Gaseous homeostasis and the circle system. Factors influencing anaesthetic gas exchange

A mathematical model of a subject breathing from a circle system has been used to follow the course of anaesthetic uptake during the simulated administration of 60% nitrous oxide, 2% halothane and 2% methoxyflurane, under non-rebreathing conditions and with fresh gas flows to the circle system of between 8 and 0.25 litre min-1. Compared with the non-rebreathing state, the use of a circle system reduced the initial rate of increase of alveolar towards fresh gas anaesthetic concentration, and the rate of increase in body anaesthetic content. The degree of reduction became more marked as fresh gas flow was reduced, and as agents of increasing blood solubility were used. These effects of a circle system were influenced by the volume of the circle system and the composition of gas initially present within the system. When the circle system was in use there were increases in the magnitude of both the concentration effect and the second gas effect which were related to the magnitude of fresh gas flow. The use of a circle system augmented the effects of changes in cardiac output and reduced the effects of changes in ventilation on the alveolar concentrations of the anaesthetic. These influences of a circle system were also dependent on the magnitude of fresh gas flow. The degree of augmentation of the effects of cardiac output decreased with increasing blood solubility of the agent in use, whilst the limitation of the effects of ventilation was greatest with the agent of highest blood solubility. Both under non-rebreathing conditions and with the circle system in use, the effects of cardiac output and ventilation were greater with 2% nitrous oxide than with 60% nitrous oxide, and were also greater when gases were given separately than when administered in combination.

Liver function following hypovolemic hypotension in rats anaesthetized with halothane or enflurane

Rats which had approximately 25-30% of their calculated blood volume removed were exposed to halothane (1%) or enflurane (2%) in 33% oxygen for 30 min. Hepatic function was evaluated by determining, at various time intervals, serum activities of glutamic-oxalacetic and glutamic-pyruvic transaminase, acid phosphatase and gamma-glutamyl-transpeptidase. In this model serum enzyme activities and animal mortality were significantly increased when hypovolemic hypotension was induced during halothane anaesthesia. The same events did not occur in bleeding animals anaesthetized with enflurane. The marked disparity in hepatic dysfunction and mortality between halothane and enflurane-anaesthetized rats during hypovolemic hypotension may be explained by the more pronounced decrease of oxygen available for the liver and production of reductive toxic intermediates in animals exposed to halothane.

The partial pressure of isoflurane or halothane does not affect their solubility in rabbit blood or brain or human brain: inhaled anesthetics obey Henry's Law

We tested the possibility that the solubility of halothane or isoflurane in rabbit blood or human or rabbit brain does not obey Henry's Law. We measured the blood/gas and brain/gas partition coefficients for both anesthetics at approximately 1 MAC and at 0.01 MAC at 37 degrees C. The partition coefficients determined at the high vs low partial pressures did not differ. We conclude that the solution of isoflurane and halothane in blood and brain obeys Henry's Law.

Effects of anticonvulsant agents on halothane-induced liver injury in human subjects and experimental animals

In order to evaluate the clinical implication of experimental studies on halothane-induced liver damage in phenobarbital-treated rats, we studied the clinical records of 315 consecutive patients who underwent brain surgery with halothane anesthesia. After exclusion of subjects with a history of alcoholism or antecedent chronic liver disease, clinical data of 279 patients with normal preoperative transaminase activities were analyzed. The incidence of halothane-induced liver injury was significantly higher in the subjects given phenobarbital than in those with no phenobarbital medication (7/100 vs. 1/179, p less than 0.01). To determine if other anticonvulsant compounds can influence halothane-induced liver injury, rats were pretreated with diphenylhydantoin or valproic acid prior to exposure to halothane under hypoxic conditions for comparison with phenobarbital. The degree of halothane hepatotoxicity assessed from ALT activities and morphological alterations was of the decreasing order of phenobarbital greater than controls = diphenylhydantoin greater than valproic acid, and a similar order was observed in the extent of reductive metabolism of halothane. These results indicate that patients pretreated with phenobarbital may be at a greater risk of halothane-induced liver damage, and that treatment with valproic acid and diphenylhydantoin lead to the production of toxic intermediates of halothane to a lesser extent than treatment with phenobarbital does.

