Nitrous oxide as an inhalation anesthetic
INTRODUCTION Pubmed - Nitrous Oxide and the Inhalation Anesthetics - 2008
General anaesthesia is a drug-induced state that is characterised by an absence of perception to all sensations. In 1920, Guedel described ether anaesthesia according to 4 stages, each of which reflects greater depression of brain function: Stage I: analgesia; Stage II: delirium; Stage III: surgical anaesthesia; and Stage IV: medullary paralysis and death. Today, these stages are merely of historical interest; they are too unpredictable and inconsistent to be attributed to modern-day general anaesthetics.
Performance of surgery usually requires an immobilised patient who is amnesic for the procedure and does not exhibit an excessive autonomic response to surgical stimulation (blood pressure and heart rate). Most anaesthesiologists agree that the anaesthetic state reflects a collection of component changes in behaviour and perception, such as immobilisation, unconsciousness and attenuation of autonomic responses to stimulation. Many of the agents classified by convention as general anaesthetics do not accomplish all of these components. The inhalation agents come closest but only at doses that depress the brain to such a degree that vital functions become jeopardized.
It is most common to use combination regimens in providing general anaesthesia: a potent sedative-hypnotic or inhalation agent is used to produce unconsciousness, analgesia is provided with an opioid, and muscle relaxation is produced with the use of a neuromuscular blocking agent.
Nitrous oxide (N2O) has been used for well over 150 years in clinical dentistry for its analgesic and anxiolytic properties. This small inorganic chemical molecule has effects of analgesia, anxiolysis, and anesthesia that are of great clinical interest.
The analgesic effect of N2O is opioid in nature, and, like morphine, may involve a myriad of neuromodulators in the spinal cord. The anxiolytic effect of N2O, on the other hand, resembles that of benzodiazepines and may be initiated at selected subunits of the γ-aminobutyric acid type A (GABAA) receptor. Similarly, the anesthetic effect of N2O may involve actions at GABAA receptors and possibly at N-methyl-D-aspartate receptors as well. Today the common medical uses of N2O include balanced anesthesia during surgery, wherein N2O is frequently combined with other general anesthetic drugs and non-anesthetic preoperative drugs, and analgesia for the relief of pain in clinical as well as emergency situations.
ANALGESIA Pubmed - Advances in understanding the Actions of Nitrous Oxide - 2007
Subanesthetic concentrations of N2O produce only analgesic and anxiolytic effects without unconsciousness.
N2O Antinociceptive Action
30% of N2O was deemed to be as effective as 10–15 mg of morphine. A large number of studies have established an important role for opioid receptors in the periaqueductal gray (PAG) area of the midbrain in pain modulation. The N2O-induced analgesic effect could be completely ablated after lesioning the PAG in the rat or following microinjection of opioid antagonists into the PAG. The results clearly implicate opioid receptors in mediation of N2O-induced antinociception and analgesia. There are multiple opioid receptors that are capable of mediating pain relief, and the specific subtypes of opioid receptors that mediate the antinociceptive effects of N2O appear to depend on various factors including the species and/or strain, the regions of the brain, and the experimental noxious stimulus.
What subtype of opioid receptor is involved?
The κ opioid receptor subtype appears to be involved; this hypothesis is supported by:
- observations that N2O-induced antinociception was antagonized by supraspinal and spinal pretreatment with antisera against the endogenous κ opioid ligand, dynorphin (DYN).
- reports of morphine-tolerant animals being cross-tolerant to N2O. It is important to notice that cross-tolerance was unilateral (N2O-tolerant animals were not cross-tolerant to morphine) so N2O might work through stimulating the neuronal release of endogenous opioid peptides.
Chronic treatment with N2O results in a tolerance that is attributable to excessive depletion of endogenous opioid peptide stores, such that a subsequent exposure to N2O is unable to release sufficient quantities of opioid peptides to cause antinociception. It appears that N2O evokes its antinociceptive effect through the supraspinal release of various DYNs, which are the endogenous ligands of κ opioid receptors, and spinal release of DYNs and ME.
