Pharmacokinetics of intravenous anaesthetics: implications for clinical use

Ketamine and Its Emergence in the Field of Neurology

The quest for a safe and effective anesthetic medication in the mid-20th century led to the discovery of CI-581, which was later named ketamine. Ketamine was labeled a “dissociative anesthetic” due to the state of sensory deprivation that it induces in the subjects receiving it. Although it enjoyed widespread use at the beginning of the Vietnam war, its use rapidly waned due to its psychedelic effect and it became more popular as a recreational drug, and in the field of veterinary medicine. However, as we gained more knowledge about its multiple sites of action, it has reemerged as a useful anesthetic/analgesic agent. In the last decade, the field of neurology has witnessed the growing use of ketamine for the treatment of several neurological conditions including migraine, status epilepticus, stroke, and traumatic brain injury (TBI). Ketamine acts primarily as a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist. The binding of ketamine to NMDA receptors leads to decreased frequency and duration of Ca⁺² channel opening and thus inhibits glutaminergic transmission. This mechanism has proven to be neuroprotective in several neurological conditions. Ketamine does not increase intracranial pressure (ICP), and it maintains cerebral perfusion pressure (CPP) by increasing cerebral blood flow. Ketamine has also been shown to inhibit massive slow waves of neurological depolarizations called cortical spreading depolarizations (CSD), usually seen during acute neurological injury and are responsible for further neurological deterioration. Unlike other anesthetic agents, ketamine does not cause cardiac or respiratory suppression. All these favorable mechanisms and cerebral/hemodynamic actions have led to increased interest among clinicians and researchers regarding the novel uses of ketamine. This review will focus on the use of ketamine for various neurological indications.

Oral management of the patient with end-stage liver disease and the liver transplant patient

The patient with end-stage liver disease who is in need of a liver transplant should have a pretransplant dental evaluation. Such a patient faces lifelong immunosuppression with an increased risk of infection. This article discusses both the need for control of oral diseases before liver transplantation and guidelines for oral care in the immediately postoperative and long-term transplant patient. Specific indications for antibiotic prophylaxis and antibiotic regimens are presented; in addition, adverse reactions and side effects of immunosuppressant drugs are discussed. Pertinent drug interactions salient to the dental management of patients with end-stage liver disease are reviewed, and specific management recommendations for these patients are presented.

Droperidol elimination after cardiopulmonary bypass surgery

A high-dose (0.75 to 2.8 mg/kg) pharmacokinetic study of droperidol was undertaken in patients during the recovery phase after cardiac surgery involving hypothermic cardiopulmonary bypass (CPB). The elimination half-life of droperidol in these patients, determined from concentration-time data obtained after CPB, was significantly prolonged relative to previously reported mean values in younger surgical patients not undergoing CPB and receiving lower doses of the drug (0.05-0.20 mg/kg). On stratification of the patients by droperidol dose, there was an inverse correlation between the size of the dose and the elimination half-life of droperidol: mean half-life decreased as mean dose increased. This difference in elimination half-life was not related to the duration of the CPB procedure, or the total anesthetic time, both of which were not significantly different between the patient groups receiving the three different doses of droperidol. The magnitude or duration of hypothermia after CPB did not differ between the three patient groups. The differences in half-lives are more likely due to the clinical condition of the patients, such that the patients who received the higher doses of droperidol were also judged clinically to be less ill and thus eliminated droperidol more efficiently. This hypothesis, however, could not be supported due to the small number of patients studied. The results obtained in this study indicate that droperidol elimination is significantly prolonged after high-dose administration to elderly patients undergoing hypothermic CPB procedures during cardiac surgery.