Oxygen concentrations required for reductive defluorination of halothane by rat hepatic microsomes

The free oxygen concentrations required for reductive defluorination of halothane by rat hepatic microsomes from control and phenobarbital- (PB) and polychlorinated biphenyl-(PCB) treated animals were determined. Halothane-exposed microsomes from treated rats generated measurable levels of fluoride ion after 30 min incubations with oxygen concentrations of 5% or less. Microsomes from control animals produced fluoride only if the free oxygen concentration was 2% or less. During anoxic (0% oxygen) incubations, defluorination rates of 2.10 +/- 0.17, 5.55 +/- 0.38, and 5.46 +/- 0.30 nmol fluoride X mg protein-1 X 30 min-1 were observed for microsomes from control, PB, and PCB rats, respectively. Normalizing the maximal rates of defluorination to the microsomal cytochrome P-450 content yielded values of 5.14 +/- 1.40, 3.70 +/- 0.15, and 2.38 +/- 0.26 nmol fluoride X nmol cytochrome P-450-1 X 30 min-1 for control, PB, and PCB microsomes, respectively. Oxygen concentrations required for reductive metabolism of halothane by isolated rat hepatic microsomes are close to normal physiologic free oxygen concentrations of 4-5% reported for centrilobular areas of the rat liver. Thus even slight reductions in hepatic oxygenation during anesthetic exposure could lead to enhanced reductive biotransformation, an observation found in rat models of halothane-associated hepatic injury. The large differences among the treatment groups in the rates of fluoride ion generated per nanomole cytochrome P-450 indicate that enzyme induction regimens disproportionately increase those isozymes of hepatic cytochrome P-450 that are not involved with the reductive defluorination of halothane.

The extent of metabolism of inhaled anesthetics in humans

To determine the percentage of anesthetic metabolized and to assess the role of metabolism in the total elimination of inhaled anesthetics, the authors administered isoflurane, enflurane, halothane, and methoxyflurane simultaneously, for 2 h, to nine healthy patients. Total anesthetic uptake during the 2 h of washin and total recovery of unchanged anesthetic in exhaled gases during 5 to 9 days of washout were measured, and from these the per cent of anesthetic uptake that was recovered was calculated. Of the isoflurane taken up, 93 +/- 4% (mean +/- SE) was recovered. To compensate for factors other than metabolism that limit complete recovery of unchanged anesthetic, the percentage recovery of each anesthetic was normalized to the percentage recovery of isoflurane (which it was assumed undergoes no metabolism). Deficits in normalized recovery were assumed to be due to metabolism of the anesthetics. The resulting estimates of metabolism of anesthetic taken up were: enflurane 8.5 +/- 1.0%, halothane 46.1 +/- 0.9%, and methoxyflurane 75.3 +/- 1.6%. These results indicate that elimination is primarily via the lungs for isoflurane and enflurane, equally via the lungs and via metabolism for halothane, and primarily via metabolism for methoxyflurane.

Perspectives in the pathogenesis of halothane-induced hepatitis

The evolution of today's accepted theories regarding the pathogenesis of halothane-induced hepatitis (HIH) is traced from a selection of the extensive literature covering this rare condition. Initial postulates indicted an immunological mechanism. The development of a laboratory model (starved, phenobarbitone-pretreated, hypoxic rat) uncovered two other 'triggers': highly reactive (toxic) free-radical intermediates generated from a reductive metabolic pathway; and hypoxic stress alone. None of these proposed mechanisms incontrovertibly excludes the other and complex interrelationships may exist. Continuing research utilizing sophisticated analytical tools and new HIH animal models suggests that the field remains incompletely explored.

Pharmacokinetics of inhaled anesthetics in humans: measurements during and after the simultaneous administration of enflurane, halothane, isoflurane, methoxyflurane, and nitrous oxide

To determine the relative washin and washout characteristics of isoflurane, enflurane, halothane, and methoxyflurane, we administered all four anesthetics simultaneously (total = 1.1 MAC) to nine healthy patients for 2 hr. Concentrations of anesthetics in end-tidal gases were measured during washin and for 5-9 days during washout. Multiexponential (multicompartment) models were fit to the washin and washout curves using least-squares analysis. Slowly equilibrating compartments could only be identified during washout. For 27 of the 36 data sets, five-compartment models fit the washout curves significantly better than four-compartment models. The time constant for our first compartment is consistent with that predicted for washout of the lungs. Time constants for the second, third, and fifth compartments were consistent with current data for blood flows and solubilities of vessel-rich, muscle, and fat tissue groups, respectively. The fourth compartment has a time constant that lies between the time constants predicted for muscle and fat.

The effect of cimetidine on anesthetic metabolism and toxicity

Because the H2-receptor antagonist cimetidine has been shown to inhibit drug metabolism, the effects of cimetidine on anesthetic metabolism and toxicity were investigated in a rat model. Cimetidine decreased inorganic plasma fluoride production after methoxyflurane administration both in 21% oxygen (P less than 0.001) and in 100% oxygen (P less than 0.001). Phenobarbital produces an increased fluoride formation after methoxyflurane anesthesia, and this fluoride formation is also reduced by cimetidine (P less than 0.005). There was no significant difference between the plasma fluoride levels in rats anesthetized with halothane or enflurane. Although cimetidine inhibited the in vivo defluorination of methoxyflurane, fluoride levels were still within the nephrotoxic range, and cimetidine is not likely to play a role as part of a preanesthetic regimen that would permit the increased clinical use of methoxyflurane. Cimetidine also inhibited the oxidative metabolism of halothane; cimetidine decreased (P less than 0.05) trifluoroacetic acid concentrations after halothane anesthesia in 21% oxygen and in 100% oxygen and decreased (P less than 0.05) bromide concentrations after halothane anesthesia in 100% oxygen. Trifluoroacetic acid levels were less (P less than 0.02) after halothane anesthesia in 14% oxygen as compared with 100% oxygen, indicating a reduction in oxidative metabolism under hypoxic conditions. However, bromide concentrations were maximal after halothane anesthesia in 21% oxygen, and significantly (P less than 0.001) less after halothane anesthesia in 14% and 100% oxygen. Bromide production, therefore, seems to be inhibited by both hypoxia and hyperoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Alcohol increases the solubility of anaesthetics in the liver