Involvement of Nitric Oxide in N2O Antinociception
N2O antinociception was antagonized in dose-related fashion by a series of L-arginine analogs that competitively inhibit NO synthase. Later studies demonstrated that N2O-induced antinociception was more specifically antagonized by pretreatment with a selective inhibitor of neuronal NOS. Nitric oxide also appears to play a key role in opioid peptide release. If N2O antinociception is caused by stimulated release of DYNs, which then activate κ opioid receptors, and if NO appears not to influence κ opioid receptor or signal transduction, then it is possible that NO influences the stimulated release of DYNs.
FIG. 1: Mechanism of N2O-induced analgesia: influence of N2O on descending inhibitory pathways. N2O induces release of endogenous opioid peptides (EOP) that activate opioid receptors on GABAergic pontine nuclei. This pathway, in turn, activates descending noradrenergic system in the dorsal horn of the spinal cord that directly or indirectly inhibits (through a GABA interneuron) nociceptive processing at the level of the primary afferent and second-order neurons that transmit sensory signals up the ascending nociceptive pathway.
Descending Pathways Activated by N2O
The release of endogenous opioid peptides and the subsequent stimulation of opioid receptors activate descending pathways that modulate nociceptive processing in the spinal cord. There are several steps to this process:
1. The stimulation of opioid receptors by N2O-released opioid peptides inhibits the inhibitory GABA-ergic pathway, thus causing disinhibition of the descending noradrenergic pathway.
2. The disinhibited noradrenergic pathway appears to modulate spinal nociceptive processing by 2 divergent pathways:
a. One population of α2 adrenergic receptors is located on second-order neurons in the pain pathway;
b. Another population is located on inhibitory GABA interneurons in the spinal cord.
FIG. 2: Putative neuronal pathway for the analgesic effect of nitrous oxide (N2O). (A) Nitrous oxide causes activation of opioidergic neurons through hypothalamic release of corticotrophin-releasing factor (CRF), which provokes the release of endogenous opioids at their terminals in the periaqueductal gray region of midbrain. Opioid receptors on GABAergic interneurons are stimulated, resulting in an inhibition of these inhibitory interneurons. In turn, this results in a disinhibition of excitatory neurons of the noradrenergic descending inhibitory neurons in the medulla–pons region. The descending noradrenergic neuron releases norepinephrine at its terminals in the spinal cord, which stimulate at least two species of adrenergic receptors, namely, α1 subtypes on GABAergic interneurons or α2B-adrenergic receptors located postsynaptically on the second-order neuron. The effect of stimulating these two sets of receptors in the dorsal horn of the spinal cord decreases firing of the second-order neuron and results in a reduction in pain impulses ascending into the supraspinal regions. (B) The effects of addition of a GABA-ergic agent, which activates postsynaptic GABAA receptors on the pain pathway, preventing activation of the noradrenergic descending inhibitory neurons by nitrous oxide (signified by the “no entry” sign). Black triangle= excitatory synapse; white triangle= inhibitory synapse; black oval= nucleus of an active cell; white oval= nucleus of an inactive cell. AR = adrenoceptor; ExNT = excitatory neurotransmitters; ExR = receptors for excitatory neurotransmitters; GABA = γ-aminobutyric acid; GABA-R = γ-aminobutyric acid receptor; LC = locus ceruleus; NE = norepinephrine; Op = opioid peptides; Op-R = opioid receptor. Pubmed - Biologic Effects of Nitrous Oxide. A Mechanistic and Toxicologic Review. 2008
ANXIOLYSIS
In dentistry, subanesthetic concentrations of N2O are routinely used to produce moderate sedation for dental surgery in anxious patients. The ease of its administration, its wide margin of safety, its analgesic and anxiolytic effects, and, most of all, its rapid reversibility make it an ideal drug for use in children. There is evidence that the relaxation and relief from anxiety during inhalation of N2O is a specific anxiolytic effect that is independent of the analgesic action of N2O. The mechanisms involved are not yet completely understood, but the benzodiazepine/GABA receptor role is thought to be relevant. N2O evokes patterns of behavioral response that are reminiscent of the effects of benzodiazepines in different animal models of experimental anxiety. N2O- and benzodiazepine-induced anxiolytic-like behaviors were equally sensitive to antagonism by the benzodiazepine binding site blocker flumazenil.
The signaling pathway for the anxiolytic-like activity
Because benzodiazepines work through facilitation of GABAergic inhibitory neurotransmission, research was conducted to determine involvement of GABAA receptors in N2O anxiolysis. These findings indicate that GABAA receptors mediate the anxiolytic-like effects caused by chlordiazepoxide and N2O activation of benzodiazepine receptors.
