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M J Laster

Publications and source records attributed to M J Laster.

At least 37 records · Page 2Linked to original sources

Acetaminophen predisposes to renal and hepatic injury from compound A in the fasting rat.

Results of previous studies of Compound A, a degradation product of sevoflurane, suggested that decreases in glutathione stores may increase potential Compound A nephrotoxicity. By depleting these stores, fasting and various drugs may augment such nephrotoxicity. To test this possibility, we pretreated fasted Fisher rats with intraperitoneal 0 (vehicle only), 250, 500, or 1000 mg/kg of acetaminophen, a commonly used drug that depletes glutathione stores. After pretreatment, we administered Compound A for 3 h at concentrations ranging from 0 to 200 ppm. The larger doses of acetaminophen predisposed to greater renal and hepatic injury. For example, at 100 ppm Compound A, no rats had renal cortical injury when given vehicle only or 250 mg/kg acetaminophen, but 90% (9 of 10 rats) had injury at 500 mg/kg and 100% (13 of 13) at 1000 mg/kg. Similarly, at 100 ppm Compound A, hepatic injury was not evident with vehicle only or 250 mg/kg, but occurred in 30% of rats at 500 mg/kg, and in 69% at 1000 mg/kg. Given the considerable differences between humans and rats, and given the large doses of acetaminophen required, the clinical relevance of these findings is unclear. If clinically relevant, circumstances producing glutathione depletion (e.g., ingestion of drugs such as acetaminophen, or nutritional deficiencies) may predispose to renal or hepatic injury from Compound A in patients given sevoflurane at low fresh gas flow rates.

Acetaminophen↗

Biotransformation of halothane, enflurane, isoflurane, and desflurane to trifluoroacetylated liver proteins: association between protein acylation and hepatic injury.

In susceptible patients, halothane, enflurane, isoflurane, and desflurane can produce severe hepatic injury by an immune response directed against reactive anesthetic metabolites covalently bound to hepatic proteins. The incidence of hepatotoxicity appears to directly correlate with anesthetic metabolism catalyzed by cytochrome P450 2E1 to trifluoroacetylated hepatic proteins. In the present study, we examined whether the extent of acylation of hepatic proteins in rats by halothane, enflurane, isoflurane, and desflurane correlated with reported relative rates of metabolism. After pretreatment with the P450 2E1 inducer isoniazid, five groups of 10 rats breathed 1.25 minimum alveolar anesthetic concentration (MAC) of halothane, enflurane, isoflurane, or desflurane in oxygen, or oxygen alone, each for 8 h. Immunochemical analysis of livers harvested 18 h after anesthetic exposure showed tissue acylation (greatest to least) after exposure to halothane, enflurane, or isoflurane. Reactivity was not different between isoflurane as compared to desflurane or oxygen alone. An enzyme-linked immunosorbent assay showed halothane reactivity was significantly greater than that of enflurane, isoflurane, desflurane, or oxygen, and that enflurane reactivity was significantly greater than desflurane or oxygen. Sera from patients with a clinical diagnosis of halothane hepatitis showed antibody reactivity against hepatic proteins from rats exposed to halothane or enflurane. No reactivity was detected in rats exposed to isoflurane, desflurane, or oxygen alone. These results indicate that production of acylated proteins may be an important mediator of anesthetic-induced hepatotoxicity.

Acylation↗

Convulsant activity of nonanesthetic gas combinations.