Liver/gas partition coefficients for isoflurane, enflurane, halothane and methoxyflurane increased two-fold in rats killed 16 h after a single injection of 15% ethanol 7 g kg-1. In contrast, blood/gas and brain/gas partition coefficients did not change. Chronic (21 days) ingestion of ethanol increased liver/gas partition coefficients four-fold, although this increase was largely attributable to nutritional changes rather than to a direct effect of ethanol. Only minimal changes (usually not more than 15%) occurred in blood/gas and brain/gas partition coefficients. On account of this effect of ethanol on anaesthetic solubility in the liver, the ingestion of ethanol may modestly increase uptake of anaesthetic during the induction of anaesthesia.

Control of end-tidal halothane concentration. Part B: Verification in dogs

Conventional anaesthetic techniques do not allow for the automatic control of end-tidal halothane concentration and, therefore, brain concentration cannot be predicted. In this study, eight dogs were ventilated with halothane in oxygen using a new closed-loop anaesthetic breathing system which provided a constant end-tidal concentration. During the first 60 min the end-tidal concentration was maintained at 0.87 vol% (1 MAC). Then followed 60 min of halothane wash-out and a further 120-min period of halothane at 1.74 vol% (2 MAC). Halothane concentrations were measured in the inspired and expired air, and in the arterial, cerebral venous and mixed venous blood. Haemodynamic and respiratory variables were measured. The system reached 95% of the target end-tidal concentration within 6 min without over-shooting. After 2 h of wash-in, significant gradients still persisted between end-tidal, arterial and cerebral venous blood concentrations. Measured uptake differed from theoretically calculated uptake by 18.3-57.6%, depending on the model used. Measured arterial and cerebral venous concentrations differed from theoretically calculated values by 7% and 17.5%, respectively. It was shown that the required end-tidal concentrations can be obtained rapidly and accurately, and that brain tissue concentrations can be predicted within certain limits.

Starvation increases the solubility of volatile anaesthetics in rat liver

We evaluated the effect of starvation on anaesthetic solubility in tissues involved in lipid transport (blood) or metabolism (liver) and in a tissue not involved in either (brain). The liver/gas partition coefficients of isoflurane, enflurane, halothane and methoxyflurane in rats increased by 15-20% after 6 h of starvation and reached a maximum increase of 35-42% after 24 h of starvation. After 48 h of starvation the coefficients had returned to control values. Blood/gas and brain/gas partition coefficients were not changed or were inconsistently changed by starvation. The maximum change in blood or brain solubility was 14% (at 6 h), and 29 of 32 mean values changed less than 10%.

Halothane and isoflurane anesthesia in pediatric outpatients

Halothane or isoflurane was used to induce anesthesia in children scheduled for outpatient surgical procedures. Both agents were administered at predetermined rates until comparable concentrations in end-expired air were reached. Induction of anesthesia, as well as the time taken before tracheal intubation was possible, was protracted in patients given isoflurane. In the recovery period, the times taken to respond to pharyngeal suction, to tracheal extubation, and to the first cry were similar for both anesthetic agents.

Effects of combined extradural blockade and general anaesthesia on indocyanine green clearance and halothane metabolism

Cardiovascular effects, indocyanine green clearance and the reductive metabolism of halothane were compared in 10 patients having peripheral surgery who received either general anaesthesia or general anaesthesia plus extradural blockade. Bupivacaine or saline was given extradurally 1 h before anaesthesia with nitrous oxide:oxygen (70:30) and halothane (end-tidal concentration 0.35%). Indocyanine green clearance was measured before (stage I) and after (stage II) bupivacaine or saline, during anaesthesia before surgery (stage III), and during surgery (stage IV). Reductive metabolism of halothane was assessed by measurement of exhaled 2-chloro-1,1,1-trifluoroethane. In the general anaesthesia plus extradural blockade group, arterial pressure was decreased to 58% (SD = 6) of stage I values over stages II-IV, whereas no changes were observed in the general anaesthesia group. There was no significant difference in indocyanine green clearance or the reductive metabolism of halothane, despite the occurrence of significant decreases in arterial pressure.

