5-Alpha-dihydroprogesterone formation in human placenta from 5alpha-pregnan-3beta/alpha-ol-20-ones and 5-pregnan-3beta-yl-20-one sulfate

5Alpha-dihydroprogesterone (5alpha-DHP) is the immediate precursor of 5alpha-pregnan-3alpha-ol-20-one, a potent anxiolytic/anesthetic agent in all vertebrate animals tested, including humans. The levels of 5alpha-DHP in the plasma of pregnant women are very high; and during the third trimester of pregnancy, the blood production rate of this steroid may exceed 100 mg/24 h. 5Alpha-DHP in maternal plasma, however, cannot be accounted for totally by the metabolism of maternal plasma progesterone. This study was conducted to evaluate the possibility that 5alpha-DHP is synthesized in placenta from 5alpha-pregnan-3alpha/beta-ol-20-ones delivered to the trophoblast via the fetal umbilical blood. In incubations of placental minces with radiolabelled 5alpha-pregnan-3alpha/beta-ol-20-ones, there is extensive epimerization and the intermediate, 5alpha-DHP, is the major product. In other incubations, 5alpha-pregnan-3beta-ol-20-one-sulfate was hydrolysed and the liberated 5alpha-pregnan-3beta-ol-20-one was converted to 5alpha-DHP by homogenates of placental tissue, but 5alpha-pregnan-3beta-ol-20-one-sulfate was not. The oxidation of 5alpha-pregnan-3alpha/beta-ol-20-ones was concentrated in microsome-enriched preparations of placental tissue and the apparent Kms for 5alpha-pregnan-3alpha-ol-20-one and 5alpha-pregnan-3beta-ol-20-one were 3.6 microM and 78 nM, respectively. The Vmaxs for 5alpha-DHP formation from 5alpha-pregnan-3alpha-ol-20-one and 5alpha-pregnan-3beta-ol-20-one were, respectively, 336 pmol/min/mg protein and 9.7 nmol/min/mg protein. These oxidation reactions were supported by both NAD+ and NADP+. We suggest that progesterone, which enters the umbilical circulation from its site of synthesis in the syncytiotrophoblast, is metabolized in the fetus to 5alpha-pregnan-3alpha/beta-ol-ones and to 5alpha-pregnan-3alpha/beta-yl-20-one sulfates. These metabolites of progesterone, 5alpha-pregnan-3alpha/beta-ol-20-one and 5alpha-pregnan-3beta-yl-20-one sulfate, formed in the fetus, serve as plasma-borne substrates for trophoblast formation of 5alpha-DHP. Because of the hemochorioendothelial nature of human placentation, 5alpha-DHP secreted from the trophoblast will preferentially enter the maternal compartment, thus constituting a maternal plasma progesterone-independent source of 5alpha-DHP.

Entropy-driven interactions of anesthetics with membrane proteins

Thermodynamic analysis of anesthetic effects on Ca2+-ATPase activity was performed to evaluate the feasibility of anesthetic binding and gain insight into the molecular events underlying the anesthetic-enzyme interactions. The Ca2+-ATPases, integral membrane proteins vital in cellular Ca2+ regulation, are suitable models for investigation of the mechanism of anesthetic action on membrane proteins that are targeted by the anesthetics. Ca2+-ATPase of plasma membrane, PMCA, and SERCA1 in the intracellular sarcoplasmic reticulum membrane were used to study two general anesthetics: halothane, a halogenated two-carbon alkane; and propofol, an intravenous, strongly lipophilic-substituted phenol. Interactions of both anesthetics result in a negative Gibbs free energy change, which in both enzymes is more favorable for the more lipophilic propofol than halothane. Temperature dependence (more negative change in Gibbs free energy at increased temperature) is in agreement with predominantly nonpolar interactions. The interactions are entropy-driven, characterized by positive enthalpy which is overcompensated by positive entropy changes. This is in contrast to the reported in literature enthalpy-driven anesthetic binding to soluble proteins. The possible contributions to the observed positive entropy change are discussed including displacement of ordered water molecules by anesthetic binding in nonpolar cavities in the membrane proteins and subtle structural rearrangements.