Propofol assay in biological fluids in pregnant women

A rapid, accurate and sensitive UV derivative method was described for measuring the Propofol concentration in some biological fluids. Furthermore two alternative procedures, a gaschromatographic and a colorimetric, were also defined, and the results of the three methods, when applied on blood samples spiked with known amounts of analyte, were compared. The samples were preliminary purified by a solid phase extraction on octadecyl C18 cartridge. The UV derivative method was applied to a pharmacokinetic study on pregnant women undergoing cesarean sections. After an induction dose administration of 2.5 mg kg-1, the maternal and umbilical vein blood were found to have comparable concentrations of propofol, with a mean half like of about 3.5 min; on the contrary no detectable levels of the drug were found in amniotic fluid. The drug recoveries > 98% and the response was linear over the range 0.05-40 micrograms ml-1.

A new hypothesis for electrical transmission in the mammalian central nervous system

It has been previously shown that the anatomical synapse does not exist in the living central nervous system, and that receptors are unlikely to be unique molecules sited on particular locations in or on the cell membrane. These conclusions and the rapidity of action of intravenously administered substances necessitated the adumbration of a new electrical hypothesis for the mechanism whereby signals pass from one part of the brain and spinal cord to another. The hypothesis proposed implies some testable predictions.

Anaesthesia for resection of lesions of the trachea and main bronchi using the neodymium yttrium aluminium garnet (Nd YAG) laser. A report of 75 treatments in 52 patients

The management of anaesthesia and outcome of 75 treatments in 52 patients are discussed. Total intravenous anaesthetic techniques were used with venturi ventilation via a rigid bronchoscope. A number of complications occurred, but there was no death or major surgical complication. All complications except one were apparent intraoperatively or in the recovery room. As the considerable risk of intraoperative hypoxia was considered to outweigh the minimal risk of intratracheal fire or explosion, no attempt was made to limit inspired oxygen concentration (FIO2) during the application of the laser. The results of ventilation/perfusion studies and a recent diagnostic bronchoscopy were very helpful in patient selection.

Pharmacokinetic implications for the clinical use of propofol

Propofol, the recently marketed intravenous induction agent for anaesthesia, is chemically unrelated to earlier anaesthetic drugs. This highly lipophilic agent has a fast onset and short, predictable duration of action due to its rapid penetration of the blood-brain barrier and distribution to the CNS, followed by redistribution to inactive tissue depots such as muscle and fat. On the basis of pharmacokinetic-pharmacodynamic modelling, a mean blood-brain equilibration half-life of only 2.9 minutes has been calculated. In most studies, the blood concentration curve of propofol has been best fitted to a 3-compartment open model, although in some patients only 2 exponential phases can be defined. The first exponential phase half-life of 2 to 3 minutes mirrors the rapid onset of action, the second (34 to 56 minutes) that of the high metabolic clearance, whereas the long third exponential phase half-life of 184 to 480 minutes describes the slow elimination of a small proportion of the drug remaining in poorly perfused tissues. Thus, after both a single intravenous injection and a continuous intravenous infusion, the blood concentrations rapidly decrease below those necessary to maintain sleep (around 1 mg/L), based on both the rapid distribution, redistribution and metabolism during the first and second exponential phases (more than 70% of the drug is eliminated during these 2 phases). During long term intravenous infusions cumulative drug concentrations and effects might be expected, but even then the recovery times do not appear to be much delayed. The liver is probably the main eliminating organ, and renal clearance appears to play little part in the total clearance of propofol. On the other hand, because the total body clearance may exceed liver blood flow, an extrahepatic metabolism or extrarenal elimination (e.g. via the lungs) has been suggested. Approximately 60% of a radiolabelled dose of propofol is excreted in the urine as 1- and 4-glucuronide and 4-sulphate conjugates of 2.6-diisopropyl 1,4-quinol, and the remainder consists of the propofol glucuronide. Thus for hepatic and renal diseases, co-medication, surgical procedure, gender and obesity do not appear to cause clinically significant changes in the pharmacokinetic profile of propofol, but the decrease in the clearance value in the elderly might produce higher concentrations during a long term infusion, with an increased drug effect. In addition, the lower induction dose observed in relation to increased age might be partly explained by a smaller central volume of distribution.(ABSTRACT TRUNCATED AT 400 WORDS)