N2O- and benzodiazepine-induced anxiolytic-like effects are also sensitive to antagonism by inhibition of NOS. These findings suggest that NO plays a key role in the anxiolytic signaling mechanism downstream from the benzodiazepine/GABAA receptor complex. The soluble 3′, 5′ -cyclic guanosine monophosphate (cGMP)-dependent pathway has been identified by many studies as the main signal transduction pathway of NO. cGMP is known to act upon several different targets: cGMP-dependent protein kinases (PKG), cGMP-fated cation channels, or cGMP-regulated phosphodiesterase. N2O-induced anxiolytic-like behavior was significantly attenuated by inhibitors of PKG but not cAMP-dependent protein kinase (PKA).
Although there is evidence that stimulation of GABAA receptors activates an anxiolytic signaling pathway that includes an enzyme sequence of NOS, soluble guanylyl cyclase, and PKG, how N2O acts at the molecular level to stimulate the BZ binding site and GABAA receptor is not yet known. Similarly to how N2O activates opioid receptors, it is plausible that N2O may induce neuronal release of endogenous benzodiazepine factors that then stimulate the GABAA receptor.
FIG. 3 Mechanism of N2O-induced anxiolysis. N2O is thought to cause activation of the benzodiazepine (BZ) binding site as its effects are blocked by flumazenil. This action facilitates γ-aminobutyric acid (GABA) activation of its binding site, resulting in chloride ion influx. The increased chloride ion concentration in the neuron might cause activation of calmodulin (CaM), which then activates the enzyme nitric oxide synthase (NOS). NOS converts the amino acid L-arginine (L-Arg) to L-citrulline (L-Cit) and NO, which stimulates the enzyme soluble guanylyl cyclase producing the second messenger cyclic guanosine monophosphate (cyclic GMP). The cyclic GMP, in turn, stimulates a cyclic GMP-dependent protein kinase (PKG) that leads to the anxiolytic drug effect.
ANESTHESIA
N2O was the first drug used for surgical anesthesia. N2O by itself would require high volume percentage and hyperbaric conditions to achieve anesthesia in 50% of subjects. Therefore, because of its low potency, in clinical practice, N2O is generally used to reduce the minimum alveolar concentration of a second inhalation agent for anesthesia and increase the rate of induction (i.e. the second gas effect) and to provide or augment the analgesic component of general anesthesia.
General anesthetics like N2O have long been hypothesized to act in a nonspecific manner on neuronal membranes, alter membrane fluidity, and influence ion channels. More recently, it has been suggested that general anesthetics might act on one or more superfamilies of ligand-gated ion channels that include GABAA, glycine, nicotinic acetylcholine, 5-hydroxytryptamine3, and glutamate receptors. The GABAA receptor is considered to be a prime target. N-methyl-D-aspartate (NMDA)-type glutamate receptors have recently emerged as a possible target of inhalation anesthetic drugs. Studies in cultured rat hippocampal neurons revealed that N2O inhibited NMDA-activated currents in a dose-dependent manner but had no effect on GABA-activated currents. N2O also inhibited excitotoxic neurodegeneration that is mediated through NMDA receptors. Similar to other NMDA antagonists, the neurotoxic effect of N2O is age-dependent and is sensitive to attenuation by GABA-ergic drugs. It is suggested that a common property of NMDA receptor antagonism may underlie the similar pharmacological profiles of N2O and ketamine. The two drugs, indeed, produce synergistic neurotoxicity when used together.
SUMMARY
It is apparent from the above discussion that N2O has multiple mechanisms of action that underlie its varied pharmacological properties. Current research indicates that the analgesic effect of N2O appears to be initiated by stimulated neuronal release of endogenous opioid peptides, with subsequent activation of opioid receptors and descending GABA and noradrenergic pathways that modulate nociceptive processing at the spinal level. The anxiolytic effect of N2O involves activation of the GABA receptor through the benzodiazepine binding site, although whether N2O acts directly or indirectly upon the latter targets remains uncertain. The anxiolytic pathway that is stimulated includes a segment that involves a sequence of 3 key enzymes, NOS, soluble guanylyl cyclase, and PKG. The anesthetic effect of N2O appears to be caused by inhibition of NMDA glutamate receptors and removing its excitatory influence in the nervous system.
