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

Pain sensations to the cold pressor test in normally menstruating women: comparison with men and relation to menstrual phase and serum sex steroid levels

The role of gonadal hormones on pain sensations was investigated in normally menstruating women (n = 16) using the cold pressor test. Tolerance time, pain threshold, and pain intensity were examined once a week during a 4-wk period, and serum concentrations of 17beta-estradiol and progesterone were determined at each test session, which were classified into the early follicular phase, late follicular phase, early luteal phase, and late luteal phase, as determined by the first day of menses and the actual hormone levels recorded. A group of men (n = 10) of the same age interval was examined for comparison. The data show that pain threshold was reduced during the late luteal phase compared with the late follicular phase, and hormone analyses showed significant positive correlation between the progesterone concentration and lowered pain threshold and increasing pain intensity. Hormone analysis also showed an interaction between S-estradiol and S-progesterone on pain intensity, demonstrating that the increased perceived pain intensity that was associated with high progesterone concentrations was significantly reduced with increasing levels of estradiol. While no statistically significant sex differences in pain measurements were found, women displayed much more pronounced, and statistically significant, session-to-session effects than men, with increased pain threshold and decreased pain intensity with each test session. Hence, these data suggest that the changes in the serum concentration of gonadal hormones that occur during the menstrual cycle influence pain sensations elicited by noxious tonic cold stimulation and show that adaptation to the cold pressor test may be sex dependent.

[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)

Recovery after total intravenous anaesthesia using combined midazolam/alfentanil infusion and reversal with flumazenil

One of the major problems with total intravenous anaesthesia (TIVA) is postoperative sedation, possibly with respiratory depression. The aim of the present study was to evaluate the recovery characteristics after TIVA using a continuous infusion of a mixture of midazolam and alfentanil with flumazenil reversal before extubation. This method was compared to balanced anaesthesia using midazolam, alfentanil and nitrous oxide without flumazenil reversal. The degree of sedation was measured by reaction time test, Glasgow Coma Scale, cipher copying test and subtraction test. We found significantly faster reaction times postoperatively in the TIVA group (n = 15) compared to the balanced group (n = 13), despite larger doses of both midazolam (median 21 mg versus 9 mg) and alfentanil (median 5.9 mg versus 4.5 mg). The other tests revealed no difference between the groups. One patient became resedated after flumazenil. We conclude that the TIVA technique described here resulted in slightly better recovery characteristics, offering a usable alternative to balanced anaesthesia.

Quantitative analysis of serum methohexital by GLC using capillary column and nitrogen-selective detection

A gas chromatographic method for routine quantitation of methohexital in plasma samples is reported. One-step extraction in organic phase, the use of a fused silica capillary column, and nitrogen-selective detection permit simple, precise, and sensitive determination of methohexital in plasma. A linear relationship is described between peak height ratio and methohexital concentrations ranging from 0.125 to 50.0 micrograms/ml (r = 0.998). The sensitivity limit of the assay was 6 ng/ml in plasma. No interfering peak was observed with numerous other drugs. The procedure was successfully applied to the determination of pharmacokinetic parameters of methohexital after IV administration or continuous infusion in a child and an adult.

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.

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.

Anaesthesia for extracorporeal shock-wave lithotripsy. A comparison of propofol and methohexitone infusions during high frequency jet ventilation

A continuous infusion of propofol 2.0-2.5 mg/kg for induction followed by 9 mg/kg/hour for the first 30 minutes and 6 mg/kg/hour thereafter, was compared with methohexitone 1.5 mg/kg for induction followed by 4.8 mg/kg/hour thereafter for maintenance of anaesthesia in a randomised study of 40 patients who underwent extracorporeal shock-wave lithotripsy using high frequency jet ventilation (HFJV). Systolic blood pressure was significantly lower in the propofol group after induction of anaesthesia, tracheal intubation, placement in the semirecumbent position in the hoist, bath immersion and after 5, 10 and 30 minutes of treatment. Diastolic blood pressure was significantly lower in the propofol group after intubation, placement in the hoist, bath immersion and after 5, 10 and 15 minutes of treatment. Heart rate was significantly lower in the propofol group after induction, intubation, placement in the hoist and bath immersion. There was no significant difference in the quality of induction between the two groups. Quality of maintenance of anaesthesia was judged to be poor in six out of 20 patients who received methohexitone compared with one out of 20 who received propofol but this difference did not reach statistical significance. There was no significant difference between the recovery times for the two groups but six out of 20 patients who received methohexitone were judged to have a poor recovery from anaesthesia compared with none in the propofol group (p less than 0.05). Propofol was associated with excellent induction, maintenance and recovery characteristics but it had a propensity to produce greater decreases in blood pressure, which were most marked when the patient was placed in the semirecumbent position in the hoist.

Computer applications in intravenous anaesthetic administration

The place of computerization in intravenous anaesthesia delivery: Although total intravenous anaesthesia may have advantages over inhalational anaesthesia in certain circumstances, it has drawbacks from the point of view of feedback control. The ideal agent is not available, although di-isopropylphenol holds promise. There is an undefinable end-point. Inadequate dosage produces the extremely unpleasant phenomenon of intra-operative awareness. Future developments, it seems, should include the development of suitable intravenous agents and transducers of anaesthetic depth, rather than increasingly complicated control systems. It is extremely unlikely that a computer will replace the anaesthetist in the foreseeable future. The anaesthetist is still required for, amongst other things, specifying the desired depth of anaesthesia and varying it during the operation, and for responding to unforeseen crises. It may be hoped that, by liberating the anaesthetist from those tasks which can be automated, more time can be devoted to patient monitoring and other aspects of anaesthetic care, thereby improving patient safety. There is an undoubted place for computerized delivery of anaesthesia in teaching (particularly teaching pharmacokinetic principles) and in research (for standardization of anaesthetic depth).

Pentobarbitone formation during thiopentone infusion

The plasma concentrations and elimination half-life of pentobarbitone were determined in 14 surgical patients receiving a continuous, exponentially decreasing, infusion of thiopentone (mean total dose, 1.05 g; SD 0.34; mean duration of infusion 2.4 hours, SD 0.7) as the primary anaesthetic agent. The plasma pentobarbitone concentration increased gradually, to reach a maximum of 1.49 micrograms/ml (SD 0.61) at the end of the thiopentone infusion, which was 15.5 per cent (SD 6.04) of the plasma thiopentone concentration. The elimination half-life of pentobarbitone measured over the following 70 hours in nine of the patients was 34.3 hours (SD 8.2), which is within the range of values reported previously in several studies in which pentobarbitone was administered directly to volunteers. It was concluded that the formation of this active metabolite during 2-3 hour thiopentone infusions was unlikely to be of clinical relevance, but that significant concentrations may occur with longer thiopentone infusions.