Use of yohimbine to reverse prolonged effects of xylazine hydrochloride in a horse being treated with chloramphenicol

A 1-year-old Standardbred gelding had received xylazine hydrochloride (0.75 to 1.00 mg/kg [0.34 to 0.45 mg/lb] of body weight, IV) during 2 surgeries for debridement of a wound. The horse was given chloramphenicol (55 mg/kg [25 mg/lb], PO, q 6 h) for 5 days, and was anesthetized a third time with xylazine (0.75 mg/kg, IM). Five hours after administration of xylazine, the horse remained markedly sedated and had clinical signs of gaseous distention of the large bowel (bloat) requiring trocharization. Administration of yohimbine (0.03 mg/kg [0.01 mg/lb], i.v.) eliminated signs of sedation within 5 minutes. Moderate flatulence developed, and gastrointestinal sounds could be heard within all 4 abdominal quadrants within 15 minutes of yohimbine administration. The remainder of recovery was unremarkable. Xylazine induces bradycardia and decreases gastrointestinal motility in addition to causing sedation, muscle relaxation, and analgesia. Chloramphenicol can inhibit oxidase activity of cytochrome P-450 and inhibit metabolism and elimination of drugs such as xylazine.

Different distribution of fluorinated anesthetics and nonanesthetics in model membrane: a 19F NMR study

Despite their structural resemblance, a pair of cyclic halogenated compounds, 1-chloro-1,2,2-trifluorocyclobutane (F3) and 1,2-dichlorohexafluorocyclobutane (F6), exhibit completely different anesthetic properties. Whereas the former is a potent general anesthetic, the latter produces no anesthesia. Two linear compounds, isoflurane and 2,3-dichlorooctofluorobutane (F8), although not a structural pair, also show the same anesthetic discrepancy. Using 19F nuclear magnetic spectroscopy, we investigated the time-averaged submolecular distribution of these compounds in a vesicle suspension of phosphatidylcholine lipids. A two-site exchange model was used to interpret the observed changes in resonance frequencies as a function of the solubilization of these compounds in membrane and in water. At clinically relevant concentrations, the anesthetics F3 and isoflurane distributed preferentially to regions of the membrane that permit easy contact with water. The frequency changes of these two anesthetics can be well characterized by the two-site exchange model. In contrast, the nonanesthetics F6 and F8 solubilized deeply into the lipid core, and their frequency change significantly deviated from the prediction of the model. It is concluded that although anesthetics and nonanesthetics may show similar hydrophobicity in bulk solvents such as olive oil, their distributions in various regions in biomembranes, and hence their effective concentrations at different submolecular sites, may differ significantly.

Titration calorimetry of anesthetic-protein interaction: negative enthalpy of binding and anesthetic potency

Anesthetic potency increases at lower temperatures. In contrast, the transfer enthalpy of volatile anesthetics from water to macromolecules is usually positive. The transfer decreases at lower temperature. It was proposed that a few selective proteins bind volatile anesthetics with negative delta H, and these proteins are involved in signal transduction. There has been no report on direct estimation of binding delta H of anesthetics to proteins. This study used isothermal titration calorimetry to analyze chloroform binding to bovine serum albumin. The calorimetrically measured delta H cal was -10.37 kJ.mol-1. Thus the negative delta H of anesthetic binding is not limited to signal transduction proteins. The binding was saturable following Fermi-Dirac statistics and is characterized by the Langmuir adsorption isotherms, which is interfacial. The high-affinity association constant, K, was 2150 +/- 132 M-1 (KD = 0.47 mM) with the maximum binding number, Bmax = 3.7 +/- 0.2. The low-affinity K was 189 +/- 3.8 M-1 (KD = 5.29 mM), with a Bmax of 13.2 +/- 0.3. Anesthetic potency is a function of the activity of anesthetic molecules, not the concentration. Because the sign of delta H determines the temperature dependence of distribution of anesthetic molecules, it is irrelevant to the temperature dependence of anesthetic potency.