Most nonanesthetics (inhaled compounds that neither cause anesthesia when given alone nor decrease the partial pressure of a known inhaled anesthetic required to produce anesthesia) and transitional compounds (inhaled compounds that are less potent than would be predicted by the Meyer-Overton hypothesis) cause convulsions. A possible exception is the perfluoroalkane series of nonanesthetics. The present study tested whether perfluoroalkanes do provide an exception. Further, we tested whether the convulsant effects of nonanesthetic and transitional compounds were additive. The nonanesthetic perfluoropropane caused convulsions at 7.5 +/- 0.7 atm (mean +/- SD). Convulsions also were produced by perfluorocyclobutane (0.976 +/- 0.002 atm), 1,2-dichlorotetrafluoroethane (0.358 +/- 0.011 atm), 2,3-dichlorooctafluorobutane (0.085 +/- 0.007 atm), 1,2-dichlorohexafluorocyclobutane (0.055 +/- 0.007 atm), and flurothyl (0.00156 +/- 0.00039 atm). Of these, 1,2-dichlorotetrafluoroethane is a transitional compound, the remainder being nonanesthetics. The combination of flurothyl plus 1,2-dichlorohexafluorocyclobutane gave evidence of antagonism (a 17% +/- 21% deviation from additivity; P < 0.05), whereas the combination of 1,2-dichlorotetrafluoroethane plus 2,3-dichlorooctafluorobutane gave evidence of synergy (a -13% +/- 8% deviation from additivity; P < 0.05). The combinations of perfluoropropane plus perfluorocyclobutane (-4% +/- 15%), and perfluoropropane plus 1,2-dichlorohexafluorocyclobutane (-1% +/- 26%) did not produce results that deviated significantly from additivity. We conclude that pairs of these compounds either produce convulsions in an additive manner, a finding consistent with (but not proving) a common mode of action; or deviate modestly from additivity, a finding suggesting that at least a portion of the mechanistic basis for convulsions might differ, particularly for flurothyl plus other nonanesthetics, or for the combination of non-anesthetics and transitional compounds.

Animals↗

Maturation decreases ethanol minimum alveolar anesthetic concentration (MAC) more than desflurane MAC in rats.

The potency of conventional inhaled anesthetics increases with increasing age: the 50% effective dose (minimum alveolar anesthetic concentration [MAC]) for anesthesia in the neonatal animal or human exceeds MAC in the young adult by approximately 30% to 60%. We tested whether this relationship also applies to the alkanols, using ethanol as a representative alkanol. We found that the MAC of ethanol in neonatal rats was 1.86 times (86% greater than) the MAC for adult rats, based on ethanol partial pressures determined from brain specimens. In contrast, the MAC of desflurane in neonatal rats was 1.19 times (19% greater than) the MAC for adult rats, less than one-fourth the 86% found for ethanol. These differences must be explained by any unitary theory of narcosis. Alternatively, the mechanistic basis for alkanol versus conventional inhaled anesthetics may differ in part or whole.

Age Factors↗

Anesthetic potencies of n-alkanols: results of additivity and solubility studies suggest a mechanism of action similar to that for conventional inhaled anesthetics.

The mechanism by which n-alkanols produce anesthesia and the characteristics relevant to those mechanisms (e.g., lipid solubilities versus potencies) remain unknown. Accordingly, we determined potencies (minimum alveolar anesthetic concentration [MAC]) and solubilities of normal methanol, ethanol, butanol, hexanol, and octanol. We also determined the additivity of these alkanols with a conventional anesthetic (desflurane) and the additivity of methanol with butanol. Finally, we determined whether alkanol metabolism influences alkanol potencies. MAC for methanol, ethanol, butanol, hexanol, and octanol (0.00200, 0.000989, 0.000133, 0.0000214, and 0.00000117 atm, respectively) increased with an increasing solubility in olive oil (olive oil/gas partition coefficients 48.6, 108, 1,650, 11,600, and 93,500, respectively) and octanol (octanol/gas partition coefficients 163, 1,150, 22,900, 135,000, and 4,140,000) to give a product of MAC x solubility for olive oil approximately 10 times less (values of 0.10-0.25) than that expected from the Meyer-Overton hypothesis (compared with conventional inhaled anesthetics). There was less deviation for octanol, but the results were more variable. Inhibition of methanol and butanol metabolism by 4-methylpyrazole did not alter MAC. Methanol, ethanol, butanol, hexanol, and octanol had approximately additive anesthetic effects with desflurane, with some small but statistically significant deviations both above and below additivity. In the presence of 0.5 MAC of desflurane, we needed to add 0.4-0.6 MAC of each alkanol to inhibit the movement of 50% of the rats in response to noxious stimulation. Similarly, the effects of methanol and butanol were additive (with each other). The saline/gas partition coefficient for each alkanol was high (3700, 2650, 1400, 900, and 709 for methanol through octanol), which indicates high polarity. We conclude that the potent anesthetic effects of normal alkanols may result from an affinity to both polar and nonpolar phases. Our finding of additivity of alkanols with each other is consistent with a common mechanism of action. Similarly, the finding of additivity or slight deviations from additivity for alkanols with desflurane is consistent with mechanisms of action that have much in common.

