The pharmacokinetics of ketamine following intramuscular injection to F344 rats

Ketamine is a glutamate N-methyl-D-aspartate receptor antagonist that is a rapid-acting dissociative anesthetic. It has been proposed as an adjuvant treatment along with other drugs (atropine, midazolam, pralidoxime) used in the current standard of care (SOC) for organophosphate and nerve agent exposures. Ketamine is a pharmaceutical agent that is readily available to most clinicians in emergency departments and possesses a broad therapeutic index with well-characterized effects in humans. The objective of this study was to determine the pharmacokinetic profile of ketamine and its active metabolite, norketamine, in F344 rats following single or repeated intramuscular administrations of subanesthetic levels (7.5 mg/kg or 30 mg/kg) of ketamine with or without the SOC. Following administration, plasma and brain tissues were collected and analyzed using a liquid chromatography-mass spectrometry method to quantitate ketamine and norketamine. Following sample analysis, the pharmacokinetics were determined using non-compartmental analysis. The addition of the current SOC had a minimal impact on the pharmacokinetics of ketamine following intramuscular administration and repeated dosing at 7.5 mg/kg every 90 minutes allows for sustained plasma concentrations above 100 ng/mL. The pharmacokinetics of ketamine with and without the SOC in rats supports further investigation of the efficacy of ketamine co-administration with the SOC following nerve agent exposure in animal models.

Pharmacokinetics of ketamine and xylazine in young and old Sprague-Dawley rats

To compare the pharmacokinetics of coadministered intraperitoneal ketamine and xylazine in young (8 to 10 wk; n = 6) and old rats (2 to 2.4 y; n = 6), blood samples obtained at 15 and 30 min and 1, 2, and 4 h after drug administration were analyzed by HPLC-tandem mass spectrometry. In both groups, the withdrawal reflex was absent during anesthesia and was present at 1.1 (± 0.2) and 2.6 (± 0.7) h after drug administration in young and old rats, respectively, with the first voluntary movement at 1.5 ± 0.2 and 4.9 ± 1.0 h. Drug availability of ketamine and xylazine was 6.0 and 6.7 times greater, respectively, in old than young rats. The rate constant of elimination of both drugs was greatly decreased and the elimination half-life was significantly greater in old compared with young rats. In conclusion, age and associated factors affect the availability of ketamine and xylazine when coadministered to attain clinical anesthesia, changing the pharmacokinetics of these drugs and prolonging anesthesia duration and recovery times with aging. Compared with their young counterparts, aged rats required much higher doses to attain a similar level of anesthesia. Finally, the long half-life of both ketamine and xylazine, when coadministered to old rats, may be a factor in research protocols because residual plasma concentrations could still be present for as long as 3 and 5 d, respectively, after administration.

[Toxicokinetics of ketamine in rabbits]

OBJECTIVE

To investigate the toxicokinetics profiles of ketamine and its main metabolite norketamine in rabbits.

METHODS

The rabbits were administered orally the hydrochloride of ketamine with a dose of 0.15 g/kg. The serum and urine samples were collected before administration and at different time points after drug administration. The concentrations of ketamine and norketamine were determined by GC-NPD and GC-MS. Compartment model and toxicokinetics parameters were simulated and calculated by WinNorLin program. Changes of important vital signs of rabbits were recorded during the experiment.

RESULTS

The mean serum concentration-time profile of ketamine and norketamine were fitted to a two-compartment open model with first order kinetics. The kinetic equation of ketamine and norketamine were p(t) = 121.760 e(-0.0025t) +0.980 e(-0.002t) +4.579 e(-0.021 t) and p(t) = 640.919 e(-0.03 t) +1.023 e(-0.001 t) +9.784 e (-0.031 t), respectively. The peak time and the peak concentration of ketamine in serum were (40.950 +/- 12.098) min and (9.015 +/- 1.344) microg/mL, respectively. The elimination half-time of ketamine in rabbits was (430.370 +/- 28.436) min. The serum and urine showed a middle relation in concentrations of ketamine during 30-240 min after drug administration. After oral administration ketamine to rabbits, the toxic symptom on the rabbits occurred at 30 min and disappeared after 120 min.