Biochemical studies on the metabolic activation of halogenated alkanes

This paper reviews recent investigations by Slater and colleagues into the metabolic activation of halogenated alkanes in general and carbon tetrachloride in particular. It is becoming increasingly accepted that free radical intermediates are involved in the toxicity of many such compounds through mechanisms including lipid peroxidation, covalent binding, and cofactor depletion. Here we describe the experimental approaches that are used to establish that halogenated alkanes are metabolized in animal tissues to reactive free radicals. Electron spin resonance spectroscopy is used to identify free-radical products, often using spin-trapping compounds. The generation of specific free radicals by radiolytic methods is useful in the determination of the precise reactivity of radical intermediates postulated to be injurious to the cell. The enzymic mechanism of the production of such free radicals and their subsequent reactions with biological molecules is studied with specific metabolic inhibitors and free-radical scavengers. These combined techniques provide considerable insight into the process of metabolic activation of halogenated compounds. It is readily apparent, for instance, that the local oxygen concentration at the site of activation is of crucial importance to the subsequent reactions; the formation of peroxy radical derivatives from the primary free-radical product is shown to be of great significance in relation to carbon tetrachloride and may be of general importance. However, while these studies have provided much information on the biochemical mechanisms of halogenated alkane toxicity, it is clear that many problems remain to be solved.

Risk assessment extrapolations and physiological modeling

The process of assessing the risk associated with human exposure to environmental chemicals inevitably relies on a number of assumptions, estimates and rationalizations. One of the more challenging aspects of risk assessment involves the need to extrapolate beyond the range of conditions used in experimental animal studies to predict anticipated human risks. The most obvious extrapolation required is that from the tested animal species to humans; but others are also generally required, including extrapolating from high dose to low dose, from one route of exposure to another and from one exposure timeframe to another. Several avenues are available for attempting these extrapolations, ranging from the assumption of strict correspondence of dose to the use of statistical correlations. One promising alternative for conducting more scientifically sound extrapolations is that of using physiologically based pharmacokinetic models that contain sufficient biological detail to allow pharmacokinetic behavior to be predicted for widely different exposure scenarios. In recent years, successful physiological models have been developed for a variety of volatile and nonvolatile chemicals, and their ability to perform the extrapolations needed in risk assessment has been demonstrated. Techniques for determining the necessary biochemical parameters are readily available, and the computational requirements are now within the scope of even a personal computer. In addition to providing a sound framework for extrapolation, the predictive power of a physiologically based pharmacokinetic model makes it a useful tool for more reliable dose selection before beginning large-scale studies, as well as for the retrospective analysis of experimental results.

Transient inhibitory effect of isoflurane upon oxidative halothane metabolism

The duration of inhibition of halothane oxidative metabolism by isoflurane was studied in rats exposed for 2 hr to an anesthetic concentration of isoflurane (0.6% inspired), followed by a 2-hr exposure to a subanesthetic concentration of halothane (0.06% inspired), starting either 0.5, 4, or 24 hr after the end of the isoflurane exposure. Other rats were exposed to halothane, isoflurane, or a mixture of both. Tissue levels of total nonvolatile fluorine were used as a measure of oxidative metabolism of halothane and hepatic levels of 1,1,1-trifluoro-2-chloroethane and 1,1-difluoro-2-chloroethylene as a measure of reductive metabolism of halothane. Isoflurane administered simultaneously with or 30 min prior to halothane significantly inhibited oxidative metabolism of halothane, but this inhibition was transient and was no longer apparent when halothane was administered 4 or 24 hr after the end of isoflurane anesthesia. The reductive metabolism of halothane was unaffected. This study suggests that isoflurane may transiently modify the action of some drugs administered during the perianesthesia period by inhibiting their oxidative metabolism. Differences in elimination kinetics of nonvolatile fluorine-containing metabolites after isoflurane and halothane exposure suggest the presence of an unidentified isoflurane metabolite.

Chemistry and biology of spin-trapping radicals associated with halocarbon metabolism in vitro and in vivo

The spin-trapping method is introduced and discussed. Some chemistry of nitroxides and nitrones is reviewed. Pattern recognition of ESR spectra of nitroxides is outlined. Factors controlling the magnitude of hyperfine splitting constants are mentioned. Methods of assigning spin adducts are listed. Review articles in the literature are referenced. Results in the electrochemical reduction of halocarbons are presented and some parallels with superoxide chemistry shown. Various speculative reactions are given. The in vitro and in vivo experiments where halocarbon radicals have been detected by spin trapping are reviewed and some new results reported. A comparison for different animals is added.