Molecular sieves: an alternative method of carbon dioxide removal which does not generate compound A during simulated low-flow sevoflurane anaesthesia

Molecular sieves are used in industry to 'scrub' industrial gases. We examined, during simulated low-flow closed system anaesthesia, (1) the carbon dioxide adsorbing potential of molecular sieves and (2) the reactivity of the sieves compared to soda lime using sevoflurane as an indicator. A low-flow anaesthetic system containing 13X molecular sieves was connected to a model lung. End-tidal concentrations of CO2 were measured continuously at an O2 flow of 800 ml.min-1 and a CO2 flow of 200 ml.min-1. In the second study, sevoflurane (FE'sevo 1.7%) was added to the system after which samples were taken from the inspiratory limb of the anaesthetic system. This experiment was performed both during carbon dioxide removal with soda lime and with the molecular sieves. The samples were stored in gas-tight syringes and analysed by gas chromatography. The temperature of both absorbents was measured throughout the study. The molecular sieves adsorbed carbon dioxide (20%) efficiently for a period of 5 h. There was a gradual increase from the baseline of 4.4% to 4.5, 5.4, and 6.0% at 90, 180, and 300 min, respectively. When sevoflurane was added to the system, compound A was detected at the start of both experiments. However, when soda lime was used the concentrations of compound A increased 10-fold after 2.5 h compared with baseline values. No increase in compound A was observed when molecular sieves were used for carbon dioxide removal. The highest mean (SD) temperature of the molecular sieves was 41.5 (3.2) degrees C. Molecular sieves are effective adsorbents of carbon dioxide when used in a simulated low-flow, closed anaesthetic system.(ABSTRACT TRUNCATED AT 250 WORDS)

[Cytochrome P450-dependent monooxygenase system and anesthetics]

The cytochrome P450-dependent monooxygenases constitute the primary enzyme system responsible for the oxidative metabolism of a variety of xenobiotics and endogenous compounds including drugs, carcinogens, fatty acids and hormones. The monooxygenase system consists of multiple forms of P450 enzymes, NADPH-cytochrome reductase and phospholipids. The level sof P450s and associated monooxygenase activities are subject to be regulated by many environmental, physiological, and pathological factors. Inhalation and intravenous anesthetics are all metabolized through these biotransformation enzymes. The pharmacokinetic properties as well as the toxicity of the anesthetics are closely related to the inducing or inhibitory status of the monooxygenase isozymes. To understand the role of cytochrome P450-monooxygenases in drug metabolism is essential for us to handle the drug-to-drug interactions and adverse effects.

Relationship of inspired anesthetic concentration to plasma concentration and urinary excretion of sevoflurane metabolites in rats

In patients, plasma concentrations of sevoflurane metabolites may be independent of inspired sevoflurane concentration over a defined dose range. In contrast, studies using rabbits have found that plasma concentrations and urinary excretion of fluoride ion are dose-dependent up to 3% inspired sevoflurane. We measured sevoflurane metabolite concentrations in adult male Sprague-Dawley rats and related them to inspired sevoflurane concentrations. When plasma concentrations and urinary excretion of metabolites were measured in vivo, they were dependent on inspired anesthetic concentration at concentrations less than 1.25%, but became less dose-dependent at higher anesthetic concentrations. Sevoflurane metabolism by precision-cut liver slices in vitro became dose-independent at more than 10-30 microM sevoflurane. No evidence of substrate inhibition was observed. These data provide evidence that sevoflurane metabolite concentrations are almost independent of inspired anesthetic concentration over at least part of the clinically used concentration range.

Analysis of sevoflurane degradation products in vapor phase samples

Sevoflurane degradation products were measured by GC-flame ionization detection in vapor phase samples using manual and automated injection methods. Sample handling techniques allowed the transfer and storage of samples for up to 72 h. Compound A, fluoromethyl 2,2-difluoro-1-(trifluoromethyl)vinyl ether, was the major vapor phase degradation product formed in simulated clinical conditions. Recoveries of 4-32 ppm (v/v) compound A concentrations using the manual method were in the range of 88-117% (n = 12, mean = 102%, R.S.D. = 9%).

Clinical sevoflurane metabolism and disposition. II. The role of cytochrome P450 2E1 in fluoride and hexafluoroisopropanol formation

BACKGROUND

Sevoflurane is metabolized to free fluoride and hexafluoroisopropanol (HFIP). Cytochrome P450 2E1 is the major isoform responsible for sevoflurane metabolism by human liver microsomes in vitro. This investigation tested the hypothesis that P450 2E1 is predominantly responsible for sevoflurane metabolism in vivo. Disulfiram, which is converted in vivo to a selective inhibitor of P450 2E1, was used as a metabolic probe for P450 2E1.

