Metabolism by rat hepatic microsomes of fluorinated ether anesthetics following ethanol consumption

The possibility that the metabolism of volatile inhalational anesthetics is altered following chronic ethanol consumption was investigated in male Fischer 344 rats. The hepatic microsomal defluorination rates of methoxyflurane, enflurane, and sevoflurane were determined for pair-fed rats receiving ethanol with normal caloric or with 50% of normal caloric intake. For comparison, the effects of phenobarbital treatment on anesthetic defluorination rates also were examined. Fourteen days of ad libitum consumption of 16% ethanol resulted in maximal defluorination rates of the above anesthetics. No overt signs of ethanol toxicity were observed. Ethanol-treated rats with a normal caloric intake had significantly increased microsomal defluorination rates per mg protein compared with pair-fed control rats as follows: methoxyflurane, 190% of control; enflurane, 298% of control; and sevoflurane, 301% of control. Ethanol-treated animals with 50% of normal caloric intake showed similar elevations in microsomal defluorination rates when compared with pair-fed controls. Phenobarbital treatment significantly increased the rate of methoxyflurane defluorination (673% of control), whereas the rates of sevoflurane defluorination (127% of control) and enflurane defluorination (86% of control) were not altered significantly. Phenobarbital treatment increased the microsomal content of cytochrome P-450, while ethanol treatment did not. This study demonstrated that regardless of total caloric intake, chronic ethanol consumption increases defluorination of inhalation anesthetics in Fischer 344 rats. It also illustrated that the two enzyme-inducing agents are unique with respect to the degree to which they enhance anesthetic defluorination.

Enflurane, isoflurane, and methoxyflurane metabolism in rat hepatic microsomes from ethanol-treated animals

The effects of ethanol on the metabolism of enflurane, isoflurane, and methoxyflurane were investigated to determine if alterations in biotransformation of these agents occur as a result of this treatment. In vitro incubations of hepatic microsomes from rats pretreated with 10 days' ethanol vapor inhalation revealed a fourfold increase in inorganic fluoride from enflurane when compared with incubations of microsomes from unpretreated rats and from phenobarbital-pretreated rats. Isoflurane, while metabolized to a lesser extent than enflurane, showed a similar stimulation of metabolism. Methoxyflurane, while metabolized to a greater extent than either enflurane or isoflurane, had lesser fluoride production by the microsomes from ethanol-pretreated rats than microsomes from phenobarbital-pretreated rats, but greater fluoride production than that found in microsomes from unpretreated rats. Ethanol pretreatment did not alter the levels of cytochrome P-450 which is the enzyme responsible for such metabolism. This suggests that the altered metabolism involves either a specific P-450 isozyme or an unidentified enzyme.

Intravenous benzodiazepines as anaesthetic agents: pharmacokinetics and clinical consequences

Despite extensive and numerous pharmacokinetic studies on benzodiazepines, the published pharmacokinetic data do not adequately explain the clinical differences found between different benzodiazepine derivatives after intravenous administration. Especially, correlations between initial drug responses and distributional changes of the benzodiazepines are limited. However, during the elimination phase some relationships exist between the kinetic and dynamic phenomena. Age, sex, diseases and concomitantly given drugs cause clinically important alterations in the pharmacokinetics of benzodiazepines. Generally these anxiolytics and sedatives should be considered as adjuvants to general anaesthesia, but not primarily as routine induction agents. The major reasons for this limitation are a high variability in drug response, a relatively slow onset of action and long-lasting residual effects. However, benzodiazepines have many important advantages (see Table 5) when used as intravenous inducing agents of general anaesthesia.

Limitations of the sodium fusion assay for fluorinated metabolites

A method involving sodium fusion and analysis by fluoride electrode, which has been used for the determination of total fluorinated metabolites of fluorine-containing drugs in physiologic fluids, is shown to be inapplicable to volatile fluorinated metabolites of volatile fluorinated anesthetic agents. The yields of inorganic fluoride from the volatile metabolites 2,2,2-trifluoroethanol and trifluoroacetaldehyde were less than 2% of expected values, whether these metabolites were present in water, buffer, urine, or hepatic microsomes, although reliable results were obtained for the relatively nonvolatile sodium trifluoroacetate. It is proposed that the assay does not provide a valid method for determining total fluorinated metabolites of volatile or nonvolatile fluorine-containing drugs, but may be of some use in determining a single nonvolatile fluorinated metabolite of a volatile fluorinated drug.

Pharmacokinetics of inhalation anesthetics: a three-compartment linear model

The evolution of mathematical models of the uptake of the inhaled anesthetic agents has produced increasingly complex models in which researchers have attempted to incorporate more and more data on the effects of anesthetics on the processes of respiration, circulation, and metabolism. One result of this evolution has been to limit the application of these models due to the large amount of data required by the model and the need for a large digital computer to generate a solution. The purpose of this study is to show that a three-compartment linear model, using only the solubility of an anesthetic in water and oil, may br used to predict the uptake of a volatile anesthetic with sufficient accuracy for practical purposes. Only a programmable hand calculator is needed for the solution. Due to the simplicity of this model, compared with previously described models, it should prove useful in understanding the kinetics of gas uptake by the body.

