Pharmacokinetics of methohexitone following intravenous infusion in humans

Therapeutic Drug Monitoring of Pentobarbital: Experience at an Academic Medical Center

BACKGROUND

Pentobarbital is used for management of intractable seizures and for reducing elevated intracranial pressure. Dosing of pentobarbital can be aided by therapeutic drug monitoring (TDM). There is no commercially available automated assay for measurement of pentobarbital serum/plasma concentrations; consequently, chromatography-based assays are often used.

METHODS

Pentobarbital TDM was studied over a 14-year period at an academic medical center. 154 patients (94 adult, 60 pediatric) were identified who had pentobarbital levels ordered at least once during a hospital encounter. Chart review included patient diagnosis, indication for pentobarbital therapy, recent or concomitant medication with other barbiturates, patient disposition, organ donation, pentobarbital dosing changes, and neurosurgical procedures. Pentobarbital serum/plasma concentrations were determined on an automated clinical chemistry platform with a laboratory-developed test adapted from a urine barbiturates immunoassay.

RESULTS

Chart review showed therapeutic use of pentobarbital generally consistent with previously published literature. The most common errors observed involved confusion in barbiturate names (eg, mix-up of pentobarbital and phenobarbital in test ordering or in provider notes) that seemed to have minimal impact on TDM effectiveness, with pentobarbital serum/plasma concentrations generally within target ranges. The laboratory-developed pentobarbital immunoassay showed cross-reactivity with phenobarbital and butalbital that was eliminated by alkaline and heat pretreatment. The immunoassay was linear to 20 mcg/mL and correlated closely with gas chromatography-mass spectrometry measurements at a reference laboratory.

CONCLUSIONS

Pentobarbital TDM can be performed by immunoassay on an automated clinical chemistry platform, providing an alternative to chromatography-based methods. Confusion in barbiturate names is common, especially pentobarbital and phenobarbital.

Post-electroconvulsive therapy recovery and reorientation time with methohexital and ketamine: a randomized, longitudinal, crossover design trial

OBJECTIVES

Methohexital, a barbiturate anesthetic commonly used for electroconvulsive therapy (ECT), possesses dose-dependent anticonvulsant properties, and its use can interfere with effective seizure therapy in patients with high seizure thresholds. Ketamine, an N-methyl-d-aspartate antagonist with epileptogenic properties not broadly used for ECT inductions, is a commonly used induction agent for general anesthesia. Recent studies suggest that the use of ketamine is effective in allowing successful ECT treatment in patients with high seizure thresholds without an increase in adverse effects. In this preliminary study, we directly compared the recovery and reorientation times of subjects receiving ketamine and methohexital for ECTs.

METHODS

Twenty patients were randomized in a crossover design to receive methohexital and ketamine for ECT inductions in alternating fashion in 6 trials. Primary outcome measures were recovery time (voluntary movement, respiratory effort, blood pressure, consciousness, and O2 saturation) and reorientation time. Secondary outcome measures were individual recovery variables, adverse effect occurrence, and seizure duration.

RESULTS

Overall recovery time was not significantly different between the 2 treatment arms (F(1, 17) = 0.72; P = 0.41). Reorientation time was faster in the methohexital arm (F(1, 17) = 9.23; P = 0.007).

CONCLUSION

Ketamine inductions resulted in higher number of adverse effects, higher subject dropout rates, and a longer reorientation time with respect to methohexital inductions. No significant difference in postanesthesia recovery time was found between the ketamine and methohexital arms. Intolerability to ketamine affected a significant proportion of subjects and suggests that ketamine should remain as an alternative or adjunctive agent for patients with high seizure thresholds.

Methohexital in total intravenous anesthesia during intraoperative neurophysiological monitoring

Total intravenous anesthesia (TIVA) is usually recommended during spinal surgery when transcranial motor evoked potentials (tcMEPs) are used to monitor. A shortage of propofol has prompted a search for an alternative sedative-hypnotic agent. We explored the use of methohexital as an alternative. TIVA was provided for two adult patients having spinal surgery using an infusion of methohexital. TcMEPs and somatosensory evoked potentials were acquired to monitor neurological function and electroencephalogram was used to titrate the methohexital dose. Two cases are presented in which the anesthesia and monitoring that was provided were successful. These cases indicate that methohexital can be a suitable alternative to propofol in some patients.

Transient improvement of essential tremor during electroconvulsive therapy

We present a patient who demonstrated transient yet significant improvement of essential tremor (ET) during electroconvulsive therapy. To our knowledge, this is the first such report. The improvement lasted during the course of electroconvulsive therapy and was of a similar magnitude to that which she had experienced on a first-line medication for ET. We discuss the potential pathophysiological implications of this observation in light of recent histopathologic findings in patients with ET.

Mechanistic Approaches to Volume of Distribution Predictions: Understanding the Processes

PURPOSE

To use recently developed mechanistic equations to predict tissue-to-plasma water partition coefficients (Kpus), apply these predictions to whole body unbound volume of distribution at steady state (Vu(ss)) determinations, and explain the differences in the extent of drug distribution both within and across the various compound classes.