Immunochemical evidence of trifluoroacetylated cytochrome P-450 in the liver of halothane-treated rats

Four hours after the administration of halothane to phenobarbital-pretreated rats, subcellular fractions of liver were isolated and the proteins in the fractions were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis, transferred to nitrocellulose sheets, and immunochemically stained with anti-trifluoroacetylated antibodies. The microsomal fraction contained the highest level of trifluoroacetylated adducts. Its major trifluoroacetylated component was immunochemically identified as a phenobarbital-inducible form of cytochrome P-450 (54 kDa), whereas the other observed trifluoroacetylated protein fraction (59 kDa) was not identified. The plasma membrane fraction also contained a 54-kDa trifluoroacetylated adduct, which was immunochemically related to the 54-kDa cytochrome P-450. Microsomes from untreated rats that were administered halothane contained only the 59-kDa trifluoroacetylated protein fraction. The specificity of the immunochemical staining for the bound oxidative metabolite of halothane was confirmed by the finding that rats treated with deuterated halothane had considerably less stained liver proteins than did those treated with halothane. These results suggest that the CF3COX oxidative metabolite of halothane is so reactive that it binds predominantly to the cytochrome P-450 that produced it.

Uptake of volatile anaesthetics in children

The uptake of halothane is known to be more rapid in children than in adults, but comparable clinical data regarding other inhalational anaesthetics are not available. In this study, the rates of uptake of halothane, enflurane, isoflurane and methoxyflurane were compared in children of different ages. Expired (FE') and inspired (FI) vapour concentrations were measured with an infrared analyser, and FE'/FI ratios were used to determine rates of uptake. Uptake rates of halothane, enflurane and methoxyflurane were more rapid in the younger than in the older children, but age had no effect on the uptake of isoflurane which was uniformly rapid in all the children studied.

[Uptake and elimination of isoflurane and halothane by children and adults]

The rate of increase of alveolar concentrations (FA/FI) of isoflurane and halothane was studied in children and adults during general anaesthesia and controlled ventilation. After 30 min of body equilibrium, elimination curves of the volatile anaesthetics were determined by measurement of alveolar (FA/FA0; infrared technique) and venous concentrations (gas chromatography). The distribution and elimination half-times (t1/2 alpha, t1/2 beta), clearance (Cl), volume of central and peripheral compartment (V1, Vz) and the volume of distribution at steady state (Vss) were calculated from the intercepts and slopes of a two-compartment model. During the uptake of anaesthetic concentrations of isoflurane and halothane, the FA/FI ratio of each gas was found to rise significantly faster in children than in adults. The reason for the more rapid approach to equilibrium in children seems to be related to physiological differences. Irrespective of age, uptake of isoflurane was more rapid than that of halothane, as it is less soluble. Similarly, isoflurane was eliminated from the lung or blood faster than halothane. Moreover, anaesthetic wash-out in children differed from that in adults. In the paediatric age group t1/2 beta under isoflurane was shorter than in adults, whereas halothane excretion took longer in children. This could be accounted for by the larger volumes of distribution Vz and Vss in the young, due to higher organ affinity of halothane. From our data we conclude that age significantly affects uptake and elimination of volatile anaesthetics and the control of anaesthesia is easiest with isoflurane in paediatric patients.

Comparison of the requirements for hepatic injury with halothane and enflurane in rats

A rat model of enflurane-associated hepatotoxicity was compared with the halothane-hypoxia (HH) model (adult male rats, phenobarbital induction, 1% halothane, 14% O2, for 2 hr). The enflurane-hypoxia heating (EHH) model involved exposing phenobarbital-pretreated male adult rats to 1.5-1.8% enflurane at 10% O2 for 2 hr with external heating to help maintain body temperature. Exposure to either anesthetic without temperature support led to a decrease in body temperature of 7-9 degrees C, while heating the animals during anesthesia resulted in only a 0.5-2 degree decrease. Reducing the oxygen tension to 10% O2 combined with heating the animals during exposure produced significant decreases in the oxidative metabolism of both halothane and enflurane as compared to exposures of 14% O2. The same conditions also caused a significant increase in the reductive metabolism of halothane, indicating that a severe hepatic hypoxia or anoxia occurs during anesthesia at 10% O2 with external heating. The time course of lesion development in the HH model paralleled results obtained with an oral dose of CCl4: gradual progression of necrosis up to 24 hr. EHH resulted in a classic hypoxic/anoxic injury with elevated serum glutamate pyruvate transaminase values and a watery vacuolization of centrilobular hepatocytes immediately after exposure. The HH model required phenobarbital pretreatment of the rats for expression of hepatic injury; EHH did not. Heating of the animals during anesthesia exposure was necessary for enflurane-induced hepatoxicity but had little effect on the HH model. Exposure to 5% O2 without anesthetic mimicked EHH in both requirements for and type of hepatic injury.

