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Studies have demonstrated that OSA patients are more likely to develop bacterial pneumonia and exhibit a higher risk of invasive pneumococcal disease. The risk intensifies with the severity of OSA, influencing hospitalization rates and the need for intensive care. OSA is also associated with an increased risk of contracting influenza and suffering more severe disease, potentially necessitating hospitalization. Similarly, OSA contributes to increased COVID-19 disease severity, reflected by higher rates of hospitalization, longer hospital stays, and a higher incidence of acute respiratory failure. The effect of OSA on mortality rates from these infections is, however, somewhat ambiguous. Finally, we explored antibiotic therapy for OSA patients with LRTIs, addressing care settings, empirical regimens, risks, and pharmacokinetic considerations. Given the substantial burden of OSA and its significant interplay with acute LRTIs, enhanced screening, targeted vaccinations, and optimized management strategies for OSA patients should be prioritized.

Obstructive Sleep Apnea, Obesity Hypoventilation Syndrome, and Pulmonary Hypertension: A State-of-the-Art Review

The pathophysiological interplay between sleep-disordered breathing (SDB) and pulmonary hypertension (PH) is complex and can involve a variety of mechanisms by which SDB can worsen PH. These mechanistic pathways include wide swings in intrathoracic pressure while breathing against an occluded upper airway, intermittent and/or sustained hypoxemia, acute and/or chronic hypercapnia, and obesity. In this review, we discuss how the downstream consequences of SDB can adversely impact PH, the challenges in accurately diagnosing and classifying PH in the severely obese, and review the limited literature assessing the effect of treating obesity, obstructive sleep apnea, and obesity hypoventilation syndrome on PH.

Unveiling cytokine charge disparity as a potential mechanism for immune regulation

Cytokines are small signaling proteins that regulate the immune responses to infection and tissue damage. Surface charges of cytokines determine their in vivo fate in immune regulation, e.g., half-life and distribution. The overall negative charges in the extracellular microenvironment and the acidosis during inflammation and infection may differentially impact cytokines with different surface charges for fine-tuned immune regulation via controlling tissue residential properties. However, the trend and role of cytokine surface charges has yet to be elucidated in the literature. Interestingly, we have observed that most pro-inflammatory cytokines have a negative charge, while most anti-inflammatory cytokines and chemokines have a positive charge. In this review, we extensively examined the surface charges of all cytokines and chemokines, summarized the pharmacokinetics and tissue adhesion of major cytokines, and analyzed the link of surface charge with cytokine biodistribution, activation, and function in immune regulation. Additionally, we identified that the general trend of charge disparity between pro- and anti-inflammatory cytokines represents a unique opportunity to develop precise immune modulation approaches, which can be applied to many inflammation-associated diseases including solid tumors, chronic wounds, infection, and sepsis.

Immune Dysregulation in Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is a disease whose incidence increase with age and is characterised by chronic inflammation and significant immune dysregulation. Inhalation of toxic substances cause oxidative stress in the lung tissue as well as airway inflammation, under the recruitment of chemokines, immune cells gathered and are activated to play a defensive role. However, persistent inflammation damages the immune system and leads to immune dysregulation, which is mainly manifested in the reduction of the body's immune response to antigens, and immune cells function are impaired, further destroy the respiratory defensive system, leading to recurrent lower respiratory infections and progressive exacerbation of the disease, thus immune dysregulation play an important role in the pathogenesis of COPD. This review summarizes the changes of innate and adaptive immune-related cells during the pathogenesis of COPD, aiming to control COPD airway inflammation and improve lung tissue remodelling by regulating immune dysregulation, for further reducing the risk of COPD progression and opening new avenues of therapeutic intervention in COPD.

CO2 exposure drives a rapid pH response in live adult Drosophila

CO2 anesthesia is the most common method for immobilizing Drosophila for research purposes. But CO2 exposure has consequences-it can impact fertility, behavior, morphogenesis, and cytoskeletal dynamics. In this respect, Drosophila is an outstanding model for studying the impact of CO2 exposure on tissues. In this study we explored the response of intracellular pH (pHi) to a one-minute CO2 pulse using a genetically encoded, ubiquitously expressed pH sensor, tpHusion, to monitor pHi within a live, intact, whole fly. We compared wild-type flies to flies lacking Imaginal disc growth factors (Idgfs), which are chitinase-like proteins that facilitate developmental processes and the innate immune response. Morphogenetic and cytoskeletal defects in Idgf-null flies are enhanced after CO2 exposure. We found that pHi drops sharply within seconds of the beginning of a CO2 pulse and recovers over several minutes. The initial profile was nearly identical in control and Idgf-null flies but diverged as the pHi returned to normal. This study demonstrates the feasibility of monitoring pH in live adult Drosophila. Studies exploring pH homeostasis are important for understanding human pathologies associated with pH dysregulation.

Myeloid Zfhx3 deficiency protects against hypercapnia-induced suppression of host defense against influenza A virus

Hypercapnia, elevation of the partial pressure of CO2 in blood and tissues, is a risk factor for mortality in patients with severe acute and chronic lung diseases. We previously showed that hypercapnia inhibits multiple macrophage and neutrophil antimicrobial functions and that elevated CO2 increases the mortality of bacterial and viral pneumonia in mice. Here, we show that normoxic hypercapnia downregulates innate immune and antiviral gene programs in alveolar macrophages (AMØs). We also show that zinc finger homeobox 3 (Zfhx3) - a mammalian ortholog of zfh2, which mediates hypercapnic immune suppression in Drosophila - is expressed in mouse and human macrophages. Deletion of Zfhx3 in the myeloid lineage blocked the suppressive effect of hypercapnia on immune gene expression in AMØs and decreased viral replication, inflammatory lung injury, and mortality in hypercapnic mice infected with influenza A virus. To our knowledge, our results establish Zfhx3 as the first known mammalian mediator of CO2 effects on immune gene expression and lay the basis for future studies to identify therapeutic targets to interrupt hypercapnic immunosuppression in patients with advanced lung disease.

Hypercapnia and its relationship with respiratory infections

INTRODUCTION

Hypercapnia is developed in patients with acute and/or chronic respiratory conditions. Clinical data concerning hypercapnia and respiratory infections interaction is limited.

AREAS COVERED

Currently, the relationship between hypercapnia and respiratory infections remains unclear. In this review, we summarize studies on the effects of hypercapnia on models of pulmonary infections to clarify the role of elevated CO2 in these pulmonary pathologies. Hypercapnia affects different cell types in the alveoli, leading to changes in the immune response. In vitro studies show that hypercapnia downregulates the NF-κβ pathway, reduces inflammation and impairs epithelial wound healing. While in vivo models show a dual role between short- and long-term effects of hypercapnia on lung infection. However, it is still controversial whether the effects observed under hypercapnia are pH dependent or not.

