Pentobarbitone formation during thiopentone infusion

The effects of cardiopulmonary bypass on plasma concentrations and protein binding of methohexital and thiopental

The effects of cardiopulmonary bypass (CPB) on plasma concentrations and protein binding of methohexital and thiopental were studied during continuous infusions in two groups of ten cardiac surgical patients. Patients were administered an infusion regimen designed to produce a stable total plasma concentration at 5 mg/L for methohexital and 10 mg/L for thiopental. Prior to the commencement of CPB the mean (+/-SD) total plasma methohexital concentration was 5.00 +/- 0.69 mg/L. This fell to 3.12 +/- 0.89 mg/L at two minutes after commencement of CPB, and rose to 4.67 +/- 1.11 mg/L by 75 minutes after commencement of CPB. The unbound fraction rose from 27.1 +/- 5.1% to 42.8 +/- 9.2% at five minutes after the start of CPB, and gradually decreased to 32.1 +/- 4.9% by 75 minutes. The unbound concentration (1.37 +/- 0.32 mg/L) was unaffected by the onset of CPB, being 1.51 +/- 0.49 mg/L at 75 minutes after the start of CPB. Thiopental followed a similar pattern to methohexital, with the total plasma thiopental concentration falling from 9.22 +/- 0.73 mg/L to 4.90 +/- 0.83 mg/L at two minutes after commencement of CPB, and rising again to 7.13 +/- 1.03 mg/L 75 minutes later. During the same period the unbound fraction of thiopental rose from 16.1 +/- 2.5% to 30.3 +/- 7.3% five minutes after the start of CPB, and fell gradually to 22.8 +/- 5.8% after 75 minutes. The unbound concentration (1.51 +/- 0.21 mg/L) was again unchanged by the onset of CPB, being 1.71 +/- 0.29 mg/L at 75 minutes. Plasma protein binding of both drugs correlated strongly with plasma albumin concentration, which decreased by 40% during CPB. It is concluded that hemodilution caused the reduction in total drug concentration and protein binding at the onset of CPB, but that the decrease in protein binding counteracted the dilution of unbound drug, resulting in a stable unbound concentration throughout CPB, and that this effect may be common for barbiturates.

Thiopental inhibits glycine receptor function in acutely dissociated rat spinal dorsal horn neurons

Whole-cell patch-clamp was used to assess the modulatory effect of thiopental (Thio) on glycine (Gly) receptor in mechanically dissociated rat spinal dorsal horn neurons. It was found that Thio inhibited the amplitude, accelerated the desensitization and prolonged the deactivation of Gly-induced currents (IGly) in a concentration-dependent manner. In addition, a rebound current occurred after washout of the co-application of Gly and Thio in most neurons tested. Moreover, the inhibitory effect of Thio was not the result of cross-inhibition between Gly and GABAA receptors. Furthermore, taurine-induced currents, a low-affinity agonist for Gly receptors, were also markedly inhibited by Thio in a similar way to IGly. These results indicate that Thio suppresses Gly receptor function and suggest that Thio anesthetic actions might not be mediated by Gly receptors. We speculate that the weak muscle relaxation and the limited analgesic effects observed during Thio anesthesia may attribute to its inhibitory effects on Gly receptors.

Barbiturates for acute neurological and neurosurgical emergencies--do they still have a role?

A number of clinical studies have reported poor clinical outcomes for patients treated with barbiturate therapy in acute neurological and neurosurgical emergencies. Barbiturate therapy, as currently practised with thiopentone and pentobarbitone at least, is also associated with a prolonged post-infusion period of clinical unresponsiveness. Hence, the popularity of barbiturate therapy for sedation of critically ill neurological and neurosurgical patients has declined over the past decade. A retrospective study of traumatic brain injury patients treated at the Royal North Shore Hospital, Sydney, with high-dose thiopentone therapy between 1987 and 1997 has found disappointing results with a 1-month mortality outcome of 50% (14 of 28 patients). Nevertheless, barbiturate therapy remains a consideration for patients with severe cranial trauma in whom preferred treatments have failed to control intracranial or cerebral perfusion pressures. More favourable results ( approximately 10% 1-month mortality rate) were encountered for patients with refractory vasospasm complicating subarachnoid haemorrhage or intracerebral haemorrhage complicating supratentorial arteriovenous malformation resection. A well designed, prospective and randomised controlled trial may be of value in further determining the role of barbiturate therapy in acute neurovascular emergencies refractory to standard therapy.