MECHANISM OF INHIBITION OF METHIONINE SYNTHASE
Aside from the biologic actions of nitrous oxide transducing its anesthetic and analgesic actions, nitrous oxide also affects methionine synthase function. Nitrous oxide oxidizes the cobalt I (Co+) form of cobalamin (vitamin B12) to Co3+:
Co+ + N2O + 2H+ -> Co3+ + N2 + H2O
A further reaction rapidly ensues:
Co 3+ + Co+ -> 2Co2+
However, an alternative reaction has also been proposed to account for nitrous oxide’s oxidation of cobalamin involving the generation of hydroxyl radicals that irreversibly oxidize cobalamin:
Co+ + N2O + H+-> Co 3+ + N2 + OH-
FIG. 4 By either of the above mechanisms, the resulting oxidized cobalt cation prevents cobalamin (vitamin B12) acting as a coenzyme for methionine synthase. Methionine synthase is a ubiquitous cytosolic enzyme that plays a crucial role in the generation of methyl groups, (via the active intermediary s-adenosylmethionine) for the synthesis of DNA, RNA, myelin, and catecholamines, among other products. After methyl group donation, s-adenosylmethionine is converted to homocysteine, which can then reenter the methionine pathway or be metabolized to cystathionine. In addition to cobalamin, methionine synthase requires 5-methyltetrahydrofolate to function as a coenzyme. One carbon transfer by folates plays a crucial role in the biosynthesis of pyrimidines and purines and in serine and glycine metabolism. This latter interconversion between serine and glycine produces the methyl groups to be added to homocysteine to produce methionine. The methyl group is initially bound to tetrahydrofolate, to give 5,10-methylenetetrahydrofolate, which is then reduced to 5-methylenetetrahydrofolate. This pathway is important because it represents the only way to produce 5-methylenetetrahydrofolate. The methyl group is then transferred to cobalamin, producing methylcobalamin, the final methyl group donor for methionine synthesis. (See also fig.5 )
This pathway is critical to cellular function, and decreased methionine synthase activity can result in both genetic and protein aberrations. Certain patient groups may be particularly susceptible to reduced methionine synthase activity, including those deficient in cobalamin, such as patients with pernicious anemia or ileal disease, alcoholics, the elderly, and the malnourished.
At anesthetic concentrations in rats, methionine synthase activity is inhibited rapidly; 50% nitrous oxide exposure decreased methionine synthase activity within 30 min, and the activity was virtually undetectable after 6 h. Because nitrous oxide readily crosses the placenta, this inhibition has been recorded in both the fetus and the dam, with 60 min of nitrous oxide (50%) inhibiting methionine synthase activity to 11% and 18% in maternal and fetal livers in rats, respectively (rats are more sensitive to nitrous oxide–induced methionine synthase inhibition than humans are.) Nonetheless, patients anesthetized with nitrous oxide (70%) exhibited a reduction in methionine synthase activity measured in liver biopsies, with a time to half methionine synthase activity of 46 min. After 200 min of nitrous oxide anesthesia, methionine synthase activity approaches zero. Typically, the enzyme’s function will recover within 3–4 days after exposure to nitrous oxide. Due to this effect, the duration of nitrous oxide exposure is correlated with increased homocysteine levels, which can be prevented by preoperative vitamin B complex therapy.
An important enzyme in the folate cycle, 5,10-methylenetetrahydrofolate reductase (MTHFR), also plays an important role in the conversion of homocysteine to methionine by generating 5-methyltetrahydrofolate. Two single-nucleotide polymorphisms are known (677 cytosine–thymidine and 1298 adenosine–cytosine) and are associated with reduced enzyme activity. Mutations in this gene are relatively common, homozygosity for many of these mutations is associated with increased homocysteine levels and reduction in MTHFR enzyme activity. Interesting recent data show that patients who receive nitrous oxide anesthesia with a homozygous mutation in MTHFR have a greater postoperative increase in plasma homocysteine levels than heterozygotes or wild type.