Alcohols↗

Minimum alveolar anesthetic concentration values for the enantiomers of isoflurane differ minimally.

Results of in vivo and in vitro studies of the anesthetic potencies of the enantiomers (optical isomers) of isoflurane provide various results ranging from no difference to differences of nearly two fold. A finding of a difference in anesthetic requirement in the whole animal has particular relevance to theories of anesthetic mechanisms of action because it suggests that anesthesia may result from a specific anesthetic-receptor interaction. This led to our decision to redetermine the minimum alveolar anesthetic concentration (MAC) of (+)-S and (-)-R enantiomers of isoflurane in 12 Sprague-Dawley rats (six per group). The (+)-S enantiomer gave a MAC of 0.0144 +/- 0.0012 atm (i.e., 1.44% +/- 0.12% at 1 atm pressure; mean +/- SD) and the (-)-R enantiomer gave a MAC of 0.0169 +/- 0.0020 atm. Although the 17% greater value for the (-)-R enantiomer is qualitatively consistent with previous results the difference is not significant (P = 0.06), and the absolute difference is smaller than that found by a previous study. However, given the small sample size, our power to define a small significant difference is limited. Regardless of statistical significance, our results do not confirm the conclusion that interaction with a specific receptor is important to the mechanism of action of inhaled anesthetics.

Anesthetics, Inhalation↗

Baralyme dehydration increases and soda lime dehydration decreases the concentration of compound A resulting from sevoflurane degradation in a standard anesthetic circuit.

UNLABELLED: Soda lime and Baralyme brand carbon dioxide absorbents degrade sevoflurane to CF2 = C(CF3)OCH2F, a potentially nephrotoxic vinyl ether called Compound A. Dehydration of these absorbents increases both the degradation of sevoflurane to Compound A and the degradation of Compound A. The balance between sevoflurane degradation and Compound A degradation determines the concentration of Compound A issuing from the absorbent (the net production of Compound A). We studied the effect of dehydration on the net production of Compound A in a simulated anesthetic circuit. Mimicking continuing oxygen delivery for 1, 2, or 3 days after completion of an anesthetic, we directed a "conditioning" fresh gas flow of 5 L/min or 10 L/min retrograde through fresh absorbent in situ in a standard absorbent system for 16, 40, and/or 64 h. The conditioned absorbent was subsequently used (without mixing of the granules) in a standard anesthetic circuit in which a 3-L rebreathing bag substituted for the lung. Metabolism was mimicked by introducing 250 mL/min carbon dioxide into the "lung," and the lung was ventilated with a minute ventilation of 10 L/ min. At the same time, we introduced sevoflurane in a fresh gas inflow of 2 L/min at a concentration sufficient to produce an inspired concentration of 3.2%. Because of increased sevoflurane destruction by the absorbent, progressively longer periods of conditioning (dehydration) and/or higher inflow rates increased the delivered (vaporizer) concentration of sevoflurane required to sustain a 3.2% concentration. Dehydration of Baralyme increased the inspired concentration of Compound A by up to sevenfold, whereas dehydration of soda lime markedly decreased the inspired concentration of Compound A. IMPLICATIONS: Economical delivery of modern inhaled anesthetics requires rebreathing of exhaled gases after removal of carbon dioxide. However, carbon dioxide absorbents (Baralyme/soda lime) may degrade anesthetics to toxic substances. Baralyme dehydration increases, and soda lime dehydration decreases, degradation of the inhaled anesthetic sevoflurane to the toxic substance, Compound A.

Anesthetics, Inhalation↗

Effects of inhaled nonimmobilizer, proconvulsant compounds on desflurane minimum alveolar anesthetic concentration in rats.

