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

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 incompletely prevents isoflurane-induced defects in murine neurodevelopment

BACKGROUND

Commonly used anesthetics have been shown to disrupt neurodevelopment in preclinical models. It has been proposed that such anesthesia-induced neurotoxicity is mediated by apoptotic neurodegeneration in the immature brain. Low dose carbon monoxide (CO) exerts cytoprotective properties and we have previously demonstrated that CO inhibits isoflurane-induced apoptosis in the developing murine brain. Here we utilized anti-apoptotic concentrations of CO to delineate the role of apoptotic neurodegeneration in anesthesia-induced neurotoxicity by assessing the effect of CO on isoflurane-induced defects in neurodevelopment.

METHODS

C57Bl/6 mouse pups underwent 1-hour exposure to 0ppm (air), 5ppm, or 100ppm CO in air with or without isoflurane on postnatal day 7. Cohorts were evaluated 5-7weeks post exposure with T-maze cognitive testing followed by social behavior assessment. Brain size, whole brain cellular content, and neuronal density in primary somatosensory cortex and hippocampal CA3 region were measured as secondary outcomes 1-week or 5-7weeks post exposure along with 7-day old, unexposed controls.

RESULTS

Isoflurane impaired memory acquisition and resulted in abnormal social behavior. Low concentration CO abrogated anesthetic-induced defects in memory acquisition, however, it also resulted in impaired spatial reference memory and social behavior abnormalities. Changes in brain size, cellular content, and neuronal density over time related to the age of the animal and were unaffected by either isoflurane or CO.

CONCLUSIONS

Anti-apoptotic concentrations of CO incompletely prevented isoflurane-induced defects in neurodevelopment, lacked concentration-dependent effects, and only provided protection in certain domains suggesting that anesthesia-related neurotoxicity is not solely mediated by activation of the mitochondrial apoptosis pathway.

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.

Carbon monoxide modulates cytochrome oxidase activity and oxidative stress in the developing murine brain during isoflurane exposure

Commonly used anesthetics induce widespread neuronal degeneration in the developing mammalian brain via the oxidative-stress-associated mitochondrial apoptosis pathway. Dysregulation of cytochrome oxidase (CcOX), the terminal oxidase of the electron transport chain, can result in reactive oxygen species (ROS) formation. Isoflurane has previously been shown to activate this enzyme. Carbon monoxide (CO), as a modulator of CcOX, is of interest because infants and children are routinely exposed to CO during low-flow anesthesia. We have recently demonstrated that low concentrations of CO limit and prevent isoflurane-induced neurotoxicity in the forebrains of newborn mice in a dose-dependent manner. However, the effect of CO on CcOX in the context of anesthetic-induced oxidative stress is unknown. Seven-day-old male CD-1 mice underwent 1h exposure to 0 (air), 5, or 100ppm CO in air with or without isoflurane. Exposure to isoflurane or CO independently increased CcOX kinetic activity and increased ROS within forebrain mitochondria. However, exposure to CO combined with isoflurane paradoxically limited CcOX activation and oxidative stress. There were no changes seen in steady-state levels of CcOX I protein, indicating post-translational modification of CcOX as an etiology for changes in enzyme activity. CO exposure led to differential effects on CcOX subunit I tyrosine phosphorylation depending on concentration, while combined exposure to isoflurane with CO markedly increased the enzyme phosphorylation state. Phosphorylation of tyrosine 304 of CcOX subunit I has been shown to result in strong enzyme inhibition, and the relative reduction in CcOX kinetics following exposure to CO combined with isoflurane may have been due, in part, to such phosphorylation. Taken together, the data suggest that CO modulates CcOX in the developing brain during isoflurane exposure, thereby limiting oxidative stress. These CO-mediated effects could have implications for the development of low-flow anesthesia in infants and children to prevent anesthesia-induced oxidative stress.

Subclinical carbon monoxide limits apoptosis in the developing brain after isoflurane exposure

BACKGROUND

Volatile anesthetics cause widespread apoptosis in the developing brain. Carbon monoxide (CO) has antiapoptotic properties, and exhaled endogenous CO is commonly rebreathed during low-flow anesthesia in infants and children, resulting in subclinical CO exposure. Thus, we aimed to determine whether CO could limit isoflurane-induced apoptosis in the developing brain.

METHODS

Seven-day-old male CD-1 mouse pups underwent 1-hour exposure to 0 (air), 5, or 100 ppm CO in air with or without isoflurane (2%). We assessed carboxyhemoglobin levels, cytochrome c peroxidase activity, and cytochrome c release from forebrain mitochondria after exposure and quantified the number of activated caspase-3 positive cells and TUNEL positive nuclei in neocortex, hippocampus, and hypothalamus/thalamus.