EXPERT OPINION

The role of hypercapnia is still a controversial debate. Hypercapnia could play a beneficial role in mechanically ventilated models, by lowering the inflammation produced by the stretch condition. But it could be detrimental in infectious scenarios, causing phagocyte dysfunction and lack of infection control. Further data concerning hypercapnia on respiratory infections is needed to elucidate this interaction.

Lowering PCO2 With Noninvasive Ventilation Is Associated With Improved Survival in Chronic Hypercapnic Respiratory Failure

BACKGROUND

Chronic hypercapnic respiratory failure is associated with high mortality. Although previous work has demonstrated a mortality improvement with high-intensity noninvasive ventilation in COPD, it is unclear whether a PCO2 reduction strategy is associated with improved outcomes in other populations of chronic hypercapnia.

METHODS

The objective of this study was to investigate the association between PCO2 reduction (by using transcutaneous PCO2 as an estimate for PaCO2 and survival in a broad population of individuals treated with noninvasive ventilation for chronic hypercapnia. We hypothesized that reductions in PCO2 would be associated with improved survival. Therefore, we performed a cohort study of all the subjects evaluated from February 2012 to January 2021 for noninvasive ventilation initiation and/or optimization due to chronic hypercapnia at a home ventilation clinic in an academic center. We used multivariable Cox proportional hazard models with time-varying coefficients and PCO2 as a time-varying covariate to test the association between PCO2 and all-cause mortality and when adjusting for known cofounders.

RESULTS

The mean ± SD age of 337 subjects was 57 ± 16 years, 37% women, and 85% white. In a univariate analysis, survival probability increased with reductions in PCO2 to < 50 mm Hg after 90 d, and these remained significant after adjusting for age, sex, race, body mass index, diagnosis, Charlson comorbidity index, and baseline PCO2 . In the multivariable analysis, the subjects who had a PaCO2 < 50 mm Hg had a reduced mortality risk of 94% between 90 and 179 d (hazard ratio [HR] 0.06, 95% CI 0.01-0.50), 69% between 180 and 364 d (HR 0.31, 95% CI 0.12-0.79), and 73% for 365-730 d (HR 0.27, 95% CI 0.13-0.56).

CONCLUSIONS

Reduction in PCO2 from baseline for subjects with chronic hypercapnia treated with noninvasive ventilation was associated with improved survival. Management strategies should target the greatest attainable reductions in PCO2 .

Mild and moderate chronic hypercapnia elicit distinct transcriptomic responses of immune function in cardiorespiratory nuclei

Chronic hypercapnia (CH) is a hallmark of respiratory-related diseases, and the level of hypercapnia can acutely or progressively become more severe. Previously, we have shown time-dependent adaptations in steady-state physiology during mild (arterial Pco2 ∼55 mmHg) and moderate (∼60 mmHg) CH in adult goats, including transient (mild CH) or sustained (moderate CH) suppression of acute chemosensitivity suggesting limitations in adaptive respiratory control mechanisms as the level of CH increases. Changes in specific markers of glutamate receptor plasticity, interleukin-1ß, and serotonergic modulation within key nodes of cardiorespiratory control do not fully account for the physiological adaptations to CH. Here, we used an unbiased approach (bulk tissue RNA sequencing) to test the hypothesis that mild or moderate CH elicits distinct gene expression profiles in important brain stem regions of cardiorespiratory control, which may explain the contrasting responses to CH. Gene expression profiles from the brain regions validated the accuracy of tissue biopsy methodology. Differential gene expression analyses revealed greater effects of CH on brain stem sites compared with the medial prefrontal cortex. Mild CH elicited an upregulation of predominantly immune-related genes and predicted activation of immune-related pathways and functions. In contrast, moderate CH broadly led to downregulation of genes and predicted inactivation of cellular pathways related to the immune response and vascular function. These data suggest that mild CH leads to a steady-state activation of neuroinflammatory pathways within the brain stem, whereas moderate CH drives the opposite response. Transcriptional shifts in immune-related functions may underlie the cardiorespiratory network's capability to respond to acute, more severe hypercapnia when in a state of progressively increased CH.NEW & NOTEWORTHY Mild chronic hypercapnia (CH) broadly upregulated immune-related genes and a predicted activation of biological pathways related to immune cell activity and the overall immune response. In contrast, moderate CH primarily downregulated genes related to major histocompatibility complex signaling and vasculature function that led to a predicted inactivation of pathways involving the immune response and vascular endothelial function. The severity-dependent effect on immune responses suggests that neuroinflammation has an important role in CH and may be important in the maintenance of proper ventilatory responses to acute and chronic hypercapnia.

Hypercapnia increases ACE2 expression and pseudo-SARS-CoV-2 entry in bronchial epithelial cells by augmenting cellular cholesterol

Patients with chronic lung disease, obesity, and other co-morbid conditions are at increased risk of severe illness and death when infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Hypercapnia, the elevation of CO2 in blood and tissue, commonly occurs in patients with severe acute and chronic lung disease, including those with pulmonary infections, and is also associated with high mortality risk. We previously reported that hypercapnia increases viral replication and mortality of influenza A virus infection in mice. We have also shown that culture in elevated CO2 upregulates expression of cholesterol synthesis genes in primary human bronchial epithelial cells. Interestingly, factors that increase the cholesterol content of lipid rafts and lipid droplets, platforms for viral entry and assembly, enhance SARS-CoV-2 infection. In the current study, we investigated the effects of hypercapnia on ACE2 expression and entry of SARS-CoV-2 pseudovirus (p-SARS-CoV-2) into airway epithelial cells. We found that hypercapnia increased ACE2 expression and p-SARS-CoV-2 uptake by airway epithelium in mice, and in cultured VERO and human bronchial epithelial cells. Hypercapnia also increased total cellular and lipid raft-associated cholesterol in epithelial cells. Moreover, reducing cholesterol synthesis with inhibitors of sterol regulatory element binding protein 2 (SREBP2) or statins, and depletion of cellular cholesterol, each blocked the hypercapnia-induced increases in ACE2 expression and p-SARS-CoV-2 entry into epithelial cells. Cigarette smoke extract (CSE) also increased ACE2 expression, p-SARS-CoV-2 entry and cholesterol accumulation in epithelial cells, an effect not additive to that of hypercapnia, but also inhibited by statins. These findings reveal a mechanism that may account, in part, for poor clinical outcomes of SARS-CoV-2 infection in patients with advanced lung disease and hypercapnia, and in those who smoke cigarettes. Further, our results suggest the possibility that cholesterol-lowering therapies may be of particular benefit in patients with hypercapnia when exposed to or infected with SARS-CoV-2.

