Evaluation of infusion regimens for thiopentone as a primary anaesthetic agent

Interaction of thiopental with esomeprazole in critically ill patients

INTRODUCTION

Thiopental is a thiobarbiturate given in the case of brain injuries to reduce intracranial pressure and to manage cerebral ischemia. A pharmacokinetic model has been described previously in critically ill patients with a different therapeutic strategy. New treatment options prompted us to investigate if drug-drug interactions occur. A new model is proposed describing the influence of concomitant administration of esomeprazole on the distribution of thiopental.

METHOD

The study population comprised 52 critically ill patients (body weight 47.1-114 kg) aged 18-78 years who had been admitted into the critical care unit for treatment of intracranial hypertension. A total mean dose of 282.8 ± 172.7 mg/kg was given in 96 ± 72 h. Pharmacokinetic analysis was performed by using a nonlinear mixed-effect population model.

RESULT

A one-compartment open model with first-order elimination identified two covariates, namely, body weight on clearance and volume of distribution, and the administration of esomeprazole on volume of distribution. The mean values (% relative standard error) for total clearance (CL) and for central volume of distribution (Vd) in patients with and without concomitant esomeprazole were 5.3 L/h (9.2 %) and 256.1 (6.4 %) and 153.2 l (19.2 %), respectively.

CONCLUSION

Based on these results, we conclude that concomitant administration of esomeprazole increases the volume of distribution and the half-live of thiopental. This drug-drug interaction should be considered when a target concentration has to be reached.

Pethidine and skin warming to prevent shivering during endovascular cooling

We tested the efficacy of pethidine and cutaneous warming to prevent shivering during percutaneous cooling in unanaesthetized patients. Ten patients scheduled for cranial neurosurgery received pethidine infusion and skin warming. The Setpoint internal heat-exchanging catheter was inserted and cooling to 33.5 degrees C was started. Clonidine and chlorpromazine were given as "rescue medication" to treat shivering. General anaesthesia was planned to be induced after cooling was complete. Rewarming was initiated at dural closure. Three patients successfully completed the protocol, cooling to 33.8 degrees C at a median rate of 3.6 (range: 3.4-3.8) degrees C/h. Two patients cooled to 33.8 degrees C but required treatment for shivering (cooling rate: 2.9 [2.8-3.1] degrees C/h). Four patients failed to cool adequately because of refractory shivering (cooling rate: 20 [1.5-2.9] degrees C/h). One patient did not shiver and yet failed to cool adequately (cooling rate: 0.76 degrees C/h). Rewarming was at a rate of 26 (1.2-4.3) degrees C/h. There were no significant complications arising from catheter placement. The combination of skin warming and pethidine was not reliable enough to be recommended for use during endovascular cooling in unanaesthetized patients.

Buspirone and meperidine synergistically reduce the shivering threshold

Mild hypothermia (i.e., 34 degrees C) may prove therapeutic for patients with stroke, but it usually provokes shivering. We tested the hypothesis that the combination of buspirone (a serotonin 1A partial agonist) and meperidine synergistically reduces the shivering threshold (triggering tympanic membrane temperature) to at least 34 degrees C while producing little sedation or respiratory depression. Eight volunteers each participated on four randomly-assigned days: 1) large-dose oral buspirone (60 mg); 2) large-dose IV meperidine (target plasma concentration of 0.8 microg/mL); 3) the combination of buspirone (30 mg) and meperidine (0.4 microg/mL); and 4) a control day without drugs. Core hypothermia was induced by infusion of lactated Ringer's solution at 4 degrees C. The control shivering threshold was 35.7 degrees C +/- 0.2 degrees C. The threshold was 35.0 degrees C +/- 0.8 degrees C during large-dose buspirone and 33.4 degrees C +/- 0.3 degrees C during large-dose meperidine. The threshold during the combination of the two drugs was 33.4 degrees C +/- 0.7 degrees C. There was minimal sedation on the buspirone and combination days and mild sedation on the large-dose meperidine day. End-tidal PCO2 increased approximately 10 mm Hg with meperidine alone. Buspirone alone slightly reduced the shivering threshold. The combination of small-dose buspirone and small-dose meperidine acted synergistically to reduce the shivering threshold while causing little sedation or respiratory toxicity.