Stereoselective interaction of thiopentone enantiomers with the GABA(A) receptor

1. As pharmacokinetic differences between the thiopentone enantiomers seem insufficient to explain the approximately 2 fold greater potency for CNS effects of (-)-S- over (+)-R-thiopentone, this study was performed to determine any enantioselectivity of thiopentone at the GABA(A) receptor, the primary receptor for barbiturate hypnotic effects. 2. Two electrode voltage clamp recording was performed on Xenopus laevis oocytes expressing human GABA(A) receptor subtype alpha1beta2gamma2 to determine relative differences in potentiation of the GABA response by rac-, (+)-R- and (-)-S-thiopentone, and rac-pentobarbitone. Changes in the cellular environment pH and in GABA concentrations were also evaluated. 3. With 3 microM GABA, the EC50 values were (-)-S-thiopentone (mean 26.0+/-s.e.mean 3.2 microM, n=9 cells) >rac-thiopentone (35.9+/-4.2 microM, n=6, P=0.1) >(+)-R-thiopentone (52.5+/-5.0 microM, n=8, P<0.02) >rac-pentobarbitone (97.0+/-11.2 microM, n=11, P<0.01). Adjustment of environment pH to 7.0 or 8.0 did not alter the EC50 values for (+)-R- or (-)-S-thiopentone. 4 Uninjected oocytes responded to >100 microM (-)-S- and R-thiopentone. This direct response was abolished by intracellular oocyte injection of 1,2-bis(2-aminophenoxy)ethane-N, N,N1,N1-tetraacetic acid (BAPTA), a Ca2+ chelating agent. With BAPTA, the EC50 values were (-)-S-thiopentone (20.6+/-3.2 microM, n=8) <(+)-R-thiopentone (36.2+/-3.2 microM, n=9, P<0.005). 5 (-)-S-thiopentone was found to be approximately 2 fold more potent than (+)-R-thiopentone in the potentiation of GABA at GABA(A) receptors expressed on Xenopus oocytes. This is consistent with the differences in potency for CNS depressant effects found in vivo.

Pharmacokinetics of thiopental and pentobarbital enantiomers after intravenous administration of racemic thiopental

We studied the pharmacokinetics of thiopental enantiomers in 14 healthy patients aged 37-73 yr receiving racemic thiopental by intravenous (IV) bolus or IV infusion. Plasma concentration of each enantiomer was measured by chiral high-performance liquid chromatography. After IV bolus, the total plasma clearance (CL) (295 +/- 132 mL/min) and volume of distribution at steady state (Vss) (139 +/- 38.5 L) of R-thiopental were significantly greater than those of S-thiopental (230 +/- 104 mL/min and 114 +/- 47.5 L, respectively). The plasma unbound fraction (fu) was determined by ultrafiltration of plasma from six healthy volunteers. The fu of R-thiopental (12.4% +/- 0.6%) was significantly greater than that of S-thiopental (10.0% +/- 1.0%). When the CL and Vss of the two enantiomers were corrected for the difference in mean fu, there were no significant differences between enantiomers for these variables. As the 20%-30% difference between the enantiomers in total CL and total Vss could be accounted for by stereoselectivity in fu, these differences are not likely to be clinically significant. During 105-180 min IV infusion of racemic thiopental to the other patients, there was no difference between enantiomers in mean plasma concentrations of total or unbound thiopental or total pentobarbital, a major metabolite of thiopental (P > 0.05). Therefore, it is appropriate to relate pharmacodynamic effects to racemic plasma concentrations of thiopental during IV infusion of racemic thiopental.

High-performance liquid chromatographic determination of thiopentone enantiomers in sheep plasma

An HPLC method was developed to determine the plasma concentrations of R(+)- and S(-)-thiopentone for pharmacokinetic studies in sheep. The method required separation of the thiopentone enantiomers from the corresponding pentobarbitone enantiomers which are usually present as metabolites of thiopentone. Phenylbutazone was used as an internal standard. After acidification, the plasma sample were extracted with a mixture of ether and hexane (2:8). The solvent was evaporated to dryness and the residues were reconstituted with sodium hydroxide solution (pH 10). The samples were chromatographed on a 100 mm x 4 mm I.D. Chiral AGP-CSP column. The mobile phase was 4.5% 2-propanol in 0.1 M phosphate buffer (pH 6.2) with a flow-rate of 0.9 ml/min. This gave k' values of 1.92, 2.92, 5.71, 9.30 and 11.98 for R(+)-pentobarbitone, S(-)-pentobarbitone, R(+)-thiopentone, S(-)-thiopentone, and phenylbutazone, respectively. At detection wavelength of 287 nm, the limit of quantitation was 5 ng/ml for R(+)-thiopentone and 6 ng/ml for S(-)-thiopentone. The inter-day coefficients of variation at concentrations of 0.02, 0.1 and 8 micrograms/ml were, respectively, 4.8, 4.4 and 3.5% for R(+)-thiopentone and, respectively, 5.0, 4.3 and 3.9% for S(-)-thiopentone (n = 6 each enantiomer). At the same concentrations, the intra-day coefficients of variation from six sets of replicates (measured over six days) were, respectively, 8.0, 8.0 and 8.8% for R(+)-thiopentone and 8.8, 7.4 and 9.6% for S(-)-thiopentone. Linearity over the standard range, 0.01-40 micrograms/ml, was shown by correlation coefficients > 0.998. This method has proven suitable for pharmacokinetic studies of thiopentone enantiomers after administration of rac-thiopentone in human plasma also and would be suitable for pharmacokinetic studies of the pentobarbitone enantiomers.

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.

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.