Anesthesia-Related Carbon Monoxide Exposure: Toxicity and Potential Therapy

James Watt, of Steam Engine Fame, Offered Inhaled Carbon Monoxide for Putative Therapeutic Action

James Watt (1736-1819) is remembered as a steam engine innovator and industrial magnate. A polymath, he was also a hands-on contributor to the Medical Pneumatic Institution of Thomas Beddoes. Watt recruited Humphry Davy, who there discovered analgesic action of inhaled nitrous oxide in 1799. Watt also built pneumatic equipment, and he introduced a gas mixture, dubbed hydro-carbonate, as a medical tonic. The bioactive component was carbon monoxide, a readily-lethal inhibitor of the transport and utilization of respiratory oxygen. Despite appreciable toxicity, carbon monoxide is an endogenous product of heme catabolism, and low doses of the gas are under laboratory investigation for therapeutic purposes. However, Watt's hydro-carbonate constituted a setback in the development of pharmacologically useful gases.

Carbon monoxide ameliorates lipopolysaccharide-induced acute lung injury via inhibition of alveolar macrophage pyroptosis

Carbon monoxide (CO) has been reported to exhibit a therapeutic effect in lipopolysaccharide (LPS)-induced acute lung injury (ALI). However, the precise mechanism by which CO confers protection against ALI remains unclear. Pyroptosis has been recently proposed to play an essential role in the initiation and progression of ALI. Thus, we investigated whether pyroptosis is involved in the protection of CO against ALI and its underlying mechanism. First, an LPS-induced ALI mouse model was established. To determine the role of pyroptosis, we evaluated histological changes and the expression levels of cleaved caspase-11, N-gasdermin D (GSDMD), and IL-1β in lung tissues, which are the indicators of pyroptosis. Inhalation of CO exhibited protective effects on LPS-induced ALI by decreasing TNF-α and IL-10 expression and ameliorating pathological changes in lung tissue. In vitro, CO significantly reduced the expression of cleaved caspase-11, N-GSDMD, IL-1β, and IL-18. In addition, it increased nuclear factor E2-related factor 2 (NRF-2) expression in a time-dependent manner in RAW 264.7 cells and decreased N-GSDMD expression. The expression of cleaved GSDMD and release of LDH were increased after treatment with a specific NRF-2 inhibitor, ML385, indicating that NRF-2 mediates the inhibition of pyroptosis by CO. Taken together, these results demonstrated that CO upregulated NRF-2 to inhibit pyroptosis and subsequently ameliorated LPS-induced ALI.

Biodegradable cascade nanocatalysts enable tumor-microenvironment remodeling for controllable CO release and targeted/synergistic cancer nanotherapy

Gas therapy as an emerging therapeutic modality for cancer treatment is still facing critical challenges such as precise delivery and controllable release of therapeutic gas. Herein, we report a "tumor-microenvironment remodeling" strategy for in situ sustained release of CO gas and magnetic resonance imaging (MRI)-monitored targeted/synergistic cancer gas/starvation nanotherapy by engineering cascade biodegradable nanocatalyst. The nanocatalyst integrates the enzyme catalyst glucose oxidase (GOD) and H₂O₂-sensitive molecule manganese carbonyl (MnCO) entrapped biodegradable hollow mesoporous organosilica nanoparticles (HMONs). Especially, GOD is initially exploited as a gatekeeper, followed by surface engineering with arginine-glycine-aspartic acid (RGD) for specifically targeting α_vβ₃ integrin-overexpressed cancer cells. The GOD is dissociated under reduced pH to release the loaded MnCO, and sequentially produce gluconic acid and H₂O₂ to remodel the TME for facilitating the in situ generation of CO/Mn²⁺. As systematically demonstrated both at cellular level and in an animal tumor xenograft model, the engineered nanocatalyst achieves superior theranostics performance via combinatorial CO gas and starving-like nanotherapy. This work provides an effective strategy for augmenting CO-mediated antitumor efficacy by remodeling the tumor microenvironment.

Predicting cost of inhalational anesthesia at low fresh gas flows: impact of a new generation carbon dioxide absorbent

It is well known that low fresh gas flows result in lower cost of inhalational agents. A new generation of carbon dioxide absorbents allows low flow anesthesia with all anesthetics but these new compounds are more expensive. This study examines the cost of inhalational anesthesia at different fresh gas flows combined with the cost of absorbent. The cost of sevoflurane and desflurane is lower at low fresh gas flows. Paradoxically the cost of isoflurane is cheaper at 2 L/min than at lower fresh gas flows due to increased cost of carbon dioxide absorbent. Therefore low fresh gas flows should be used when feasible with sevoflurane and desflurane, but higher fresh gas flows up to 2 L/min may be more economical with isoflurane during maintenance phase of anesthesia.

Carbon monoxide-triggered health effects: the important role of the inflammasome and its possible crosstalk with autophagy and exosomes

Carbon monoxide (CO) has long been known as a "silent killer" because of its ability to bind hemoglobin (Hb), leading to reduced oxygen carrying capacity of Hb, which is the main cause of CO poisoning (COP) in humans. Emerging studies suggest that mitochondria is a key target of CO action that can impact key biological processes, including apoptosis, cellular proliferation, inflammation, and autophagy. Despite its toxicity at high concentrations, CO also exhibits cyto- and tissue-protective effects at low concentrations in animal models of organ injury and disease. Specifically, CO modulates the production of pro- or anti-inflammatory cytokines and mediators by regulating the NLRP3 inflammasome. Given that human diseases are strongly associated with inflammation, a deep understanding of the exact mechanism is helpful for treatment. Autophagic factors and inflammasomes interact in various situations, including inflammatory disease, and exosomes might function as the bridge between the inflammasome and autophagy activation. Thus, the interplay among autophagy, mitochondrial dysfunction, exosomes, and the inflammasome may play pivotal roles in the health effects of CO. In this review, we summarize the latest research on the beneficial and toxic effects of CO and their underlying mechanisms, focusing on the important role of the inflammasome and its possible crosstalk with autophagy and exosomes. This knowledge may lead to the development of new therapies for inflammation-related diseases and is essential for the development of new therapeutic strategies and biomarkers of COP.