CONCLUSION

The toxicokinetics parameters and kinetic equation of ketamine and norketamine in rabbits may provide the theoretical basis for forensic identification of reasonable specimen collection and inferring the time of oral administration ketamine from the ketamine concentration in serum.

Subanesthetic dose of ketamine decreases prefrontal theta cordance in healthy volunteers: implications for antidepressant effect

BACKGROUND

Theta cordance is a novel quantitative electroencephalography (QEEG) measure that correlates with cerebral perfusion. A series of clinical studies has demonstrated that the prefrontal theta cordance value decreases after 1 week of treatment in responders to antidepressants and that this effect precedes clinical improvement. Ketamine, a non-competitive antagonist of N-methyl-D-aspartate (NMDA) receptors, has a unique rapid antidepressant effect but its influence on theta cordance is unknown.

METHOD

In a double-blind, cross-over, placebo-controlled experiment we studied the acute effect of ketamine (0.54 mg/kg within 30 min) on theta cordance in a group of 20 healthy volunteers.

RESULTS

Ketamine infusion induced a decrease in prefrontal theta cordance and an increase in the central region theta cordance after 10 and 30 min. The change in prefrontal theta cordance correlated with ketamine and norketamine blood levels after 10 min of ketamine infusion.

CONCLUSIONS

Our data indicate that ketamine infusion immediately induces changes similar to those that monoamineric-based antidepressants induce gradually. The reduction in theta cordance could be a marker and a predictor of the fast-acting antidepressant effect of ketamine, a hypothesis that could be tested in depressive patients treated with ketamine.

Ketamine disposition in children presenting for procedural sedation and analgesia in a children's emergency department

BACKGROUND

The aim of this study was to describe ketamine pharmacokinetics in children to simulate time-concentration profiles to predict duration of concentrations associated with anesthesia, arousal and analgesia.

METHODS

Children presenting for painful procedures in the Emergency Dept were given ketamine 1-1.5 mgxkg(-1) i.v. Blood was assayed for ketamine on 3-6 occasions (median 3) over the subsequent 14-152 min (median 28.5). A population pharmacokinetic analysis was undertaken by using nonlinear mixed effects models (NONMEM). Simulation was used to predict time-concentration profiles in this cohort

RESULTS

There were 188 observations from 54 children (age 8.3 sd 3.5 years, weight 32.5 sd 15.6 kg). A two-compartment (central, peripheral) linear disposition model fitted data better than a one-compartment model. Population parameter estimates and their between subject variability (BSV), standardized to a 70-kg person using allometric models, were central volume (V1) 38.7 (BSV 64%) l.70 kg(-1), peripheral volume of distribution (V2) 102 (51.7%) l.70 kg(-1), clearance (CL) 90 (38.1%) l.h(-1) 70 kg(-1) and intercompartment clearance (Q) 215 (19%) l.h(-1) 70 kg(-1). At 10 min half of the children given 1 mgxkg(-1) will have a serum concentration below 0.75 mgxl(-1). This is a concentration associated with 'awakening' in adults. However, almost all the children will still have a serum concentration above 0.1 mgxl(-1), a level associated with analgesia in adults.

CONCLUSIONS

Ketamine 1 mgxkg(-1) i.v. provides satisfactory serum concentrations for children undergoing sedation for painful procedures of <5-min duration and produces concentrations associated with analgesic effect for more than 10 min. Clearance increases with decreasing age in children. The relationship between serum concentration and effect is poorly defined in children.

Characteristics of the relationship between plasma ketamine concentration and its effect on the minimum alveolar concentration of isoflurane in dogs

OBJECTIVE

To characterize the shape of the relationship between plasma ketamine concentration and minimum alveolar concentration (MAC) of isoflurane in dogs.