UNLABELLED: Anesthetics depress the central nervous system, whereas nonimmobilizers (previously called nonanesthetics) and transitional compounds having the same physical properties (e.g., solubility in lipid) do not produce anesthesia (nonimmobilizers) or are less potent anesthetics than might be predicted from their lipophilicity (transitional compounds). Potential explanations for the absent or decreased anesthetic effect of nonimmobilizer and transitional compounds include the theories that the nonimmobilizers are devoid of anesthetic effect and that transitional compounds have a decreased capacity to produce anesthesia; that the effects of these compounds are not apparent because the concentrations examined are too low; or that anesthesia, or lack thereof, results from a balance between depression and excitation (all nonimmobilizer and transitional compounds produce convulsions). To examine these issues further, we tested the effect of various multiples of the convulsive 50% effective dose (ED50) of three nonimmobilizers and one transitional compound on the minimum alveolar anesthetic concentration (MAC) of desflurane in rats. The nonimmobilizer 2,3-dichlorooctafluorobutane (NI-1), from 0.7 to 1.1 times its convulsive ED50, increased the MAC of desflurane by 14%-27%, but at 1.6 times its convulsive ED50 caused no change in MAC; the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (NI-2) did not change MAC at concentrations up to its convulsant ED50, but it increased MAC by 25% and 36% at 1.3 and 1.7 times its convulsant ED50, respectively. The nonimmobilizer flurothyl (NI-3) decreased the MAC of desflurane by 20% +/- 6% (mean +/- SD) at 0.5 times its convulsant ED50, but it caused no change at higher partial pressures (up to 7.8 times its convulsant ED50), and the transitional compound CF3CCl2-O-CF2Cl (T-1) significantly decreased MAC by 16% +/- 7% at 0.8 times its convulsant ED50, but the 6%-8% decreases in MAC at 0.4 and 1.6 times its convulsant ED50 were not significant. Thus, neither nonimmobilizer nor transitional compounds produced a consistent dose-related effect on the MAC of desflurane, and any changes were small. These results suggest that the excitation produced by transitional compounds or nonimmobilizers does not explain their limited ability or inability to produce anesthesia. The data are consistent with a decreased anesthetic efficacy of transitional compounds and the lack of efficacy of nonimmobilizers. IMPLICATIONS: Inhaled compounds that do not cause anesthesia (nonimmobilizers) are used to test theories of anesthetic action. Their use presumes that a trivial explanation, such as cancelling stimulatory and depressant effects, does not explain the absence of anesthesia. The present results argue against such an explanation.

Anesthetics↗

Dose-related biochemical markers of renal injury after sevoflurane versus desflurane anesthesia in volunteers.

UNLABELLED: Sevoflurane (CH2F-O-CH[CF3]2) reacts with carbon dioxide absorbents to produce Compound A (CH2F-O-C[=CF2][CF3]). Because of concern about the potential nephrotoxicity of Compound A, the United States package label (but not that of several other countries) for sevoflurane recommends the use of fresh gas flow rates of 2 L/min or more. We previously demonstrated in humans that a 2-L/min flow rate delivery of 1.25 minimum alveolar anesthetic concentration (MAC) sevoflurane for 8 h can injure glomeruli (i.e., produce albuminuria) and proximal tubules (i.e., produce glucosuria and urinary excretion of alpha-glutathione-S-transferase [alpha-GST]). The present report extends this investigation to fasting volunteers given 4 h (n = 9) or 2 h (n = 7) of 1.25 MAC sevoflurane versus desflurane at 2 L/min via a standard circle absorber anesthetic system (all subjects given both anesthetics). Markers of renal injury (urinary creatinine, albumin, glucose, alpha-GST, and blood urea nitrogen) did not reveal significant injury after anesthesia with desflurane. Sevoflurane degradation with a 2-L/min fresh gas inflow rate produced average inspired concentrations of Compound A of 40 +/- 4 ppm (mean +/- SD, 8-h exposure [data from previous study]), 42 +/- 2 ppm (4 h), and 40 +/- 5 ppm (2 h). Relative to desflurane, sevoflurane given for 4 h caused statistically significant transient injury to glomeruli (slightly increased urinary albumin and serum creatinine) and to proximal tubules (increased urinary alpha-GST). Other measures of injury did not differ significantly between anesthetics. Neither anesthetic given for 2 h at 1.25 MAC produced injury. We conclude that 1.25 MAC sevoflurane plus Compound A produces dose-related glomerular and tubular injury with a threshold between 80 and 168 ppm/h of exposure to Compound A. This threshold for renal injury in normal humans approximates that found previously in normal rats. IMPLICATIONS: Human (and rat) kidneys are injured by a reactive compound (Compound A) produced by degradation of the clinical inhaled anesthetic, sevoflurane. Injury increases with increasing duration of exposure to a given concentration of Compound A. The response to Compound A has several implications, as discussed in the article.

