The mechanisms of carbon monoxide production by inhalational agents

A call for immediate climate action in anesthesiology: routine use of minimal or metabolic fresh gas flow reduces our ecological footprint

PURPOSE

Climate change is a global threat, and inhalational anesthetics contribute to global warming by altering the photophysical properties of the atmosphere. On a global perspective, there is a fundamental need to reduce perioperative morbidity and mortality and to provide safe anesthesia. Thus, inhalational anesthetics will remain a significant source of emissions in the foreseeable future. It is, therefore, necessary to develop and implement strategies to minimize the consumption of inhalational anesthetics to reduce the ecological footprint of inhalational anesthesia.

SOURCE

We have integrated recent findings concerning climate change, characteristics of established inhalational anesthetics, complex simulative calculations, and clinical expertise to propose a practical and safe strategy to practice ecologically responsible anesthesia using inhalational anesthetics.

PRINCIPAL FINDINGS

Comparing the global warming potential of inhalational anesthetics, desflurane is about 20 times more potent than sevoflurane and five times more potent than isoflurane. Balanced anesthesia using low or minimal fresh gas flow (≤ 1 L·min-1) during the wash-in period and metabolic fresh gas flow (0.35 L·min-1) during steady-state maintenance reduces CO2 emissions and costs by approximately 50%. Total intravenous anesthesia and locoregional anesthesia represent further options for lowering greenhouse gas emissions.

CONCLUSION

Responsible anesthetic management choices should prioritize patient safety and consider all available options. If inhalational anesthesia is chosen, the use of minimal or metabolic fresh gas flow reduces the consumption of inhalational anesthetics significantly. Nitrous oxide should be avoided entirely as it contributes to depletion of the ozone layer, and desflurane should only be used in justified exceptional cases.

Carbon Monoxide Poisoning

Carbon monoxide is a colorless, odorless, highly toxic gas primarily produced through the incomplete combustion of organic material. Carbon monoxide binds to hemoglobin and other heme molecules, causing tissue hypoxia and oxidative stress. Symptoms of carbon monoxide poisoning can vary from a mild headache to critical illness, which can make diagnosis difficult. When there is concern for possible carbon monoxide poisoning, the diagnosis can be made via blood co-oximetry. The primary treatment for patients with carbon monoxide poisoning is supplemental oxygen, usually delivered via a nonrebreather mask. Hyperbaric oxygen can also be used, but the exact indications are controversial.

Investigation and Possibilities of Reuse of Carbon Dioxide Absorbent Used in Anesthesiology

Absorbents used in closed and semi-closed circuit environments play a key role in preventing carbon dioxide poisoning. Here we present an analysis of one of the most common carbon dioxide absorbents—soda lime. In the first step, we analyzed the composition of fresh and used samples. For this purpose, volumetric and photometric analyses were introduced. Thermal properties and decomposition patterns were also studied using thermogravimetric and X-ray powder diffraction (PXRD) analyses. We also investigated the kinetics of carbon dioxide absorption under conditions imitating a closed-circuit environment.

Anesthesia-Related Carbon Monoxide Exposure: Toxicity and Potential Therapy

Exposure to carbon monoxide (CO) during general anesthesia can result from volatile anesthetic degradation by carbon dioxide absorbents and rebreathing of endogenously produced CO. Although adherence to the Anesthesia Patient Safety Foundation guidelines reduces the risk of CO poisoning, patients may still experience subtoxic CO exposure during low-flow anesthesia. The consequences of such exposures are relatively unknown. In contrast to the widely recognized toxicity of high CO concentrations, the biologic activity of low concentration CO has recently been shown to be cytoprotective. As such, low-dose CO is being explored as a novel treatment for a variety of different diseases. Here, we review the concept of anesthesia-related CO exposure, identify the sources of production, detail the mechanisms of overt CO toxicity, highlight the cellular effects of low-dose CO, and discuss the potential therapeutic role for CO as part of routine anesthetic management.