Hypercapnia alters mitochondrial gene expression and acylcarnitine production in monocytes

CO2 is produced during aerobic respiration. Normally, levels of CO2 in the blood are tightly regulated but pCO2 can rise (hypercapnia, pCO2  > 45 mmHg) in patients with lung diseases, for example, chronic obstructive pulmonary disease (COPD). Hypercapnia is a risk factor in COPD but may be of benefit in the context of destructive inflammation. The effects of CO2 per se, on transcription, independent of pH change are poorly understood and warrant further investigation. Here we elucidate the influence of hypercapnia on monocytes and macrophages through integration of state-of-the-art RNA-sequencing, metabolic and metabolomic approaches. THP-1 monocytes and interleukin 4-polarized primary murine macrophages were exposed to 5% CO2 versus 10% CO2 for up to 24 h in pH-buffered conditions. In hypercapnia, we identified around 370 differentially expressed genes (DEGs) under basal and about 1889 DEGs under lipopolysaccharide-stimulated conditions in monocytes. Transcripts relating to both mitochondrial and nuclear-encoded gene expression were enhanced in hypercapnia in basal and lipopolysaccharide-stimulated cells. Mitochondrial DNA content was not enhanced, but acylcarnitine species and genes associated with fatty acid metabolism were increased in hypercapnia. Primary macrophages exposed to hypercapnia also increased activation of genes associated with fatty acid metabolism and reduced activation of genes associated with glycolysis. Thus, hypercapnia elicits metabolic shifts in lipid metabolism in monocytes and macrophages under pH-buffered conditions. These data indicate that CO2 is an important modulator of monocyte transcription that can influence immunometabolic signaling in immune cells in hypercapnia. These immunometabolic insights may be of benefit in the treatment of patients experiencing hypercapnia.

Myeloid Zfhx3 Deficiency Protects Against Hypercapnia-induced Suppression of Host Defense Against Influenza A Virus

Hypercapnia, elevation of the partial pressure of CO 2 in blood and tissues, is a risk factor for mortality in patients with severe acute and chronic lung diseases. We previously showed that hypercapnia inhibits multiple macrophage and neutrophil antimicrobial functions, and that elevated CO 2 increases the mortality of bacterial and viral pneumonia in mice. Here, we show that normoxic hypercapnia downregulates innate immune and antiviral gene programs in alveolar macrophages (AMØs). We also show that zinc finger homeobox 3 (Zfhx3), mammalian ortholog of zfh2, which mediates hypercapnic immune suppression in Drosophila , is expressed in mouse and human MØs. Deletion of Zfhx3 in the myeloid lineage blocked the suppressive effect of hypercapnia on immune gene expression in AMØs and decreased viral replication, inflammatory lung injury and mortality in hypercapnic mice infected with influenza A virus. Our results establish Zfhx3 as the first known mammalian mediator of CO 2 effects on immune gene expression and lay the basis for future studies to identify therapeutic targets to interrupt hypercapnic immunosuppression in patients with advanced lung diseases.

Graphical abstract

Association between rate of change in PaCO2 and functional outcome for patients with hypercapnia after out-of-hospital cardiac arrest: Secondary analysis of a randomized clinical trial

BACKGROUND

Normocapnia is suggested for post resuscitation care. For patients with hypercapnia after cardiac arrest, the relationship between rate of change in partial pressure of carbon dioxide (PaCO₂) and functional outcome was unknown.

METHODS

This was the secondary analysis of Resuscitation Outcomes Consortium (ROC) amiodarone, lidocaine, and placebo (ALPS) trial. Patients with at least 2 PaCO₂ recorded and the first indicating hypercapnia (PaCO₂ > 45 mmHg) after return of spontaneous circulation (ROSC) were included. The rate of change in PaCO₂ was calculated as the ratio of the difference between the second and first PaCO₂ to the time interval. The primary outcome was modified Rankin Score (mRS), dichotomized to good (mRS 0-3) and poor (mRS 4-6) outcomes at hospital discharge. The independent relationship between rate of change in PaCO₂ and outcome was investigated with multivariable logistic regression model.

RESULTS

A total of 746 patients with hypercapnia were included for analysis, of which 264 (35.4%) patients had good functional outcome. The median rate of change in PaCO₂ was 4.7 (interquartile range [IQR] 1.7-12) mmHg per hour. After adjusting for confounders, the rate of change in PaCO₂ (odds ratio [OR] 0.994, confidence interval [CI] 0.985-1.004, p = 0.230) was not associated the functional outcome. However, rate of change in PaCO₂ (OR 1.010, CI 1.001-1.019, p = 0.029) was independently associated with hospital mortality.

CONCLUSIONS

For OHCA patients with hypercapnia on admission, the rate of change in PaCO₂ was not independently associated with functional outcome; however, there was a significant trend that higher decreased rate was associated with increased hospital mortality.

Hypercapnia alters stroma-derived Wnt production to limit β-catenin signaling and proliferation in AT2 cells

Persistent symptoms and radiographic abnormalities suggestive of failed lung repair are among the most common symptoms in patients with COVID-19 after hospital discharge. In mechanically ventilated patients with acute respiratory distress syndrome (ARDS) secondary to SARS-CoV-2 pneumonia, low tidal volumes to reduce ventilator-induced lung injury necessarily elevate blood CO2 levels, often leading to hypercapnia. The role of hypercapnia on lung repair after injury is not completely understood. Here - using a mouse model of hypercapnia exposure, cell lineage tracing, spatial transcriptomics, and 3D cultures - we show that hypercapnia limits β-catenin signaling in alveolar type II (AT2) cells, leading to their reduced proliferative capacity. Hypercapnia alters expression of major Wnts in PDGFRα+ fibroblasts from those maintaining AT2 progenitor activity toward those that antagonize β-catenin signaling, thereby limiting progenitor function. Constitutive activation of β-catenin signaling in AT2 cells or treatment of organoid cultures with recombinant WNT3A protein bypasses the inhibitory effects of hypercapnia. Inhibition of AT2 proliferation in patients with hypercapnia may contribute to impaired lung repair after injury, preventing sealing of the epithelial barrier and increasing lung flooding, ventilator dependency, and mortality.

Clinical characteristics, physiological features, and outcomes associated with hypercapnia in patients with acute hypoxemic respiratory failure due to COVID-19---insights from the PRoVENT-COVID study

PURPOSE

We determined the incidence of hypercapnia and associations with outcome in invasively ventilated COVID-19 patients.

METHODS

Posthoc analysis of a national, multicenter, observational study in 22 ICUs. Patients were classified as 'hypercapnic' or 'normocapnic' in the first three days of invasive ventilation. Primary endpoint was prevalence of hypercapnia. Secondary endpoints were ventilator parameters, length of stay (LOS) in ICU and hospital, and mortality in ICU, hospital, at day 28 and 90.

RESULTS

Of 824 patients, 485 (58.9%) were hypercapnic. Hypercapnic patients had a higher BMI and had COPD, severe ARDS and venous thromboembolic events more often. Hypercapnic patients were ventilated with lower tidal volumes, higher respiratory rates, higher driving pressures, and with more mechanical power of ventilation. Hypercapnic patients had comparable minute volumes but higher ventilatory ratios than normocapnic patients. In hypercapnic patients, ventilation and LOS in ICU and hospital was longer, but mortality was comparable to normocapnic patients.