Adult↗

Quantitative differences in the production and toxicity of CF2=BrCl versus CH2F-O-C(=CF2)(CF3) (compound A): the safety of halothane does not indicate the safety of sevoflurane.

UNLABELLED: Carbon dioxide absorbents degrade both halothane and sevoflurane to toxic unsaturated compounds (CF2=CBrCl and CH2F-O-C[=CF2][CF3] [i.e., Compound A], respectively). Given the long history of safe administration of halothane, comparable toxicities of these degradation products would imply a similar safety of sevoflurane. We therefore examined CF2=CBrCl in the context of four issues relevant to previous studies of the toxicity of Compound A: 1) reactivity of the degradation product in vitro; 2) rate of its production in vitro; 3) its in vivo toxicity; 4) importance of the beta-lyase pathway to the toxicity in vivo. We found the following. 1) CF2=CBrCl is less reactive than Compound A, degrading in human serum albumin at one-fifth the rate of Compound A. 2) Over a 3-h period of "anesthesia," a standard circle system containing Baralyme (Allied Healthcare Products, Inc., St. Louis, MO) produces 30 times as much Compound A from a minimum alveolar anesthetic concentration (MAC) concentration of sevoflurane as CF2=CBrCl from a MAC concentration of halothane; with soda lime, the difference is 60-fold. Correcting for differences in uptake of halothane versus sevoflurane decreases the differences to 20-40 times. 3) For a 3-h administration to rats, the partial pressure of Compound A causing minimal renal injury or necrosis of half the affected tubule cells exceeds the partial pressure of CF2=CBrCl causing minimal injury or necrosis of half the affected tubule cells by a factor of approximately 4-6. Thus, the ratio of production (Item 2 above) to the partial pressure causing injury with CF2=CBrCl is approximately a quarter of that ratio for Compound A. 4) Compounds that block the beta-lyase pathway either do not change (acivicin) or decrease (aminooxyacetic acid; AOAA) renal injury from CF2=CBrCl in rats, whereas these compounds increase (acivicin) or do not change (AOAA) injury from Compound A. We conclude that the safety of halothane cannot be used to support the safety of sevoflurane. IMPLICATIONS: Carbon dioxide absorbents degrade halothane and sevoflurane to unsaturated compounds nephrotoxic to rats. Relative to sevoflurane's degradation product, halothane's degradation product has less toxicity relative to production, less reactivity, and a different mechanism of injury. The clinical absence of halothane nephrotoxicity does not necessarily indicate a similar absence for sevoflurane.

Absorption↗

Dehydration of Baralyme increases compound A resulting from sevoflurane degradation in a standard anesthetic circuit used to anesthetize swine.