MATERIALS AND METHODS

Vu(ss) values were predicted for 92 structurally diverse compounds in rats and 140 in humans by two approaches. The first approach incorporated Kpu values predicted for 13 tissues whereas the second was restricted to muscle.

RESULTS

The prediction accuracy was good for both approaches in rats and humans, with 64-78% and 82-92% of the predicted Vu(ss) values agreeing with in vivo data to within factors of +/-2 and 3, respectively.

CONCLUSIONS

Generic distribution processes were identified as lipid partitioning and dissolution where the former is higher for lipophilic unionised drugs. In addition, electrostatic interactions with acidic phospholipids can predominate for ionised bases when affinities (reflected by binding to constituents within blood) are high. For acidic drugs albumin binding dominates when plasma protein binding is high. This ability to explain drug distribution and link it to physicochemical properties can help guide the compound selection process.

Evaluation of the human serum albumin column as a discovery screening tool for plasma protein binding

A total of 69 compounds with a variety of chemical structures were assayed using a human serum albumin column in combination with UV and mass spectrometric detection. A moderate correlation, R(2)=0.661, between the plasma protein binding, determined by traditional techniques of equilibrium dialysis or ultrafiltration, and chromatographic retention factor (k'/k'+1) was observed. Disparity between the regression line and numerous samples was observed across the entire range of plasma protein binding. Attempts to discriminate between compounds from the data set to achieve better correlation based physico-chemical properties were unsuccessful. Good agreement was observed for retention times obtained with UV detection with mobile phase containing phosphate buffer and mass spectrometric detection with mobile phase containing acetate buffer. Essentially identical data were obtained for compounds analyzed in singlet or cassette for minimally or highly bound (>90% bound) compounds. Analysis of cassettes containing compounds with plasma protein binding greater than 90% did not cause column overload, even at analyte concentrations up to 100 microg/ml. Diverse results were obtained when chromatographic retention was used to rank order various classes of compounds. Better correlation with ordering from known binding was obtained when a compound class contained a wide range of protein binding, in contrast to when compounds within a given class were all highly bound.

Elimination of methohexitone after long-term, high-dose infusion in patients with critically elevated intracranial pressure

OBJECTIVE

To determine the plasma elimination of methohexitone in patients with critically elevated intracranial pressure (ICP) who received the drug in high doses for several days.

DESIGN

Drug-monitoring study.

SETTING

Intensive care unit at a university hospital.

PATIENTS

Twelve intensive care unit patients with brain injuries who received methohexitone as a final therapeutic approach after routine therapy had proved to be insufficient in controlling critically elevated ICP.

MEASUREMENTS AND MAIN RESULTS

Plasma samples were taken during methohexitone infusion, before cessation, and in distinct, short increments after discontinuation of the infusion. Methohexitone was determined in plasma by reverse-phase high-pressure liquid chromatography and photometric detection. The median duration of infusion of methohexitone was 137 hrs (minimum, 27 hrs; maximum, 445 hrs), with a median infusion rate of 62.5 microg/kg/min (minimum, 22.5 microg/kg/min; maximum, 116.2 microg/kg/min). Plasma concentrations of methohexitone at burst suppression under concomitant analgesic sedation ranged from 1.6 to 17.3 microg/mL (median, 4.7 microg/mL). After cessation of methohexitone infusion, the decline of plasma concentrations followed a biexponential function. Clearance rates, volume of distribution at steady state, context-sensitive half-time, and initial and terminal elimination half-times were calculated. Pharmacokinetic data showed remarkable interindividual variability that could not be correlated to the infusion rate, to the duration of the infusion, or to obvious differences in physiology or the disease states of these patients. Even in patients with high plasma concentrations who received the drug for a considerable length of time, the initial decline in plasma concentration was exponential, indicating redistribution.

CONCLUSIONS

We conclude that the elimination kinetics of methohexitone after long-term, high-dose infusion in critically ill patients with brain injuries may favor the use of methohexitone over thiopentone for controlling critically elevated ICP by allowing for a more timely neurologic examination after cessation.

Effective management of spinal pain in one hundred seventy-seven patients evaluated for manipulation under anesthesia

OBJECTIVE

To demonstrate that manipulation under anesthesia (MUA), a conservative treatment modality, is both safe and efficacious in the treatment of both acute and chronic spinal pain disorders in appropriately selected patients. MUA can be safely used to treat pain arising from the cranial, cervical, thoracic, and lumbar spine, as well as the sacroiliac and pelvic region.

SETTING

An ambulatory surgical center.

SUBJECTS

The treatment group consisted of 177 patients between ages 17 and 65 years. Evaluation followed a treatment algorithm created by the authors as a multidisciplinary approach to patient selection, evaluation, treatment, and timing of specialized referral, in consideration of previously published algorithms. Prior forms of treatment, both conservative and surgical in nature, had failed in these patients.