CONCLUSION

Hypercapnia occurs often in invasively ventilated COVID-19 patients. Main differences between hypercapnic and normocapnic patients are severity of ARDS, occurrence of venous thromboembolic events, and a higher ventilation ratio. Hypercapnia has an association with duration of ventilation and LOS in ICU and hospital, but not with mortality.

Carbon Dioxide Sensing by Immune Cells Occurs through Carbonic Anhydrase 2-Dependent Changes in Intracellular pH

CO₂, the primary gaseous product of respiration, is a major physiologic gas, the biology of which is poorly understood. Elevated CO₂ is a feature of the microenvironment in multiple inflammatory diseases that suppresses immune cell activity. However, little is known about the CO₂-sensing mechanisms and downstream pathways involved. We found that elevated CO₂ correlates with reduced monocyte and macrophage migration in patients undergoing gastrointestinal surgery and that elevated CO₂ reduces migration in vitro. Mechanistically, CO₂ reduces autocrine inflammatory gene expression, thereby inhibiting macrophage activation in a manner dependent on decreased intracellular pH. Pharmacologic or genetic inhibition of carbonic anhydrases (CAs) uncouples a CO₂-elicited intracellular pH response and attenuates CO₂ sensitivity in immune cells. Conversely, CRISPR-driven upregulation of the isoenzyme CA2 confers CO₂ sensitivity in nonimmune cells. Of interest, we found that patients with chronic lung diseases associated with elevated systemic CO₂ (hypercapnia) display a greater risk of developing anastomotic leakage following gastrointestinal surgery, indicating impaired wound healing. Furthermore, low intraoperative pH levels in these patients correlate with reduced intestinal macrophage infiltration. In conclusion, CO₂ is an immunomodulatory gas sensed by immune cells through a CA2-coupled change in intracellular pH.

Risks and Benefits of Ultra-Lung-Protective Invasive Mechanical Ventilation Strategies with a Focus on Extracorporeal Support

Lung-protective ventilation strategies are the current standard of care for patients with acute respiratory distress syndrome (ARDS) in an effort to provide adequate ventilatory requirements while minimizing ventilator-induced lung injury. Some patients may benefit from ultra-lung-protective ventilation, a strategy that achieves lower airway pressures and tidal volumes than the current standard. Specific physiological parameters beyond severity of hypoxemia, such as driving pressure and respiratory system elastance, may be predictive of those most likely to benefit. Since application of ultra-lung-protective ventilation is often limited by respiratory acidosis, extracorporeal membrane oxygenation (ECMO) or extracorporeal carbon dioxide removal (ECCO2R), which remove carbon dioxide from blood, are attractive options. These strategies are associated with hematological complications, especially when applied at low blood flow rates with devices designed for higher blood flows, and a recent large randomized, controlled trial failed to show a benefit from an ECCO2R-facilitated ultra-lung-protective ventilation strategy. Only in patients with very severe forms of ARDS has the use of an ultra-lung-protective ventilation strategy - accomplished with ECMO - been suggested to have a favorable risk-to-benefit profile. In this Critical Care Perspective, we address key areas of controversy related to ultra-lung-protective ventilation, including the trade-offs between minimizing ventilator-induced lung injury and the risks from strategies to achieve this added protection. In addition, we suggest which patients might benefit most from an ultra-lung-protective strategy and propose areas of future research.

Mechanisms of Hypercapnia-Induced Endoplasmic Reticulum Dysfunction

Protein transcription, translation, and folding occur continuously in every living cell and are essential for physiological functions. About one-third of all proteins of the cellular proteome interacts with the endoplasmic reticulum (ER). The ER is a large, dynamic cellular organelle that orchestrates synthesis, folding, and structural maturation of proteins, regulation of lipid metabolism and additionally functions as a calcium store. Recent evidence suggests that both acute and chronic hypercapnia (elevated levels of CO₂) impair ER function by different mechanisms, leading to adaptive and maladaptive regulation of protein folding and maturation. In order to cope with ER stress, cells activate unfolded protein response (UPR) pathways. Initially, during the adaptive phase of ER stress, the UPR mainly functions to restore ER protein-folding homeostasis by decreasing protein synthesis and translation and by activation of ER-associated degradation (ERAD) and autophagy. However, if the initial UPR attempts for alleviating ER stress fail, a maladaptive response is triggered. In this review, we discuss the distinct mechanisms by which elevated CO₂ levels affect these molecular pathways in the setting of acute and chronic pulmonary diseases associated with hypercapnia.

Mortality and Healthcare Use of Patients with Compensated Hypercapnia

Rationale: Acute hypercapnic respiratory failure has been shown to be associated with worse outcomes for various disease states, but less is known about patients with compensated hypercapnic respiratory failure. Although these patients have a normal pH, it remains unknown whether chronically elevated carbon dioxide partial pressure (Pco2), irrespective of etiology, puts patients at risk of adverse events. Objectives: To understand the burden of and clinical factors associated with morbidity and mortality in patients with compensated hypercapnic respiratory failure. Methods: We performed a query of the electronic medical record to identify patients hospitalized at the University of Michigan from January 1 to December 31, 2018, who had compensated hypercapnia, by using a Pco2 ⩾ 50 mm Hg and a pH of 7.35-7.45 on arterial blood gas . We obtained demographic and clinical data from the electronic medical record. Survival probabilities for Pco2 subgroups (50.0-54.9, 55.0-64.9, and ⩾65.0 mm Hg) were determined by using the Kaplan-Meier product limit estimator. Cox proportional hazard models were constructed to test the association between Pco2 and all-cause mortality. Results: We identified 491 patients with compensated hypercapnia. The mean age was 60.5 ± 16.2 years. Patients were 57.4% male and 86.2% white. The mean pH and Pco2 were 7.38 ± 0.03 and 58.8 ± 9.7 mm Hg, respectively. There was a total of 1,030 hospitalizations, with 44.4% of patients having two or more admissions. The median numbers of cumulative hospital and intensive care unit days were 21.0 (interquartile range [IQR], 11.0-38.0) and 7.0 (IQR, 3.0-14.0) days, respectively. Two hundred seventeen patients (44.2%) died over a median of 592 days. In univariate analysis, every 5-mm Hg increase in Pco2 was associated with a higher risk of all-cause death (hazard ratio, 1.09; 95% confidence interval [CI]: 1.03-1.16; P = 0.004). This association was maintained after adjusting for the age, sex, body mass index (BMI), and Charlson comorbidity index (hazard ratio of 1.09 for every 5-mm Hg increase in Pco2; 95% CI: 1.02-1.16; P = 0.009). There was a statistically significant interaction between the Pco2 and the BMI in relation to mortality (P = 0.01 for the interaction term). Conclusions: Patients with compensated hypercapnic respiratory failure have high mortality and healthcare use, with higher Pco2 being associated with worse survival. Hypercapnic patients with obesity have a higher risk of death with increases in Pco2.

