Background: Oxygen therapy usually exposes patients to hyperoxia, which induces injuries in the lung, the heart, and the brain. The gut and its microbiome play key roles in critical illnesses, but the impact of hyperoxia on the gut and its microbiome remains not very clear. We clarified the time- and dose-dependent effects of hyperoxia on the gut and investigated oxygen-induced gut dysbiosis and explored the underlying mechanism of gut injury by transcriptome analysis.Methods: The C57BL/6 mice were randomly divided into the control group and nine different oxygen groups exposed to hyperoxia with an inspired O₂ fraction (FiO₂) of 40, 60, and 80% for 24, 72, and 168 h (7 days), respectively. Intestinal histopathological and biochemical analyses were performed to explore the oxygen-induced gut injury and inflammatory response. Another experiment was performed to explore the impact of hyperoxia on the gut microbiome by exposing the mice to hyperoxia (FiO₂ 80%) for 7 days, with the 16S rRNA sequencing method. We prolonged the exposure (up to 14 days) of the mice to hyperoxia (FiO₂ 80%), and gut transcriptome analysis and western blotting were carried out to obtain differentially expressed genes (DEGs) and signaling pathways related to innate immunity and cell death.Results: Inhaled oxygen induced time- and dose-dependent gut histopathological impairment characterized by mucosal atrophy (e.g., villus shortening: 80% of FiO₂ for 24 h: P = 0.008) and enterocyte death (e.g., apoptosis: 40% of FiO₂ for 7 days: P = 0.01). Administered time- and dose-dependent oxygen led to intestinal barrier dysfunction (e.g., endotoxemia: 80% of FiO₂ for 72 h: P = 0.002) and potentiated gut inflammation by increasing proinflammatory cytokines [e.g., tumor necrosis factor alpha (TNF-α): 40% of FiO₂ for 24 h: P = 0.003)] and reducing anti-inflammatory cytokines [Interleukin 10 (IL-10): 80% of FiO₂ for 72 h: P < 0.0001]. Hyperoxia induced gut dysbiosis with an expansion of oxygen-tolerant bacteria (e.g., Enterobacteriaceae). Gut transcriptome analysis identified 1,747 DEGs and 171 signaling pathways and immunoblotting verified TLR-4, NOD-like receptor, and apoptosis signaling pathways were activated in oxygen-induced gut injury.Conclusions: Acute hyperoxia rapidly provokes gut injury in a time- and dose-dependent manner and induces gut dysbiosis, and an innate immune response is involved in an oxygen-induced gut injury.

Molecular oxygen is typically delivered to patients via oxygen inhalation or extracorporeal membrane oxygenation (ECMO), potentially resulting in systemic hyperoxia from liberal oxygen inhalation or localized hyperoxia in the lower body from peripheral venoarterial (VA) ECMO. Consequently, this exposes the gastrointestinal tract to excessive oxygen levels. Hyperoxia can trigger organ damage due to the overproduction of reactive oxygen species and is associated with increased mortality. The gut and gut microbiome play pivotal roles in critical illnesses and even small variations in oxygen levels can have a dramatic influence on the physiology and ecology of gut microbes. Here, we reviewed the emerging preclinical evidence which highlights how excessive inhaled oxygen can provoke diffuse villous damage, barrier dysfunction in the gut, and gut dysbiosis. The hallmark of this dysbiosis includes the expansion of oxygen-tolerant pathogens (e.g., Enterobacteriaceae) and the depletion of beneficial oxygen-intolerant microbes (e.g., Muribaculaceae). Furthermore, we discussed potential impact of oxygen on the gut in various underlying critical illnesses involving inspiratory oxygen and peripheral VA-ECMO. Currently, the available findings in this area are somewhat controversial, and a consensus has not yet to be reached. It appears that targeting near-physiological oxygenation levels may offer a means to avoid hyperoxia-induced gut injury and hypoxia-induced mesenteric ischemia. However, the optimal oxygenation target may vary depending on special clinical conditions, including acute hypoxia in adults and neonates, as well as particular patients undergoing gastrointestinal surgery or VA-ECMO support. Last, we outlined the current challenges and the need for future studies in this area. Insights into this vital ongoing research can assist clinicians in optimizing oxygenation for critically ill patients.