Carbon monoxide and anesthesia-induced neurotoxicity

The majority of commonly used anesthetic agents induce widespread neuronal degeneration in the developing mammalian brain. Downstream, the process appears to involve activation of the oxidative stress-associated mitochondrial apoptosis pathway. Targeting this pathway could result in prevention of anesthetic toxicity in the immature brain. Carbon monoxide (CO) is a gas that exerts biological activity in the developing brain and low dose exposures have the potential to provide neuroprotection. In recent work, low concentration CO exposures limited isoflurane-induced neuronal apoptosis in a dose-dependent manner in newborn mice and modulated oxidative stress within forebrain mitochondria. Because infants and children are routinely exposed to low levels of CO during low-flow general endotracheal anesthesia, such anti-oxidant and pro-survival cellular effects are clinically relevant. Here we provide an overview of anesthesia-related CO exposure, discuss the biological activity of low concentration CO, detail the effects of CO in the brain during development, and provide evidence for CO-mediated inhibition of anesthesia-induced neurotoxicity.

Performance of a new carbon dioxide absorbent, Yabashi lime® as compared to conventional carbon dioxide absorbent during sevoflurane anesthesia in dogs

In the present study, we compare a new carbon dioxide (CO2) absorbent, Yabashi lime(®) with a conventional CO2 absorbent, Sodasorb(®) as a control CO2 absorbent for Compound A (CA) and Carbon monoxide (CO) productions. Four dogs were anesthetized with sevoflurane. Each dog was anesthetized with four preparations, Yabashi lime(®) with high or low-flow rate of oxygen and control CO2 absorbent with high or low-flow rate. CA and CO concentrations in the anesthetic circuit, canister temperature and carbooxyhemoglobin (COHb) concentration in the blood were measured. Yabashi lime(®) did not produce CA. Control CO2 absorbent generated CA, and its concentration was significantly higher in low-flow rate than a high-flow rate. CO was generated only in low-flow rate groups, but there was no significance between Yabashi lime(®) groups and control CO2 absorbent groups. However, the CO concentration in the circuit could not be detected (≤5ppm), and no change was found in COHb level. Canister temperature was significantly higher in low-flow rate groups than high-flow rate groups. Furthermore, in low-flow rate groups, the lower layer of canister temperature in control CO2 absorbent group was significantly higher than Yabashi lime(®) group. CA and CO productions are thought to be related to the composition of CO2 absorbent, flow rate and canister temperature. Though CO concentration is equal, it might be safer to use Yabashi lime(®) with sevoflurane anesthesia in dogs than conventional CO2 absorbent at the point of CA production.

Carboxyhaemoglobin formation and ECG changes during hysteroscopic surgery, transurethral prostatectomy and tonsillectomy using bipolar diathermy

Diathermy is known to produce a mixture of waste products including carbon monoxide. During transcervical hysteroscopic surgery, carbon monoxide might enter the circulation leading to the formation of carboxyhaemoglobin. In 20 patients scheduled for transcervical hysteroscopic resection of myoma or endometrium, carboxyhaemoglobin was measured before and at the end of the surgical procedure, and compared with levels measured in 20 patients during transurethral prostatectomy, and in 20 patients during tonsillectomy. Haemodynamic data, including ST-segment changes, were recorded. Levels of carboxyhaemoglobin increased significantly during hysteroscopic surgery from median (IQR [range]) 1.0% (0.7-1.4 [0.5-4.9])% to 3.5% (2.0-6.1 [1.3-10.3]%, p < 0.001), compared with levels during prostatectomy or tonsillectomy. Significant ST-segment changes were observed in 50% of the patients during hysteroscopic surgery. Significant correlations were observed between the increase in carboxyhaemoglobin and the maximum ST-segment change (ρ = -0.707, p < 0.01), between the increase in carboxyhaemoglobin and intravasation (ρ = 0.625; p < 0.01), and between intravasation and the maximum ST-segment change (ρ = -0.761; p < 0.01). The increased carboxyhaemoglobin levels during hysteroscopic surgery appear to be related to the amount of intravasation and this could potentially be a contributing factor to the observed ST-segment changes.