UNLABELLED: In a model anesthetic circuit, dehydration of Baralyme brand carbon dioxide absorbent increases degradation of sevoflurane to CF2=C(CF3)OCH2F, a nephrotoxic vinyl ether called Compound A. In the present study, we quantified this increase using "conditioned" Baralyme in a circle absorbent system to deliver sevoflurane anesthesia to swine. Mimicking continuing oxygen delivery for 2 days after completion of an anesthetic, we directed a conditioning fresh gas flow of 5 L/min retrograde through fresh absorbent in situ in a standard absorbent system for 40 h. The conditioned absorbent was subsequently used (without mixing of the granules) in a standard anesthetic circuit to deliver sevoflurane to swine weighing 78 +/- 2 kg. The initial inflow rate of fresh gas flow was set at 10 L/min with the vaporizer at 8% to achieve the target end-tidal concentration of 3.0%-3.2% sevoflurane in approximately 20 min. The flow was later decreased to 2 L/min, and the vaporizer concentration was decreased to sustain the 3.0%-3.2% value for a total of 2 h (three pigs) or 4 h (eight pigs). Inspired Compound A increased over the first 30 +/- 60 min to a peak concentration of 357 +/- 49 ppm (mean +/- SD), slowly decreasing thereafter to 74 +/- 6 ppm at 4 h. The average concentration over 2 h was 208 +/- 25 ppm, and the average concentration over 4 h was 153 +/- 19 ppm. Pigs were killed 1 or 4 days after anesthesia. The kidneys from pigs anesthetized for both 2 h and 4 h showed mild inflammation but little or no tubular necrosis. These results suggest that dehydration of Baralyme may produce concentrations of Compound A that would have nephrotoxic effects in humans in a shorter time than would be the case with normally hydrated Baralyme. IMPLICATIONS: The vapor known as Compound A can injure the kidney. Dehydration of Baralyme, a standard absorbent of carbon dioxide in inhaled anesthetic delivery systems, can cause a 5- to 10-fold increase in Compound A concentrations produced from the inhaled anesthetic, sevoflurane, given at anesthetizing concentrations in a conventional anesthetic system.

Anesthesia, Closed-Circuit↗

compound A induces sister chromatid exchanges in Chinese hamster ovary cells.

BACKGROUND: Compound A [CF2 = C(CF3)OCH2F], a degradation product of sevoflurane [(CF3)2CHOCH2F], is a vinyl ether and may be an alkylating agent. Thus it is a potential genotoxin. METHODS: The capacity of compound A to produce sister chromatid exchanges was measured in Chinese hamster ovary cells with and without metabolic activation. Concentrations of 11 to 468 ppm compound A were applied for 2 h, the Chinese hamster ovary cells were incubated for a further 34 h in the presence of bromodeoxyuridine, and then colcemid was added to produce arrest in metaphase. Coded slides of cells were examined blindly, and 50 chromosome spreads were counted for each test concentration. RESULTS: The lowest concentration of compound A applied without metabolic activator (27 ppm) significantly increased (P < 0.001) sister chromatid exchanges, and increasing concentrations of compound A increased the incidence of exchanges. Metabolic activation did not increase the incidence of exchanges. CONCLUSIONS: Compound A increases sister chromatid exchanges at concentrations (27 ppm) obtained in low-flow systems when sevoflurane is used at concentrations approximating minimum alveolar concentration.

Anesthetics, Inhalation↗

Recovery and kinetic characteristics of desflurane and sevoflurane in volunteers after 8-h exposure, including kinetics of degradation products.

BACKGROUND: Desflurane and sevoflurane permit speedier changes in anesthetic partial pressures than do older halogenated anesthetics. The authors determined the kinetic characteristics of desflurane and sevoflurane and those of compound A [CH2F-O-C(=CF2)(CF3)], a nephrotoxic degradation product of sevoflurane. METHODS: Volunteers received 1.25 minimum alveolar concentration of desflurane or sevoflurane, each administered for 8 h in a fresh gas inflow of 2 l/min. Inspired (F(I)) and end-tidal (F(A)) concentrations of anesthetic and compound A were measured during administration, and F(A) relative to F(A0) (the last end-tidal concentration during administration) during elimination. The indices of recovery were also measured. RESULTS: The ratio F(I)/F(A) rapidly approached 1.0, with values greater for sevoflurane (desflurane 1.06 +/- 0.01 vs. sevoflurane 1.11 +/- 0.02, mean +/- SD). The ratio F(A)/F(I) for compound A was approximately 0.8. The F(A)/F(A0) ratio decreased slightly more rapidly with desflurane than with sevoflurane, and objective measures indicated faster recovery with desflurane: The initial response to command (14 +/- 4 min vs. 28 +/- 8 min [means +/- SD]) and orientation (19 +/- 4 vs. 33 +/- 9 min) was quicker, and recovery was faster as defined by results of the Digit Symbol Substitution, P-deletion, and Trieger tests. Desflurane produced less vomiting (1 [0.5, 3]; median [quartiles] episodes) than did sevoflurane (5 [2.5, 7.5] episodes). The F(A)/F(A0) ratio for compound A decreased within 5 min to a constant value of 0.1. CONCLUSIONS: These anesthetics have kinetics consistent with their solubilities. Sevoflurane's greater biodegradation probably increases F(I)/F(A) differences during anesthetic administration and decreases F(A)/F(A0) differences during elimination. The F(A) for compound A differs from F(I) by 20% (F(A)/F(I) = 0.8) because of substantial degradation. Recovery from anesthesia proceeds nearly twice as fast with desflurane than with sevoflurane. Differences in ventilation, or alveolar or tissue elimination, do not completely explain the slower recovery with sevoflurane.