Placental transfer and tissue distribution of thiopental in the pregnant rat

The pharmacokinetic profile of thiopental was studied in pregnant rats after an iv bolus dose of 15 mg/kg. The unbound concentration-time profile of the drug in maternal plasma, placenta, fetal brain, fetal carcass, and amniotic fluid was described, developing an adequate pharmacokinetic model. Maternal plasma levels of thiopental fell rapidly after injection, distributing into tissues (half-life of distribution phase averaged 3 min). Thiopental crossed the placenta and entered the fetal body (brain included) and amniotic fluid. Peak levels were seen within 10 min of injection and declined in all tissues parallel to maternal plasma (rate constant range 0.012-0.017 min-1). The concentrations of drug in the fetal unit were smaller than in the central compartment and maternal plasma. However, the absolute transfer ratios (calculated using the pharmacokinetic parameters obtained from the model) and the relative exposure ratios (as the ratio of the area under the unbound concentration-time curve in tissue to that in maternal plasma) suggested that fetuses were exposed to a potentially efficacious level of the drug. The model formulated to describe the tissue distribution of thiopental may offer a useful approach for analysis of the kinetic profile of other compounds administered during pregnancy or at delivery in rats and other species.

The volatile anesthetics, halothane, enflurane and isoflurane, influence the distribution of thiopental in man differently

In man, a change of thiopental pharmacokinetics was observed under halothane anesthesia, but not when patients were anesthetized with enflurane and isoflurane. After an initial subanesthetic dose of 50 mg thiopental, the concentrations in serum (T) were determined over 15 min (4 samples). From these T-values the pharmacokinetic parameters Vc (central volume of distribution), t1/2 alpha and Cl were established (control). 16 min after the first thiopental dose, one of the inhalation anesthetics was administered (randomized). After 45 min exposure to the respective inhalation anesthetic (2-3 MAC in combination with N2O, steady-state a second dose of 50 mg thiopental was injected and the T-values were determined again over 15 min. The T-values of the control course varied considerably; the logarithmic frequency distribution revealed two distinct subgroups of patients, A and B, with characteristic Vc and t1/2 alpha. Both subgroups were influenced by the volatile anesthetics in a similar way with regard to pharmacokinetic parameters. With halothane, Vc was decreased and t1/2 alpha was shortened. In contrast, enflurane and isoflurane did not affect the pharmacokinetic parameters.

A comparison of the effects of fentanyl and alfentanil on the sleeping times of the intravenous induction agents

The effect of increasing doses of fentanyl and alfentanil administered before four intravenous induction agents (thiopentone, methohexitone, Althesin and etomidate) on the sleeping time were studied in mice. A biphasic pattern of sleeping time was noted with the more rapidly metabolized and eliminated compounds. Possible explanations are discussed.

Quantitative analysis of common anaesthetic agents

A summary of chromatographic (e.g. gas chromatography, high-performance liquid chromatography) and non-chromatographic (e.g. radioimmunoassay, enzyme-multiplied immunoassay) analytical techniques suitable for the quantitative analysis of the most popular inhalational (halothane, methoxyflurane, enflurane, isoflurane and nitrous oxide), intravenous (barbiturate, benzodiazepines, etomidate, althesin, morphine, fentanyl, alfentanil, sufentanil, droperidol and ketamine) general and amide-type local (lidocaine, mepivacaine, etidocaine and bupivacaine) anaesthetic agents and some of their metabolites, in biological material, is described. In the case of inhalational anaesthetics attention is also payed to pollution measurement and breath-to-breath monitoring.