SYSTEMIC INFLUENCE
RESPIRATORY EFFECTS
All anaesthetic gases increase respiratory rate and diminish tidal volume. Unlike other agents, however, the increase in rate produced by nitrous oxide may actually provide a net increase in minute ventilation. Therefore, when used alone for mild to moderate sedation, nitrous oxide does not depress ventilation. However, when it is combined with sedatives or opioids that depress ventilation, a more pronounced and clinically important depression may result. Similar to other inhalation agents, nitrous oxide produces a dose-dependent depression of ventilatory drive with greater influence on the ventilatory response to hypoxemia than to hypercapnia. This is to say that if respiratory depression occurs, nitrous oxide obtunds the body’s normal response to lowered oxygen tension rather than to elevated carbon dioxide tension. Because patients who have significant chronic obstructive pulmonary disease rely almost entirely on hypoxemic drive, someone suggests that nitrous oxide should be avoided in these patients, not only because of its depression of hypoxemic drive, but also because of high oxygen concentrations delivered with nitrous oxide, which may remove the stimulus for hypoxemic drive.
CARDIOVASCULAR EFFECTS
Nitrous oxide mildly depresses myocardial contractility, but this is compensated by its ability to activate sympathetic activity. In both normal patients and those with coronary artery disease, sub-anaesthetic concentrations of nitrous oxide have little influence on cardiac output, stroke volume, and heart rate. At higher concentrations, nitrous oxide actually increases these variables, while volatile agents have the opposite influence. Any depressant influences of nitrous oxide are overshadowed by its augmentation of sympathetic tone, but this requires caution. Opioids depress sympathetic outflow, and when combined with nitrous oxide, depressant influences of nitrous oxide on myocardium could be unmasked. This may be relevant for patients who are compromised by significant degrees of heart failure. Arterial blood pressure remains stable in patients who receive sub-anaesthetic concentrations of nitrous oxide. Nitrous oxide increases venous tone, leading to increased venous return to the heart, and this contributes to the stable cardiovascular function observed with nitrous oxide. It is notable that this influence of nitrous oxide on venous tone is exploited at times to facilitate venous access during difficult venipuncture.
CONTRAINDICATIONS
TRANSFER TO GAS-FILLED SPACES
The blood/gas partition coefficient of nitrous oxide is 0.46, which is more than 30 times greater than that of nitrogen (0.014). When a patient’s inspired gas mixture is switched from air containing approximately 78% nitrogen to an anesthetic mixture containing 70% nitrous oxide, the nitrous oxide will enter gas-filled spaces more than 30 times faster than nitrogen can exit the space. As a result, the volume or pressure within such a space will increase. Although lower concentrations (30% to 50%) of nitrous oxide are used for sedation, it enters gas-filled spaces more rapidly than nitrogen can exit. Gas volume and pressure can become dangerously high within an obstructed bowel, pneumothorax, sinusitis or if the eustachian tube is inflamed. Rupture of the tympanic membrane is possible during administration of nitrous oxide, and negative pressure may lead to serous otitis possibly contributing to postoperative nausea and vomiting. Other anesthetic gases are administered in such low concentrations that their partial pressures do not lead to the effects observed with nitrous oxide.
DIFFUSION HYPOXIA
When inhalation of high concentrations of nitrous oxide is discontinued, high partial pressure in blood transfers nitrous oxide to the alveoli rapidly; this dilutes the partial pressure of oxygen in the alveoli and may lead to hypoxemia. For this reason, it is conventional practice to provide the patient with 100% oxygen during the first few minutes following discontinuation of nitrous oxide. However hypoxemia is significant for only a matter of minutes and has been documented only when high concentrations (70%) were delivered by full mask or by endotracheal tube. These conditions cannot be met with the use of conventional dental nitrous oxide machines with nasal masks, nevertheless, providing 100% oxygen toward the end of a dental appointment has other benefits, such as providing a placebo influence.
PATIENT EXPOSURE RISK
Nitrous oxide has been implicated in the adverse effects on health seen in those individuals who are chronically exposed to trace amounts of the drug. These adversities include infertility, spontaneous abortion, blood dyscrasias, and neurologic deficits. These concerns pertain only to chronic exposure; it is presumed that healthy surgical patients could receive nitrous oxide without harm.
- Immune effects: decreased proliferation of human peripheral blood mononuclear cells and increased and decreased neutrophil chemotaxis is reported. Nonetheless, there is no evidence to suggest that nitrous oxide induces more immunosuppression than other anesthetics.