Plasma fluoride and bromide concentrations during occupational exposure to enflurane or halothane

The plasma fluoride and bromide concentrations were studied in operating theatre personnel. When enflurane was used, the increase in plasma fluoride concentration could not be distinguished from normal individual variations, but the plasma bromide concentration increased significantly when halothane was used. Seven patients were exposed to enflurane in a concentration of 200 parts per million for 4 h. A significant increase in the plasma concentration of fluoride was observed. The peak concentrations of fluoride occurred during exposure and the increase lasted less than 12 h. The increase in fluoride concentration was larger at this trace concentration than reported after anaesthetic concentrations. Cerebrospinal fluid (CSF) fluoride concentration was also studied; a smaller and delayed increase was found in CSF compared to plasma.

HLA antigens in patients with unexplained hepatitis following halothane anesthesia

We examined HLA-A,B,C and DR locus antigens in 38 Japanese patients who had recovered from halothane hepatitis. The patients were divided into two subgroups, i.e. jaundice and non-jaundice groups, because the clinical features were quite different in each. DR2 was positive in 14 (58.3%) of 24 patients with jaundice, compared with 281 (33.6%) of the 837 Japanese healthy controls (chi-square with Yates' correction = 5.30, relative risk = 2.77, P less than 0.025). Conversely, Bw44 was increased in non-jaundice patients (50.0%), compared with 157 (12.7%) of the 1234 Japanese healthy controls (chi-square with Yates' correction = 13.75, relative risk = 6.86, P less than 0.001). The haplotype frequency (Hf) of Aw24-Bw52-DR2 was high in the patients with jaundice (Hf = 0.2362), while it was zero in the patients without jaundice (P less than 0.0042). These data suggest that the two groups of halothane hepatitis have different genetic backgrounds.

Immunological studies on the mechanism of halothane-induced hepatotoxicity: immunohistochemical evidence of trifluoroacetylated hepatocytes

The fulminant hepatotoxicity caused by halothane has been thought to have an immunological basis because this toxicity occurs most often after repeated administration of halothane and because sera from patients recovering from severe halothane hepatotoxicity contain antibodies that bind to the surface membranes of hepatocytes of rabbits treated with halothane. In order to determine whether the major reactive metabolite of halothane, trifluoroacetyl halide, covalently binds to hepatocytes, we have developed specific and sensitive peroxidase enzyme-linked immunosorbent assays and an indirect immunofluorescence staining method for identifying trifluoroacetylated (TFA)-hepatocytes. Liver sections prepared from rats at 4 hr after halothane administration were stained preferentially in the centrilobular region with anti-TFA serum whereas livers of control rats showed no staining. The specificity of the assay for the TFA group was confirmed by the complete inhibition of the staining with 200 microM N-epsilon-TFA-L-lysine in the diluted antiserum. On the other hand, 2 mM halothane or L-lysine did not inhibit this staining. Moreover, treatment of rats with deuterated halothane resulted in significantly less staining than did halothane. At 24 hr after halothane administration, hepatocytes isolated and stained by indirect immunofluorescence showed a linear and granular pattern on their surface membranes. These results indicate that trifluoroacetyl halide either reacts directly with constituents of the plasma membranes or with other cellular components which become incorporated into the plasma membranes.

Sex differences in halothane metabolism and hepatotoxicity in a rat model

This study was designed to investigate sex differences in halothane metabolism and hepatotoxicity in the hypoxic rat model. Phenobarbital-induced male and female rats were anesthetized with 1% halothane in 14% oxygen for two hours. Female rats were found to metabolize halothane by the oxidative pathway to a similar extent as males, but the extent of metabolism by the reductive pathway was less in females. All male rats exposed under these conditions developed confluent centrilobular hepatic necrosis. Females were less susceptible than males to the hepatotoxic effect of halothane, with responses ranging from no hepatic injury to confluent centrilobular necrosis limited to within a few cells of the central veins. This lesser susceptibility was not, however, solely due to the lesser extent of reductive metabolism in females, as lowering the inspired oxygen concentration to 12% increased the extent of reductive metabolism but did not increase the severity of the hepatic injury.

Guinea-pig model of halothane-associated hepatotoxicity in the absence of enzyme induction and hypoxia

Halothane anesthesia (1%) administered in 21% oxygen for 4 hr to an outbred strain of guinea pig in the absence of enzyme induction resulted in liver damage in 40 of the 65 animals studied. Necrosis was either confluent around the central veins or in scattered foci throughout the lobules. Damage was present on the second and third days after anesthesia. By day 7 the livers had recovered, evidenced by lack of histological changes and normal serum alanine aminotransferase activity. Administration of halothane in 14 or 80% inspired oxygen did not alter the extent or incidence of liver damage. Major end-metabolites of halothane biotransformation (2-chloro-1,1-difluoroethylene, 2-chloro-1,1,1-trifluoroethane, inorganic fluoride and trifluoroacetic acid) were identified at each oxygen concentration. The metabolic inhibitor SKF-525A significantly decreased the amounts of the volatile metabolites 2-chloro-1,1,1-trifluoroethane and 2-chloro-1,1-difluoroethylene. SKF-525A also decreased the incidence and severity of hepatic damage. Both halothane (1%) and isoflurane (1.1%) anesthesia caused similar reductions in mean arterial blood pressure. However, in contrast to halothane, isoflurane was not hepatotoxic. The results indicate that liver necrosis is unlikely to be caused by anesthesia per se, but rather by hepatotoxic metabolites of halothane. This model offers the opportunity to study the pathogenesis of halothane hepatotoxicity after the administration of halothane alone.