STUDY DESIGN

Retrospective analysis of previous data.

ANIMALS

Four healthy adult dogs.

METHODS

The MAC of isoflurane was determined at five to six different plasma ketamine concentrations. Arterial blood samples were collected at the time of MAC determination for measurement of plasma ketamine concentration. Plasma concentration/effect data from each dog were fitted to a sigmoid inhibitory maximum effect model in which MAC(c)= MAC(0) - (MAC(0)-MAC(min)) x C(gamma)/EC(50)(gamma)+C(gamma), where C is the plasma ketamine concentration, MAC(c) is the MAC of isoflurane at plasma ketamine concentration C, MAC(0) is the MAC of isoflurane without ketamine, MAC(min) is the lowest MAC predicted during ketamine administration, EC(50) is the plasma ketamine concentration producing 50% of the maximal MAC reduction, and gamma is a sigmoidicity factor. Nonlinear regression was used to estimate MAC(min), EC(50), and gamma.

RESULTS

Mean +/- SEM MAC(min), EC(50) and gamma were estimated to be 0.11 +/- 0.01%, 2945 +/- 710 ng mL(-1) and 3.01 +/- 0.84, respectively. Mean +/- SEM maximal MAC reduction predicted by the model was 92.20 +/- 1.05%.

CONCLUSIONS

The relationship between plasma ketamine concentration and its effect on isoflurane MAC has a classical sigmoid shape. Maximal MAC reduction predicted by the model is less than 100%, implying that high plasma ketamine concentrations may not totally abolish gross purposeful movement in response to noxious stimulation in the absence of inhalant anesthetics.

CLINICAL RELEVANCE

The parameter estimates reported in this study will allow clinicians to predict the expected isoflurane MAC reduction from various plasma ketamine concentrations in an average dog.

Predictive performance of the Domino, Hijazi, and Clements models during low-dose target-controlled ketamine infusions in healthy volunteers

BACKGROUND

Healthy volunteers received low-dose target-controlled infusions (TCI) of ketamine controlled by the Domino model while cognitive function tests and functional neuroimaging were performed. The aim of the current study was to assess the predictive performance of the Domino model during these studies, and compare it with that of three other ketamine models.

METHODS

Fifty-eight volunteers received ketamine administered by a TCI device on one or more occasions at target concentrations of either 50, 100, or 200 ng ml-1. At each target concentration, two or three venous blood samples were withdrawn during infusion, with a further sample after the infusion ended. Ketamine assays were performed by gas chromatography. The plasma concentration time courses predicted by the Hijazi, Clements 125, and Clements 250 models were calculated retrospectively, and the predictive performance of each of the models was assessed using Varvel methodology.

RESULTS

For the Domino model, bias, inaccuracy, wobble, and divergence were - 2.7%, 33.9%, 24.2%, and 0.1463% h-1, respectively. There was a systematic increase in performance error over time. The Clements 250 model performed best by all criteria, whereas the Hijazi model performed least well by all criteria except for bias.

CONCLUSIONS

Performance of the Domino model during control of low-dose ketamine infusions was sub-optimal. The Clements 250 model may be a better model for controlling low-dose TCI ketamine administration.

Stereoselective pharmacokinetics of ketamine and norketamine after racemic ketamine or S-ketamine administration during isoflurane anaesthesia in Shetland ponies

BACKGROUND

The arterial pharmacokinetics of ketamine and norketamine enantiomers after racemic ketamine or S-ketamine i.v. administration were evaluated in seven gelding ponies in a crossover study (2-month interval).