IMPLICATIONS

Mild hypothermia may be an effective treatment for acute stroke, but it usually triggers shivering, which could be harmful. Our results indicate that the combination of small-dose buspirone and small-dose meperidine acts synergistically to reduce the shivering threshold while causing little sedation or respiratory toxicity. This combination may facilitate the induction of therapeutic hypothermia in stroke victims.

Population pharmacokinetics of high-dose thiopental in patients with cerebral injuries

Thiopental monitoring was performed in 95 critically ill patients hospitalized for neurologic damage, High-dose thiopental was infused during long-term treatment. Total dose of 333 +/- 144 mg/kg (449 +/- 185 mg/kg for females and 302 +/- 113 mg/kg for men) were given in 125 +/- 43 hours. Plasma concentration-time data were analyzed according to a population pharmacokinetic approach with an initial group of 65 patients. Clearance (CL) and central volume of distribution (Vc) were modeled alone and under the influence of demographic covariates, assuming a two-compartment open model with first-order elimination. The final population models were as follows: CL (L/hr) = 11.7.weight (kg).age (yr)/(2136 + age2) and Vc = 1.52.weight (kg) + 44.8. Mean CL and Vc mean population estimates were 8.01 L/hr (133 ml/min or 2.02 ml/min/kg) and 145 L (2.19 L/kg). The predictive performance of the population modeling and parameters was evaluated with a bayesian fitting procedure in an independent validation set of 30 patients with similar physical and clinical characteristics. There was no statistically significant bias or imprecision between measured and predicted thiopental plasma concentrations in this validation group. Moreover, there was a good adequation (r = 0.939) between individual CL values predicted from the population formula and estimated with the bayesian approach.

Determination of (R)-(+)- and (S)-(-)-isomers of thiopentone in plasma by chiral high-performance liquid chromatography

A method for the determination of R-(+)- and (S)-(-)-isomers of thiopentone in plasma was developed. Following liquid-liquid extraction, the separation of enantiomers of thiopentone and the internal standard (racemic ketamine) was achieved by high-performance liquid chromatography on an alpha1-acid glycoprotein (AGP) column with ultraviolet detection at 280 nm. The mobile phase consisted of 20 mM KH2PO4 buffer-2-propanol-methanol (93.5:5.0:1.5) at pH 5.0. The flow-rate was 0.9 ml/min. The limit of quantification for each isomer was approximately 10 ng/ml. The assay is suitable for pharmacokinetic studies of (R)-(+)- and (S)-(-)-isomers of thiopentone, following usual bolus intravenous clinical doses of the racemic drug.

Pharmacokinetics of thiopental after single and multiple intravenous doses in critical care patients

Thiopental was administered to neurosurgical patients for cerebral protection and its pharmacokinetic parameters were determined after a single bolus of 540, 1000 or 1500 mg (3 subjects) or after multiple doses of 250 mg (5 subjects) and 500 mg (2 subjects) every two hours for up to 7 days. The data were analysed by a two- or three-compartment model and linear kinetics. After a single iv bolus, the mean initial volume of distribution (V1) was 0.481 l.kg-1, and the steady-state volume of distribution (Vss) was 2.16 l.kg-1. The distribution (t1/2 alpha) and elimination (t1/2 beta) half-lives were 0.590 and 5.89 h, respectively, and the mean residence time (MRT) was 7.44 h. The clearance was 5.41 ml.min-1.kg-1. With repeated injections, the pharmacokinetic parameters for each patient were estimated taking into account all administered doses and blood samples, which were taken whenever possible daily at steady state and after the last dose. The variability observed in the pharmacokinetic parameters of thiopental reflected by the coefficient of variation (CV%) was wide but was of similar magnitude within patients (CVintra) as it was between patients (CVinter). The steady-state trough plasma concentration (Cmin obs) ranged from 4.8 to 30 mg.l-1 (mean 16.0 mg.l-1 and median 14.3 mg.l-1). Peak concentrations (Cmax obs) ranged from 8.35 to 45 mg.l-1 (25.4 mg.l-1, and median 23.3 mg.l-1). The values of V1 and Vss were similar to those obtained after a single dose. For V1, the mean was 0.333 l.kg-1. The mean Vss was 2.68 l.kg-1, with a CVintra of 12.6 to 56% and a CVinter of 13.2%. A shorter distribution half-life t1/2 alpha was noted on multiple dosing; the mean value was 0.122 h. The elimination half-life t1/2 beta and the mean residence time became longer due to a decrease in clearance. For t1/2 beta the mean value was 16.3 h. The mean MRT was 21.9 h, CVintra 9.19 to 48.5%, and the CVinter 35.3%. The mean clearance was 2.16 ml.min-1.kg-1, CVintra 7.28 to 25.5%, and the CVinter 20.4%. This value is 50% lower than after a single dose. Identification of the kinetic parameters of thiopental allows simulation of the effects of doses on subsequent plasma levels and will permit a priori prediction of day to day adjustment of drug dosage.