INTERVENTION

Patients underwent three sequential manipulations under intravenous sedation, followed by 4 to 6 weeks of skilled spinal manipulation and therapeutic modalities.

OUTCOME MEASURES

Data regarding changes in Visual Analog Scale (VAS), range of motion, medication needs, and return to work status were used to document progress. All patients had follow-up for 6 months.

RESULTS

On average, VAS ratings improved by 62.2% in those patients with cervical pain problems. On average, VAS ratings improved by 60.1% in those patients with lumbar pain problems. There was a near-complete reversal in patients out of work before MUA (68.6%) and those returning to unrestricted activities at 6 months after MUA (64.1%). There was a 58.4% reduction in the percentage of patients requiring prescription pain medication from the pre-MUA period to 6 months after MUA. Additionally, 24.0% of the treatment group required no medication at 6 months after MUA.

CONCLUSION

A multidisciplinary approach to evaluation and treatment, including MUA, offers patient benefits above and beyond what can be obtained through the individual providers working alone.

The cost-effectiveness of methohexital versus propofol for sedation during monitored anesthesia care

We designed this study to test the hypothesis that methohexital is a cost-effective alternative to propofol for sedation during local anesthesia. Sixty consenting women undergoing breast biopsy procedures under local anesthesia were randomly assigned to receive an infusion of either propofol (50 microg x kg(-1) x min(-1)) or methohexital (40 microg x kg(-1) x min(-1)). The sedative infusion rate was titrated to maintain an observer's assessment of alertness/sedation (OAA/S) score of 3 (with 1 = awake/alert to 5 = asleep). Fentanyl 25 microg i.v. was administered as a "rescue" analgesic during the operation. We assessed the level of sedation (OAA/S score), vital signs, time to achieve an OAA/S score of 3 at the onset and a score of 1 after discontinuing the infusion, discharge times, perioperative side effects, and patient satisfaction. The direct cost of methohexital was lower than that of propofol, based on the milligram dosage infused during the operation. The sedative onset (to achieve an OAA/S score of 3) and the recovery (to return to an OAA/S score of 1) times, as well as discharge times, did not differ between the two groups. Patients receiving methohexital had a significantly lower incidence of pain on initial injection compared with those receiving propofol (10% vs 23%). Because the use of methohexital (29.4 +/- 2.7 microg x kg(-1) x min(-1)) for sedation during breast biopsy procedures has a similar efficacy and recovery profile to that of propofol (36.8 +/- 15.9 microg x kg(-1) x min(-1)) and is less costly based on the amount infused, it seems to be a cost-effective alternative to propofol for sedation during local anesthesia. However, when the cost of the drug infused and drug wasted was calculated, there was no difference in the overall drug cost.

IMPLICATIONS

When administered to maintain a stable level of sedation during local anesthesia, methohexital is an acceptable alternative to propofol. However, the overall drug costs were similar with the two drugs.

Inhibition of a fast inwardly rectifying potassium conductance by barbiturates

Whole cell voltage clamp recordings were used to study the effects of two barbiturates, methohexital and pentobarbital, on inwardly rectifying K+ currents in the plasma membrane of a rat basophilic granulocyte cell line (RBL-1). Inwardly rectifying K+ currents are responsible for maintaining the resting membrane potential in a variety of cell types including skeletal and cardiac muscle, neurons, glia, blood cells, and endothelial cells. RBL-1 cells are unusual because the inward rectifier is the only apparent voltage-dependent current in these cells. Steps to command potentials between + 80 and -120 mV evoked only this strongly rectifying, rapidly developing current at membrane potentials more hyperpolarized than the reversal potential for K' ions. Extracellular Cs+ (10 mM) and Ba2+ (100 microM and 1 mM) blocked this current in a reversible and voltage-dependent manner. The voltage threshold for activation of the inwardly rectifying K+ current is dependent on the extracellular K+ concentration as predicted by the Nernst equation. Methohexital and pentobarbital reversibly inhibited the current in a concentration-dependent fashion with 50% inhibitory concentration (IC50) values of 145 microM and 218 microM respectively. The Hill slopes for both of these effects were approximately 1. The inhibition was not voltage dependent. These results indicate that fast inwardly rectifying K+ channels are potential molecular targets for barbiturates and could explain some of the diverse clinical effects of these drugs.

