Selective effect of althesin on the auditory evoked response in man

The auditory evoked response (AER) has been studied in six patients before the induction of general anaesthesia, during anaesthesia with nitrous oxide in oxygen and mechanical ventilation, then with a stepwise increasing rate of infusion of Althesin ranging from 18 micrograms kg-1 min 1 to 90 micrograms kg-1 min-1. The sections of the AER examined in this study were the brainstem waves I, III and V and the early cortical waves Pa and Nb. There were dose-related changes in latency and amplitude of waves Pa and Nb, latency increasing and amplitude diminishing with increasing concentrations of alphaxalone as measured in venous blood. In contrast, there were no changes in either latency or amplitude of the brainstem waves. This is consistent with other work suggesting that Althesin has little effect on neuronal function below the level of the superior colliculus.

Age and the solubility of volatile anesthetics in ovine tissues

To determine the effect of age on the solubility of volatile anesthetics in tissues, we measured the blood/gas and tissue/gas partition coefficients of isoflurane, enflurane, halothane, and methoxyflurane in vitro at 37 degrees C in newborn lambs and postpartum adult sheep. The tissue specimens examined were brain, heart, liver, kidney, muscle, and fat. Hematocrit and serum concentrations of albumin, globulin, cholesterol, and triglycerides were measured. The blood/gas partition coefficients, hematocrit, and the serum concentrations of albumin, globulin, cholesterol, and triglycerides in the newborn lambs did not differ from those in the adult sheep. The tissue/blood partition coefficients [the ratio of (tissue/gas)/(blood/gas)] in newborn lambs were 28% [mean value for the four anesthetics] less than those in the adults. The tissue/blood partition coefficients for enflurane and methoxyflurane in newborn tissues were significantly less (P less than 0.05) than those for halothane and isoflurane. We conclude that the blood/gas partition coefficients in sheep do not change significantly with age, and that the time required for equilibration of volatile anesthetics (particularly enflurane and methoxyflurane) in newborn tissues is probably less than in adult sheep.

An electroencephalographic comparison of alfentanil with other narcotics and with thiopental

Using aperiodic analysis, we compared the EEG produced by alfentanil with the EEGs produced by two other opiates--fentanyl and sufentanil--on the one hand and with the EEG produced by a barbiturate--thiopental--on the other hand. Alfentanil and thiopental were injected over 1 minute; fentanyl and sufentanil were injected over 10 to 15 minutes. From the aperiodic analysis we derived up to seven single-number variables computed over 30- or 60-second epochs. All the opiates induced EEGs that were qualitatively similar to each other, although the maximum or minimum values tended to be greater and the time course more rapid with alfentanil than with the other two opiates. This finding may have been related to the fact that we injected relatively more alfentanil and administered it more rapidly. The EEGs produced by alfentanil and thiopental differed markedly, both qualitatively and quantitatively. The total power at 1 Hz and cumulative power at 3 Hz went to higher peak values with alfentanil, the latter tending to decrease with thiopental. The total number of waves per epoch went to lower peak values with alfentanil; there was little change with thiopental. The frequency below which 90% of the power resides went to considerably lower peak values with alfentanil than with thiopental. Finally, total power at 10 to 12 Hz (alpha waves) and average power at 17 to 19 Hz (beta waves) went to very high peak values with thiopental, but decreased with alfentanil.(ABSTRACT TRUNCATED AT 250 WORDS)

Age and nature of operation influence the pharmacokinetics of midazolam

The pharmacokinetics of midazolam were studied in surgical patients given 0.3 mg kg-1 i.v. for either the induction of anaesthesia, or postoperative sedation following cardiopulmonary bypass. The short elimination half-life of midazolam (2.4 h in patients less than 50 yr undergoing minor surgery) was significantly, although not markedly, prolonged with age (4.1 h in patients greater than 50 yr undergoing minor procedures) and by the nature of the operation (3.8 h after major operative procedures). These changes were the result of alterations in clearance and volume of distribution with age, and in volume of distribution with nature of operation.

Quantitative analysis of common anaesthetic agents

A summary of chromatographic (e.g. gas chromatography, high-performance liquid chromatography) and non-chromatographic (e.g. radioimmunoassay, enzyme-multiplied immunoassay) analytical techniques suitable for the quantitative analysis of the most popular inhalational (halothane, methoxyflurane, enflurane, isoflurane and nitrous oxide), intravenous (barbiturate, benzodiazepines, etomidate, althesin, morphine, fentanyl, alfentanil, sufentanil, droperidol and ketamine) general and amide-type local (lidocaine, mepivacaine, etidocaine and bupivacaine) anaesthetic agents and some of their metabolites, in biological material, is described. In the case of inhalational anaesthetics attention is also payed to pollution measurement and breath-to-breath monitoring.

