Automated gas chromatography for studies of midazolam pharmacokinetics

Plasma midazolam concentrations following single therapeutic doses can be quantitated using electron-capture gas-liquid chromatography. After addition of a benzodiazepine analogue as an internal standard, samples are extracted with benzeneisoamyl alcohol (98.5:1.5). The organic extract is evaporated to dryness, reconstituted, and chromatographed on an SP-2250 liquid phase. The automatic sampler allows up to 100 chromatographic analyses per 24-h period. The sensitivity limits are 2-3 ng of midazolam per ml of plasma, and the variation of identical samples is 7 per cent or less. The method was used in a study of six females who received single intravenous doses of midazolam for purposes of anesthetic induction prior to minor gynecologic procedures. Mean (+/- SE) kinetic variables for midazolam were: volume of central compartment, 0.37 (+/ 0.06 l/kg; total volume of distribution, 1.72 (+/- 0.05) l/kg: initial distribution half-life, 7.2 (+/- 1.6) min; elimination half-life, 2.5 (+/- 0.2) h; total clearance, 8.1 (+/- 0.52) ml.min-1.kg-1.

Pharmacodynamics of minaxolone, a new steroidal anesthetic

Studies were designed to examine blood concentration-response relationships for minaxolone administered to sheep by either i.v. infusion or bolus injection in both clinical doses and overdoses. Responses examined included effects on sensory nervous, cardiovascular and respiratory systems. The ratio of fatal blood concentrations to anesthetic blood concentrations (therapeutic ratio) was 3 to 4 without respiratory support other than endotracheal intubation to maintain a patent airway. Minaxolone consistently depressed respiration rate, raised PACO2 and depressed PAO2. Death resulted from respiratory failure. With intermittent positive pressure ventilation, the therapeutic ratio was raised to 30 to 40. Duration of anesthesia after bolus doses was directly related to the logarithm of the dose. Cardiovascular effects were minimal with clinical doses and mild overdoses and included slight tachycardia, slightly elevated blood pressure and increased total peripheral resistance with little or no change in cardiac output. With gross overdoses, stable hypotension and depressed cardiac output resulting from decreased stroke volume occurred. Its excellent anesthetic properties and lack of deleterious effects at normal doses on the cardiovascular system commend minaxolone for further evaluation.

Infusions of minaxolone to supplement nitrous oxide-oxygen anaesthesia. A comparison with althesin

In 19 patients, minaxolone citrate, a water-soluble steroid anaesthetic agent, has been used as a continuous infusion to supplement nitrous oxide anaesthesia. The minimum infusion rate (MIR) was found to be 11.3 micrograms kg-1 min-1 for patients premedicated with morphine 10 mg i.m. and breathing spontaneously 66% nitrous oxide in oxygen. The cardiovascular and respiratory effects were similar to those described in a previous study for Althesin administered under comparable conditions. Recovery from minaxolone was prolonged compared with Althesin, and this may be related both to the water-solubility of the drug, and to its greater apparent volume of distribution.

Pharmacokinetics in laboratory animals of ICI 35 868, a new i.v. anaesthetic agent

Blood concentrations of ICI 35 868 have been determined in rat, pig, rabbit and cat after single i.v. injections. In all species the initial distribution volume was greater than blood volume and the overall distribution volume was large. Half-lives of the distribution phase were extremely short (1-6 min) and the terminal half-lives were also short (16-55 min). In all species examined a correlation existed between the systemic blood concentration of ICI 35 868 and duration of sleep, with concentrations in the range 1-4 micrograms ml-1 being effective in producing unconsciousness. No changes in pharmacokinetics or in the effective concentration occurred on repeated administration or after infusion.

Routine direct injection gas--liquid chromatographic procedure for the analysis of volatile halogenated anaesthetics in whole blood using a new external injection port

This communication describes the design and construction of a new external injection port for the direct gas--liquid chromatographic analysis of volatile compounds in whole blood. Aliquots (4--40 microliter) of EDTA anti-coagulated blood containing the volatile compound and a weighed quantity of the internal standard, isobutanol, were injected into the disposable glass wool filter of the carrier gas stream directly onto the chromatographic column. Typical data are presented from chromatography performed with dual 6 ft. x 2 mm I.D. glass columns containing Chromosorb 101 programmed from 110--180 degrees at 6 degrees/min and the external injection port maintained at 180 degrees. The method eliminated the problems usually associated with direct injection methods and permitted the accurate analysis of halothane, methoxyflurane, diethyl ether and ethanol over the approximate range 1--100 mg%. Using this analytical procedure the distribution of halothane between the cells and plasma of human blood at 4 degrees was found to be 2.0 +/- 0.2.