METHODS

Anaesthesia was induced with isoflurane in oxygen via a face-mask and then maintained at each pony's individual MAC. Racemic ketamine (2.2 mg kg(-1)) or S-ketamine (1.1 mg kg(-1)) was injected in the right jugular vein. Blood samples were collected from the right carotid artery before and at 1, 2, 4, 8, 16, 32, 64, and 128 min after ketamine administration. Ketamine and norketamine enantiomer plasma concentrations were determined by capillary electrophoresis. Individual R-ketamine and S-ketamine concentration vs time curves were analysed by non-linear least square regression two-compartment model analysis using PCNonlin. Plasma disposition curves for R-norketamine and S-norketamine were described by estimating AUC, C(max), and T(max). Pulse rate (PR), respiratory rate (R(f)), tidal volume (V(T)), minute volume ventilation (V(E)), end-tidal partial pressure of carbon dioxide (PE'(CO(2))), and mean arterial blood pressure (MAP) were also evaluated.

RESULTS

The pharmacokinetic parameters of S- and R-ketamine administered in the racemic mixture or S-ketamine administered separately did not differ significantly. Statistically significant higher AUC and C(max) were found for S-norketamine compared with R-norketamine in the racemic group. Overall, R(f), V(E), PE'(CO(2)), and MAP were significantly higher in the racemic group, whereas PR was higher in the S-ketamine group.

CONCLUSIONS

Norketamine enantiomers showed different pharmacokinetic profiles after single i.v. administration of racemic ketamine in ponies anaesthetised with isoflurane in oxygen (1 MAC). Cardiopulmonary variables require further investigation.

Pharmacokinetics of S(+) ketamine derived from target controlled infusion

BACKGROUND

A computer controlled infusion device for S(+) ketamine was used in combination with a Diprifusor device to provide anaesthesia for 20 ASA I or II patients undergoing elective colonoscopy. The aim of the study was to assess the performance of the pharmacokinetic model for S(+) ketamine used in the delivery algorithm of the device.

RESULTS

It was observed that during the first 30 min of infusion there was systematic underprediction by the delivery system of the measured levels of S(+) ketamine. New pharmacokinetic constants were derived from the observed data which provided, on pharmacokinetic simulation, improved prediction of the measured values of S(+) ketamine. Prospective application of this modified model for S(+) ketamine in a further nine study patients was performed and the pharmacokinetic performance of the model was reassessed. The data from all 29 patients was subsequently used to calculate the population distribution of S(+) ketamine clearance. The distribution was found to be normal only in the logarithmic domain. In the normal domain the mode of S(+) ketamine clearance was found to be 35.8 ml kg(-1) min(-1) with 5 and 95% confidence limits of, respectively, 11.5 and 111.1 ml kg(-1) min(-1).

CONCLUSION

It was necessary to modify the original published pharmacokinetic parameters incorporated into the S(+) ketamine delivery system in order to simulate improved PK performance during short procedures (<1 h duration) where propofol was concurrently administered. This improved performance was confirmed in a further prospective study.

Effect of intravenous administration of ketamine on the minimum alveolar concentration of isoflurane in anesthetized dogs

OBJECTIVE

To determine the effect of 6 plasma ketamine concentrations on the minimum alveolar concentration (MAC) of isoflurane in dogs.

ANIMALS

6 dogs.

PROCEDURE

In experiment 1, the MAC of isoflurane was measured in each dog and the pharmacokinetics of ketamine were determined in isoflurane-anesthetized dogs after IV administration of a bolus (3 mg/kg) of ketamine. In experiment 2, the same dogs were anesthetized with isoflurane in oxygen. A target-controlled IV infusion device was used to administer ketamine and to achieve plasma ketamine concentrations of 0.5, 1, 2, 5, 8, and 11 microg/mL by use of parameters obtained from experiment 1. The MAC of isoflurane was determined at each plasma ketamine concentration, and blood samples were collected for ketamine and norketamine concentration determination.