Elimination kinetics of thiopentone in mothers and their newborn infants

The elimination kinetics of thiopentone was studied in 7 newborns delivered by Caesarean section and in their mothers who had received the drug for induction of anaesthesia. At delivery, 4-9 min after induction, drug concentrations in cord blood were half those in material blood. The mean half-life of thiopentone in the newborns was about double that in their mothers (15 vs 7 h) confirming a disposition similar to other barbiturates. For the first time renal clearance of thiopentone was estimated in the newborn; 0.074 ml/h/kg. Only 0.0007% (about 2 micrograms) of the maternal dose was recovered in the urine of newborns over 36 h. Pentobarbitone, an active metabolite, was not detected in any specimen. The findings demonstrate the reliability of current anaesthesiological technique (thiopentone-succinylcholine-oxygen) with minimal fetal exposure to the drug.

Intravenous benzodiazepines as anaesthetic agents: pharmacokinetics and clinical consequences

Despite extensive and numerous pharmacokinetic studies on benzodiazepines, the published pharmacokinetic data do not adequately explain the clinical differences found between different benzodiazepine derivatives after intravenous administration. Especially, correlations between initial drug responses and distributional changes of the benzodiazepines are limited. However, during the elimination phase some relationships exist between the kinetic and dynamic phenomena. Age, sex, diseases and concomitantly given drugs cause clinically important alterations in the pharmacokinetics of benzodiazepines. Generally these anxiolytics and sedatives should be considered as adjuvants to general anaesthesia, but not primarily as routine induction agents. The major reasons for this limitation are a high variability in drug response, a relatively slow onset of action and long-lasting residual effects. However, benzodiazepines have many important advantages (see Table 5) when used as intravenous inducing agents of general anaesthesia.

Chlormethiazole (Heminevrin), pethidine and nitrous oxide as compared to halothane for general anesthesia

The anaesthetic and postanaesthetic course in a group of gynaecological patients anaesthetized with chlormethiazole (Heminevrin) was investigated and the results compared to a similar group of patients anaesthetized with halothane. Both drugs were used as the main anaesthetic agent in the respective regimes, supplemented by nitrous oxide/oxygen and muscle relaxants. Because chlormethiazole is devoid of analgetic effects, the importance of using pethidine in combination with chlormethiazole is emphasized. An advantage of using chlormethiazole is that it can serve as both an induction and maintenance agent. The plasma concentrations of chlormethiazole were studied in seven patients. For induction, the mean concentration was 4.5 micrograms/ml. The mean concentration on waking at the termination of operation was 1.3 micrograms/ml. No serious side effects were encountered in either treatment. The results suggested that chlormethiazole in combination with an analgetic drug and nitrous oxide could be suitable in elderly patients, although occasionally less effective in the young.

Bioavailability, pharmacokinetics, and analgesic activity of ketamine in humans

The pharmacokinetics of ketamine in analgesic doses after intravenous, intramuscular, and oral administration was investigated in healthy volunteers. Plasma ketamine concentration-time curves were fitted by a two-compartment open model with a terminal half-life of 186 min. Absorption after intramuscular injection was rapid and the bioavailability was 93%. However, only 17% of an oral dose was absorbed because of extensive first-pass metabolism. Simultaneous measurements of the elevation of pain threshold in an ischemic exercise test showed a marked effect for 15-60 min after intramuscular injection, but little or no effect after the oral solution. Pain threshold elevation occurred at plasma ketamine concentrations above 160 ng/ml.

Blood disoprofol levels in anesthetised patients. Correlation of concentrations after single or repeated doses with hypnotic activity

The blood concentrations of disoprofol (Diprivan) after single intravenous doses of 1, 2 or 3 mg/kg have been examined in a subpopulation from previously reported clinical studies. The linear relationship between sleep time and dose could be explained by the linearity of the pharmacokinetics at these doses. After a single injection the awakening concentration was independent of dose, with a mean value of 1.04 micrograms/ml. No acute tolerance occurred with disoprofol. On repeated 1 mg/kg bolus injections the sleeping time rose initially but stabilised after four doses. The waking concentration was independent of the number of doses administered. The clinical findings fitted an agent with a very rapid distribution phase and a short elimination half-life.