Protein binding of methohexital. Study of parameters and modulating factors using the equilibrium dialysis technique

This paper describes the parameters that characterize methohexital-albumin binding and the influence of physiological or analytical factors on this binding. Two useful and reproducible methods for measuring the free concentration-equilibrium dialysis (ED) and ultrafiltration (UF)-are described and their performances are compared. Methohexital binds exclusively to albumin according to a two-class binding model. The first is a saturable class site of high affinity constant (KA = 11 200 M-1) and a number of sites per albumin molecule of 1. The second is a non-saturable site of poorer affinity (nKA = 810 M-1). The bound fraction of methohexital in the therapeutic range and at physiological albumin concentration is 86.7 +/- 0.9% in isolated albumin solution. In serum, it ranges from 80 to 84.5%, according to subjects (n = 6). Binding is inhibited by the presence of endogenous compounds of serum (for a given albumin concentration the bound fraction decreases from 90.3% in isolated albumin solution to 82.6% in serum), probably by free fatty acids. An increase in the bound fraction is observed when the pH is increased from 7 to 9. This phenomenon may be explained by a higher affinity of the drug towards the basic (B-form) conformation of the albumin molecule, in analogy with the close barbiturate thiopental. A decrease in the bound fraction against temperature is shown, as though binding forces diminished with increase in temperature. Indeed, the binding modification is less pronounced in the presence of serum endogenous compounds. As expected, there is no evidence of any effect of heparin anticoagulant on the bound fraction. Methohexital binding is strongly modified by the albumin concentration; the bound fractions change from 67 to 91% in the albumin range 150-900 microM.

Comparison of the pharmacokinetics of methohexital during cardiac surgery with cardiopulmonary bypass and vascular surgery

The aim of this study was to assess the pharmacokinetics of methohexital (ME) in major vascular surgery (VASC) and to compare these data with the pharmacokinetics of ME during hypothermic cardiopulmonary bypass (HCPB) (temperature: 28 degrees C) and normothermic cardiopulmonary bypass (NCPB) (temperature: 37 degrees C). An ME bolus (2 mg/kg) was administered to 8 VASC patients at the start of surgery and to 11 HCPB patients and 11 NCPB patients at the start of cardiopulmonary bypass (CPB). Twenty-one arterial blood samples were withdrawn over the following 24 hours for ME assays. All of the patients were given similar anesthesia (fentanyl, diazepam) and muscle relaxation (pancuronium). In the VASC group, ME total body clearance (TBC) was 6 +/- 2 mL/kg/min (mean +/- SD), which is less than in previous studies. When comparing HCPB and NCPB groups, elimination half-life (T1/2), TBC, volume of distribution (VD), area under the curve (AUC), and mean residence time (MRT) were similar. When comparing VASC and CPB patients, TBC and VD were greater in CPB patients than in VASC patients; thus, T1/2 (equal to 0.693 x VD/TBC) was similar. AUC was smaller in CPB patients because of hemodilution, but MRT was similar. It is concluded that ME clearance is lower in patients undergoing major vascular surgery than in healthy patients. The temperature and the duration of CPB do not seem to substantially influence the pharmacokinetics of ME when a bolus is administered. Parameters such as AUC, TBC, and VD appear modified by hemodilution during CPB; however, T1/2 and MRT, which allow comparisons between CPB and non-CPB patients, were similar in these patients.

Premedication with metoclopramide decreases the frequency of methohexital induced hiccup

Metoclopramide is one of many drugs that have been recommended for the treatment of intractable hiccup. Methohexital may produce hiccup during induction of general anesthesia. 211 women received methohexital for induction and maintenance of general anesthesia for short gynaecological procedures. All the patients were premedicated with fentanyl, diazepam and atropine. 109 patients were randomly selected to receive metoclopramide before induction of anesthesia; the remaining 102 patients served as a control group, and were anesthetized without metoclopramide premedication. The frequency of hiccup was compared between the two groups. 7 patients had hiccup in the metoclopramide premedicated group, as compared to 17 patients in the control group. This difference was statistically significant. We conclude that metoclopramide reduces the frequency of methohexital induced hiccup, and recommend that metoclopramide be routinely used for the premedication of methohexital injection.

[Pharmacokinetics of methohexital given by constant rate intravenous infusion]

The pharmacokinetic characteristics of a constant rate methohexitone infusion were studied in young ASA 1 patients undergoing maxillofacial surgery. They were randomly assigned to two groups; group M patients (n = 7) were given 9 mg.kg-1.h-1 of methohexitone for one hour, and group MF patients (n = 7) 9 mg.kg-1.h-1 of methohexitone with 7 micrograms.kg-1.h-1 of fentanyl, also for one hour. Blood samples for determining methohexitone concentrations were obtained at various times, from before the start of the methohexitone infusion up to 19 h afterwards. In twelve patients, a two-compartment model was appropriate to characterize the decrease of methohexitone concentration; for the other two (one in each group), a three-compartment model was applied. There were no statistically significant differences between the two groups. Elimination half-life in group M was 3.22 +/- 1.96 h, and total plasma clearance 8.54 +/- 2.8 ml.kg-1.min-1. The wide variations in pharmacokinetic parameters between subjects may explain some unpredictable variations in duration of action of methohexitone. Fentanyl did not modify methohexitone pharmacokinetics, which remained of the first order. However, it potentiated the barbiturate's action: extubation was only possible after stopping the infusion for 39.4 min +/- 22 min in group MF, and 15.4 min +/- 6 min in group M (p less than 0.01). At that time, plasma concentrations were respectively 3.12 +/- 0.99 mg.l-1 (group MF) and 5.71 +/- 2.09 mg.l-1 (group M), (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

The efficacy and safety of methohexital in the emergency department

STUDY OBJECTIVE

To determine the efficacy and safety of methohexital sodium (MTX) in emergency department patients.