Use of a continuous infusion of althesin in neuroanaesthesia. Changes in cerebral blood flow, cerebral metabolism, the EEG and plasma alphaxalone concentration

Ten patients with small supratentorial tumours were studied during craniotomy. Cerebral blood flow (CBF) was measured in the contralateral hemisphere by a modification of the Kety and Schmidt technique using xenon-133 i.v. With an Althesin infusion rate of 0.2 ml kg-1 h-1, CBF was 24.4 +/- 5.4 ml min-1/100 g and CMRO2 1.87 +/- 0.44 ml min-1/100 g at PaCO2 4.1 +/- 0.7 kPa (mean +/- SD). During constant infusion rates of Althesin, steady values of CMRO2 were obtained, while an increase in infusion rate of 150% was associated with an increase in plasma alphaxalone concentration, a decrease in CMRO2 and a tendency of further EEG suppression. However, great inter- and intraindividual variations were present, and correlations between CMRO2, EEG activity and plasma alphaxalone concentration were weak.

Hematocrit and the solubility of volatile anesthetics in blood

To clarify the effect of hematocrit on the solubility of volatile anesthetics in blood, we measured the blood-gas partition coefficients of isoflurane, enflurane, halothane, and methoxyflurane concurrently at 37 degrees C in blood from four adults. We measured the blood-gas partition coefficients in the plasma (hematocrit 0%) and packed red cell fractions (hematocrit 80%), and in four mixtures of these two fractions (hematocrits 10%, 25%, 40%, and 55%). The mixtures were prepared by recombining appropriate amounts of plasma and packed red cells from each adult. As hematocrit increased, the blood-gas partition coefficient of isoflurane decreased linearly (P less than 0.01), whereas that of enflurane increased linearly (P less than 0.05). The partition coefficient for isoflurane in plasma was 20% greater than that in packed red cells, whereas the partition coefficient for enflurane in plasma was 10% less than that in packed cells. The blood-gas partition coefficients of halothane and methoxyflurane did not change significantly between measurements in plasma and packed red cells. We conclude that hematocrit exerts a statistically significant effect on the blood-gas partition coefficient of isoflurane and enflurane.

Accumulation of midazolam after repeated dosage in patients receiving mechanical ventilation in an intensive care unit

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Age and solubility of volatile anesthetics in blood

The more rapid rate of rise of alveolar anesthetic partial pressure in children compared with adults may be explained in part by an increasing solubility of volatile anesthetics in blood with age. To investigate this possibility, the authors measured the blood-gas partition coefficients of isoflurane, enflurane, halothane, and methoxyflurane in four groups of fasting subjects: 10 full-term newborns (at delivery), 11 children (3-7 years old), 11 adults (20-40 years old), and 10 elderly adults (75-85 years old). The blood-gas partition coefficients were greatest in adults: isoflurane 1.46, enflurane 2.07, halothane 2.65, and methoxyflurane 16.0; and least in newborns: 1.19, 1.78, 2.14, 13.3, respectively. The blood-gas partition coefficients in children (1.28, 1.78, 2.39, 15.0, respectively), which were intermediate between those in newborns (P less than 0.005) and those in adults (P less than 0.005), were not significantly different from those in elderly adults (1.29, 1.79, 2.41, 15.0, respectively). The blood-gas partition coefficients of both isoflurane and enflurane correlated directly with the serum albumin and triglyceride concentrations; that of halothane correlated directly with the serum cholesterol, albumin, triglyceride, and globulin concentrations; and that of methoxyflurane correlated directly with the serum cholesterol, albumin, and globulin concentrations. The authors conclude that age significantly affects blood-gas partition coefficients, and the lower blood-gas partition coefficients in children explain in part the more rapid rise of alveolar anesthetic partial pressure in this age group.

Placental transfer of midazolam and its metabolite 1-hydroxymethylmidazolam in the pregnant ewe

Plasma concentrations of midazolam and its metabolites were measured in maternal and fetal blood after intravenous administration of 90 mg of midazolam to seven pregnant ewes in whom sampling catheters had been chronically implanted. Midazolam and its metabolite 1-hydroxymethylmidazolam crossed the placenta of the pregnant ewe. The fetal/maternal plasma concentration ratio of midazolam and the metabolite was 0.15. The distribution and elimination half-lives of midazolam and the metabolite were identical in both maternal and fetal circulations. The major compound found in the urine was the metabolite 1-hydroxymethylmidazolam glucuronide.

Influence of sample site on blood concentrations of ICI 35868

Simultaneous blood samples from an artery, a peripheral vein and a central vein were analysed for ICI 35868 concentrations following an induction dose of 2.0 mg kg-1 administered i.v. over 60 s, to five patients before cardiac surgery. Up to 60 s after the end of the administration of the drug there were wide differences in drug concentration between the sampling sites. Thus, any attempt to correlate effect with blood concentration over this early period would be problematic. From 60 s there were no significant differences in drug concentration between the three sites. Thus, as long as the mixing period is allowed for, peripheral venous sampling provides an acceptable alternative to arterial puncture in studies to correlate drug effect and concentration and for pharmacokinetic investigations.