Midazolam minimally impairs thermoregulatory control

Perioperative hypothermia usually results largely from pharmacologic inhibition of normal thermoregulatory control. Midazolam is a commonly used sedative and anesthetic adjuvant whose thermoregulatory effects are unknown. We therefore tested the hypothesis that midazolam administration impairs thermoregulatory control. Eight volunteers were studied on 2 days each, once without drug and once at a target total plasma midazolam concentration of 0.3 micrograms/mL (corresponding to administration of approximately 40 mg over approximately 4 h). Each day, skin and core temperatures were increased sufficiently to provoke sweating, and then reduced to elicit peripheral vasoconstriction and shivering. We mathematically compensated for changes in skin temperature using the established linear cutaneous contributions to control of each response. From these calculated thresholds (core temperatures triggering responses at a designated skin temperature of 34 degrees C), we determined the thermoregulatory effects of midazolam. The sweating threshold was decreased approximately 0.3 degrees C by midazolam administration: 37.3 +/- 0.2 degrees C vs 37.0 +/- 0.3 degrees C (P = 0.0004, paired t-test). Midazolam decreased the core temperature that triggered vasoconstriction somewhat more: 37.1 +/- 0.2 degrees C vs 36.3 +/- 0.5 degrees C (P = 0.0002). Similarly, midazolam decreased the shivering threshold: 35.9 +/- 0.3 degrees C vs 35.3 +/- 0.6 degrees C (P = 0.03). The sweating-to-vasoconstriction (interthreshold) range, therefore, increased from 0.2 +/- 0.1 degrees C to 0.7 +/- 0.3 degrees C (P = 0.002). Although statistically significant, this relatively small increase contrasts markedly with the 3-5 degrees C interthreshold ranges produced by clinical doses of volatile anesthetics, propofol, and opioids.(ABSTRACT TRUNCATED AT 250 WORDS)

Do barbiturates impair zymosan-induced granulocyte function?

PURPOSE

The dose-response relationship of commercially available preparations of methohexital, pentobarbital, phenobarbital, and thiopental and their respective drug-free solutions on granulocyte function was investigated to evaluate whether suppression of neutrophil chemiluminescence is mediated by the barbiturates themselves or by their drug-free solutions. Furthermore, it was assessed whether suppression of chemiluminescence is due to an interaction mainly with neutrophils or to free radical scavenging.

METHODS

The dose-response effects of the four barbiturates on granulocyte function were tested by zymosan-induced neutrophil chemiluminescence and, in addition, in a cell-free chemiluminescence system.

RESULTS

Methohexital and pentobarbital did not influence zymosan-induced neutrophil chemiluminescence, whereas phenobarbital and thiopental decreased neutrophil chemiluminescence in a dose-dependent fashion. Nonphysiological osmolality (531 mosmol/kg) caused this impaired neutrophil chemiluminescence at the greatest concentration of phenobarbital. Thiopental solely suppressed neutrophil chemiluminescence drug specifically. Because thiopental also reduced chemiluminescence generated in a cell-free system, free radical scavenging might contribute to the impaired neutrophil chemiluminescence observed with thiopental.

CONCLUSIONS

With the exception of thiopental, barbiturates do not impair oxygen radical production during phagocytosis of neutrophils.