Estimation of time to compromised tenability in fires: is it time to change paradigms?

The ISO standard 13571 estimates the time to the compromised tenability of people in enclosed fires. This is understood as the time which must be available for the structural design to pass an evacuation, or an escape paradigm for the evacuation of burning buildings. As with all emergency response planning values, such once-in-a-lifetime events cannot readily be validated side-by-side. Consequently, risk assessors must refer to animal-based reference data fitting the scenario of concern closely. The analysis detailed in this paper used the concentration × time (Cxt)-matrix of point of departures (PODs) from rats acutely exposed to carbon monoxide (CO), which is amongst the most abundant toxic fire gases. The objective of the analysis was to clarify whether the time- and effect-adjusted nonlethal threshold concentration LCt₀₁ × 1/3 from acute rat inhalation studies is suited to model thresholds characterizing any 'impairment of escape' in humans. Modeled outcomes are compared with published reference data from human volunteers exposed at the similar C × t's of CO at 800 ppm × 1-h and 100 ppm × 8-h. These exposure durations match the maximum escape duration of 1-h considered in the ISO standard 13571 and standards enforcing occupational exposure limits of 8-h duration. The reference PODs indicative of 'impairment of escape' in healthy adults relied on C × t's below those eliciting any loss of motor function or psychoneurological functions. The comparison of the LCt₀₁ × 1/3 based modeled outcomes from rats match favorably with the effect-based PODs from humans. Consistent with published evidence from humans, carboxyhemoglobin (COHb) saturation-a biomarker of exposure rather than of effect-failed to reliably predict effect-based outcomes. Unlike the LCt₀₁ × 1/3 threshold approach, the COHb-based median approach used by ISO TS 13571 is inconsistent with human evidence and both over- and under-estimates the CO-related potency for causing incapacitation at non-toxic and critically-toxic C × 's, respectively. In summary, it seems timely that the ISO TS 13571 standard pays attention to scientific progress in relevant toxicity information and refinements to scientific methods shown to adequately predict human risks.

Carbon monoxide in intensive care medicine-time to start the therapeutic application?!

Carbon monoxide (CO) is not only known as a toxic gas due to its characteristics as an odorless molecule and its rapid binding to haem-containing molecules, thus inhibiting the respiratory chain in cells resulting in hypoxia. For decades, scientists established evidence about its endogenously production in the breakdown of haem via haem-oxygenase (HO-1) and its physiological effects. Among these, the modulation of various systems inside the body are well described (e.g., anti-inflammatory, anti-oxidative, anti-apoptotic, and anti-proliferative). Carbon monoxide is able to modulate several extra- and intra-cellular signaling molecules leading to differentiated response according to the specific stimulus. With our growing understanding in the way CO exerts its effects, especially in the mitochondria and its intracellular pathways, it is tempting to speculate about a clinical application of this substance. Since HO-1 is not easy to induce, research focused on the application of the gaseous molecule CO by itself or the implementation of carbon monoxide releasing molecules (CO-RM) to deliver the molecule at a time- and dose dependently safe way to any target organ. After years of research in cellular systems and animal models, summing up data about safety issues as well as possible target to treat in various diseases, the first feasibility trials in humans were established. Up-to-date, safety issues have been cleared for low-dose carbon monoxide inhalation (up to 500 ppm), while there is no clinical data regarding the injection or intake of any kind of CO-RM so far. Current models of human research include sepsis, acute lung injury, and acute respiratory distress syndrome as well as acute kidney injury. Carbon monoxide is a most promising candidate in terms of a therapeutic agent to improve outbalanced organ conditions. In this paper, we summarized the current understanding of carbon monoxide’s biology and its possible organ targets to treating the critically ill patients in tomorrow’s ICU.

The Role of Heme Oxygenase-1 in Remote Ischemic and Anesthetic Organ Conditioning

The cytoprotective effects of the heme oxygenase (HO) pathway are widely acknowledged. These effects are mainly mediated by degradation of free, pro-oxidant heme and the generation of carbon monoxide (CO) and biliverdin. The underlying mechanisms of protection include anti-oxidant, anti-apoptotic, anti-inflammatory and vasodilatory properties. Upregulation of the inducible isoform HO-1 under stress conditions plays a crucial role in preventing or reducing cell damage. Therefore, modulation of the HO-1 system might provide an efficient strategy for organ protection. Pharmacological agents investigated in the context of organ conditioning include clinically used anesthetics and sedatives. A review from Hoetzel and Schmidt from 2010 nicely summarized the effects of anesthetics on HO-1 expression and their role in disease models. They concluded that HO-1 upregulation by anesthetics might prevent or at least reduce organ injury due to harmful stimuli. Due to its clinical safety, anesthetic conditioning might represent an attractive pharmacological tool for HO-1 modulation in patients. Remote ischemic conditioning (RIC), first described in 1993, represents a similar secure option to induce organ protection, especially in its non-invasive form. The efficacy of RIC has been intensively studied herein, including on patients. Studies on the role of RIC in influencing HO-1 expression to induce organ protection are emerging. In the first part of this review, recently published pre-clinical and clinical studies investigating the effects of anesthetics on HO-1 expression patterns, the underlying signaling pathways mediating modulation and its causative role in organ protection are summarized. The second part of this review sums up the effects of RIC.