METHODS

Twenty-one patients within 30% of ideal body weight, who provided institutional review board-approved informed consent and were randomized to receive disulfiram (500 mg oral, n = 11) or nothing (control, n = 10) the night before surgery, were evaluated. All patients received sevoflurane (2.7% end-tidal, 1.3 MAC) in oxygen for 3 h after propofol induction. Thereafter, sevoflurane was discontinued, and anesthesia was maintained with propofol, fentanyl, and nitrous oxide. Blood sevoflurane concentrations during anesthesia and for 8 h thereafter were measured by gas chromatography. Plasma and urine fluoride and total (unconjugated plus glucuronidated) HFIP concentrations were measured by an ion-selective electrode and by gas chromatography, respectively, during anesthesia and for 96 h postoperatively.

RESULTS

Patient groups were similar with respect to age, weight, sex, case duration, and intraoperative blood loss. The total sevoflurane dose, measured by cumulative end-tidal sevoflurane concentrations (3.7 +/- 0.1 MAC-h; mean +/- SE), total pulmonary uptake, and blood sevoflurane concentrations, was similar in both groups. In control patients, plasma fluoride and HFIP concentrations were increased compared to baseline values intraoperatively and postoperatively for the first 48 and 60 h, respectively. Disulfiram treatment significantly diminished this increase. Plasma fluoride concentrations increased from 2.1 +/- 0.3 microM (baseline) to 36.2 +/- 3.9 microM (peak) in control patients, but only from 1.7 +/- 0.2 to 17.0 +/- 1.6 microM in disulfiram-treated patients (P < 0.05 compared with control patients). Peak plasma HFIP concentrations were 39.8 +/- 2.6 and 14.4 +/- 1.1 microM in control and disulfiram-treated patients (P < 0.05), respectively. Areas under the plasma fluoride- and HFIP-time curves also were diminished significantly to 22% and 20% of control patients, respectively, by disulfiram treatment. Urinary excretion of fluoride and HFIP was similarly significantly diminished in disulfiram-treated patients. Cumulative 96-h fluoride and HFIP excretion in disulfiram-treated patient was 1,080 +/- 210 and 960 +/- 240 mumol, respectively, compared to 3,950 +/- 560 and 4,300 +/- 540 mumol in control patients (P < 0.05).

CONCLUSIONS

Disulfiram, an effective P450 2E1 inhibitor, substantially decreased fluoride ion and HFIP production during and after sevoflurane anesthesia. These results suggest that P450 2E1 is a predominant P450 isoform responsible for human sevoflurane metabolism in vivo.

Actions of general anaesthetics on a neuronal nicotinic acetylcholine receptor in isolated identified neurones of Lymnaea stagnalis

1. Completely isolated identified neurones from the right parietal ganglion of the pond snail Lymnaea stagnalis were studied under two-electrode voltage-clamp. Neuronal nicotinic acetylcholine receptor currents were studied at low acetylcholine (ACh) concentrations (< or = 200 nM). At these levels, control currents were non-desensitizing and proportional to the square of the ACh concentration. 2. IC50 concentrations were determined for the steady-state inhibition of the ACh-activated current by 31 general anaesthetics plus the non-anaesthetic alcohol n-tridecanol. The general anaesthetics included inhalational agents, n-alcohols, n-alkane-(alpha,omega)-diols, cycloalcohols and an n-alkane. 3. Anaesthetic inhibition was independent of voltage and consistent with two anaesthetic-binding sites on the receptor. 4. IC50 concentrations for inhibiting the neuronal nicotinic ACh receptor correlated well (r = 0.97) with EC50 concentrations for general anaesthesia. The maximum deviation from the line of identity was less than fourfold. The inhalational agents tended to be more potent as inhibitors of the ACh receptor than as general anaesthetics, while the alcohols and diols were less potent. 5. The inhibition of the ACh-induced current by the homologous series of n-alcohols exhibited a cutoff at the same position (just after dodecanol) as found for the induction of general anaesthesia in tadpoles. 6. Polarity profile maps of the anaesthetic-binding sites on the neuronal nicotinic ACh receptor were calculated from IC50 concentrations for the homologous series of n-alcohols and n-alkane-(alpha,omega)-diols. They reveal amphiphilic sites with apolar regions capable of accommodating the hydrocarbon chains of n-alcohols as large as decanol. A striking resemblance was found to profiles previously calculated from data for tadpole general anaesthesia.