RESULTS

Actual mean +/- SD plasma ketamine concentrations were 1.07 +/- 0.42 microg/mL, 1.62 +/- 0.98 microg/mL, 3.32 +/- 0.59 microg/mL, 4.92 +/- 2.64 microg/mL, 13.03 +/- 10.49 microg/mL, and 22.80 +/- 25.56 microg/mL for target plasma concentrations of 0.5, 1, 2, 5, 8, and 11 microg/mL, respectively. At these plasma concentrations, isoflurane MAC was reduced by 10.89% to 39.48%, 26.77% to 43.74%, 25.24% to 84.89%, 44.34% to 78.16%, 69.62% to 92.31%, and 71.97% to 95.42%, respectively. The reduction in isoflurane MAC was significant, and the response had a linear and quadratic component. Salivation, regurgitation, mydriasis, increased body temperature, and spontaneous movements were some of the adverse effects associated with the high plasma ketamine concentrations.

CONCLUSIONS AND CLINICAL RELEVANCE

Ketamine appears to have a potential role for balanced anesthesia in dogs.

Determining the plasma concentration of ketamine that enhances epidural bupivacaine-and-morphine-induced analgesia

N-methyl-D-aspartate (NMDA) receptor antagonists enhance opioid-induced analgesia. The plasma concentration of ketamine, an NMDA receptor antagonist that enhances epidural morphine-and-bupivacaine-induced analgesia, is not known. We examined 24 patients with lung carcinoma or metastatic lung tumor who underwent video-assisted thoracic surgery in a placebo-controlled, double-blind manner 4 h after emergence from anesthesia. The morphine + ketamine group (n = 8) and morphine + placebo group (n = 8) received 5 mL volume of 2.5 mg morphine and 0.25% bupivacaine and the placebo + ketamine group (n = 8) received 5 mL volume of saline and 0.25% bupivacaine epidurally at the end of skin closure. Four hours after this anesthesia, in the morphine + ketamine and placebo + ketamine groups, ketamine was administered to successively maintain a stable plasma ketamine concentration of 0, 10, 20, 30, 40, and 50 ng/mL by a target-controlled infusion device, and patients assessed the levels of pain at rest, pain on coughing, somnolence (drowsiness), and nausea using a 100-mm visual analog scale (VAS). In the morphine + placebo group, a placebo (saline) was similarly administered instead of ketamine. In the morphine + ketamine group, the VAS scores for pain at rest and pain on coughing significantly decreased on ketamine administration at a plasma concentration of 20 ng/mL or larger compared with the respective baseline VAS scores (P < 0.05 each). In the placebo + ketamine group, the VAS scores for pain at rest and pain on coughing did not significantly change at any plasma concentration of ketamine as compared to the morphine + placebo group. In the morphine + ketamine group, a plasma concentration of ketamine larger than 20 ng/mL did not further reduce VAS scores for pain at rest and pain on coughing. The VAS scores for drowsiness were comparable among the three groups at any plasma concentration of ketamine. Ketamine at a plasma concentration of 20 ng/mL or larger may enhance epidural morphine-and-bupivacaine-induced analgesia. As an adjunct with epidural morphine-and-bupivacaine and considering the safety of small doses, the minimal plasma concentration of ketamine given IV may be approximately 20 ng/mL.

Ketamine disrupts frontal and hippocampal contribution to encoding and retrieval of episodic memory: an fMRI study

The N-methyl-D-aspartate (NMDA) receptor antagonist ketamine produces episodic memory deficits. We used functional magnetic resonance imaging to characterize the effects of ketamine on frontal and hippocampal responses to memory encoding and retrieval in healthy volunteers using a double-blind, placebo-controlled, randomized, within-subjects comparison of two doses of intravenous ketamine. Dissociation of the effects of ketamine on encoding and retrieval processes was achieved using two study-test cycles: in the first, items were encoded prior to drug infusion and retrieval tested, during scanning, on drug; in the second, encoding was scanned on drug, and retrieval tested once ketamine plasma levels had declined. We additionally determined the interaction of ketamine with the depth of processing that occurred at encoding. A number of effects upon task-dependent activations were seen. Overall, our results suggest that left frontal activation is augmented by ketamine when elaborative semantic processing is required at encoding. In addition, successful encoding on ketamine is supplemented by additional non-verbal processing that is incidental to task demands. The effects of ketamine at retrieval are consistent with impaired access to accompanying contextual features of studied items. Our findings show that, even when overt behaviour is unimpaired, ketamine has an impact upon the recruitment of key regions in episodic memory task performance.