DESIGN

A consecutive case series; all ED patients who received MTX from July 1989 through July 1990 were studied in a prospective manner.

SETTING

A university hospital ED.

PARTICIPANTS

All 102 adult and pediatric patients who received MTX.

INTERVENTIONS

Emergency physicians trained in the use of MTX administered the drug. A study form was completed that provided data on dosage and response, indications for use, final diagnosis, hemodynamic and respiratory parameters, and an efficacy survey.

RESULTS

Average cumulative dose of MTX was 1.6 +/- 1.3 mg/kg. The average duration of action per dose was 7.6 +/- 5.0 minutes. Leading indications for use were orthopedic procedures (54), endotracheal intubation (18), head computed tomography scan (ten), and wound care (eight). Hemodynamic changes were minimal; the average change in systolic blood pressure five minutes after MTX was -1.8 +/- 20 mm Hg. Average heart rate change was 3.0 +/- 20 beats. Percent respiratory depression was 17 +/- 24%. There was no correlation between MTX dose and the degree of respiratory depression. Narcotics and/or benzodiazepines were given with MTX in 92 cases, with no significant increase in respiratory depression. No major complications attributable to MTX were identified.

CONCLUSION

MTX, when used by trained personnel who adhere to an established protocol, appears to be a safe and effective drug in selected ED patients.

Pharmacokinetics of methohexital during cardiovascular surgery with cardiopulmonary bypass

The pharmacokinetics of methohexital after intravenous bolus administration was studied during cardiovascular surgery with cardiopulmonary bypass. The effect of body temperature (normothermia and hypothermia) during cardiopulmonary bypass on methohexital pharmacokinetics was investigated. The pharmacokinetic data obtained were compared with those from vascular surgery without cardiopulmonary bypass. A marked decrease in plasma methohexital concentrations and therefore in area under curve and a significant increase in clearance and in volume of distribution were observed in the cardiopulmonary bypass groups compared to the vascular surgery group without cardiopulmonary bypass. However, the elimination half-life and the mean residence time were similar in the 2 groups. Furthermore, the study shows that body temperature during cardiopulmonary bypass does not influence methohexital pharmacokinetics.

Effect of upper abdominal surgery and cirrhosis upon the pharmacokinetics of methohexital

The pharmacokinetic profile of methohexital was studied in cirrhotic patients (n = 8), patients undergoing upper abdominal surgery (n = 8) and orthopaedic patients under general anaesthesia (n = 8). The total plasma clearance of methohexital was unchanged in cirrhotics: 54 +/- 22 l.h-1 (mean +/- s.d.) as well as in patients undergoing upper abdominal surgery: 60 +/- 14 l.h-1 in comparison to orthopaedic surgery: 70 +/- 24 l.h-1. The central volume and total volume of distribution and the distribution and elimination half-lives were similar between the three groups. Despite its hepatic dependent elimination, methohexital elimination kinetics were unchanged in patients undergoing upper abdominal surgery and in cirrhosis. Owing to the high hepatic extraction ratio of methohexital, its elimination should be influenced by the hepatic blood flow. The unchanged elimination kinetics presently observed in patients with cirrhosis or those undergoing upper abdominal surgery suggest that the hepatic blood flow is less diminished than expected in these patients.

Metabolites of methohexitone do not contribute to its prolonged action on the central nervous system

Methohexitone has been reported to have a prolonged action on the central nervous system (CNS) despite its relatively short elimination half-life. A hydroxy metabolite of methohexitone was identified, purified and isolated from the urine collected from eight surgical patients and five mongrel dogs anaesthetized with methohexitone. Using a rotarod device, the CNS-activity of the human and canine metabolites was tested in mice and compared to that of methohexitone. In this test, the metabolites showed CNS-depressant activity, but the ED50 (dose needed to affect 50% of the animals) of the metabolites was ten times higher than the ED50 of the parent drug. Because in patients, no traces of the hydroxy metabolite could be found in blood, and in dogs the plasma concentration of the metabolite was about two-thirds of that of the parent compound, it is unlikely that the residual CNS-effects of methohexitone are due to its major metabolite, hydroxymethohexitone.