Comparison of thiopentone and diazepam as induction agents of anaesthesia for Caesarean section

Clinical effects of thiopentone (3 mg/kg body weight) and diazepam (0.3 mg/kg) were compared for elective caesarean sections in two groups of 43 and 39 women, respectively. They were given general anaesthesia in left lateral tilt. The systolic, diastolic and mean arterial pressures were moderately elevated at onset of surgery in both groups, probably indicating light anaesthesia. Later, a gradual decrease to the preinduction levels was observed. The mean injection-delivery (I-D) interval was 344 s in the thiopentone group and 339 s in the diazepam group. The I-D intervals were shorter than 10 min in 39 of the cases in the thiopentone group and 38 cases in the diazepam group. Low Apgar scores at 1 min (6 or less) occurred in five of the neonates in each group, while all had normal Apgar scores at 5 min ( 7 or mor) As judged by the Apgar scores and the acid-base status of umbilical cord blood, the effects of the induction agents on the neonatal condition were indistinguishable in the two groups. In the thiopentone group, unpleasant recollections were reported in 5 out of 40 patients (12.5%) compared to none in the diazepam group. Diazepam-nitrous oxide anaesthesia is well accepted by the mothers and is alternative to supplementing thiopentone induction with a volatile gas for patients who have previous experienced wakefulness or express fear of awareness. The main drawback with diazepam induction, however, is the slow induction of sleep. Harmful drug effects on the neonates must be expected if the dose has to be increased in order to ensure sleep.

Antiemetic effectiveness of intraoperatively administered droperidol in pediatric strabismus outpatient surgery--preliminary report of a controlled study

Low doses (0.05 mg/kg) of intravenously administered droperidol were given intraoperatively to randomly assigned pediatric strabismus patients in a controlled double-masked paradigm. No difference between control and treatment groups in the severity of vomiting was noted in the postanesthesia recovery room, but such a difference was probably present in the hospital rooms during the interval between room arrival and the meeting of hospital discharge criteria. Administration of the drug did not appear to produce somnolence sufficient to delay postoperative recovery.

Comparison of the placental transfer of thiopental and diazepam in caesarean section

Drug concentrations were measured in whole blood obtained from mother and child after induction of general anaesthesia with thiopental or diazepam and delivery by Caesarean section. In 27 cases given thiopental 3 mg/kg intravenously the 5-min child/mother concentration ratio rose with increasing injection-delivery (I-D) interval up to 8-10 min. The concentration in the newborn at 2 h showed a similar trend. In 30 cases given diazepam 0.3 mg/kg for sleep induction, there were some low values in cases delivered within 4 min after the injection. However, higher neonatal concentrations and child/mother ratios were observed when the operation lasted 4-5 min, and there was no further increase in cases with longer I-D intervals. There is evidence to suggest that net transfer to the fetus proceeds at a slower rate with thiopental than with diazepam. However, in the present series of low risk elective Caesarean sections, there was no appreciable difference between the induction agents with regard to their effect on the newborn infant.

Failure of thiopental to modify global anoxic injury

To corroborate thiopental protection from cerebral anoxia after cardiopulmonary arrest, 23 sedated, curarized, adult dogs were asphyxiated by plugging the endotracheal tube. Cardiopulmonary resuscitation (CPR) was started 7 minutes after electrocortical silence. Twelve animals received no other treatment (controls), 10 regained consciousness and spontaneous respirations, but remained decerebrate, blind, unable to drink or feed. Two dogs returned to a normal neurologic state. Ten dogs were treated with thiopental after CPR, 7 received 15 mg/kg the first minute, followed by 23 mg/kg over 1 hour; 3 received 60 mg/kg in the first 3 minutes, followed by 30 mg/kg over 1 hour. Except for 1 dog in the low-dose group that recovered neurologically, thiopental-treated dogs showed no neurological or survival improvement over the controls.