Anaesthetic drugs in the elderly

The outcome of surgery in the elderly depends on the skill of the surgeon and the expertise of the anaesthetist caring for the patient. This anaesthetic proficiency depends initially on the application of the detailed knowledge of the drugs used during anaesthesia, and on the understanding of the way changes in the elderly patients' physiology with ageing affects both kinetic and dynamic facets of pharmacology. However, the very diversity of the elderly population makes their precise response to drugs unpredictable, and, therefore, clear, unequivocal guidance on practice is limited. The important changes in both physiology and pharmacokinetics with ageing are discussed before covering the specific drugs used in current anaesthetic practice in the United Kingdom. This review covers the drugs used primary for the induction and maintenance of anaesthesia, and only covers related fields where there are changes in recent practice that raise issues of importance during surgery. This review, therefore, deals mainly with the anaesthetic vapours and intravenous drugs, and with neuromuscular blocking agents. The restriction of space and the enormity of the subject demand that the areas covered in this review are solely limited to the major classes of drugs used routinely in general anaesthetic practice in the United Kingdom. The detailed pharmacokinetics and pharmacodynamics of drugs also used in the post-operative period, such as the wide variety of opiates, and of drugs mainly used in general medical practice, but occasionally used as premedicants or supplements during anaesthesia, have been monitored only in passing.

Potency of lipid and protein formulation of 5 alpha-pregnanolone at induction of anaesthesia and the corresponding regional brain distribution

We have studied the anaesthetic potencies of 5 alpha-pregnanolone albumin solution (PAS) and 5 alpha-pregnanolone Intralipid emulsion (PLE) at equivalent concentrations in male rats using an EEG threshold method. The criterion of anaesthesia was burst suppression of the EEG of 1 s or more (the "silent second" (SS)) as a sign of deep anaesthesia. The potency of the two formulations was assessed by comparing the threshold doses of 5 alpha-pregnanolone at three dose rates (1.0, 2.0 and 3.0 mg kg-1 min-1). We found that SS was initiated in all rats after infusions of PAS, while no SS could be induced in rats after infusion of PLE at a larger dose. A higher concentration of 5 alpha-pregnanolone was found in all brain and peripheral tissues of PAS-treated rats than in those treated with PLE. In rats with PAS-induced anaesthesia (3.0 mg kg-1 min-1), the highest concentrations were detected in striatum (mean 19.40 (SD 1.21) ng mg-1). Although there was a small insignificant reduction in threshold doses with dose rates at 2.0-3.0 mg kg-1 min-1, the tissue concentrations in striatum, frontal cortex and occipital cortex were found to be significantly increased. We conclude that PAS was more potent than PLE in inducing anaesthesia. Brain distribution of 5 alpha-pregnanolone varied regionally in a manner similar to the variation in GABAA receptor sensitivity to this neuroactive steroid.

Degradation products of sevoflurane during low-flow anaesthesia

Low-flow (1 litre min-1) sevoflurane anaesthesia was used in 16 patients undergoing laparoscopic cholecystectomy (group LSC, n = 8) or tympanoplasty (group TP, n = 8), and concentrations of sevoflurane degradation products were measured. Degradation products in the circuit were measured hourly, and end-tidal carbon dioxide concentration, inspired and end-tidal sevoflurane concentrations, and carbon dioxide elimination were monitored. The only degradation product detected was CF2=C(CF3)-O-CH2F (compound A). The mean maximum concentrations of compound A were 21.6 (SEM 1.6) ppm and 19.6 (0.8) ppm in the LSC and TP groups, respectively (ns). The maximum temperatures of soda lime were 46.4 (0.5) degrees C and 44.8 (0.5) degrees C, respectively (P < 0.05). Hourly end-tidal sevoflurane concentrations and concentrations of sevoflurane degradation products were the same for both groups. Carbon dioxide elimination was the same for both groups 1 h after the start of anaesthesia, but was higher in group LSC after 2 h (P < 0.05). Intraperitoneal carbon dioxide insufflation associated with laparoscopic cholecystectomy had no effect on the concentration of sevoflurane degradation products.