Postmortem blood ketamine distribution in two fatalities

Despite the reported increased use of ketamine as a recreational drug, relatively few fatalities attributed to ketamine poisoning have been documented. Two recent fatalities in which ketamine was detected are described and compared with cases previously reported in the scientific literature. Concentrations of ketamine were measured in the heart and femoral blood samples using gas chromatography with nitrogen phosphorus detection. Ketamine concentrations in a 26-year-old man whose death was attributed to ketamine intoxication were 6.9 and 1.8 mg/L in heart and femoral blood, respectively. In this case, the ketamine concentration detected in the heart blood is in agreement with the lowest concentration reported in the literature, in which ketamine intoxication was ruled as the cause of death and no other drugs were present. Ketamine concentrations in a 20-year-old man, whose death was attributed to asthma and ketamine was considered an incidental finding, were 1.6 and 0.6 mg/L in heart and femoral blood, respectively. Marked differences between heart and femoral blood ketamine concentrations were observed in both of the reported cases. This may be indicative of incomplete distribution prior to death and/or postmortem redistribution of ketamine.

Chronopharmacological studies of ketamine in normal and NMDA epsilon1 receptor knockout mice

BACKGROUND

The effectiveness and toxicity of many drugs depends on the dosing-time schedule, relative to the circadian rhythms of biochemical, physiological, and behavioural processes. Previous studies have found chronopharmacology of ketamine, which is a N-methyl-d-aspartate (NMDA) receptor antagonist. The in vivo contribution of the NMDA receptor epsilon1 subunit (NR2A) in this effect is unclear.

METHODS

In the present study, daily variations in the hypnotic effect of ketamine were determined in wild-type mice and NMDA epsilon1 knockout (KO) mice.

RESULTS

The effect of ketamine had a definite daily variation in wild-type mice. No significant difference in blood concentration was observed at different dosing times (10:00 and 22:00). In NMDA receptor epsilon1 KO mice, the hypnotic effect of ketamine was weaker than in wild-type mice and there was no dependence on the time of administration. Significant pharmacokinetic differences were not observed between wild-type and KO mice.

CONCLUSIONS

The enhanced hypnotic effect in the active phase of the circadian cycle is likely a result of changes with the time of day in the susceptibility of the central nervous system to ketamine. Knockout of the NMDA receptor epsilon1 subunit gene markedly reduced the effect of ketamine, and eliminated the time-dependent sensitivity to ketamine.

An Investigation to Dissociate the Analgesic and Anesthetic Properties of Ketamine Using Functional Magnetic Resonance Imaging

Background

Anatomic sites within the brain, which activate in response to noxious stimuli, can be identified with the use of functional magnetic resonance imaging. The aim of this study was to determine whether the analgesic effects of ketamine could be imaged.

Methods

Ketamine was administered to eight healthy volunteers with use of a target-controlled infusion to three predicted plasma concentrations: 0 (saline), 50 (subanalgesic), and 200 ng/ml (analgesic, subanesthetic). Volunteers received noxious thermal stimuli and auditory stimuli and performed a motor task within a 3-T human brain imaging magnet. Activation of brain regions in response to noxious and auditory stimuli and during the motor task was compared with behavioral measures.

Results

The analgesic subanesthetic dose of ketamine significantly reduced the pain scores, and this matched a decrease in activity within brain regions that activate in response to noxious stimuli, in particular, the insular cortex and thalamus. A different pattern of activation was observed in response to an auditory task. In comparison, smaller behavioral and imaging changes were found for the motor paradigm. The lower dose of ketamine gave similar but smaller nonsignificant effects.

Conclusion

The analgesic effect can be measured within a more global effect of ketamine as shown by auditory and motor tasks, and the analgesia produced by ketamine occurs with a smaller degree of cortical processing in pain-related regions.