Toxicity and biotransformation of volatile halogenated anesthetics in rat hepatocyte suspensions

This study examines the toxicity (as measured by reduced intracellular K+) and metabolism (defluorination) of halothane and enflurane in rat hepatocytes in suspension (RHS) with regards to O2 tension, time, and concentration. In 95% O2 halothane is more toxic than enflurane when RHS are exposed to 5-20 microliters of these anesthetics. At these levels halothane is not metabolized while enflurane is metabolized. At 21% O2 a similar pattern was seen with regards to toxicity. However, metabolism of halothane rapidly reached an elevated level while that of enflurane is reduced when compared to 95% O2. Thus toxicity of halothane and enflurane at these dose levels appears to be unrelated to metabolism and due solely to a solvent effect.

Reductive metabolism of halothane in children

The volatile halothane metabolites, 2-chloro-1,1,1,-trifluoroethane and 2-chloro-1,1-difluoroethylene, were identified in the exhaled breath of ten children during halothane anaesthesia. Although there was considerable variation among children in the concentration and time-course of metabolite exhalation, exhaled concentrations of these metabolites were of a similar magnitude to those reported in adults. This result suggests that the lower incidence of halothane hepatitis in children compared with adults is not due to a poor ability to metabolise halothane by the reductive pathway.

Accumulation in murine amniotic fluid of halothane and its metabolites

The distribution of radioactivity in pregnant mice was registered at 0, 4, and 24 hrs after a 10 min. period of inhalation of 14C-halothane. Autoradiographic methods were used to allow to distinguish between the distribution of volatile (non-metabolize) halothane, water-soluble metabolites, and firmly tissue-bound metabolites. While volatile radioactivity was seen predominantly at short survival intervals, e.g. in body fat, blood, brain and liver, metabolites accumulated with time. Peak values occurred at 4 hrs in most organs (measured with liquid scintillation as well). The most remarkable findings were the high concentrations of radioactivity in amniotic fluid (and the ocular fluids of adults) with peak values at 4 hrs and rather high concentrations still prevailing at 24 hrs after inhalation. It is assumed that this activity represents only partly volaile halothane and mostly non-volatile metabolites. High activity of metabolites was seen in the neuroepithelium of the embryo in early gestation. Firmly tissue-bound metabolites, still remaining after washing the tissues with trichloroacetic acid and organic solvents, were found in the nasal mucosa, trachea and bronchial tree and in (presumably centrilobular) zones of the liver of adults after inhalation and 5-day old mice after intraperitoneal injection, indicating the formation of reactive metabolites in these organs. Firmly tissue-bound activity was not observed in the corresponding foetal organs.

Isoflurane does not prevent hepatic injury produced by halothane in rats

We speculated that the inhibitory effect of isoflurane on the metabolism of halothane might reduce hepatic injury produced by halothane. To test this hypothesis we pretreated male rats with phenobarbital and 24 hr later exposed them to one of three types of anesthesia. One group of rats was anesthetized with 0.6 MAC isoflurane in 21-25% oxygen for 20 min, followed by exposure to 0.3 MAC isoflurane and 0.3 MAC halothane either in 9% oxygen for 46 min (n = 5) or in 12% oxygen for 60 min (n = 11). A second group of rats received 0.3 MAC halothane in 9% oxygen for either 46 min (n = 12) or in 12% oxygen for 60 min (n = 10). The third group received 0.3 MAC isoflurane in 9% oxygen for either 46 min (n = 12) or for 120 min (n = 8). The rats were killed 24 hr after the exposures and liver slides prepared. Histologic examination revealed that rats treated with isoflurane plus halothane, or with halothane alone showed a significant hepatic injury (P less than 0.005) when compared with those treated with isoflurane. Thus isoflurane failed to protect the liver from halothane-induced injury.

Do general anaesthetics act by competitive binding to specific receptors?

Most proteins are insensitive to the presence of anaesthetics at concentrations which induce general anaesthesia, while some are inhibited by certain agents but not others. Here we show that, over a 100,000-fold range of potencies, the activity of a pure soluble protein (firefly luciferase) can be inhibited by 50% at anaesthetic concentrations which are essentially identical to those which anaesthetize animals. This identity holds for inhalational agents (such as halothane, methoxyflurane and chloroform), aliphatic and aromatic alcohols, ketones, ethers and alkanes. This finding is all the more striking in view of the fact that the inhibition is shown to be competitive in nature, with anaesthetic molecules competing with substrate (luciferin) molecules for binding to the protein. We show that the anaesthetic-binding site can accommodate only one large, but more than one small, anaesthetic molecule. The obvious mechanism suggested by our results is that general anaesthetics, despite their chemical and structural diversity, act by competing with endogenous ligands for binding to specific receptors.