Hypercapnia-induces IRE1α-driven Endoplasmic Reticulum-associated Degradation of the Na,K-ATPase β-subunit

Acute respiratory distress syndrome (ARDS) is often associated with elevated levels of CO2 (hypercapnia) and impaired alveolar fluid clearance. Misfolding of the Na,K-ATPase (NKA), a key molecule involved in both alveolar epithelial barrier tightness and in resolution of alveolar edema, in the endoplasmic reticulum (ER) may decrease plasma membrane (PM) abundance of the transporter. Here, we investigated how hypercapnia affects the NKA β-subunit (NKA-β) in the ER. Exposing murine precision-cut lung slices (PCLS) and human alveolar epithelial A549 cells to elevated CO2 levels led to a rapid decrease of NKA-β abundance in the ER and at the cell surface. Knockdown of ER alpha-mannosidase I (MAN1B1) and ER degradation enhancing alpha-mannosidase like protein 1 by siRNA or treatment with the MAN1B1 inhibitor, kifunensine rescued loss of NKA-β in the ER, suggesting ER-associated degradation (ERAD) of the enzyme. Furthermore, hypercapnia activated the unfolded protein response (UPR) by promoting phosphorylation of inositol-requiring enzyme 1α (IRE1α) and treatment with a siRNA against IRE1α prevented the decrease of NKA-β in the ER. Of note, the hypercapnia-induced phosphorylation of IRE1α was triggered by a Ca2+-dependent mechanism. Additionally, inhibition of the inositol trisphosphate receptor decreased phosphorylation levels of IRE1α in PCLS and A549 cells, suggesting that Ca2+ efflux from the ER might be responsible for IRE1α activation and ERAD of NKA-β. In conclusion, here we provide evidence that hypercapnia attenuates maturation of the regulatory subunit of NKA by activating IRE1α and promoting ERAD, which may contribute to impaired alveolar epithelial integrity in patients with ARDS and hypercapnia.

Effect of acute isooxic hypercapnia on oxidative activity of systemic neutrophils in endotoxemic rabbits

Introduction

Whether carbon dioxide (CO₂) affects systemic oxidative phenomena under conditions of endotoxemia is not sufficiently clarified. The study aimed to assess the impact of moderate acute hypercapnia on the respiratory burst of circulating neutrophils in mechanically ventilated endotoxemic rabbits.

Material and methods

Twenty-four endotoxemic rabbits were mechanically ventilated with standard or CO₂-enriched gas mixture in order to obtain isooxic hypercapnia. At a baseline point and following 180 min of hypercapnic ventilation, luminol-dependent chemiluminescence (CL) of circulating neutrophils and serum 2-thiobarbituric acid reactive substance (TBARS) concentrations were measured. Throughout the study, leukocyte and neutrophil counts, pH status, circulatory parameters and body temperature were also assessed.

Results

Following 180 min of hypercapnic ventilation, opsonized zymosan (OZ)-stimulated neutrophils showed lower CL vs. the control group (p = 0.004). Other parameters studied were not affected.

Conclusions

Short-term isooxic hypercapnia in endotoxemic rabbits preserves circulating neutrophil count pattern and reactive oxygen species (ROS) generation, but it may reduce phagocytosis.

Influence of Intermittent Hypoxia/Hypercapnia on Atherosclerosis, Gut Microbiome, and Metabolome

Obstructive sleep apnea (OSA), a common sleep disorder characterized by intermittent hypoxia and hypercapnia (IHC), increases atherosclerosis risk. However, the contribution of intermittent hypoxia (IH) or intermittent hypercapnia (IC) in promoting atherosclerosis remains unclear. Since gut microbiota and metabolites have been implicated in atherosclerosis, we examined whether IH or IC alters the microbiome and metabolome to induce a pro-atherosclerotic state. Apolipoprotein E deficient mice (ApoE-/- ), treated with IH or IC on a high-fat diet (HFD) for 10 weeks, were compared to Air controls. Atherosclerotic lesions were examined, gut microbiome was profiled using 16S rRNA gene amplicon sequencing and metabolome was assessed by untargeted mass spectrometry. In the aorta, IC-induced atherosclerosis was significantly greater than IH and Air controls (aorta, IC 11.1 ± 0.7% vs. IH 7.6 ± 0.4%, p < 0.05 vs. Air 8.1 ± 0.8%, p < 0.05). In the pulmonary artery (PA), however, IH, IC, and Air were significantly different from each other in atherosclerotic formation with the largest lesion observed under IH (PA, IH 40.9 ± 2.0% vs. IC 20.1 ± 2.6% vs. Air 12.2 ± 1.5%, p < 0.05). The most differentially abundant microbial families (p < 0.001) were Peptostreptococcaceae, Ruminococcaceae, and Erysipelotrichaceae. The most differentially abundant metabolites (p < 0.001) were tauro-β-muricholic acid, ursodeoxycholic acid, and lysophosphoethanolamine (18:0). We conclude that IH and IC (a) modulate atherosclerosis progression differently in distinct vascular beds with IC, unlike IH, facilitating atherosclerosis in both aorta and PA and (b) promote an atherosclerotic luminal gut environment that is more evident in IH than IC. We speculate that the resulting changes in the gut metabolome and microbiome interact differently with distinct vascular beds.

Hypercapnia in the critically ill: insights from the bench to the bedside

Carbon dioxide (CO2) has long been considered, at best, a waste by-product of metabolism, and at worst, a toxic molecule with serious health consequences if physiological concentration is dysregulated. However, clinical observations have revealed that 'permissive' hypercapnia, the deliberate allowance of respiratory produced CO2 to remain in the patient, can have anti-inflammatory effects that may be beneficial in certain circumstances. In parallel, studies at the cell level have demonstrated the profound effect of CO2 on multiple diverse signalling pathways, be it the effect from CO2 itself specifically or from the associated acidosis it generates. At the whole organism level, it now appears likely that there are many biological sensing systems designed to respond to CO2 concentration and tailor respiratory and other responses to atmospheric or local levels. Animal models have been widely employed to study the changes in CO2 levels in various disease states and also to what extent permissive or even directly delivered CO2 can affect patient outcome. These findings have been advanced to the bedside at the same time that further clinical observations have been elucidated at the cell and animal level. Here we present a synopsis of the current understanding of how CO2 affects mammalian biological systems, with a particular emphasis on inflammatory pathways and diseases such as lung specific or systemic sepsis. We also explore some future directions and possibilities, such as direct control of blood CO2 levels, that could lead to improved clinical care in the future.