Background: Inhaled oxygen is the first-line therapeutic approach for maintaining tissue oxygenation in critically ill patients, but usually exposes patients to damaging hyperoxia. Hyperoxia adversely increases the oxygen tension in the gut lumen which harbors the trillions of microorganisms playing an important role in host metabolism and immunity. Nevertheless, the effects of hyperoxia on gut microbiome and metabolome remain unclear, and metagenomic and metabolomics analysis were performed in this mouse study.Methods: C57BL/6 mice were randomly divided into a control (CON) group exposed to room air with fractional inspired oxygen (FiO₂) of 21% and a hyperoxia (OXY) group exposed to FiO₂ of 80% for 7 days, respectively. Fecal pellets were collected on day 7 and subjected to metagenomic sequencing. Another experiment with the same design was performed to explore the impact of hyperoxia on gut and serum metabolome. Fecal pellets and blood were collected and high-performance liquid chromatography with mass spectrometric analysis was carried out.Results: At the phylum level, hyperoxia increased the ratio of Firmicutes/Bacteroidetes (p = 0.049). At the species level, hyperoxia reduced the abundance of Muribaculaceae bacterium Isolate-037 (p = 0.007), Isolate-114 (p = 0.010), and Isolate-043 (p = 0.011) etc. Linear discriminant analysis effect size (LEfSe) revealed that Muribaculaceae and Muribaculaceae bacterium Isolate-037, both belonging to Bacteroidetes, were the marker microbes of the CON group, while Firmicutes was the marker microbes of the OXY group. Metagenomic analysis using Kyoto Encyclopedia of Genes and Genomes (KEGG) and Carbohydrate-Active enZYmes (CAZy) revealed that hyperoxia provoked disturbances in carbohydrate and lipid metabolism. Fecal metabolomics analysis showed hyperoxia reduced 11-dehydro Thromboxane B2-d4 biosynthesis (p = 1.10 × 10⁻¹¹). Hyperoxia blunted fecal linoleic acid metabolism (p = 0.008) and alpha-linolenic acid metabolism (p = 0.014). We showed that 1-docosanoyl-glycer-3-phosphate (p = 1.58 × 10⁻¹⁰) was the most significant differential serum metabolite inhibited by hyperoxia. In addition, hyperoxia suppressed serum hypoxia-inducible factor-1 (HIF-1, p = 0.007) and glucagon signaling pathways (p = 0.007).Conclusion: Hyperoxia leads to gut dysbiosis by eliminating beneficial and oxygen strictly intolerant Muribaculaceae with genomic dysfunction of carbohydrate and lipid metabolism. In addition, hyperoxia suppresses unsaturated fatty acid metabolism in the gut and inhibits the HIF-1 and glucagon signaling pathways in the serum.

The mammalian colon lumen exists in a highly anaerobic environment (oxygen partial pressure (PO ₂ ) < 1 mmHg), which promotes the growth of beneficial obligate anaerobes (OA) while limiting the expansion of pathogenic facultative anaerobes (FA). Gut dysbiosis is associated with a wide range of human diseases, and is often characterized by an overgrowth of FA, particularly those in the Enterobacteriaceae family. Oxygen (O ₂ ) plays a crucial role in bacterial physiology and ecology, and increased O ₂ availability is a key driver of gut dysbiosis. O ₂ therapy is commonly used for hypoxic patients, either through inhalation or extracorporeal membrane oxygenation (ECMO), both of which can expose the gut to excess O ₂ , known as hyperoxia. Hyperoxia leads to the overproduction of reactive O ₂ species, resulting in organ injury and worsening clinical outcomes. Viewing gut dysbiosis from an ecological perspective highlights the disruption of host mechanisms that regulate the gut microbiota, particularly in the context of antibiotic use and a western (low fiber) diet, where physiological hypoxia in the colonic epithelium is compromised. This review extends that perspective to O ₂ therapy in acute care, discussing the rationale and experimental evidence linking hyperoxia to gut dysbiosis, with a focus on venoarterial (VA)-ECMO support as a potential contributor. Understanding these mechanisms could help clinicians optimize O ₂ management during therapy.

Central venous catheters (CVCs) often cause life-threatening complications, especially CVC-related bloodstream infection (CVC-BSI) and catheter-related thrombosis (CRT). Here, we report an unusual case of misplaced CVC-induced emphysematous thrombophlebitis, a rare but potentially lethal form of CRT and CVC-BSI characterized by both thrombosis and gas formation. A 48-year-old male presented to the emergency room of a local hospital with sudden-onset headache and coma for 4 h. Computed tomography (CT) revealed right basal ganglia hemorrhage, so emergency decompressive craniotomy was performed and a CVC was inserted through the right subclavian vein for fluid resuscitation during anesthesia. Two days later, the patient was transferred to the intensive care unit of our hospital for further critical care. On day 9 after CVC insertion, the patient suddenly developed fever and hypotension. Point-of-care ultrasound (POCUS) demonstrated thrombosis and dilatation of the right internal jugular vein (IJV) filled with thrombosis. Ultrasonography also revealed that the CVC tip had been misplaced into the IJV and was surrounded by gas bubbles, which manifested as hyperechoic lines with dirty shadowing and comet-tail artifacts. Further CT scan confirmed air bubbles surrounding the CVC in the right neck. The final diagnosis was septic emphysematous thrombophlebitis induced by a misplaced CVC and ensuing septic shock. The responsible CVC was removed immediately. The patient received fluid resuscitation, intravenous noradrenaline, and a 10-d ultra-broad spectrum antibiotic treatment to combat septic shock. Both CVC and peripheral venous blood cultures yielded methicillin-resistant Staphylococcus cohnii. The patient was gradually weaned off vasopressors and the symptoms of redness and swelling in the right neck subsided within 7 d. Emphysematous thrombophlebitis is a fulminant and life-threatening CVC-BSI associated with thrombosis and gas formation in the vein. A misplaced CVC may facilitate the development of emphysematous thrombophlebitis. POCUS can easily identify the artifacts produced by gas and thrombosis, facilitating rapid diagnosis at the bedside.