In vivo spin-trap study on anaerobic dehalogenation of halothane

Radical formation in vivo by anaerobic dehalogenation of halothane is described in this paper. The radicals were stabilized by spin-trapping and assayed by electron spin resonance spectrometry. The radical adducts were formed by inhalation of halothane in vivo and increased with decrease in inspired oxygen concentration. Following administration of the spin-trap, the expired concentration of CF2CHCl and CF3CH2Cl which are the anaerobic metabolites of halothane decreased, but bilious trifluoroacetate which are aerobic did not change. These results strongly suggest that radical intermediates are produced in anaerobic dehalogenation of halothane to CF2CHCl and CF3CH2Cl.

Effect of O2 tension on the bioactivation and metabolism of aliphatic halides by primary rat-hepatocyte cultures

The covalent binding of CCl4 and CF3CHBrCl to cultured rat hepatocytes was enhanced by anoxic conditions, indicative of enhanced reductive biotransformation. The covalent binding of CHCl3 and CHCl2CH2Cl decreased as the O2 concentration was decreased, indicating a preference for oxidative metabolism. Lower O2 tensions decreased the formation of polar water-soluble metabolites with CHCl3 and CHCl2CH2Cl, while CCl4 and CF3CHBrCl were unaffected. The results indicate that primary cultured hepatocytes can reductively biotransform and bioactivate aliphatic halides under anoxic conditions.

Elimination of nitrous oxide accelerates elimination of halothane: reversed second gas effect

The effect of nitrous oxide on the elimination of halothane was studied in 10 patients ranging in age from 20 to 50 years. After establishing a stable baseline (inspired halothane concentration: 0.85%, end-tidal halothane concentration: 0.75%), halothane administration was stopped and the rate of decrease in alveolar concentration of halothane (FE/ FE0 , FE: measured end-tidal concentration of halothane; FE0 : the endtidal concentration immediately preceding the cessation of halothane administration) was measured continuously. The rate of decrease in FE/ FE0 was more rapid when nitrous oxide (70%) is discontinued abruptly and replaced by the same concentration of nitrogen (Part 2) than when the nitrous oxide is continued (Part 1). One minute and a half after the cessation of halothane administration, FE/ FE0 was 0.38 +/- 0.05 (mean +/- SD) in Part 2 and 0.45 +/- 0.04 in Part 1 (P less than 0.01). In Part 2, the fall in the alveolar concentration of halothane was accompanied by a decrease in alveolar carbon dioxide from 4.27 +/- 0.01% to 4.16 +/- 0.01% at 1.5 min and an increase in the mean expired tidal volume from 522 +/- 39 ml to 557 +/- 29 ml. The authors conclude that the elimination of nitrous oxide accelerates the elimination of halothane both by dilution and by an increased expired ventilation.

Minimum alveolar concentrations (MAC) of halothane and nitrous oxide in swine

To compare anesthetic effects using a swine model, we needed to know the minimum alveolar concentrations (MAC) of halothane and nitrous oxide that produce anesthesia in the pig. This information does not exist in literature. Furthermore, MAC varies considerably among species: by more than 60% for halothane, and by more than 200% for nitrous oxide. Therefore, using eight young swine, we determined mean (+/- SEM) MAC values for halothane (1.25 +/- 0.04% of one atmosphere) and nitrous oxide (277 +/- 18% of one atmosphere). These values are higher than values reported for other mammals. Factors possibly accounting for this variability include interspecies differences, age, body temperature, increased sympathetic activity, and differences in methodology.

Inhibitory effect of isoflurane upon oxidative metabolism of halothane

The effect of isoflurane upon halothane metabolism was studied in rats exposed to mixtures of subanesthetic concentrations of isoflurane (0.015-0.32%) and halothane (0.062%). The extent of halothane metabolism was determined from concentrations of halothane and its metabolites (total nonvolatile fluorine, 1,1,1-trifluoro-2-chloroethane and 1,1-difluoro-2-chloroethylene) in tissues of rats. At the end of exposures lasting 3 hr, distribution of halothane in tissues indicated that the bioavailability of halothane in liver was unaffected by exposure to isoflurane. Isoflurane, however, significantly inhibited the oxidative metabolism of halothane, as indicated by reduced concentrations of total nonvolatile fluorine in tissues. The inhibition is concentration-dependent. Isoflurane enhances the reductive metabolism of halothane, as indicated by increased concentrations of volatile metabolites in liver. Exposure to nitrous oxide (2.2-50%) had no effect on halothane metabolism. The mechanism of the inhibitory effect remains to be explained.