The pharmacology of propofol

A review of the pharmacology of propofol, a new IV anesthetic agent, is presented. Solubilized in a soybean emulsion, propofol is one of a series of sterically hindered phenols that exhibit anesthetic activity. Induction of anesthesia with propofol may be associated with pain on injection, apnea, and a reduction in arterial blood pressure (BP) and cardiac output. Caution should be ascribed to its use in patients with coronary artery disease, where these effects may have the potential for producing myocardial ischemia. The hemodynamic responses to laryngoscopy and intubation are attenuated. The pharmacokinetic profile suggests suitability as an infusion for either maintenance of anesthesia or sedation. Use of propofol as an infusion during surgery may result in a further reduction in cardiac output, particularly with the concomitant administration of adjuvant increments of fentanyl. The ventilatory response to CO2 is depressed during such an infusion. The high clearance of propofol suggests that even after a prolonged infusion, recovery should be rapid. This finding has been confirmed in a series of studies establishing propofol as an ideal agent for use in a total IV anesthetic technique. Both the quality and speed of recovery, together with the absence of emetic sequelae, support the use of propofol in an outpatient setting. Propofol appears to have no long-term effect on adrenocortical function and appears safe for use in patients with acute intermittent porphyria and susceptibility to malignant hyperpyrexia.

Pharmacokinetics and haemodynamic effects of prolonged methohexitone infusion

1. The use of continuous infusion anaesthesia has only been of interest since the development of short-acting, less cumulative and less toxic drugs. 2. This study aimed to compare pharmacokinetics and haemodynamic effects during and after long time methohexitone constant rate infusion. Sixteen patients were given either 60 or 90 micrograms kg-1 min-1 methohexitone during 14 h. Blood samples were taken hourly during this time and 12 h following the end of infusion. 3. Infusion period was analysed by a single exponential model; post-infusion time showed a three compartment model, the intermediate phase parameters corresponding to those of the infusion period. 4. Methohexitone was haemodynamically well tolerated; prolonged infusion decreases oxygen consumption, mainly by a decrease in oxygen demand. 5. Many patients remained unconscious for unacceptably long periods of time after post-operative sedation by methohexitone.

Comparison of two and ten per cent rectal methohexitone for induction of anaesthesia in children

Plasma methohexitone concentrations were determined in 30 children, aged one to six years, who received 25 mg.kg-1 rectal methohexitone as either a two per cent or ten per cent solution for induction of anaesthesia. Venous blood samples were obtained 15, 30, 45 and 120 minutes following drug administration. Twenty-six of 30 children were asleep within fifteen minutes. Mean plasma methohexitone concentrations were 5.39, 4.42, 3.40 and 1.54 micrograms.ml-1 at 15, 30, 45 and 120 minutes following administration of two per cent methohexitone. Use of the ten per cent solution resulted in mean plasma methohexitone concentrations of 3.81, 3.12, 2.31 and 1.07 micrograms.ml-1 at the same time intervals. Plasma methohexitone concentrations were significantly higher at 15, 30 and 45 minutes following administration of two per cent methohexitone, when compared to the ten per cent solution.

Affinities of barbiturates for the GABA-receptor complex and A1 adenosine receptors: a possible explanation of their excitatory effects

The effects of barbiturates on the GABA-receptor complex and the A1 adenosine receptor were studied. At the GABA-receptor complex the barbiturates inhibited the binding of [35S]t-butylbicyclophosphorothionate ([35S]TBPT) and enhanced the binding of [3H]diazepam. Kinetic and saturation experiments showed that both effects were allosteric. Whereas all barbiturates caused complete inhibition of [35S]TBPT binding, they showed varying degrees of maximal enhancement of [3H]diazepam binding; (+/-)methohexital was identified as the most efficacious compound for this enhancement. At the A1 adenosine receptor all barbiturates inhibited the binding of [3H]N6-phenylisopropyladenosine ([3H]PIA) in a competitive manner. The comparison of the effects on [3H]diazepam and [3H]PIA binding showed that excitatory barbiturates interact preferentially with the A1 adenosine receptor, and sedative/anaesthetic barbiturates with the GABA-receptor complex. It is speculated that the interaction with these two receptors might be the basis of the excitatory versus sedative/anaesthetic properties of barbiturates.

Clinical pharmacokinetic considerations in the treatment of increased intracranial pressure

Life-threatening increased intracranial pressure can be reversed by a variety of drugs. These compounds all have some disadvantages, producing rebound effects, severe coma or cardiovascular depression and electrolyte imbalance. However, reduction of intracranial pressure is a prerequisite for recovery and the benefits of treatment outweigh the risks. Dexamethasone is rapidly eliminated, the short half-life (about 3 hours) indicating that dosage intervals should be kept small. As yet, however, its therapeutic efficacy has not been clearly demonstrated. Therefore, an association between pharmacokinetics and pharmacodynamics cannot be established. Osmotic diuretics are the most widely used agents for reduction of intracranial pressure. Pharmacokinetics show a very close relationship to changes in serum osmolality, but there are large variations in the clearance. For the use of osmotics, the blood-brain barrier must be intact. Osmotic diuretics may lead to intracerebral oedema or to acute renal failure as serum osmolality increases. Considering the pharmacokinetics of each drug, and the dynamics of intracerebral pressure and osmolality, an intermittent, individually titrated dosage should be administered, with simultaneous monitoring of intracranial pressure. Frusemide (furosemide) can be used as an adjunct, to enhance the effect of osmotic diuretics. Its pharmacokinetics are limited by renal function, depending on age as well as on the extent of renal impairment. Altered renal elimination of concomitantly administered drugs, and electrolyte imbalances should be anticipated when diuretics are used. Barbiturates are certain to decrease intracranial pressure in humans by an as yet unknown mechanism. Their administration is recommended for patients that do not respond to conventional therapy. As barbiturates can result in deep coma, knowledge of their pharmacokinetics is of great importance for recovery. Following single doses, pentobarbitone has a relatively long elimination half-life (about 22 hours). However, after repeated administration for several days, its elimination may be enhanced due to autoinduction. Thiopentone kinetics are characterised by distribution and redistribution into deep peripheral compartments. Administration of high and frequent doses leads to considerably delayed recovery. This is not true for methohexitone, which shows comparable pharmacokinetics after single and multiple dose administration. Elimination depends on liver blood flow. Thus, recovery from methohexitone-coma is rapid. Rapid elimination is also an important characteristic of etomidate and alphaxalone/alphadolone, two non-barbiturate hypnotics.(ABSTRACT TRUNCATED AT 400 WORDS)