The current status of continuous noninvasive measurement of total, carboxy, and methemoglobin concentration

Intraoperative early detection of anemia, identifying toxic levels of carboxyhemoglobin after carbon monoxide exposure and titrating drug dosage to prevent toxic levels of methemoglobin are important goals. The pulse oximeter works by illuminating light into the tissue and sensing the amount of light absorbed. The same methodology is used by laboratory hemoglobinometers to measure hemoglobin concentration. Because both devices work in the same way, efforts were made to modify the pulse oximeter to also measure hemoglobin concentration. Currently there are 2 commercial pulse oximeters (Masimo Rainbow SET and OrSense NBM-200MP) that measure total hemoglobin concentration and one (Masimo) that also measures methemoglobin and carboxyhemoglobin. In this review, we describe the peer-reviewed literature addressing the accuracy of these monitors.

Desflurane: a clinical update of a third-generation inhaled anaesthetic

Available volatile anaesthetics are safe and efficacious; however, their varying pharmacology provides small but potentially clinically important differences. Desflurane is one of the third-generation inhaled anaesthetics. It is the halogenated inhaled anaesthetic with the lowest blood and tissue solubilities, which promotes its rapid equilibration and its rapid elimination following cessation of administration at the end of anaesthesia. The low fat solubility of desflurane provides pharmacological benefits, especially in overweight patients and in longer procedures by reducing slow compartment accumulation. A decade of clinical use has provided evidence for desflurane's safe and efficacious use as a general anaesthetic. Its benefits include rapid and predictable emergence, and early recovery. In addition, the use of desflurane promotes early and predictable extubation, and the ability to rapidly transfer patients from the operating theatre to the recovery area, which has a positive impact on patient turnover. Desflurane also increases the likelihood of patients, including obese patients, recovering their protective airway reflexes and awakening to a degree sufficient to minimise the stay in the high dependency recovery area. The potential impact of the rapid early recovery from desflurane anaesthesia on intermediate and late recovery and resumption of activities of daily living requires further study.

Carbon monoxide re-breathing during low-flow anaesthesia in infants and children

BACKGROUND

Carbon monoxide (CO) has been detected within anaesthesia breathing systems. One potential source in this setting is exhaled endogenous CO. We hypothesized that CO is re-breathed during low-flow anaesthesia (LFA) in infants and children.

METHODS

Twenty children (age 2 months-7 yr) undergoing general anaesthesia were evaluated in a prospective observation study. LFA was established for 60 min followed by high-flow anaesthesia (HFA) for the next 60 min. Exhaled and inspired CO were measured every 5 min within the breathing circuit. Carboxyhaemoglobin (COHb%) was measured at baseline, at 60 min, after LFA, and at 120 min, after HFA.

RESULTS

CO concentrations increased during LFA. Inspired CO peaked at 14 ppm. During HFA, exhaled CO levels remained constant whereas inspired CO decreased markedly. Exhaled and inspired CO during HFA differed significantly from LFA. The trajectory of change in exhaled and inspired CO was most closely associated with the fresh-gas flow (FGF):minute ventilation ratio. COHb% significantly increased in children <2 yr of age at 60 min after LFA and remained increased.

CONCLUSIONS

LFA increased exhaled and inspired CO and increased COHb% in children <2 yr of age. Thus, LFA resulted in re-breathing of exhaled CO and exposure, especially in the youngest children. Re-breathing exhaled gas during LFA could pose a risk for an acute CO exposure in patients who have elevated COHb and high baseline levels of exhaled CO. If practitioners match or exceed minute ventilation with FGF to avoid LFA, CO re-breathing can be limited.