- Hematologic toxicity: Inhibition of methionine synthase may lead to significant hematologic complications such as megaloblastic anemia. Indeed, even short periods of nitrous oxide exposure (2–6 h) in seriously ill patients can cause megaloblastic bone marrow changes. Another vulnerable group at risk from further methionine synthase suppression may be the elderly, because up to 20% are deficient in cobalamin and substrates for methionine synthase. These patients are at potential risk even from exposure to short durations of nitrous oxide. Nonetheless, there is no evidence that individuals who are not deficient in cobalamin or folate are vulnerable to hematologic complications of nitrous oxide if exposed for less than 6 h.
- Neurologic effects: reduced methionine synthase function may lead to myelinopathies such as subacute combined neurodegeneration of the spinal cord, which has also been reported to occur with long-term nitrous oxide abuse. More severe neurologic complications have been reported after nitrous oxide exposure in patients with rare congenital gene deficiencies.
- Myocardial effects: nitrous oxide has been associated with increased myocardial risk in the perioperative period. However, although plausible, causation between any increased risk from nitrous oxide and increased homocysteine levels is unproven. A further exploration showed a correlation between nitrous oxide exposure, increased homocysteine level, and impaired flow-mediated dilatation of the brachial artery. The duration of nitrous oxide exposure can be correlated with this marker of endothelial dysfunction, increasing the likelihood of causation.
NITROUS OXIDE MAY NOT BE SUITABLE FOR PATIENTS AFFECTED BY DOWN SYNDROME
Pubmed - Homocysteine Metabolism in Children with Down Syndrome: In Vitro Modulation - 2001
Pubmed - The MTR A2756G polymorphism is associated with an increase of plasma homocysteine concentration in Brazilian individuals with Down syndrome. - 2007
With its long history of safety in medicine and dentistry, nitrous oxide sedation can be used safely for almost all patients routinely treated in the ambulatory dental setting, in fact nitrous oxide is considered the safest of all the modalities available for sedation in dentistry. However, similar to any other pharmacologic agent, nitrous oxide may not be suitable for all patients.
Because of its hematologic toxicity, nitrous oxide can be considered virtually dangerous in particular for patients affected by Down syndrome. The plasma profile of the metabolites in the methionine/homocysteine pathway in patients with DS could be indeed consistent with a functional folate deficiency secondary to overexpression of the CBS gene. The specific metabolites and metabolic pathways affected by CBS are summarized in figure 5. A 157% increase in CBS enzyme activity has been previously documented in individuals with DS and has been associated with reduced levels of homocysteine.
An increase in the transsulfuration pathway via CBS overexpression indirectly deprives the methionine synthase reaction of one of its precursors, homocysteine, while, at the same time, it promotes the accumulation of its other precursor, 5-methyltetrahydrofolate (5-MTHF). The decrease in the methionine synthase activity thus creates the well-established “methyl trap,” as a consequence of the one-way kinetics of 5-MTHF synthesis (fig. 5). More important, a decrease in methionine synthase activity reduces the conversion of 5-MTHF to tetrahydrofolate (THF), the metabolically active form of folate, required for synthesis of nucleotides for RNA and DNA synthesis. Because of the methyl trap, an intracellular functional folate deficiency can exist in presence of normal, or even elevated, serum folate and B12 levels. The patients with DS had significantly altered plasma levels of each of the metabolites, in the methionine/homocysteine pathway and also in the CBS-mediated transsulfuration pathway. Plasma levels of homocysteine and methionine are significantly decreased, whereas plasma cystathionine is increased, demonstrating that the product of the CBS reaction is elevated, despite the presence of adaptive mechanisms to down-regulate gene expression and enzyme activity. Finally, the significant decrease in plasma glutathione in the patients with DS is consistent with reports of intracellular oxidative-stress increase due to overexpression of the Cu-Zn superoxide dismutase gene (SOD), which is also located on chromosome 21.
FIG. 5 Overview of interactive and interdependent reactions involved in cellular one-carbon metabolism, with emphasis on the two major metabolic functions of these pathways: normal DNA synthesis/repair and normal cellular methylation reactions. These two major functions intersect at the folate/B12–dependent methionine synthase reaction, which regenerates methionine from homocysteine and, at the same time, generates metabolically active THF for DNA/RNA nucleotide synthesis. Two genes (CBS and SOD) on chromosome 21 that are overexpressed in individuals with DS are shown in circles. Arrows indicate direct and indirect alterations in metabolites, induced by CBS overexpression in individuals with DS.