Threshold hypnotic concentration of methohexitone

Methohexitone was administered to 8 healthy adult volunteers as a microprocessor controlled infusion that generated 3 cycles of linearly increasing plasma levels with an anticipated slope of 0.2 microgram.ml-1.min-1. When a deep unconscious state was obtained, as indicated by burst suppression in the EEG, the infusion was stopped and then restarted when the volunteer was fully orientated. Frequent venous blood samples were obtained during and after the infusions to evaluate the threshold concentration at induced sleep and the return of orientation, at the loss and return of the eye lid reflex and corneal reflex, and the appearance and disappearance of EEG burst suppression patterns. From the first to the third infusion cycle only a slight and insignificant increase in the mean threshold concentrations was observed so the plasma levels were averaged over all three infusion cycles. The concentrations (microgram/ml) found were: asleep 3.39 and orientated 3.35, loss 4.42 and recurrence 4.32 of eye lid reflex, loss 6.51 and recurrence 5.18 of corneal reflex, and appearance 10.7 and disappearance 9.3 of burst suppression. Acute tolerance that would have led to a significant increase in threshold concentration from the first to the last infusion cycle was not demonstrated. If induced sleep and the appearance of EEG burst suppression are considered as clinical endpoints of anaesthesia, the therapeutic window of methohexitone covers a mean venous serum concentration range of 3.4 to 10.7 micrograms/ml.

A comparison of the effects of fentanyl and alfentanil on the sleeping times of the intravenous induction agents

The effect of increasing doses of fentanyl and alfentanil administered before four intravenous induction agents (thiopentone, methohexitone, Althesin and etomidate) on the sleeping time were studied in mice. A biphasic pattern of sleeping time was noted with the more rapidly metabolized and eliminated compounds. Possible explanations are discussed.

The pharmacokinetics of methohexital in young and elderly subjects

The pharmacokinetics of methohexital were investigated in ten young adult volunteers and in seven young and seven elderly patients. The latter two groups underwent enflurane and nitrous oxide anesthesia and surgery. Each subject received a bolus dose of 2 mg/kg of methohexital intravenously. Plasma levels of the drug were measured for 8 h after injection by gas chromatography using a nitrogen detector. Anesthesia (combined with surgery) and increase in age did not separately affect the kinetics of the drug; however, the elimination half-life was longer in the elderly patients group than in the young non-anesthetized volunteers.

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.

Choice reaction time. A method of measuring postoperative psychomotor performance decrements

Postoperative psychomotor performance changes were measured in three groups of thirteen subjects using a portable serial four-choice reaction time apparatus. Anaesthesia consisted of either thiopentone, nitrous oxide and halothane (thiopentone subjects) or methohexitone and fentanyl/nitrous oxide and incremental methohexitone (methohexitone subjects). Tests were performed on the day before operation and after 2, 4, 6, 8 and approximately 24 hours recovery on both groups of patients and at approximately the same times on nonoperated control patients. Control subjects' mean four-choice reaction time decreased steadily with practice such that the difference between first and last tests was statistically significant. Mean four-choice reaction times of thiopentone and methohexitone subjects increased from 515 to 550 ms and 552 to 600 ms respectively after 2 hours recovery. Subsequently four-choice reaction times decreased in both groups although methohexitone subjects were still significantly slower than controls after 6 hours. Thiopentone subjects were slower than controls at 2, 4 and 8 hours recovery. There was no difference between the three groups of subjects after 24 hours recovery.

Prolonged recovery after intravenous diazepam

Prolonged recovery from intravenous diazepam after the day of administration has been examined in 2,468 consecutive patients who underwent oral surgery and general dentistry. Patients ranged in age from 2 to 94 years and received a mean diazepam dose of 19 mg, either by itself or with small amounts of methohexital sodium. Recovery was assessed by use of a questionnaire, and the answers were related to sexual gender, age, weight, dosage, procedure, duration, level of apprehension, and results of the sedation.

