Fluoride concentrations in urine of delivery ward personnel following exposure to low concentrations of methoxyflurane

Midwives and other delivery ward personnel exposed to methoxyflurane do not have measurable traces of the agent in expired air when examined soon after exposure. This may imply a rapid uptake of the anesthetic. If this is the case, then the products of the metabolism of methoxyflurane, such as fluoride, may appear in the urine of such personnel. The present study investigated urinary fluoride levels in 24 delivery ward personnel and compared the values found after methoxyflurane/nitrous oxide analgesia with those measured in the same individuals after exposure to nitrous oxide alone. A highly significant difference was observed. Thus it would appear that, in spite of an apparently adequate system of environmental ventilation, there is a significant uptake of methoxyflurane by delivery ward personnel when this agent is employed for obstetrical analgesia.

Metabolism of synthane: comparison with in vivo and in vitro defluorination of other halogenated hydrocarbon anaesthetics

Aerobic defluorination of the inhalation anaesthetic agent, synthane, was compared with that of methoxyflurane, enflurane and halothane and with two other anaesthetics, isoflurane and sevoflurane. In vitro, in microsomes prepared from phenobarbitone-induced and control livers, synthane and halothane were not defluorinated. The relative order of defluorination of the other anaesthetics was methoxyflurane greater than enflurane greater than isoflurane. In vivo, following 4 h of 1.2% (MAC) synthane anaesthesia, urinary inorganic fluoride excretion was increased by only a trivial amount and only in phenobarbitone-treated rats; polyuria was not observed. Synthane is the least metabolized of the fluorinated ether anaesthetics; its administration will not result in inorganic fluoride nephropathy. An index of nephrotoxic potential for fluorinated anaesthetic agents was formulated utilizing in vitro fluoride production data and oil: gas partition coefficients.

Effect of phenytoin (DPH) treatment on methoxyflurane metabolism in rats

The toxicity and metabolism of the fluorinated anesthetic methoxyflurane were compared in Fischer 344 rats pretreated with phenytoin or phenobarbital. Treatment with either drug potentiated the polyuric effects of methoxyflurane by more than 100%. Also, serum inorganic fluoride (F-) levels and urinary F- excretions after methoxyflurane exposure were comparable in phenytoin- and phenobarbital-treated rats, a 26 to 49% increase as compared to rats treated with methoxyflurane alone. In vitro, 10-fold increases in the rate of hepatic microsomal methoxyflurane defluorination were observed after treatment of rats with either phenytoin or phenobarbital. Kinetic studies with microsomes demonstrated inhibition of methoxyflurane defluorination in the presence of phenytoin. Defluorination of three additional fluorinated ether anesthetics, enflurane, isoflurane and sevoflurane, also was examined in vitro. Phenytoin and phenobarbital treatment resulted in similar enhancement of defluorination of the latter two anesthetics, but not enflurane. Phenytoin and phenobarbital treatment increase defluorination of fluorinated ether anesthetics to approximately the same extent in vitro and in vivo in Fischer 344 rats.

The effects of deuteration on the metabolism of halogenated anesthetics in the rat

The authors studied the effects of substituting deuterium for hydrogen in several volatile anesthetics on their metabolism in the Fischer rat. Substitution of deuterium in the ethyl portion of methoxyflurane increased the metabolic production of fluoride ion by 19 per cent when administered at a concentration of 0.05 per cent. Total replacement of hydrogen by deuterium resulted in a 29 per cent decrease in the amount of fluoride produced, while deuteration of only the methoxyl group produced a 33 per cent decrease in fluoride produced. Deuteration of halothane resulted in a 15 or 26 per cent decrease in serum bromide at 0.75 per cent or 1.0 per cent, respectively. Deuteration in the ethyl portions of enflurane and two experimental agents, CF2HOCF2CFBrH and CF2HOCF2CCl2H resulted in 65, 76, and 29 per cent decreases in urinary fluoride, respectively. Anesthesia with deuterated chloroform at a concentration of 0.36 per cent produced a 35 per cent decrease in serum glutamic pyruvic transaminase (SGPT). It is concluded that deuteration of volatile anesthetics changes their metabolism, in most cases producing decreases in metabolism. This effect may lessen the organ toxicity believed to occur with some of these anesthetics.