Management of Chronic Respiratory Failure in Chronic Obstructive Pulmonary Disease: High-Intensity and Low-Intensity Ventilation

A significant body of literature supports the benefit of noninvasive ventilation (NIV) for acute hypercapnia in the setting of exacerbations of chronic obstructive pulmonary disease (COPD). In those with severe COPD with chronic hypercapnic respiratory failure, however, the role of NIV has been more controversial. This article reviews the physiologic basis for considering NIV in patients with COPD, summarizes existing evidence supporting the role of NIV in COPD, highlights the patient population and ventilatory approach most likely to offer benefit, and suggests a potential clinical pathway for managing patients.

Elevated CO2 modulates airway contractility

Carbon dioxide (CO₂), a primary product of oxidative metabolism, can be sensed by eukaryotic cells eliciting unique responses via specific signalling pathways. Severe lung diseases such as chronic obstructive pulmonary disease are associated with hypoventilation that can lead to the elevation of CO₂ levels in lung tissues and the bloodstream (hypercapnia). However, the pathophysiological effects of hypercapnia on the lungs and specific lung cells are incompletely understood. We have recently reported using combined unbiased molecular approaches with studies in mice and cell culture systems on the mechanisms by which hypercapnia alters airway smooth muscle contractility. In this review, we provide a pathophysiological and mechanistic perspective on the effects of hypercapnia on the lung airways and discuss the recent understanding of high CO₂ modulation of the airway contractility.

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.

Elevated CO2 Levels Delay Skeletal Muscle Repair by Increasing Fatty Acid Oxidation

Muscle dysfunction often occurs in patients with chronic obstructive pulmonary diseases (COPD) and affects ventilatory and non-ventilatory skeletal muscles. We have previously reported that hypercapnia (elevated CO₂ levels) causes muscle atrophy through the activation of the AMPKα2-FoxO3a-MuRF1 pathway. In the present study, we investigated the effect of normoxic hypercapnia on skeletal muscle regeneration. We found that mouse C2C12 myoblasts exposed to elevated CO₂ levels had decreased fusion index compared to myoblasts exposed to normal CO₂. Metabolic analyses of C2C12 myoblasts exposed to high CO₂ showed increased oxidative phosphorylation due to increased fatty acid oxidation. We utilized the cardiotoxin-induced muscle injury model in mice exposed to normoxia and 10% CO₂ for 21 days and observed that muscle regeneration was delayed. High CO₂-delayed differentiation in both mouse C2C12 myoblasts and skeletal muscle after injury and was restored to control levels when cells or mice were treated with a carnitine palmitoyltransfearse-1 (CPT1) inhibitor. Taken together, our data suggest that hypercapnia leads to changes in the metabolic activity of skeletal muscle cells, which results in impaired muscle regeneration and recovery after injury.

Humidified Warmed CO2 Treatment Therapy Strategies Can Save Lives With Mitigation and Suppression of SARS-CoV-2 Infection: An Evidence Review

The coronavirus disease (COVID-19) outbreak has presented enormous challenges for healthcare, societal, and economic systems worldwide. There is an urgent global need for a universal vaccine to cover all SARS-CoV-2 mutant strains to stop the current COVID-19 pandemic and the threat of an inevitable second wave of coronavirus. Carbon dioxide is safe and superior antimicrobial, which suggests it should be effective against coronaviruses and mutants thereof. Depending on the therapeutic regime, CO2 could also ameliorate other COVID-19 symptoms as it has also been reported to have antioxidant, anti-inflammation, anti-cytokine effects, and to stimulate the human immune system. Moreover, CO2 has beneficial effects on respiratory physiology, cardiovascular health, and human nervous systems. This article reviews the rationale of early treatment by inhaling safe doses of warmed humidified CO2 gas, either alone or as a carrier gas to deliver other inhaled drugs may help save lives by suppressing SARS-CoV-2 infections and excessive inflammatory responses. We suggest testing this somewhat counter-intuitive, but low tech and safe intervention for its suitability as a preventive measure and treatment against COVID-19. Overall, development and evaluation of this therapy now may provide a safe and economical tool for use not only during the current pandemic but also for any future outbreaks of respiratory diseases and related conditions.

Recurrent aspiration pneumonia precipitated by obstructive sleep apnea

OBJECTIVE

The clearance of the pharynx by deglutition and the respiratory phase patterns associated with deglutition are important in protecting airways and lungs against aspiration. The deglutition and respiratory phase patterns during sleep in patients (without swallowing disorders while awake) with obstructive sleep apnea (OSA) precipitating recurrent intractable aspiration pneumonia were investigated.

METHODS

After videoendoscopic and videofluorographic examinations of swallowing showed subjects had no swallowing disorders while awake, two adults with recurrent intractable aspiration pneumonia precipitated by severe OSA were examined via time-matched digital recordings of polysomnography and surface electromyography of the muscles (thyrohyoid and suprahyoid muscles) related to swallowing and compared with the same patients before and under CPAP therapy.

RESULTS

CPAP therapy cured recurrent intractable aspiration pneumonia. Swallows following and/or followed by inspiration (uncoordinated deglutition with respiration), which were frequently observed before CPAP therapy, were markedly reduced under CPAP therapy. On the other hand, swallows following and/or followed by expiration (coordinated deglutition with respiration) markedly increased under CPAP therapy. Deglutition was related to the sleep stage. The deeper the sleep stage, the lower the deglutition frequency. Before and under CPAP therapy, swallowing was infrequent and absent for long periods. However, respiratory phase patterns associated with sleep-related deglutition in patients with OSA under CPAP therapy markedly improved.

CONCLUSIONS

In patients (without swallowing disorders while awake) with OSA precipitating recurrent intractable aspiration pneumonia, the high rate of uncoordinated deglutition with respiration (swallows following and/or followed by inspiration) during sleep were markedly reduced and the rate of coordinated deglutition with respiration (swallows following and/or followed by expiration) was markedly increased under CPAP therapy. Sleep-related deglutition and respiratory phase patterns are likely to adversely influence aspiration pneumonia in patients with OSA. CPAP therapy improved not only apnea-hypopnea during sleep and sleep quality but also sleep-related deglutition, especially respiratory phase patterns associated with deglutition in patients with OSA. CPAP therapy may decrease the risk of aspiration and greatly improve aspiration-related diseases such as aspiration pneumonia.

Hypercapnia Regulates Gene Expression and Tissue Function

Carbon dioxide (CO₂) is produced in eukaryotic cells primarily during aerobic respiration, resulting in higher CO₂ levels in mammalian tissues than those in the atmosphere. CO₂ like other gaseous molecules such as oxygen and nitric oxide, is sensed by cells and contributes to cellular and organismal physiology. In humans, elevation of CO₂ levels in tissues and the bloodstream (hypercapnia) occurs during impaired alveolar gas exchange in patients with severe acute and chronic lung diseases. Advances in understanding of the biology of high CO₂ effects reveal that the changes in CO₂ levels are sensed in cells resulting in specific tissue responses. There is accumulating evidence on the transcriptional response to elevated CO₂ levels that alters gene expression and activates signaling pathways with consequences for cellular and tissue functions. The nature of hypercapnia-responsive transcriptional regulation is an emerging area of research, as the responses to hypercapnia in different cell types, tissues, and species are not fully understood. Here, we review the current understanding of hypercapnia effects on gene transcription and consequent cellular and tissue functions.