[The closed system (author's transl)]

It is appropriate to consider dose-regulated or quantitative closed-circuit anesthesia in terms of uptake of inhalation anesthetics from a reservoir bag which is never allowed to fill completely. The method which the authors have used routinely for 8 years requires reduction of the dose as a function of the inverse square root of elapsed anaesthesia time. Nitrogen does not accumulate because there is an inevitable loss of 20 to 100 ml/min between cuff and trachea. A simpler method, which has been well received, is that of "minimal flow" anesthesia (Virtue) with a constant fresh gas flow of 300 ml/min O2 and 200 ml/min N2O. "Low-flow" techniques typically entail use of 500 ml each of O2 and N2O per min.

The pharmacokinetics of metabolism of inhalation anaesthetics. A simulation study

The non-steady state metabolism of inhalation anaesthetics during anaesthesia and recovery was simulated with a linear and non-linear whole body compartmental model. These two models were studied for nine anaesthetics during 1-MAC anaesthesias lasting 1, 4 and 8 h and recovery from 5 to 10 days. Both models demonstrated significant metabolism for several days following anaesthesia. For the linear model, both the percentage and the molar quantity of anaesthetic metabolized increased with increased duration of anaesthesia, increased anaesthetic fat solubility and increased assumed rate of hepatic metabolism. For the non-linear model, the duration of anaesthesia had little effect on the percentage metabolized but demonstrated increased molar quantities of anaesthetic metabolized with increased duration of anaesthesia and increased fat solubility. The agreement between the results obtained from the non-linear model and experimental data in the literature suggests that many inhalation anaesthetics belong to a class of xenobiotics whose biotransformation is limited by the same or similar non-linear rate-limiting step(s). A difference in the quantities of anaesthetic metabolized would be a direct consequence of the tissue solubilities of the anaesthetic.

Interaction of general anesthetics with phospholipid vesicles and biological membranes

Low concentrations of general anesthetics, including halothane, ethrane, trilene, diethyl ether and chloroform are observed to shift the phase transitions of phospholipid vesicles to lower temperatures, and from these data partition coefficients for the anesthetic between lipid and water can be calculated. In contrast to the anesthetics, high concentrations of ethanol are required to shift the phase transition of lipids and glycerol causes no effect. Above the phase transition general anesthetics alter nuclear magnetic resonance spectra of phospholipid dispersions and increase the rotational and lateral diffusion rates of fluorescent probes located in the hydrocarbon core of the bilayer, indicating that they induce disorder in the structure. In red blood cell membranes and sarcoplasmic reticulum fragments, the rotational diffusion rate of 1-phenyl-6-phenylhexatriene is increased in the presence of general anesthetics. The 220 MHz nuclear magnetic resonance spectra of sarcoplasmic reticulum reveal some resolved lines from the lecithin fatty acid protons; addition of general anesthetic increases the contribution of these peaks. The data from the NMR and fluorescence techniques lead to the conclusion that general anesthetics increase the pool size of melted lipids in the bimolecular phospholipid layers of biological membranes; this would account for the ability of general anesthetics to increase passive diffusion rates of various substances in membranes.

A latex bag tonometer for determining the anaesthetic blood/gas partition coefficient

Many techniques for measurement of the tissue/gas partition coefficient are valid. Each has specific advantages and shortcomings. All the methods require suitable analytic equipment and means for controlling temperature. The disadvantages of a particular technique, the ability to cope with them and the availability of the necessary apparatus are the determining factors in choosing the best method. The latex bag tonometer has a precision and ease which compares favourably with previously described techniques for measuring anaesthetic blood/gas partition coefficients. The technique was successfully used to show the effect of in vivo haemodilution on the halothane blood/gas partition coefficient.

A method for the gas chromatographic analysis of inhalation anaesthetics in whole blood by direct injection into a simple precolumn device

A simple heated precolumn device constructed from standard gas chromatographic components is described and evaluated. The device permits the analysis of concentrations of anaesthetic agents by direct injection of whole blood containing a suitable internal standard. A method is described for the analysis of halothane, chloroform, trichloroethylene, and methoxyflurane. The procedure is rapid and of acceptable accuracy.

Minimum blood concentration--a universal concept of anaesthetic potency

Although brain concentration of anaesthetic agents is attractive as a method of expressing anaesthetic potency, both theoretical and practical considerations limit the usefulness of this index. Therefore, the MAC concept has been utilized to develop another expression of potency: the minimum blood concentration. This takes into account the blood/gas solubility coefficient, and the fact that at equilibrium the amount of anaesthetic agent present in brain is reflected in the blood concentration, and it is applicable to both parenteral and to inhalation anaesthetic agents.

An evaluation of the Lex-O2-Con oxygen content analyser

The accuracy of the Lex-O2-Con apparatus is compared with the classical method of Van Slyke & Neill (1924) and the inaccuracies introduced by the presence of volatile anaesthetics in the sample are evaluated. The absolute levels of oxygen content depend upon the flow rate of carrier gas through the instrument.