Blood solubility of isoflurane measured by a multiple gas phase equilibration technique

Using a multiple gas phase equilibration technique, we determined the solubility of isoflurane in arterial blood at 37 degrees C obtained from patients anaesthetized with isoflurane. Goodness of fit of individual sets of data to a predicted decline of isoflurane concentration in successive equilibrated gas phases and reproducibility of results indicate that the analytical technique was acceptably accurate. In blood samples taken from twenty-three patients undergoing minor surgical procedures, the blood/gas partition coefficient of isoflurane was 1.20 +/- 0.08 (mean +/- S.D.). Individual values were not detectably related to absolute tension of isoflurane, preoperative haemoglobin concentration nor patient age. In samples taken from twenty neurosurgical patients, the blood/gas partition coefficient was slightly less, 1.14 +/- 0.09 (p less than 0.05), a difference we attribute to the effect of previously infused mannitol. In vitro, blood solubility of isoflurane was decreased by haemodilution with normal saline, mannitol 20 per cent or plasma, was increased by hypothermia (30 degrees C) and was decreased by hyperthermia (40 degrees C). We conclude that the solubility of isoflurane in human blood is approximately half that of halothane.

Pharmacokinetic evaluation of ICI 35 868 in man. Single induction doses with different rates of injection

Blood concentrations of ICI 35 868 have been measured in patients following a single bolus dose of 2 mg kg-1. Three different rates of injection of the anaesthetic agent (3-5s, 20s and 40 or 50s) were examined. Pharmacokinetic indices, derived from blood concentrations of ICI 35 868, were independent of the speed of injection. The blood profiles could be described by a two-compartment open model with a mean alpha-phase half-life of 2.5 min and a mean beta-phase half-life of 54.5 min. The mean total body clearance was 3454 ml min-1. Similar data were obtained from a 4-mg kg-1 dose. The mean recovery time (4.4 min) and concentration of ICI 35 868 at awakening (1.05 micrograms ml-1) were also independent of the rate of injection. Using the derived pharmacokinetic model, predictions of drug concentrations have been made for repeated bolus doses, or infusions, of ICI 35 868.

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.

Comparison of the cerebral function monitor and plasma concentrations of thiopentone and alphaxalone during total i.v. anaesthesia with repeated bolus doses of thiopentone and althesin

Repeated bolus doses of thiopentone or Althesin were administered to 10 patients every 240 s while cerebral electrical activity was recorded with the Cerebral Function Monitor (CFM). Peripheral venous blood samples were collected at 60 and 225 s after each bolus dose for the measurement of plasma concentrations of the drugs. Significant correlations in the range r = 0.56-0.96 (P = 0.02-0.00001) between plasma thiopentone or alphaxalone concentrations and the upper and lower edges of the CFM trace were found. For the patients with relatively poor correlations, better correlations were obtained when 60- and 225-s sample were analysed separately.

Blood disoprofol levels in anesthetised patients. Correlation of concentrations after single or repeated doses with hypnotic activity

The blood concentrations of disoprofol (Diprivan) after single intravenous doses of 1, 2 or 3 mg/kg have been examined in a subpopulation from previously reported clinical studies. The linear relationship between sleep time and dose could be explained by the linearity of the pharmacokinetics at these doses. After a single injection the awakening concentration was independent of dose, with a mean value of 1.04 micrograms/ml. No acute tolerance occurred with disoprofol. On repeated 1 mg/kg bolus injections the sleeping time rose initially but stabilised after four doses. The waking concentration was independent of the number of doses administered. The clinical findings fitted an agent with a very rapid distribution phase and a short elimination half-life.

[A microprocessor controlled infusion scheme for midazolam to achieve constant plasma levels (author's transl)]

In seven healthy volunteers a microprocessor controlled Infusion scheme for midazolam was established on the basis of pharmacokinetic analysis. The dosage regimen is composed of three different parts (BET-infusion): 1. a bolus initially injected for filling the central compartment 2. the elimination rate compensating the drug loss by elimination and 3. the transfer rate exponentially declining and balancing midazolam transfer to the peripheral compartment. --Thus, the amount of drug necessary for well-defined plasma levels can be minimized compared with other optimized dosage regimens like repetitive boluses or combinations of different infusion rates. By using this BET-infusion technique we were able to attain nearly constant therapeutic plasma levels from the beginning on (without initial overshoot). Unwanted cardiovascular and respiratory side effects could be prevented. The median of EEG-power spectra was used for monitoring EEG-background activity and for the correlation of plasma levels and pharmacological effect. The minimal plasma level producing a deep hypnotic effect with an EEG-median of less than 5 s-1 was about 0.5 microgram/ml. On the basis of these findings several modes of dosage are presented obtaining at least therapeutic midazolam concentrations of 0.6 microgram/ml for a secure hypnotic effect.