Fentanyl does not alter the "sleep" plasma concentration of thiopental

Interactions of anesthetics with their targets: non-specific, specific or both?

What makes a general anesthetic a general anesthetic? We shall review first what general anesthesia is all about and which drugs are being used as anesthetics. There is neither a unique definition of general anesthesia nor any consensus on how to measure it. Diverse drugs and combinations of drugs generate general anesthetic states of sometimes very different clinical quality. Yet the principal drugs are still considered to belong to the same class of 'general anesthetics'. Effective concentrations of inhalation anesthetics are in the high micromolar range and above, and even for intravenous anesthetics they do not go below the micromolar range. At these concentrations, many molecular and higher level targets are affected by inhalation anesthetics, fewer probably by intravenous anesthetics. The only physicochemical characteristic shared by anesthetics is the correlation of their anesthetic potencies with hydrophobicity. These correlations depend on the group of general anesthetics considered. In this review, anesthetic potencies for many different targets are plotted against octanol/water partition coefficients as measure of hydrophobicity. Qualitatively, similar correlations result, suggesting several but weak interactions with proteins as being characteristic of anesthetic actions. The polar interactions involved are weak, being roughly equal in magnitude to hydrophobic interactions. Generally, intravenous anesthetics are noticeably more potent than inhalation anesthetics. They differ considerably more between each other in their interactions with various targets than inhalation anesthetics do, making it difficult to come to a decision which of these should be used in future studies as representative 'prototypical general anesthetics'.

Effect-site concentration of propofol for recovery of consciousness is virtually independent of fentanyl effect-site concentration

Fentanyl reduces the amount of propofol necessary to prevent responses to surgical stimuli. However, opioids have relatively little effect on consciousness. We, therefore, tested the hypothesis that fentanyl minimally alters the effect-site concentration of propofol associated with awakening. Fifty women having gynecologic laparotomy with propofol anesthesia were randomly allocated into the following target effect-site fentanyl concentrations: 0.8, 1.0, 1.4, 2.0, and 3.0 ng/mL. Fentanyl was continued at the designated rate through the initial postoperative phase. The propofol effect-site concentration associated with eye opening in response to verbal command was regarded as the awakening concentration. The estimated propofol effect-site concentrations at awakening did not differ significantly among the groups and were 1.9 +/- 0.5 micro g/mL with a fentanyl effect-site concentration of 0.8 ng/mL; 1.6 +/- 0.4 micro g/mL with 1.0 ng/mL of fentanyl; 1.6 +/- 0.2 micro g/mL with 1.4 ng/mL of fentanyl; 1.7 +/- 0.4 micro g/mL with 2.0 ng/mL of fentanyl; and 1.6 +/- 0.34 micro g/mL with 3.0 ng/mL of fentanyl (mean +/- SD). Seventy percent of the subjects in the 0.8 ng/mL fentanyl group spontaneously complained of pain, whereas none of the patients in the 2 or 3 ng/mL groups did. Five (56%) of 9 women in the 3 ng/mL group had a postoperative respiratory rate <6 breaths/min. Heart rate in one of these women decreased to <40 bpm. These data suggest that the optimal fentanyl effect-site concentration in patients recovering from gynecologic laparoscopy is between 1.4 and 2.0 ng/mL.

IMPLICATIONS

The effect-site concentration for propofol at awakening was virtually independent of the fentanyl effect-site concentration over the range of 0.8 to 3.0 ng/mL; however, 0.8 ng/mL of fentanyl was associated with inadequate postoperative analgesia, and 3.0 ng/mL of fentanyl was associated with respiratory toxicity. The optimal postoperative fentanyl effect-site concentration during recovery from propofol general anesthesia for laparotomy thus appears to be near 2 ng/mL.

The interaction of eltanolone and fentanyl with special reference to logistic regression analysis

We investigated whether fentanyl decreases the serum concentrations of the steroid anesthetic eltanolone effective in producing loss of consciousness in 50% of patients (EC50induction) and in preventing movement at skin incision in 50% of patients (EC50incision). For anesthetic induction, patients received effect-site target concentrations of fentanyl 0.0, 1.5, 3.0, or 4.5 ng/mL and eltanolone 500, 750, 1000, or 1200 ng/mL. Loss of response to verbal command was assessed after 10 min. For incision, patients received effect-site target concentrations of fentanyl 0.5,1.5, 3.0, or 4.5 ng/mL and eltanolone 547-2926 ng/mL. Movement at incision was assessed at least 10 min after new targets were entered. Probability of loss of consciousness and of movement versus arterial serum concentration combinations were analyzed by logistic regression. Dixon up-down analysis was used to estimate ET50incision effective target concentration combinations. In the absence of fentanyl, anesthesia was induced in only 1 of 12 patients, which suggests that the EC50induction is >1500 ng/mL at fentanyl 0.0 ng/mL. With fentanyl (38 patients), eltanolone EC50induction was independent of fentanyl concentration, calculated as 628 ng/mL. For the incision phase (52 patients), logistic regression failed to generate a valid model. Dixon analysis (43 patients) produced an eltanolone ET50incision of 2288 ng/mL at fentanyl targets of 0.5 ng/mL, 754 ng/mL at 1.5 ng/mL, 735 ng/mL at 3.0 ng/mL, and 645 ng/mL at 4.5 ng/mL. Fentanyl reduced the serum concentration of eltanolone required to produce loss of consciousness and the target concentration of eltanolone required to prevent movement to skin incision.