Hypercapnia: An Aggravating Factor in Asthma

Asthma is a common chronic respiratory disorder with relatively good outcomes in the majority of patients with appropriate maintenance therapy. However, in a small minority, patients can experience severe asthma with respiratory failure and hypercapnia, necessitating intensive care unit admission. Hypercapnia occurs due to alveolar hypoventilation and insufficient removal of carbon dioxide (CO₂) from the blood. Although mild hypercapnia is generally well tolerated in patients with asthma, there is accumulating evidence that elevated levels of CO₂ can act as a gaso-signaling molecule, triggering deleterious effects in various organs such as the lung, skeletal muscles and the innate immune system. Here, we review recent advances on pathophysiological response to hypercapnia and discuss potential detrimental effects of hypercapnia in patients with asthma.

Correlation of Arterial CO2 and Respiratory Impedance Values among Subjects with COPD

Chronic obstructive pulmonary disease (COPD) is a respiratory illness characterized by airflow limitation and chronic respiratory symptoms with a global prevalence estimated to be more than 10% in 2010 and still on the rise. Furthermore, hypercapnic subject COPD leads to an increased risk of mortality, morbidity, and poor QoL (quality of life) than normocapnic subjects. Series of studies showed the usefulness of the forced oscillation technique (FOT) to measure small airway closure. Traditional findings suggested that hypercapnia may not be the main treating targets, but recent findings suggested that blood stream CO₂ may lead to a worse outcome. This study aimed to seek the relationship between CO₂ and small airway closure by using FOT. Subjects with COPD (n = 124; hypercapnia 22 and normocapnia 102) were analyzed for all pulmonary function values, FOT values, and arterial blood gas analysis. Student’s t-test, Spearman rank correlation, and multi linear regression analysis were used to analyze the data. COPD subjects with hypercapnia showed a significant increase in R5, R20, Fres, and ALX values, and a greater decrease in X5 value than normocapnic patients. Also, multiple linear regression analysis showed R5 was associated with hypercapnia. Hypercapnia may account for airway closure among subjects with COPD and this result suggests treating hypercapnia may lead to better outcomes for such a subject group.

Hypercapnia Suppresses Macrophage Antiviral Activity and Increases Mortality of Influenza A Infection via Akt1

Hypercapnia (HC), elevation of the partial pressure of CO2 in blood and tissues, is a risk factor for mortality in patients with severe acute and chronic lung diseases. We previously showed that HC inhibits multiple macrophage and neutrophil antimicrobial functions and increases the mortality of bacterial pneumonia in mice. In this study, we show that normoxic HC increases viral replication, lung injury, and mortality in mice infected with influenza A virus (IAV). Elevated CO2 increased IAV replication and inhibited antiviral gene and protein expression in macrophages in vivo and in vitro. HC potentiated IAV-induced activation of Akt, whereas specific pharmacologic inhibition or short hairpin RNA knockdown of Akt1 in alveolar macrophages blocked HC's effects on IAV growth and the macrophage antiviral response. Our findings suggest that targeting Akt1 or the downstream pathways through which elevated CO2 signals could enhance macrophage antiviral host defense and improve clinical outcomes in hypercapnic patients with advanced lung disease.

Extreme climate scenario and parasitism affect the Amazonian fish Colossoma macropomum

Global warming caused by greenhouse gases accumulation, in particular carbon dioxide, is the major current environmental challenge, as it will affect all life forms over the next decades. Aquaculture, a fast growing food production sector, is already facing the effects of global warming. The fish immune system is expected to be especially affected by increased temperature and carbon dioxide, mainly when associated to infectious diseases outbreaks. Here, we experimentally investigated the associated effects of an extreme climate scenario and two levels of monogenean parasitism on the hematological and immunological response of an important Amazon fish for continental aquaculture: Colossoma macropomum. Individuals of C. macropomum with low and high levels of parasitism were exposed to current and extreme climate scenarios (4.5 °C and 900 ppm CO₂ above current levels). We characterized their hematological profile using classical methods, and their immune-related gills gene expression (HSP70, IL-1β and IL-10) using quantitative real-time polymerase chain reaction (qPCR). After 7 days of exposure, we observed that exposure to extreme climate scenario caused rapid increase of parasitism intensity and likely acute inflammation, indicated by the higher expression of HSP70 and IL-1β. The IL-10 gene was downregulated in both groups exposed to extreme climate scenario, contrasting with animals exposed to current scenario. Thus, in the current scenario, the parasitized tambaqui showed a response to the tissue damage, which was not observed in the animals exposed to the extreme scenario.

Hypercapnia Impairs Na,K-ATPase Function by Inducing Endoplasmic Reticulum Retention of the β-Subunit of the Enzyme in Alveolar Epithelial Cells

Alveolar edema, impaired alveolar fluid clearance, and elevated CO2 levels (hypercapnia) are hallmarks of the acute respiratory distress syndrome (ARDS). This study investigated how hypercapnia affects maturation of the Na,K-ATPase (NKA), a key membrane transporter, and a cell adhesion molecule involved in the resolution of alveolar edema in the endoplasmic reticulum (ER). Exposure of human alveolar epithelial cells to elevated CO2 concentrations caused a significant retention of NKA-β in the ER and, thus, decreased levels of the transporter in the Golgi apparatus. These effects were associated with a marked reduction of the plasma membrane (PM) abundance of the NKA-α/β complex as well as a decreased total and ouabain-sensitive ATPase activity. Furthermore, our study revealed that the ER-retained NKA-β subunits were only partially assembled with NKA α-subunits, which suggests that hypercapnia modifies the ER folding environment. Moreover, we observed that elevated CO2 levels decreased intracellular ATP production and increased ER protein and, particularly, NKA-β oxidation. Treatment with α-ketoglutaric acid (α-KG), which is a metabolite that has been shown to increase ATP levels and rescue mitochondrial function in hypercapnia-exposed cells, attenuated the deleterious effects of elevated CO2 concentrations and restored NKA PM abundance and function. Taken together, our findings provide new insights into the regulation of NKA in alveolar epithelial cells by elevated CO2 levels, which may lead to the development of new therapeutic approaches for patients with ARDS and hypercapnia.