The plasma profile of the metabolites involved in the methionine/homocysteine pathway in patients with DS is consistent with a functional folate deficiency secondary to CBS overexpression and the folate trap. Several clinical observations suggest that patients with DS may be functionally folate deficient despite normal plasma levels of folate and B12. The significant decrease in plasma methionine levels observed in the children with DS most likely reflects a decrease in methionine synthase activity, secondary to CBS-mediated removal of its precursor, homocysteine. The 50% decrease in methionine is of clinical concern because of the central importance of methionine in maintaining protein synthesis for growth, acute-phase protein/antibody synthesis, and peptide-hormone synthesis. In addition, methionine is the sole precursor for the synthesis of SAM, the primary methyl donor for essential cellular methylation reactions (fig. 5). An adaptive metabolic response to reduced intracellular levels of methionine and SAM is the breakdown of phosphatidylcholine to choline and betaine (trimethylglycine [TMG]). The enzyme betaine-homocysteine methyltransferase (BHMT) transfers a methyl group from betaine to homocysteine and provides an important alternative route for endogenous methionine synthesis when folate is limited (or trapped). A progressive depletion of plasma choline levels via controlled folate deprivation in humans has been documented. Individuals with DS have increased sensitivity to anticholinergic drugs that can be explained by increased catabolism and depletion of choline for methionine resynthesis. The significant decreases, in both SAM and SAH, in the plasma of the patients with DS suggest a general depression in cellular methylation capacity in these patients. Further evidence for protein-methylation dysfunction is provided by the observation that individuals with DS synthesize reduced levels of methylnicotinamide after a nicotinamide load, indicating a reduced protein-methylation capacity. Finally, evidence of delayed myelination of DS neurons is consistent with reduced methylation of myelin basic protein. It has been suggested that hyperuricemia in individuals with DS is due to excessive purine synthesis resulting from overexpression oh the gene for de novo purine synthesis which is located on chromosome 21. Recent observations suggest an alternative explanation for increased uric acid production in DS. The overexpression of CBS would result in an enhanced flux of methionine metabolites down the transsulfuration pathway, resulting in excess production of adenosine from SAH hydrolysis (fig. 5). In addition, the methyl trap in individuals with DS would reduce the availability of folate for de novo synthesis of the purine ring. Therefore, the hyperuricemia in DS reflects increased adenosine production secondary to CBS overexpression and is not the result of increased de novo purine synthesis. The reduced plasma glutathione observed in patients with DS most likely reflects an adaptive antioxidant response to chronic oxidative stress, resulting from SOD overexpression (fig. 5).
Plasma Hcy is influenced by modifiable and non-modifiable factors, such as gender, vitamin status and genetic factors. The C677T and A1298C polymorphisms of the MTHFR gene are important genetic determinants of Hcy concentrations. The polymorphic alleles result in an enzyme with reduced specific activity, consequently leading to increased Hcy concentrations. The presence of genetic variants involved in folate metabolism in DS individuals may confer a survival advantage. A preferential transmission of the MTHFR 677T allele from heterozygous fathers to DS children was observed. This may represent a metabolic advantage since the overexpression of the CßS enzyme in these individuals is associated with decreased Hcy, methionine and S-adenosylmethionine concentrations.
CONCLUSIONS Pubmed - Inhalation sedation with nitrous oxide as an alternative to dental general anaesthesia for children -2003
However, nitrous oxide is used by many pediatric anesthesiologists for gaseous induction where its nonirritant quality and the second gas effect are unique attributes. Although attempts are being made to find alternatives to nitrous oxide in the labor ward, it seems that the routine use of nitrous oxide in this setting will continue for the foreseeable future. Facilities to ensure adequate scavenging and ventilation are imperative to ensure the occupational health of medical staff. Good prospective epidemiologic evidence is required to fully evaluate the risks of different anesthetics in the modern operating/anesthetic room environment. Nitrous oxide has been used for 150 years for analgesia and anesthesia and in the main has proven safe and efficacious. Finally, nitrous oxide is cheap.
Deborah Auteri, Elisabetta Garombo