The pharmacokinetics and pharmacodynamics of minaxolone

Plasma concentrations of minaxolone were measured in 15 female patients during and for up to 3 hours after a minaxolone and nitrous oxide anesthetic. Nine patients received a single dose and six patients two or three doses of minaxolone. Plasma minaxolone decay can be described by two-compartment kinetics. Distribution is rapid, with a mean half-life of 2.1 minutes, and the elimination half-life is short (47 minutes). Plasma clearance is high (1.55 l./min). Plasma levels of minaxolone at recovery were similar in patients receiving both single and multiple doses, suggesting a valid relationship between plasma level and effect. It is suggested that minaxolone may be a suitable agent for administration by continuous infusion.

[Principles of clinical pharmacokinetics in anaesthesiology (author's transl)]

The principles of clinical pharmacokinetics of intravenous and inhalation anaesthesia and the correlation with pharmacodynamics are reviewed with special reference to the commonly used anaesthetic agents. Pharmacokinetics is regarded mainly as an aid towards optimization of dosage. Dosage rules based on pharmacokinetic principles, for repeated administration and infusion are outlined for the induction, maintenance and termination of anaesthesia. Interindividual variations and biological disposition are not taken into account. The kinetic processes common to intravenous and inhalation anaesthesia are presented in the form of a multiexponential function for constant concentrations.

[Mechanism of action of intravenous regional anesthesia]

It is considered that the anesthetic solution administered according to the regional intravenous anesthesia method will be non-uniformly dispersed due to the valvular system of the veins, the competence and disposition of which varies from one individual to the other. The anesthetic solution will progress to the level of venous capillaries, but will penetrate the tissues of the blood-deprived segment in variable concentrations. The anesthetic will pass from the venous network to the extravascular space in large amounts and this is facilitated by the vasodilatation, increased vascular permeability, and the increased number of functional capillaries. The anesthetic drug when present at the level of the cells will exert its action on the cellular membrane by a process which is facilitated by low pH and hypercapnia decreasing excitability and conduction in the nerves. The anesthetic solution will exert its action both at the level of the trunk and at the level of the peripheral nerve endings. However, the major effects occur at the periphery, while trunk blockade is secondary and mostly due to consecutive ischaemic modifications.

The partial molar volumes of anesthetics in lipid bilayers

The excess volumes of mixing of benzyl alcohol, halothane, and methoxyflurane in water and in suspensions of several lipid bilayers have been determined at 25 degrees C using a novel excess volume dilatometer. The excess volumes of mixing in water were all found to be negative, whereas in lipid suspensions they were all more positive than those in water alone. From known partition coefficients the partial molar volumes of these three solutes in the lipid bilayers were calculated. These values were all close to the molar volumes of the pure anesthetics, as was a value determined for halothane in olive oil. Halothane was studied in dipalmitoylphosphatidylcholine below its phase transition, and was found to exhibit a much larger excess volume than in any other system we studied. The potency of these three anesthetics was determined in tadpoles. It was calculated that at equi-anesthetic doses these three agents caused an expansion in egg lecithin/cholesterol (2:1) bilayers of 0.21 +/- 0.015%. This result is consistent with the hypothesis that general anesthetics act by expanding membranes.

Partition equilibrium of inhalation anesthetics and alcohols between water and membranes of phospholipids with varying acyl chain-lengths

From the depression of the phase-transition temperature of phospholipid membranes, the partition coefficients of inhalation anesthetics (methoxyflurane, halothane, enflurane, chloroform and diethyl ether) and alcohols (benzyl alcohol and homologous n-alcohols up to C = 7) between phospholipid vesicle membranes and water were determined. The phospholipids used were dimyristoyl-, dipalmitoyl- and distearoylphosphatidylcholines. It was found that the difference in the acyl chain length of the three phospholipids did not affect the partition coefficients of the inhalation anesthetics and benzyl alcohol. The actions of these drugs are apparently directed mainly to the interfacial region. In contrast, n-alcohols tend to bind more tightly to the phospholipid vesicles with longer acyl chains. The absolute values of the transfer free energies of n-alcohols increased with the increase of the length of the alkyl chain of the alcohols. The increment was 3.43 kJ per each carbon atom. The numerical values of the partition coefficients are not identical when different expressions for solute concentrations (mole fraction, molality and molarity) are employed. The conversion factors among these values were estimated from the molecular weights and the partial molal volumes of the phospholipids in aqueous solution determined by oscillation densimetry.