Pre-Treatment with Ten-Minute Carbon Dioxide Inhalation Prevents Lipopolysaccharide-Induced Lung Injury in Mice via Down-Regulation of Toll-Like Receptor 4 Expression

Various animal studies have shown beneficial effects of hypercapnia in lung injury. However, in patients with acute respiratory distress syndrome (ARDS), there is controversial information regarding the effect of hypercapnia on outcomes. The duration of carbon dioxide inhalation may be the key to the protective effect of hypercapnia. We investigated the effect of pre-treatment with inhaled carbon dioxide on lipopolysaccharide (LPS)-induced lung injury in mice. C57BL/6 mice were randomly divided into a control group or an LPS group. Each LPS group received intratracheal LPS (2 mg/kg); the LPS groups were exposed to hypercapnia (5% carbon dioxide) for 10 min or 60 min before LPS. Bronchoalveolar lavage fluid (BALF) and lung tissues were collected to evaluate the degree of lung injury. LPS significantly increased the ratio of lung weight to body weight; concentrations of BALF protein, tumor necrosis factor-α, and CXCL2; protein carbonyls; neutrophil infiltration; and lung injury score. LPS induced the degradation of the inhibitor of nuclear factor-κB-α (IκB-α) and nuclear translocation of NF-κB. LPS increased the surface protein expression of toll-like receptor 4 (TLR4). Pre-treatment with inhaled carbon dioxide for 10 min, but not for 60 min, inhibited LPS-induced pulmonary edema, inflammation, oxidative stress, lung injury, and TLR4 surface expression, and, accordingly, reduced NF-κB signaling. In summary, our data demonstrated that pre-treatment with 10-min carbon dioxide inhalation can ameliorate LPS-induced lung injury. The protective effect may be associated with down-regulation of the surface expression of TLR4 in the lungs.

Elevated CO2 regulates the Wnt signaling pathway in mammals, Drosophila melanogaster and Caenorhabditis elegans

Carbon dioxide (CO2) is sensed by cells and can trigger signals to modify gene expression in different tissues leading to changes in organismal functions. Despite accumulating evidence that several pathways in various organisms are responsive to CO2 elevation (hypercapnia), it has yet to be elucidated how hypercapnia activates genes and signaling pathways, or whether they interact, are integrated, or are conserved across species. Here, we performed a large-scale transcriptomic study to explore the interaction/integration/conservation of hypercapnia-induced genomic responses in mammals (mice and humans) as well as invertebrates (Caenorhabditis elegans and Drosophila melanogaster). We found that hypercapnia activated genes that regulate Wnt signaling in mouse lungs and skeletal muscles in vivo and in several cell lines of different tissue origin. Hypercapnia-responsive Wnt pathway homologues were similarly observed in secondary analysis of available transcriptomic datasets of hypercapnia in a human bronchial cell line, flies and nematodes. Our data suggest the evolutionarily conserved role of high CO2 in regulating Wnt pathway genes.

Extrinsic acidosis suppresses glycolysis and migration while increasing network formation in pulmonary microvascular endothelial cells

Acidosis is common among critically ill patients, but current approaches to correct pH do not improve disease outcomes. During systemic acidosis, cells are either passively exposed to extracellular acidosis that other cells have generated (extrinsic acidosis) or they are exposed to acid that they generate and export into the extracellular space (intrinsic acidosis). Although endothelial repair following intrinsic acidosis has been studied, the impact of extrinsic acidosis on migration and angiogenesis is unclear. We hypothesized that extrinsic acidosis inhibits metabolism and migration but promotes capillary-like network formation in pulmonary microvascular endothelial cells (PMVECs). Extrinsic acidosis was modeled by titrating media pH. Two types of intrinsic acidosis were compared, including increasing cellular metabolism by chemically inhibiting carbonic anhydrases (CAs) IX and XII (SLC-0111) and with hypoxia. PMVECs maintained baseline intracellular pH for 24 h with both extrinsic and intrinsic acidosis. Whole cell CA IX protein expression was decreased by extrinsic acidosis but not affected by hypoxia. When extracellular pH was equally acidic, extrinsic acidosis suppressed glycolysis, whereas intrinsic acidosis did not. Extrinsic acidosis suppressed migration, but increased Matrigel network master junction and total segment length. CRISPR-Cas9 CA IX knockout PMVECs revealed an independent role of CA IX in promoting glycolysis, as loss of CA IX alone was accompanied by decreased hexokinase I and pyruvate dehydrogenase E1α expression and decreasing migration. 2-deoxy-d-glucose had no effect on migration but profoundly inhibited network formation and increased N-cadherin expression. Thus, we report that while extrinsic acidosis suppresses endothelial glycolysis and migration, it promotes network formation.

Perioperative complications in patients with sleep apnea following primary total shoulder arthroplasty: An analysis of 33,366 patients

Introduction

The study evaluated whether sleep apnea (SA) patients undergoing total shoulder arthroplasty (TSA) are at greater odds of: 1) medical complications; 2) implant-related complications; 3) readmission rates; and 4) costs.

Methods

Complications and readmissions were assessed using logistic regression analysis. Welch's t-test was used to compare CCI and cost between cohorts.

Results

33,366 patients equally distributed in both cohorts. SA increased the odds of medical [Odds-ratio (OR)]: 2.52, p < 0.001) and implant-related complications (OR: 1.43, p < 0.001). Readmission rates were similar to controls (OR: 0.99, p = 0.878), whereas costs were higher (p < 0.001).

Conclusion

SA increases complications and costs following TSA.

Hypercapnic tumor microenvironment confers chemoresistance to lung cancer cells by reprogramming mitochondrial metabolism in vitro

The tumor microenvironment has previously been reported to be hypercapnic (as high as ~84 mmHg), although its effect on tumor cell behaviors is unknown. In this study, high CO2 levels, ranging from 5% to 15%, protected lung cancer cells from anticancer agents, such as cisplatin, carboplatin and etoposide, by suppressing apoptosis. The cytoprotective effect of a high CO2 level was independent of acidosis and was due to mitochondrial metabolic reprogramming that reduced mitochondrial respiration, as assessed by oxygen consumption, oxidative phosphorylation, mitochondrial membrane and oxidative potentials, eventually leading to reduced reactive oxidant species production. In contrast, high CO2 levels did not affect cisplatin-mediated DNA damage responses or the expression of Bcl-2 family proteins. Although high CO2 levels inhibited glycolysis, this inhibition was not mechanistically involved in high CO2-mediated reductions in mitochondrial respiration, because a high CO2 concentration inhibited isolated mitochondria. A cytoprotective effect of high CO2 levels on mitochondria DNA-depleted cells was not noted, lending support to our conclusion that high CO2 levels act on mitochondria to reduce the cytotoxicity of anticancer agents. High CO2-mediated cytoprotection was also noted in a 3D culture system. In conclusion, the hypercapnic tumor microenvironment reprograms mitochondrial respiratory metabolism causing chemoresistance in lung cancer cells. Thus, tumor hypercapnia may represent a novel target to improve chemosensitivity.