Determination of end-tidal sevoflurane concentration for tracheal intubation and minimum alveolar anesthetic concentration in adults

The purpose of this study was to determine both the concentration of sevoflurane required for tracheal intubation (MACEI) and its minimum alveolar anesthetic concentration (MAC) in adults. The study group consisted of 86 elective surgical patients, ASA physical status I or II, aged 16-59 yr. There was no premedication administered. For MACEI determination, after establishing and maintaining the predetermined end-tidal concentration for 20 min, tracheal intubation was attempted using a cuffed tracheal tube without muscle relaxant or other adjuvants. Each concentration at which tracheal intubation was attempted was predetermined as follows: 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, 6.5%, and 7.0%. For MAC determination, the patients examined were basically the same as those for MACEI determination, except for those who received muscle relaxant or other adjuvants because they were "not intubated smoothly." After establishing and maintaining the predetermined end-tidal concentration for 20 min, skin incision was attempted. Each concentration at which skin incision was attempted was predetermined as follows: 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%. The MACEI of sevoflurane was 4.52% (95% confidence limits, 3.91%-5.21%), and the ED95 for tracheal intubation was 8.07%. The MAC of sevoflurane was 1.58% (95% confidence limits, 1.14%-1.98%), and the AD95 (anesthetic ED95) was 2.96%. The MACEI/MAC ratio was 2.86 (95% confidence limits, 2.63-3.43). Anesthesia induction followed by tracheal intubation can be accomplished in adults when sevoflurane is administered as a sole anesthetic, but in excess of 8% end-tidal concentration.

The specific binding site of the volatile anesthetic diiodomethane to purple membrane by X-ray diffraction

To determine the binding site of volatile anesthetics on purple membrane, diiodomethane (CH2I2) was used as a label in X-ray diffraction experiments. At less than 3 mM diiodomethane, purple membrane retains its two-dimensional crystallinity of bacteriorhodopsin, absorption spectra show only a little blue shift and the decay time of the M-intermediate becomes fast in flash photolysis experiments. These effects are similar to those of other anesthetics previously studied. The position where the anesthetic binds was identified by difference Fourier methods, and refined by model calculations. This study suggests that volatile anesthetics bind specifically to the protein-lipid interfacial region near the surface of membrane.

The pharmacology of sevoflurane in infants and children

BACKGROUND

Sevoflurane is a new volatile anesthetic with physical properties that should make it suitable for anesthesia (MAC of sevoflurane on oxygen alone and in 60% nitrous oxide, (MAC) of sevoflurane in oxygen alone and in 60% nitrous oxide, the hemodynamic, induction and emergence responses to sevoflurane and the metabolism to inorganic fluoride were studied in 90 ASA physical status 1 or 2 neonates, infants, and children.

METHODS

MAC of sevoflurane in oxygen was determined in six groups of subjects stratified according to age: full-term neonates, infants 1-6 and > 6-12 months and children > 1-3, > 3-5 and > 5-12 yr. MAC in 60% nitrous oxide was determined in a separate group of children 1-3 yr of age. After an inhalational induction, the trachea was intubated (except for neonates in whom an awake intubation was performed). MAC for each age group was determined using the Up-and-Down technique of Dixon.

RESULTS

MAC of sevoflurane in neonates, 3.3 +/- 0.2% and in infants 1-6 months of age, 3.2 +/- 0.1%, were similar; MAC in older infants 6-12 months and children 1-12 yr was constant at approximately 2.5%; MAC of sevoflurane in 60% nitrous oxide in children 1-3 yr of age was 2.0 +/- 0.2%. Systolic arterial pressure decreased significantly at 1 MAC before skin incision compared with awake values in all subjects except children 1-3 yr with 60% nitrous oxide and children 5-12 yr in oxygen, and then returned toward awake values after skin incision. Heart rate was unchanged at approximately 1 MAC sevoflurane before incision compared with awake values in all subjects except children > 3-5 and > 5-12 yr in whom heart rate increased before incision. Induction of anesthesia, particularly with respect to airway irritability, and emergence from sevoflurane anesthesia were not remarkable. The plasma concentration of inorganic fluoride reached maximum values (8.8-16.7 microM) 30 min after discontinuation of anesthesia.

