Pleural Hypercarbia After Lung Surgery Is Associated With Persistent Alveolopleural Fistulae

Is chest tube capnography effective in differentiating between true and false air leaks after minimally invasive thoracic surgery?

OBJECTIVE

Air leak (AL) is the most frequent adverse event after thoracic surgery. When AL occurs, the concentration of the principal gas in the pleural space should be similar to that of air exhaled. Accordingly, we tried to develop a new method to identify AL by analyzing pCO2 levels in the air flow from the chest drainage using capnography.

METHODS

This is a prospective observational study of 104 patients who underwent VATS surgery between January 2020 and July 2021. Digital drainage systems were used to detect AL.

RESULTS

Eighty-two patients (79%) had lung resection. Among them, 19 had post-operative day 1 air leaks (median 67 ml/min). AL patients had higher intrapleural CO2 levels (median 24 mmHg) (p < 0.001). Median chest drainage duration was 2 days (range 1.0-3.0). Univariable logistic regression showed a linear and significant association between intrapleural CO2 levels and AL risk (OR 1.26, 95% CI 1.17-1.36, p < 0.001, C index: 0.94). The Univariable Gamma model demonstrated that an elevation in CO2 levels was linked to AL on POD1 (with an adjusted mean effect of 7.006, 95% CI 1.59-12.41, p = 0.011) and extended duration of drainage placement (p < 0.001).

CONCLUSIONS

Intrapleural CO2 could be an effective tool to assess AL. The linear association between variables allows us to hypothesize the role of CO2 in the identification of AL. Further studies should be performed to identify a CO2 cutoff that will standardize the management of chest drainage.

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.

Risk factors for prolonged air leak after pulmonary surgery: A systematic review and meta-analysis

This study aimed to comprehensively identify risk factors for the occurrence of prolonged air leak (PAL) in patients undergoing pulmonary surgery. Studies were retrieved from 3 databases, including PubMed, Web of Science, and EmBase up to 13 May 2020. We performed meta-analysis using Bayesian random effect models through divergence restricting conditional tessellation (DIRECT) algorithm. The effect size was expressed as odds ratio (OR) or mean difference (MD), each with 95% credible interval (CrI). The evidence quality was evaluated. Subgroup analyses and sensitivity analyses were conducted. Thirty-nine studies with 89006 patients were finally included. Pooled PAL incidence was 15%. Of 30 risk factors, 22 were significantly associated with increased PAL incidence. Five risk factors were ultimately selected with high evidence quality: smoking history (OR 1.84, 95%CrI 1.45 to 2.31, P<0.001), preoperative steroid use (OR 1.51, 95%CrI 0.87 to 2.65, P = 0.031), lower ratio of forced expiratory volume in 1 s and forced vital capacity (OR 1.99, 95%CrI 1.22 to 3.33, P = 0.005), non-fissureless technique (OR 2.14, 95%CrI 1.31 to 3.66, P = 0.003), and pathological TNM stage III/IV (OR 1.50, 95%CrI 1.07 to 2.12, P = 0.003). Regarding the negative impact of PAL on the personal cost and postoperative recovery, the verification of previous proposed factors and investigation of recently discovered ones both implied directions for risk stratification and the establishment of an applicable prediction model.

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 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.

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.

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.

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.

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.

Pleural gas analysis for the identification of alveolopleural fistulae

PURPOSE OF REVIEW

The method for identification of alveolopleural fistulae (APF) by visual inspection of air bubbles in the chest drainage system has several limitations and suffers from poor accuracy. Here we discuss the use of a novel technique of pleural gas analysis in the identification and management of APF.

RECENT FINDINGS

We found that pleural gas analysis has higher sensitivity and specificity than visual inspection in identifying APF. Additionally, we demonstrated that intrapleural gas milieu impacts lung healing and reduction of intrapleural carbon dioxide can promote resolution of APF.

SUMMARY

Pleural gas analysis is a novel technique to identify and manage APF. Integration of gas analysis in chest drainage systems would provide a more objective method for managing chest tubes and providing a favorable pleural gas environment for lung healing.