Evaluation of polyalkylcyanoacrylate nanoparticles as a potential drug carrier: preparation, morphological characterization and loading capacity

Some physicochemical behaviours were investigated of polyethyl- (PECA) and polyisobutylcyanoacrylate (PICA), which, in recent years, have been proposed as nanoparticle colloidal systems for drug carrying. We observed the influence of preparation conditions, such as pH value and surfactant concentration, on parameters such as particle size and polymer molecular weight. Lower operating pH values (0-2) resulted in smaller nanoparticles than those prepared at pH 5.5. The polymer molecular weight was also a function of pH: low molecular weight at low pH and vice-versa. The surfactant concentration positively influenced main particle size and polymer molecular weight. These trends were independent of type of monomer; in fact, both ethyl- (ECA) and isobutyl-2-cyanoacrylate (ICA) showed the same behaviour. Loading capacity, as well as release profile, of the two polymers were evaluated using fluorescein as a model drug. Whereas both polymers showed almost the same release profile, there was a difference in the amount of encapsulated probe: higher aliquots for PICA than for PECA. Storage effects on such physicochemical parameters were also tested.

A method for implementing programmed infusion of thiopentone and methohexitone with a simple infusion pump

We have tabulated the series of steps in infusion rate required to maintain constant arterial levels of thiopentone and methohexitone. The tables are based on multiexponential equations for infusion rate, derived from plasma drug efflux studies. In each table an initial bolus is followed by nine steps in infusion rate over three hours. The tables provide rates suitable for delivery by a standard syringe pump to achieve and maintain an arterial concentration of 10 mg/l of thiopentone and 5 mg/l of methohexitone. Other desired drug concentrations can be derived from the table by simple multiplication.

Continuous infusion of methohexital and alfentanil hydrochloride for general anesthesia in outpatient third molar surgery

Three anesthetic techniques were compared in this study: 1) Intermittent Brevital boluses supplemented with fentanyl and midazolam all titrated to patient movement, 2) constant infusion of Brevital supplemented with fentanyl and midazolam all delivered in calculated mg/kg doses based on total body weight, and 3) constant infusion of methohexital (Brevital) and alfentanil (Alfenta) supplemented by midazolam (Versed), droperidol, and glycopyrolate (Robinul) delivered in calculated mg/kg doses based on lean body mass. Nitrous oxide was delivered in all cases via nasal mask in a 30% to 50% concentration. The mean total dose of Brevital in group 1 (intermittent Brevital bolus) was 0.17 mg/kg/min (SD = 0.07), group 2 (Brevital infusion) was 0.23 mg/kg/min (SD = 0.06), and group 3 (alfentanil/Brevital infusion) was 0.12 mg/kg/min (SD = 0.07). Mean total dose of alfentanil in group 3 equaled 1.58 mcg/kg/min (SD = 0.73). In group 1, 94% of the patients experienced moderate to severe movement intraoperatively. Twenty-three percent of the patients in group 2, and only 7% of group 3 exhibited moderate to severe movement. Emergence in group 3 averaged 4.5 minutes (SD = 1.6). Three patients (7%) in group 3 had postoperative nausea. Additional subjective findings in group 3 included easier airway maintenance during administration of the anesthetic, lack of unpleasant emergence phenomena such as crying, and prompt readiness for discharge. It was concluded that continuous alfentanil and Brevital infusion satisfied the objectives of safety, stability, predictability, and rapid recovery, while improving operating conditions (less patient movement) when compared with more traditional anesthetic techniques.

Intravenous anaesthetic agents

Detailed knowledge of the pharmacology of the intravenous anaesthetic agents--a relatively-small group of drugs--is necessary to achieve optimal results in a diverse patient population. The trend towards short-stay surgery requires a consideration of the speed of recovery from anaesthesia, as well as the quality of that recovery, more than ever before. New agents, particularly propofol, have expanded the potential for total intravenous anaesthesia, but technical developments in drug delivery are needed for the full realization of the properties of these drugs.