CONCLUSIONS

We conclude that sevoflurane appears to be a suitable anesthetic agent for use in neonates, infants and children undergoing < or = 1 h of anesthesia.

Anesthetic and hemodynamic interactions of dexmedetomidine and fentanyl in dogs

BACKGROUND

Anesthetic doses of dexmedetomidine (DMED), a highly selective alpha 2 agonist, are not well tolerated hemodynamically. The combination of an opioid with DMED might reduce the dosage requirements for each drug and thereby allow the same anesthetic depth to be achieved with lesser degrees of their individual side effects.

METHODS

Dogs were anesthetized with enflurane. One group (n = 5) received intravenous doses of DMED from 0.1 to 10 micrograms/kg. Two other groups of five dogs each received fentanyl 15 micrograms/kg plus 0.05 microgram.kg-1.min-1 or fentanyl 45 micrograms/kg plus 0.2 micrograms.kg-1.min-1. Thereafter, they received DMED doses of 0.03-3 micrograms/kg. After the effects of the last DMED dose were measured, atipamezole 0.3 mg/kg was infused intravenously and all measurements were repeated. Then, naloxone (1 mg/kg) was injected intravenously and a final set of measurements obtained. Anesthetic effects were assessed by determining enflurane minimum alveolar concentration (MAC). Hemodynamics and plasma fentanyl concentrations were measured at each determination of MAC.

RESULTS

DMED and fentanyl individually produced dose-related reductions of enflurane MAC. During the lower rate infusion of fentanyl (plasma fentanyl concentration 1.0 +/- 0.3 ng/ml), DMED reduced enflurane MAC more than could be attributed to a simple additive interaction. During the higher rate infusion of fentanyl (plasma fentanyl concentration 4.4 +/- 0.7 ng/ml), DMED reduced enflurane MAC to greater degrees than were achievable by fentanyl alone. DMED caused a dose-dependent increase in arterial pressure concomitantly with a decrease in cardiac output, and these changes were not modified by fentanyl. The bradycardia following DMED was augmented by fentanyl.

CONCLUSIONS

There was a positive interaction, additive or synergistic, between DMED and fentanyl with respect to their enflurane-sparing effects. The interaction allowed the same depth of anesthesia to be achieved by lower doses of all three drugs, potentially limiting the intensity of their individual side effects. However, the presence of fentanyl increased the degree of bradycardia induced by DMED.

Toxicity of compound A in rats. Effect of increasing duration of administration

BACKGROUND

An olefin called compound A (CF2 = C(CF3)OCH2F) results from the action of soda lime or Baralyme on sevoflurane. We have demonstrated that rats exposed to the olefin for 3 h died at or were injured by olefin concentrations lower than those previously reported to produce these effects. The present report examines the impact of duration of exposure to the olefin on such effects.

METHODS

Twenty-three groups of ten Wistar rats breathed 0, 12.5, 25, 50, 75, 100, 125, 150, 175, 200, 225, and 250 ppm of the olefin in oxygen for 6 or 12 h. Rats that survived were killed on day 1 or day 4 after breathing the olefin, and specimens of brain, kidney, lung, liver, and small intestine were obtained from all rats for examination by microscopy using hematoxylin and eosin stain and a stain (proliferating cell nuclear antigen) for cell growth (regeneration).

RESULTS

The lethal concentrations in 50% of rats equaled 203 +/- 4 ppm (mean +/- SE) for a 6-h exposure period and 127 +/- 9 ppm for a 12-h exposure period, and both values were less than the previously determined value of 331 +/- 7 ppm for a 3-h exposure period. Compared with results from control rats (those breathing oxygen for 6 h or 12 h), only renal and pulmonary injury were found. Pulmonary injury only occurred at near-lethal concentrations. Renal injury (defined as necrosis of the outer stripe of the outer medullary layer or corticomedullary junction necrosis) occurred at and above 25-50 ppm for 6-h and 12-h exposures, respectively, a result similar to that previously obtained with a 3-h exposure. Exposure to 25-50 ppm stimulated cell regeneration in a dose-related manner.