Enflurane and methoxyflurane metabolism at anesthetic and at subanesthetic concentrations

In an attempt to determine the importance of concentration of an anesthetic agent as a determinant of the extent of its biotransformation, we measured fluoride excretion in groups of Fischer 344 rats treated with one of several subanesthetic or an anesthetic concentration (1 MAC) of either enflurane or methoxyflurane. Anesthetic administrations (2.0% enflurane or 0.26% methoxyflurane) ranged from 0.15 hours (9 minutes) to 4.8 hours. Subanesthetic exposures, all of 48 hours duration, ranged in concentration from 0.2% enflurane to 0.0016% methoxyflurane. Greatest metabolism occurred at the lowest concentration time (MAC-hours) of subanesthetic administrations and at the shortest duration of anesthetic exposure. Increasing time in the case of anesthetizing exposures, or concentration in subanesthetic exposures, increased the amount of metabolite produced. However, the increased production of metabolite was not proportional to the increase of concentration or duration of exposure. Enzyme induction was ruled out as an important factor in the larger amount of metabolism seen during the subanesthetic exposures. Therefore, the exposure of a patient to the metabolites of an anesthetic is actually low although the anesthetic is administered at a high concentration.

Serum and urine inorganic fluoride concentrations and urine oxalate concentrations following methoxyflurane anesthesia in the dog

Plasma fluoride, urine fluoride and urine oxalate concentrations were measured before administering an anesthetic to 8 dogs, and at 0, 3, 9, 24, 48, and 72 hours following 1.5 hours of anesthesia with 1% methoxyflurane. Plasma and urine osmolalities were measured and compared with fluoride and oxalate values. Fluoride concentration increased in both plasma and urine following anesthesia when compared with the preanesthetic concentrations. Maximum mean plasma inorganic fluoride was 106.71 mumoles per liter (+/- 25.44 SE) at 9 hours after exposure to methoxyflurane was completed. By 72 hours after exposure to methoxyflurane the plasma fluoride concentration was 23.47 microM/L (+/- 5.74 SE). Mean urine inorganic fluoride concentration was highest at 9 hours after exposure to methoxyflurane and reached 6047.03 microM/L (+/- 1378.46 SE) as compared to the mean preanesthetic base-line concentration of 542.68 microM/L (+/- 132.93 SE), and the 72 hour mean urine fluoride concentration which was 1593.78 microM/L (+/- 579.46 SE). Urine oxalate concentrations, when compared with urine osmolality (mg/mOsm), increased throughout the study. The 72-hour concentration after exposure to methoxyflurane was 2.5 times the preanesthetic (mg/mOsm) oxalate concentration. Plasma osmolality did not change markedly during the study. Urine osmolalities varied between animals and collection times, but a consistent pattern did not occur. Clinical and laboratory signs of renal dysfunction were not observed in any animal during the study.

Biodegradation of halothane, enflurane and methoxyflurane

The biodegradation of halothane, enflurane and methoxyflurane was studied in 22 patients undergoing abdominal surgery, by measuring the uptake and elimination of each agent and the fluoride excretion in urine. Six control patients were anaesthetized with nitrous oxide in oxygen together with neuromuscular blocking drugs, five patients with nitrous oxide in oxygen and 0.93% halothane, five with nitrous oxide in oxygen and 1.30% enflurane, and six with nitrous oxide in oxygen and 0.31% methoxyflurane. The ratio of the fluoride excretion in urine to the total amount of fluoride contained in the amount of each anaesthetic agent absorbed during anaesthesia was estimated to be 17.7% for halothane, 2.3% for enflurane and 46.3% for methoxyflurane. The serum fluoride concentration increased to a maximum of 15.8 +/- 3.8 mumol litre-1 (mean +/- SD) at 6 h after anaesthesia with methoxyflurane, while it did not exceed 8 mumol litre-1 with the other anaesthetic agents.

Anesthetic indices--further data

Anesthetic indices for methoxyflurane, enflurane, and isoflurane in oxygen and halothane in nitrous oxide and oxygen (50:50), were determined in rats using measurements of heart and brain concentrations of the volatile agents at the endpoints of anesthesia, respiratory arrest and cardiac failure. The indices related respiratory arrest to anesthesia (respiratory index-A1r), cardiac failure to anesthesia (cardiac index-AIc) and respiratory arrest to cardiac failure (cardiorespiratory index-AIcr). Isoflurane had a significantly higher AIr (3.1) and AIc (5.7) than enflurane (AIr 1.8, AIc 3.3), methoxyflurane (AIr 2.2, AIc 3.7) and halothane in nitrous oxide and oxygen (AIr 2.4, AIc 3.7). These indices were also higher than those previously determined for halothane (AIr 2.3, AIc 3.0). Isoflurane had a higher AIcr (1.9) than halothane (1.6). Enflurane had a significantly lower AIr (1.8) than any of the other agents studied. These findings suggested a greater margin of safety for isoflurane, especially with respect to the heart, and a greater potential for respiratory depression for enflurane than for the other agents. Nitrous oxide decreased the amount of halothane necessary to produce anesthesia, but also that needed to produce respiratory arrest or cardiac failure. The addition of nitrous oxide, therefore, did not significantly enhance the overall safety of halothane anesthesia with respect to potential respiratory or cardiac depression.