Total intravenous anaesthesia

Total intravenous anaesthesia is a feasible alternative to the use of inhalational agents and the rationale behind such a technique is presented. The various intravenous agents are discussed, particularly with regard to their suitability for this form of anaesthesia, and their physiological effects compared with those of the inhalational anaesthetics. Some of the problems associated with the use of total intravenous anaesthesia are mentioned.

General kinetic and dynamic principles and their application to continuous infusion anaesthesia

The use of intravenous anaesthetic agents by continuous infusion requires knowledge of their pharmacokinetic properties. In this article, the general pharmacokinetic principles behind the use of infusions of intravenous agents are presented and the literature with regard to the individual drugs used in this way is reviewed.

[Electroencephalographic study of pediatric anesthesia with intramuscular methohexital]

Intramuscular methohexital was used in pediatric anesthesia to carry out special investigations. There seemed to be few side effects but twitches or seizures have been reported. An electroencephalographic study was performed in normal children. The electroencephalogram showed periods of increased amplitude. The authors discussed the problem of side effects following the intramuscular administration of methohexital in the normal child, and prescribed its use in the epileptic.

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.

Discriminative stimulus effects of pentobarbital in rhesus monkeys: tests of stimulus generalization and duration of action

Rhesus monkeys were trained to emit 20 or 30 consecutive responses on one lever following an IM injection of pentobarbital (10 or 18 mg/kg) and the same number of consecutive responses on another lever following an injection of saline. The required number of correct consecutive responses in both cases resulted in food delivery. When responding was reliably under the control of the presession injection, the ability of a variety of other compounds to produce pentobarbital-appropriate responding was examined. Diazepam, clobazam, methohexital, pentobarbital, and phenobarbital, given 10 or 20 min before the session, produced dose-related pentobarbital-appropriate responding in each monkey. Ethylketazocine and dextromethorphan produced responding primarily on the saline-appropriate lever, whereas codeine, cyclazocine, dextrorphan, and ketamine resulted in responding that was, on the average, intermediate between that appropriate for pentobarbital and that appropriate for saline. When tested at various times after their injection, methohexital (3.2 mg/kg) and pentobarbital (10 mg/kg) produced pentobarbital-appropriate responding within 10 min. Barbital (56 mg/kg) resulted in pentobarbital-appropriate responding only if at least 1 h intervened between the injection and the experimental session. The discriminative effects of methohexital, pentobarbital, and barbital lasted approximately 20-60, 120-240, and 480-720 min, respectively. The time-course of the discriminative stimulus effects of barbiturates in the rhesus monkey appears to parallel closely other pharmacological actions of these compounds.

Simultaneous determination of blood concentrations of methohexital and its hydroxy metabolite by gas chromatography and identification of 4'-hydroxymethohexital by combined gas--liquid chromatography--mass spectrometry

A simple, sensitive and selective method is described for the simultaneous determination of low concentrations (less than 50 ng/ml) of underivatized methohexital and its hydroxy metabolite in small (0.1 ml) samples of human and rat plasma or whole blood by gas chromatography with nitrogen-selective detection. Moreover, the main metabolite in rat and man was identified as 4'-hydroxymethohexital by comparison of chromatograms from gas--liquid chromatography (GLC) with data obtained from GLC--mass spectrometry and 1H-nuclear magnetic resonance spectrometry of this metabolite, produced both by incubating methohexital with isolated rat liver microsomes and by isolating this metabolite from rat urine.

The effect of diazepam on methohexitone short anaesthesia: a clinical double-blind investigation

A double-blind investigation was performed on 113 adult females, hospitalized for short gynaecological procedures, in order to establish the suitability of methohexitone for short procedures and whether or not the quality of methohexitone anaesthesia could be improved with a diazepam supplement. All patients were anaesthetized with methohexitone (2.0 mg/kg): 57 of the patients received a 10 mg diazepam intravenous supplement and 56 received intravenous placebo (solvent of diazepam only). Diazepam injected after induction with methohexitone clearly improved the quality of methohexitone anaesthesia (p less than 0.1 more than 0.05) and reduced the need for supplementary doses of methohexitone (p less than 0.1 more than 0.05). The incidences of both nausea and vomiting were minimized (p less than 0.01 more than 0.001). Patient acceptance was equally good in both groups. It is commented that patients may seem fully awake and oriented, notwithstanding the fact that the body still contains significant amounts of the anaesthetic agent. Ambulatory patients should never be allowed to leave the hospital unescorted, as they recover their 'street-orientation' only after 12 to 24 hours. Furthermore, the patients or their escorts should be given definite and written instructions to avoid absolutely the use of alcohol or hypnotics which could, even in small amounts, potentiate the effect of methohexitone still in the body, for at least 12 hours. It is recommended that patients should avoid driving a motor vehicle or using dangerous tools or machines for 12 to 24 hours following recovery from methohexitone anaesthesia.