Anesthetics, Inhalation↗

Nonanesthetics can suppress learning.

Nonanesthetic gases or vapors do not abolish movement in response to noxious stimuli despite partial pressures and affinities for lipids that would, according to the Meyer-Overton hypothesis, predict such abolition. We investigated whether nonanesthetics depress learning and memory (i.e., provide amnesia). To define learning, we used a "fear-potentiated startle paradigm": rats trained to associate light with a noxious stimulus (footshock) will startle more, as measured by an accelerometer, when a startle-eliciting stimulus (e.g., a noise) is paired with light than when the startle-eliciting stimulus is presented alone. We imposed light-shock pairings on 98 rats under three conditions: no anesthesia (control); 0.20, 0.29, and 0.38 times the minimum alveolar anesthetic concentration (MAC) of desflurane; or two nonanesthetics (1,2-dichloroperfluorocyclobutane and perfluoropentane) at partial pressures predicted from their lipid solubilities to be between 0.2 and 1 MAC. Desflurane produced a dose-related depression of learning with abolition of learning at 0.28 MAC. Perfluoropentane at 0.2-predicted MAC had the same effect as 0.28 MAC desflurane. 1,2-Dichloroperfluorocyclobutane at 0.5- to 1-predicted MAC abolished learning. Because nonanesthetics suppress learning but not movement (the two critical components of anesthesia), they may prove useful in discriminating between mechanisms and sites of action of anesthetics.

Anesthetics, Inhalation↗

Metabolism of compound A by renal cysteine-S-conjugate beta-lyase is not the mechanism of compound A-induced renal injury in the rat.

Compound A [CF2 = C(CF3)OCH2F], a vinyl ether produced by CO2 absorbents acting on sevoflurane, can produce corticomedullary junction necrosis (injury to the outer stripe of the outer medullary layer, i.e., corticomedullary junction) in rats. Several halogenated alkenes produce a histologically similar corticomedullary necrosis by converting glutathione conjugates of these alkenes to halothionoacetyl halides. To test whether this mechanism explained the nephrotoxicity of Compound A, we blocked three metabolic steps which would lead to formation of a halothionoacetyl halide: 1) we depleted glutathione by administering dl-buthionine-S, R-sulfoximine (BSO); 2) we blocked cysteine S-conjugate formation by administering acivicin (AT-125); and 3) we inhibited subsequent metabolism by renal cysteine conjugate beta-lyase to the nephrotoxic halothionoacetyl halides by administering aminooxyacetic acid (AOAA). These treatments were given alone or in combination to separate groups of 10 or 20 Wistar rats before their exposure to Compound A. We hypothesized that blocking these metabolic steps should decrease the injury produced by breathing 150 ppm of Compound A for 3 h. However, we found either no change or an increase in renal injury, suggesting that this pathway mediates detoxification rather than toxicity. Our findings suggest that the cysteine-S-conjugate-mediated pathway is not the mechanism of Compound A nephrotoxicity and, therefore, observed interspecies differences in the activity of this activating pathway may not be relevant in the prediction of the nephrotoxic potential of Compound A in clinical practice.

Aminooxyacetic Acid↗

Factors affecting production of compound A from the interaction of sevoflurane with Baralyme and soda lime.