Increase in anesthetic uptake, excretion, and blood solubility in man after eating

Blood-gas partition coefficients of N2O, enflurane, halothane, methoxyflurane, and isoflurane were measured on blood samples from 12 healthy male volunteers before and after eating. The solubility values determined while volunteers fasted substantiate previously reported blood-gas partition coefficients for enflurane, isoflurane, and halothane. Solubility values for methoxyflurane and N2O were slightly greater and smaller, respectively, than accepted values. The uptake and excretion of N2O, enflurane, halothane, and methoxyflurane also were measured in 6 of these subjects in the fasted and postprandial states. Subjects breathed a constant, inspired mixture containing trace concentrations of all 4 gases. Eating increased blood solubility by 17 to 24 percent for all agents except N2O. Accordingly, the rates of rise of the end-tidal enflurane, halothane, and methoxyflurane concentrations were 7 to 8 percent below control, and the rates of anesthetic uptake increased 20 to 23 percent. Simulation studies showed that the increased ventilation induced by eating opposed and, therfore, minimized the impact of increased blood solubility and cardiac output on the rate of end-tidal anesthetic rise. Changes in blood solubility did not correlate with levels of plasma triglycerides and cholesterol.

Formaldehyde formation as a metabolite of methoxyflurane

The production of formaldehyde as a metabolite of enzymatic biotransformation of methoxyflurane was observed in vitro in hepatic microsomal preparations derived from Japanese monkeys, rabbits and Wistar strain rats. Production rates of formaldehyde were 0.80-2.50 n moles/mg protein/min in the induced group which was pretreated with phenobarbitone, and 0.15-0.68 n moles/mg protein/min in the non-induced group. This reaction needed the presence of NADPH and oxygen. Formaldehyde production was almost completely inhibited by addition of 25% CO. Further degradation of formaldehyde did not occur in microsomal suspension. No detectable species difference was observed among the animals used in this study. These results indicate that the biotransformation of methoxyflurane is preceded by interaction with cytochrome p-450 in hepatic microsomes in the presence of NADPH and oxygen.

Fluoride in bone of rats anesthetized during gestation with enflurane or methoxyflurane

Fluoride concentrations in maternal and fetal bones were measured following exposure of pregnant rats to methoxyflurane and enflurane anesthesia. Fluoride content in fetal bone increased significantly only after exposure to methoxyflurane, and then only when methoxyflurane was administered after 12 days of gestation, when ossification of fetal bone begins. Fluoride concentrations in maternal bone increased following both anesthetics, except in rats exposed to methoxyflurane after 15 days of gestation, when ossification of many fetal skeletal parts is in progress and fluoride is preferentially deposited in the fetal skeleton.

Toxicity following methoxyflurane anaesthesia. IV. The role of obesity and the effect of low dose anaesthesia on fluoride metabolism and renal function

Seven obese and five normal weight patients were studied before, during and after one hour of methoxyflurane-nitrous oxide anaesthesia during peripheral surgical operations and compared with eight patients of normal weight anaesthetized with nitrous oxide-meperidine and d-tubocurare. Estimates were made of renal function, including serum and urinary electrolytes, osmolarity, uric acid, urea and creatinine. Renal clearances for the latter three substances were also calculated. Serum and urinary inorganic and organic fluoride concentrations were measured, as were renal clearances. This low dose methoxyflurane anaesthesia resulted only in a decrease in uric acid clearance among all the measures, when compared to the meperidine-nitrous oxide controls. The clearance of uric acid remained depressed for longer in the obese patients, but otherwise they did not differ from the normal weight patients. It is possible but not proven that depressed uric acid clearance may be related to the organic fluoride metabolite and an early indicator of methoxyflurane renal toxicity. The previously documented biotransformation of methoxyflurane was seen in this study. A double peak in serum inorganic fluoride was shown in all patients but one. Rather large differences in peak levels of serum inorganic fluoride occurred. The only significant difference between the obese and normal weight patients as far as fluoride metabolism was concerned was a greater variability in the serum inorganic fluoride levels in the obese patients. It would appear that the obese patient metabolizes methoxyflurane in a quantitatively if not qualitatively different fashion than the normal weight patient, perhaps because of fatty infiltration of the liver. Caution is advised in the use of methoxyflurane for more than 90 minutes of low concentration administration in view of the unpredictability of the biotransformation.