CONCLUSIONS

In rats, lethal concentrations of the olefin and concentrations producing severe renal injury are inversely related to the duration of exposure to the olefin, exceeding by two- to fourfold peak concentrations that can be obtained in clinical practice. The threshold concentrations for nephrotoxicity (i.e., minimal toxicity) equal concentrations that can be produced in clinical practice. However, even if these threshold effects in rats apply to humans, they probably would not alter renal function. Although dose-related, neither the lethal nor the toxic effects are simply a function of cumulative dose (concentration-time).

Toxicity of compound A in rats. Effect of a 3-hour administration

BACKGROUND

Soda lime converts sevoflurane to CF2 = C(CF3)OCH2F, an olefin called compound A, whose toxicity raises concerns regarding the safe administration of sevoflurane via rebreathing circuits. The present report extends the findings of a previous investigation by others of the toxicity of this olefin, and establishes concentration-response relationships for such toxicity.

METHODS

Eighteen groups of ten Wistar rats breathed 0, 25, 50, 100, 200, 300, 350, and 400 ppm of the olefin in oxygen for 3 h. The olefin concentrations were developed in a square-wave manner by injection of saturated vapor followed by a continuous delivery of dilute vapor. The lethal concentration in 50% (LC50) of animals was estimated by logistic regression. Rats were killed on day 1 or day 4 after breathing the olefin, and specimens of brain, kidney, lung, liver, and small intestine were obtained from all rats for examination using light microscopy.

RESULTS

The LC50 equaled 331 ppm (95% confidence limits +/- 13 ppm). No injury resulted to lung or small intestine in either the experimental or the control group (those breathing only oxygen for 3 h). Renal injury (necrosis of the outer strip of the outer medulla, defined in this report as corticomedullary tubular necrosis) occurred at 50 ppm and greater; hepatic injury at 350 ppm and greater; and cerebral injury only at 400 ppm.

CONCLUSIONS

The lethal concentration and the threshold for toxicity of the olefin are less than previously reported. The threshold for nephrotoxicity reaches the range of values for the olefin that have been attained in clinical practice. Further studies are required to determine whether these results in rats can be extrapolated to patients.

Anaesthetic agents and excretion in breast milk

This review is an update on anesthetic agents and their excretion into breast milk; it presents the reported effects on suckling infants, and discusses the precautions which should be considered. For most anaesthetic agents, there is very sparse information about breast milk excretion and even less published knowledge about the possible effects on the suckling infant. Generally, when an anaesthetic agent is given on a single-dose basis, there is no evidence that it is excreted in breast milk in clinically significant amounts, even if there are detectable concentrations of the drug in the milk. Most anaesthetics are rapidly cleared from the mother, and, consequently, it should be possible to allow suckling as soon as practically feasible after surgery. However, repeated administration of certain opiates and benzodiazepines has been reported to cause adverse effects in neonates, with premature neonates apparently being more susceptible. Thus, in long-term treatment with these drugs, the importance of uninterrupted breast feeding should be assessed against possible adverse drug effects in the neonate.

A simplified gas chromatographic method for quantifying the sevoflurane metabolite hexafluoroisopropanol

BACKGROUND

The results of sevoflurane biotransformation (fluoromethyl-1,1,1,3,3,3,-hexafluoro-2-propyl ether) to inorganic fluoride have been examined. However, these investigations have lacked a simplified assay for determining the primary organic metabolite, hexafluoroisopropanol. Previous attempts have involved extensive extraction steps, complicated derivatization techniques, or sophisticated detectors.

METHODS

After enzymatic hydrolysis of conjugates, hexafluoroisopropanol is detected readily using a head space gas chromatographic analysis with a flame ionization detector.

RESULTS

The gas chromatographic technique was linear from 10 to 800 microM with a correlation coefficient of 0.999. The detection limit was 10 microM in urine and 25 microM in blood.

CONCLUSIONS

This simplified approach does not require the extraction, derivatization, or mass spectrometric detectors of previous methods. As sevoflurane utilization and research increases, this assay should allow for a variety of laboratory and clinical disposition studies to be performed.