IMPLICATIONS

Fentanyl reduced the serum concentration of eltanolone required to produce loss of consciousness and the target concentration of eltanolone required to prevent movement to skin incision. Future interaction studies of this nature using logistic regression should model responses to hypnotic alone separately from responses to hypnotic-analgesic combinations.

How to manage drug interactions

Multiple drugs are used to provide anaesthesia. On average, four to six drugs are used during anaesthesia and, therefore, drug interactions are common. These interactions are primarily either pharmacokinetic or pharmacodynamic. Due to the relatively short duration of drug administration for anaesthesia, pharmacokinetic drug interactions resulting from alterations in drug metabolism do not generally produce clinically significant effects. Pharmacodynamic-drug interactions between anaesthetic drugs, however, are potentially serious. This may reflect that anaesthesia is not a single entity, but a process provided by a combination of drugs; i.e. loss of consciousness, analgesia and neuromuscular blockade. An understanding of each drug's pharmacokinetics, pharmacodynamics and drug interactions will allow clinicians to administer drugs to provide a more optimal anaesthetic.

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.

Pharmacokinetic-pharmacodynamic relationships for opioids in balanced anaesthesia

The pure mu-receptor opioid agonists fentanyl, sufentanil and alfentanil are commonly used to provide the specific anti-nociceptive component of a balanced anaesthesia technique. Trefentanil and remifentanil are new opioids with a very short duration of action. Remifentanil has an ester structure and is very rapidly metabolised by blood and tissue esterases. Different perioperative stimuli require different plasma concentrations to suppress responses of the patient. The ability of the anaesthesiologist to select a precise dosage scheme for the individual patient is impeded by the large interindividual pharmacokinetic and pharmacodynamic variability. In addition, the combination of opioids and other drugs used to produce the desired components of balanced anaesthesia may exert additive, synergistic or antagonistic effects. Knowledge of factors influencing the pharmacokinetics and pharmacodynamics is still fragmentary and often controversial. Consequently, the opioid dose needs to be adjusted according to the responses of the patient during surgery to ensure adequate anaesthesia and rapid recovery. The duration of action is not predicted by the elimination half-life alone. The decline in effect-site concentration is dependent on the complex entity of infusion duration, and pharmacokinetic and pharmacodynamic parameters. Computer simulations of infusions of varying duration have been extremely useful when selecting an opioid for a specific clinical scenario on a rational basis. Traditionally, opioids are still administered by intermittent bolus injections. A disadvantage of this method of administration is that plasma concentrations fluctuate above and below the level required for adequate anaesthesia. Computer-assisted infusion pumps make it possible to target a particular drug concentration in plasma and to maintain or change this concentration as needed. This technique provides more stable anaesthesia and a more rapid recovery of the patient.

Arterial and jugular venous bulb blood propofol concentrations during induction of anesthesia

The aim of this study was to show that blood propofol concentrations at loss of consciousness vary with the rate of administration. Eighteen patients were allocated to receive a propofol infusion at 6 or 12 mg.kg-1.h-1 (approximately 8 and 15 mg/min) for induction of anesthesia. Propofol concentrations were analyzed from simultaneous arterial and jugular bulb venous blood samples. There were no significant differences in the dose of propofol administered to induce anesthesia (0.52 mg/kg both groups). However there were significant differences between the groups in the mean induction times (309 and 156 s), and in median arterial and venous concentrations at induction. Arterial concentrations were 1.93 and 2.70 and venous 1.11 and 1.51 micrograms/mL. There were no significant differences between the groups in the area between the arterial and venous time concentration curves from start of infusion to loss of consciousness (3.14 and 3.05 micrograms.mL-1.min-1). This study confirms that a target blood concentration of propofol cannot be identified with loss of consciousness under nonsteady state conditions. Both arterial and venous blood propofol concentrations at loss of consciousness depend on the rate of administration.