Effects of hypercapnia on the lung

Gases are sensed by lung cells and can activate specific intracellular signalling pathways, and thus have physiological and pathophysiological effects. Carbon dioxide (CO₂ ), a primary product of oxidative metabolism, can be sensed by eukaryotic cells eliciting specific responses via recently identified signalling pathways. However, the physiological and pathophysiological effects of high CO₂ (hypercapnia) on the lungs and specific lung cells, which are the primary site of CO₂ elimination, are incompletely understood. In this review, we provide a physiological and mechanistic perspective on the effects of hypercapnia on the lungs and discuss the recent understanding of CO₂ modulation of the alveolar epithelial function (lung oedema clearance), epithelial cell repair, innate immunity and airway function.

Severe hypercapnia and outcome of mechanically ventilated patients with moderate or severe acute respiratory distress syndrome

PURPOSE

To analyze the relationship between hypercapnia developing within the first 48 h after the start of mechanical ventilation and outcome in patients with acute respiratory distress syndrome (ARDS).

PATIENTS AND METHODS

We performed a secondary analysis of three prospective non-interventional cohort studies focusing on ARDS patients from 927 intensive care units (ICUs) in 40 countries. These patients received mechanical ventilation for more than 12 h during 1-month periods in 1998, 2004, and 2010. We used multivariable logistic regression and a propensity score analysis to examine the association between hypercapnia and ICU mortality.

MAIN OUTCOMES

We included 1899 patients with ARDS in this study. The relationship between maximum PaCO₂ in the first 48 h and mortality suggests higher mortality at or above PaCO₂ of ≥50 mmHg. Patients with severe hypercapnia (PaCO₂ ≥50 mmHg) had higher complication rates, more organ failures, and worse outcomes. After adjusting for age, SAPS II score, respiratory rate, positive end-expiratory pressure, PaO₂/FiO₂ ratio, driving pressure, pressure/volume limitation strategy (PLS), corrected minute ventilation, and presence of acidosis, severe hypercapnia was associated with increased risk of ICU mortality [odds ratio (OR) 1.93, 95% confidence interval (CI) 1.32 to 2.81; p = 0.001]. In patients with severe hypercapnia matched for all other variables, ventilation with PLS was associated with higher ICU mortality (OR 1.58, CI 95% 1.04-2.41; p = 0.032).

CONCLUSIONS

Severe hypercapnia appears to be independently associated with higher ICU mortality in patients with ARDS.

TRIAL REGISTRATION

Clinicaltrials.gov identifier, NCT01093482.

Prolonged air leak following lobectomy can be predicted in lung cancer patients

PURPOSE

The purpose of this study was to identify the factors associated with prolonged air leak (PAL) following pulmonary lobectomy for lung cancer.

METHODS

The data of 146 patients who underwent pulmonary lobectomy for lung cancer between August 2010 and July 2015 were retrospectively reviewed. Air leaks were assessed daily by a visual evaluation and were categorized as follows: forced expiratory only (Grade 1), expiratory only (Grade 2), or continuous (Grade 3). Logistic regression analyses were performed to identify the predictors of PAL (>5 days).

RESULTS

PAL occurred in 23 patients (16%). An air leak at rest (Grade ≥ 2) was detected on postoperative day (POD) 1 in 48% of the patients with PAL and 7% of the patients without PAL. A univariate analysis demonstrated that PAL was significantly associated with male sex, a smoking history of ≥ 40 pack years, a serum albumin level of ≤4.0 mg/dL, and an air leak on POD1 (Grade ≥ 2). A multivariate analysis demonstrated that a serum albumin level of ≤4.0 mg/dL (p = 0.027) and an air leak on POD1 (p = 0.006) were independent predictors of PAL. PAL occurred in 75% of the patients with these two risk factors.

CONCLUSIONS

The preoperative serum albumin level and the presence of a visually evaluated air leak on POD1 may be useful indicators for the perioperative management of air leaks.

Suction or Nonsuction: How to Manage a Chest Tube After Pulmonary Resection

Despite several randomized trials and meta-analyses, the dilemma as to whether to apply suction after subtotal pulmonary resection has not been solved. The combination of a poorly understood pathophysiology of the air leak phenomenon and the inadequate quality of the published randomized trials is actually preventing thoracic surgeons from abandoning an empirical management of chest drains. Even digital systems do not seem to have made the difference so far. Based on the evidence of the literature, the authors propose a new air leak predictor score (ALPS) as a contributing step toward appropriateness in using intraoperative sealants, opting for an external suction and managing and chest tubes.

Air leak after lung resection: pathophysiology and patients' implications

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