Various alkali (e.g., soda lime) convert sevoflurane to CF2=C(CF3)OCH2F, a vinyl ether called "Compound A, " whose toxicity raises concerns regarding the safe administration of sevoflurane via rebreathing circuits. In the present investigation, we measured the sevoflurane degradation and output of Compound A caused by standard (13% water) Baralyme brand absorbent and standard (15% water) soda lime, and Baralyme and soda lime having various water contents (including no water). We used a flow-through system, applying a gas flow rate relative to absorbent volume that roughly equaled the rate/volume found in clinical practice. Both absorbents, at similar water contents, temperatures, and sevoflurane concentrations, produced roughly equal concentrations of Compound A. Dry and nearly dry absorbents produced less Compound A early in exposure to sevoflurane, and more later, than standard absorbents. Increases in temperature and sevoflurane concentration increased output of Compound A. Both absorbents, especially when dry, also destroyed Compound A, the concentration exiting from absorbent resulting from a complex sum of production and destruction. We conclude that the variability of concentrations of Compound A found in clinical practice may be largely explained by the inflow rate used (i.e., by rebreathing), sevoflurane concentration, and absorbent temperature and dryness. The effect of dryness is complex, with fresh dry absorbent destroying Compound A as it is made, and with dry absorbent that has been exposed to sevoflurane for a period of time providing a sometimes unusually high output of Compound A.

Absorption↗

Anesthetic and convulsant properties of aromatic compounds and cycloalkanes: implications for mechanisms of narcosis.

We examined the anesthetic and convulsant properties of 16 unfluorinated to completely fluorinated aromatic compounds, having six to nine carbon atoms (e.g., benzene to 1,3,5-tris(trifluoromethyl)benzene), and four cycloalkanes (cyclopentane to cyclooctane). Benzene, fluorobenzene, toluene, p-xylene, ethylbenzene, and cyclopentane caused excitation (twitching, jerking, and hyperactivity), and three aromatic compounds (perfluorotoluene, p-difluorotoluene and 1,3,5-tris(trifluoromethyl)benzene) and three cycloalkanes (cyclohexane, cycloheptane, and cyclooctane) produced convulsions. Cyclooctane and 1,3,5-tris(trifluoromethyl)benzene were nonanesthetics. Except for nonanesthetics and perfluorotoluene (too toxic to test for anesthetic potency), all compounds produced anesthesia or decreased the minimum alveolar anesthetic concentration of desflurane. Aromatic compounds were more potent and lipid-soluble than n-alkanes (data from previous report) and cycloalkanes. All three series increasingly disobeyed the Meyer-Overton hypothesis as molecular size increased. For a particular number of carbons (e.g., cyclohexane, n-hexane, and benzene), the deviation was cycloalkanes > or = normal alkanes > aromatic compounds. These results suggest that molecular shape (including "bulkiness") and size provide limited clues to the structure of the anesthetic site of action.

Anesthesia, Inhalation↗

Carbon monoxide production from degradation of desflurane, enflurane, isoflurane, halothane, and sevoflurane by soda lime and Baralyme.

Anecdotal reports suggest that soda lime and Baralyme brand absorbent can degrade inhaled anesthetics to carbon monoxide (CO). We examined the factors that govern CO production and found that these include: 1) The anesthetic used: for a given minimum alveolar anesthetic concentration (MAC)-multiple, the magnitude of CO production (greatest to least) is desflurane > or = enflurane > isoflurane >> halothane = sevoflurane. 2) The absorbent dryness: completely dry soda lime produces much more CO than absorbent with just 1.4% water content, and soda lime containing 4.8% or more water (standard soda lime contains 15% water) generates no CO. In contrast, both completely dry Baralyme and Baralyme with 1.6% water produce high concentrations of CO, and Baralyme containing 4.7% water produces concentrations equaling those produced by soda lime containing 1.4% water. Baralyme containing 9.7% or more water and standard Baralyme (13% water) do not generate CO.3) The type of absorbent: at a given water content, Baralyme produces more CO than does soda lime. 4) The temperature: an increased temperature increases CO production. 5) The anesthetic concentration: more CO is produced from higher anesthetic concentrations. These results suggest that CO generation can be avoided for all anesthetics by using soda lime with 4.8% (or more) water or Baralyme with 9.7% (or more) water, and by using inflow rates of less than 2-3 L/min. Such inflow rates are low enough to ensure that the absorbent does not dry out.

Adsorption↗