Clinical significance of the biotransformation of inhalation anesthetics

Inhalation anesthetics, a class of drugs formerly believed to be biologically inert, are now recognized to undergo considerable biotransformation. The viscerotoxicity of certain anesthetics on kidney and liver can be explained in terms of metabolism. The entity of "halothane hepatitis" remains mechanistically and diagnostically a mystery, but if it exists, it could be due to abnormalities of biotransformation.

Metabolism of methoxyflurane in man

Excretion of methoxyflurane was studied in 12 patients receiving anesthesia in a closed rebreathing circuit at a constant alveolar concentration of approximately 0.24 per cent. The mean methoxyflurane uptake was 18 g (range 7.6-31 g) during a mean time of anesthesia administration of 2 hours, 18 minutes (range 55-309 minutes). An average of 19 per cent of the uptake was recovered unchanged in the exhaled air after anesthesia. Urinary excretion of organic fluorine, fluoride, and oxalic acid was equivalent to 29, 7.7 and 7.1 per cent of methoxyflurane uptake, respectively. Approximately a third of the uptake remained unrecovered. It is postulated that a portion of the unrecovered drug became permanently bound to tissues and hence its excretion was delayed beyond the period of the study.

Kinetics of methoxyflurane biotransformation with reference to substrate inhibition

The kinetics of biotransformation of methoxyflurane by rat hepatic microsomes in vitro was studied. The rate of biotransformation as measured by analysis of metabolites continued to increase even at near-saturation concentrations of the anesthetic. Methoxyflurane biotransformation followed either an unbounded curve with empirical formula y = a ln(bx + 1) or an asymptotic curve with formula (see article) No substrate inhibition was observed. Total fluoride Vmax of 135.1 mmumol F-/mg protein/30' was increased to 931.9 by phenobarbital induction; free fluoride Vmax from 39.2 to 403.2. Thus, enzyme induction shifted biotransformation to the production of greater amounts of inorganic free fluoride metabolites than organic fluoride-containing metabolites. Phenobarbital induction caused qualitative as well as quantitative alteration in the biotransformation of methoxyflurane.

Methoxyflurane biotransformation and renal function following methoxyflurane administration for vaginal delivery or cesarean section

Methoxyflurane (MOF) administration for conscious analgesia during vaginal delivery (range 5 to 70 min, mean 23 min) or for anesthesia following delivery of the infant at cesarean section (range 25 to 70 min, mean 44 min) was studied in 18 healthy parturients. Serum ionic fluoride increased significantly in both groups 2 hours after discontinuing MOF with peak concentrations of 11.2 and 14.1 mumol/L in the vaginal delivery and cesarean section groups, respectively. Individual peak serum ionic fluoride levels in the 2 groups of 21 and 25 mumol/L were well below reported levels for subclinical toxicity. Significant ionic fluoride elevations in fetal umbilical venous blood (mean 5.3 mumol/L) were measured in the vaginal-delivery group. Maternal urinary ionic fluoride and oxalate were elevated 24 and 48 hours postpartum. BUN, creatinine, urine volume, and urine osmolality remained within normal range. These data indicate that hazardous elevations of serum ionic fluoride with subsequent renal dysfunction are unlikely following low-dose MOF administration for vaginal delivery or cesarean section.

[Gaschromatographic examination of the expired air after anaesthesia with halothane, enflurane and methoxyflurane (author's transl)]

The concentration of anaesthetic gases in the expired air was studied for up to 17 days in patients who had been anaesthetized with methoxyflurane, enflurane or halothane. The estimation were made by means of gaschromatography. The rate and duration of elimination of the anaesthetic via the lungs depends on the fat stores of the patient (as calculated by the body weight/stature ratio), the length of anaesthesia and the Lutz index (calculated average concentration of the inhaled anaesthetic). The concentration of anaesthetic gases in the expired air fell steeply during the first 12 hours after anaesthetic and then decreased more gradually. The drop in concentration during the first 24 hours was steepest for methoxyflurane, followed by halothane and enflurane in that order. The results are in fair agreement with the rate of metabolization of the three anaesthetic agents. The investigation showed that during the first after anaesthesia appreciable quantities of anaesthetic gases are eliminated via the lungs and, consequently, released into the air of the recovery room.