Infusions of propofol to supplement nitrous oxide-oxygen for the maintenance of anaesthesia. A comparison with halothane

The peri-operative and postoperative effects of propofol given by infusion were compared with halothane as a supplement to nitrous oxide-oxygen anaesthesia for body surface surgery in patients who breathed spontaneously. Anaesthesia was induced after opioid premedication, with either propofol 2.5 mg/kg or thiopentone 4-5 mg/kg which were followed respectively by an infusion of propofol 12 mg/kg/hour for 10 minutes and at a variable rate thereafter, or by halothane at a mean inspired concentration of 1.2%. Maintenance of anaesthesia required a median rate of infusion of propofol of 149.4 micrograms/kg/minute. The cardiovascular effects during induction and maintenance of anaesthesia were similar in the two groups. The overall incidence of side effects was low but immediate recovery was significantly faster in patients who received propofol.

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)

Hypnotics in infusion anaesthesia--with particular reference to thiopentone

Hypnotics are a group of drugs whose primary action is to produce unconsciousness. This contrasts to the opioids whose primary action is to reduce the sensation of pain and the tranquillisers whose primary action is to calm and to attenuate psychotic disease. Hypnotics of interest to the anaesthetist are the inhalational agents and intravenously administered drugs, including the barbiturates thiopentone and methohexitone, chlormethiazole, etomidate and propofol. Knowledge of the use of hypnotics to maintain anaesthesia is largely confined to the volatile anaesthetic agents, while knowledge of the use of the intravenous hypnotics is largely restricted to their use as intravenous induction agents and as sedative for regional procedures. The use of intravenous hypnotics to maintain anaesthesia requires careful control of infusion rates based on pharmacokinetic predictions. Once techniques are established, favourable operating conditions can be achieved, as well as acceptably short recovery times. With such techniques, the benefits of freedom from many of the adverse aspects of inhalational anaesthesia can be realised.

Pharmacokinetics of intravenous and oral salbutamol and its sulphate conjugate

The pharmacokinetics of salbutamol and its sulphate conjugate metabolite were investigated after intravenous and steady-state oral administration of salbutamol to 10 healthy volunteers. With intravenous administration, total plasma clearance was 480 +/- 123 ml min-1, elimination half-life was 3.86 +/- 0.83 h and apparent volume of distribution was 156 +/- 381. Urinary excretion of unchanged drug and sulphate conjugate were 64.2 +/- 7.1% and 12.0 +/- 3.1% of the dose, respectively. With oral administration, systemic availability was 0.50 +/- 0.04, and urinary excretion of unchanged drug and sulphate conjugate were 31.8 +/- 1.9% and 48.2 +/- 7.3% of the dose, respectively. The drug eliminated on the first-pass could be accounted for entirely as sulphate conjugate formed, presumably, in the intestinal wall. Renal clearance of salbutamol was 291 +/- 70 ml min-1 after intravenous and 272 +/- 38 ml min-1 after oral administration, while the renal clearance of the sulphate conjugate was 98.5 +/- 23.5 ml min-1 after oral administration. Heart rate increased with increasing plasma salbutamol concentration, although a lag was evident. The effect on heart rate was lower after 24 h continuous oral salbutamol administration.

Thiopentone levels during cardiopulmonary bypass. Changes in plasma protein binding during continuous infusion

Plasma total and unbound concentrations of thiopentone were investigated during exponentially decreasing infusions in seven patients undergoing cardiopulmonary bypass. Total plasma thiopentone concentrations reached a plateau (10.2, SD 2.1 micrograms/ml) soon after the initial bolus dose and commencement of the infusion. Concentrations were maintained until the onset of cardiopulmonary bypass, whereupon total plasma thiopentone concentration fell abruptly to 50.0 (SD 5.8) percent of the prebypass level. The unbound fraction of thiopentone increased from 16.6 (SD 1.9) percent before bypass to a maximum of 29.3 (SD 5.6) percent during bypass (p less than 0.01), decreased to 22.9 (SD 3.3) percent at the end of bypass (p less than 0.01), but was still elevated 5-7 hours later (20.5, SD 2.5 percent). The result of the changes in binding was a smaller decline in unbound thiopentone concentration at the onset of bypass to 76.4 (SD 15.7) percent of the prebypass level. Also, unbound levels returned to the prebypass level by the end of bypass, whereas total levels remained low.