RESULTS

Carboxyhemoglobin levels approximated those expected in humans after a similar time-weighted CO exposure. Isoflurane significantly increased cytochrome c peroxidase activity, cytochrome c release, the number of activated caspase-3 cells, and TUNEL positive nuclei in the forebrain of air-exposed mice. CO, however, abrogated isoflurane-induced cytochrome c peroxidase activation and cytochrome c release from forebrain mitochondria and decreased the number of activated caspase-3 positive cells and TUNEL positive nuclei after simultaneous exposure with isoflurane.

CONCLUSIONS

Taken together, the data indicate that CO can limit apoptosis after isoflurane exposure via inhibition of cytochrome c peroxidase depending on concentration. Although it is unknown whether CO directly inhibited isoflurane-induced apoptosis, it is possible that low-flow anesthesia designed to target rebreathing of specific concentrations of CO may be a desired strategy to develop in the future in an effort to prevent anesthesia-induced neurotoxicity in infants and children.

[The choice of a pediatric anesthesia ventilator]

The technology of anesthesia ventilators has substantially progressed during last years. The choice of a pediatric anesthesia ventilator needs to be led by multiple parameters: requirement, technical (pneumatic performance, velocity of halogenated or oxygen delivery), cost (purchase, in operation, preventive and curative maintenance), reliability, ergonomy, upgradability, and compatibility. The demonstration of the interest of pressure support mode during maintenance of spontaneous ventilation anesthesia makes this mode essential in pediatrics. In contrast, the financial impact of target controlled inhalation of halogenated has not be studied in pediatrics. Paradoxically, complex and various available technologies had not been much prospectively studied. Anesthesia ventilators performances in pediatrics need to be clarified in further clinical and bench test studies.

Performance validation of a modified magnetic resonance imaging-compatible temperature probe in children

INTRODUCTION

During magnetic resonance imaging (MRI), children are at risk for body temperature variations. The cold MRI environment that preserves the MRI magnet can cause serious hypothermia. On the other hand, hyperthermia may also develop because of radiofrequency-induced heating of the tissues, particularly in prolonged examinations. Because of a lack of MRI-compatible core temperature probes, temperature assessment is unreliable, and specific absorption rate-related patient heat gain must be calculated to determine the allowable scan duration. We compared an MRI-compatible temperature probe and a modification thereof to a standard esophageal core body temperature probe in children.

METHODS

Children undergoing general anesthesia were recruited, each patient serving as his/her own control. Core body temperature was measured using 3 different devices: (1) a fiberoptic MRI-compatible skin surface temperature probe (MRI-skin) located on the child's skin surface; (2) a fiberoptic MRI-compatible temperature probe modified with a single-use sleeve at the tip (MRI-core), located in the nasopharynx; and (3) a standard temperature monitor (STRD) located in the esophagus or nasopharynx. The Bland-Altman method was used for statistical analysis.

RESULTS

We enrolled 60 children aged 7.8 ± 6 years (mean ± SD) weighing 32.4 (±26.4) kg. The estimated difference between the STRD and MRI-core measurements of core temperature was 0.06°C (confidence interval [CI]: -0.02, 0.15), and between the STRD and the MRI-skin 1.19°C (CI: 0.97, 1.41). According to the Bland-Altman analysis, the 95% limits of agreement ranged from -0.9 to 3.4 and from -1.3 to 1.2 between the STRD and the MRI-skin probe and the MRI-core probe, respectively.

DISCUSSION

Our results show good agreement between standard esophageal measurements of core temperature and core temperature measured using a modified MRI-core probe during general anesthesia in a general surgical pediatric population. The ability to accurately assess core temperature in the MRI suite may safely allow longer scan times and therefore reduce repeat anesthetic exposure, improve patient safety, and enhance the quality of care in children.

Neurodevelopmental consequences of sub-clinical carbon monoxide exposure in newborn mice

Carbon monoxide (CO) exposure at high concentrations results in overt neurotoxicity. Exposure to low CO concentrations occurs commonly yet is usually sub-clinical. Infants are uniquely vulnerable to a variety of toxins, however, the effects of postnatal sub-clinical CO exposure on the developing brain are unknown. Apoptosis occurs normally within the brain during development and is critical for synaptogenesis. Here we demonstrate that brief, postnatal sub-clinical CO exposure inhibits developmental neuroapoptosis resulting in impaired learning, memory, and social behavior. Three hour exposure to 5 ppm or 100 ppm CO impaired cytochrome c release, caspase-3 activation, and apoptosis in neocortex and hippocampus of 10 day old CD-1 mice. CO increased NeuN protein, neuronal numbers, and resulted in megalencephaly. CO-exposed mice demonstrated impaired memory and learning and reduced socialization following exposure. Thus, CO-mediated inhibition of neuroapoptosis might represent an important etiology of acquired neurocognitive impairment and behavioral disorders in children.