The effect of hypothermia on myogenic motor-evoked potentials to electrical stimulation with a single pulse and a train of pulses under propofol/ketamine/fentanyl anesthesia in rabbits

In the present study, we investigated the effect of hypothermia on myogenic motor-evoked potentials (MEPs) in rabbits. The influence of stimulation paradigms to induce MEPs was evaluated. Twelve rabbits anesthetized with ketamine, fentanyl, and propofol were used for the study. Myogenic MEPs in response to electrical stimulation of the motor cortex with a single pulse and a train of three and five pulses were recorded from the soleus muscle. After the control recording of MEPs at 38 degrees C of esophageal temperature, the rabbits were cooled by surface cooling. Esophageal temperature was maintained at 35 degrees C, 32 degrees C, 30 degrees C, and 28 degrees C, and MEPs were recorded at each point. MEP amplitude to single- pulse stimulation was significantly reduced with a re-duction of core temperature to 28 degrees C compared with the control value at 38 degrees C (0.8 +/- 0.4 mV versus 2.3 +/- 0.3 mV; P < 0.05), whereas MEP amplitude to train-pulse stimulation did not change significantly during the cooling. MEP latency was increased linearly with a reduction of core temperature regardless of stimulation paradigms. In conclusion, these results indicate that a reduction of core temperature to 28 degrees C did not influence MEP amplitudes as long as a train of pulses, but not a single pulse, was used for stimulation in rabbits under propofol/ketamine/fentanyl anesthesia.

IMPLICATIONS

Intraoperative monitoring of myogenic motor-evoked potentials (MEPs) may be required under hypothermic conditions because of its neuroprotective efficacy. However, data on the influence of hypothermia on myogenic MEPs are limited. The results indicate that multipulse stimulation may be better than single-pulse stimulation when monitoring MEPs during hypothermia.

Pharmacokinetics and haemodynamics of ketamine in intensive care patients with brain or spinal cord injury

BACKGROUND

Ketamine is used as an anaesthetic agent for short surgical procedures, and as a sedative and analgesic in intensive care patients. Intensive care patients with brain or spinal cord injury may have physiological changes that could alter the pharmacokinetics of ketamine. The pharmacokinetics of ketamine have been studied in healthy volunteers and in patients undergoing different types of surgery, but no data are available in intensive care patients.

METHODS

We determined the pharmacokinetics of ketamine and its active metabolites, norketamine and dehydronorketamine, in 12 intensive care patients with brain or spinal cord injury. The effect of ketamine on haemodynamic variables was also investigated.

RESULTS

The total clearance of ketamine, mean (SD), was 36.0 (13.3) ml min(-1) kg(-1), the volume of distribution (Vbeta) was 16.0 (8.6) litre kg(-1), and the elimination half-life was 4.9 (1.6) h. Ketamine did not alter any haemodynamic variables in the patients studied.

CONCLUSIONS

Pharmacokinetic variables of ketamine in intensive care patients are greater than in healthy volunteers and in surgical patients. The increase in the volume of distribution is greater than the increase in clearance, resulting in a longer estimated half-life of ketamine in this patient group.

Protein binding of ketamine and its active metabolites to human serum

Ketamine is an anaesthetic agent extensively used in intensive care patients, and it has proved its efficacy in the management of burned patients. In these patients, alterations in serum protein binding occur that may have significant clinical implications. Scarce data were observed in the literature about the binding of ketamine to human plasma proteins, and no data about the binding of its active metabolites, norketamine (NK) and dehydronorketamine (DHNK) were found. In this study, protein binding of ketamine, NK and DHNK in human serum were determined using the ultrafiltration technique. The percentage of drug bound to serum proteins at 30 degrees C was found to be 69%, 60% and 50% for DHNK, ketamine and NK, respectively, while these percentages were 75%, 64% and 54% for DHNK, ketamine and NK respectively at 20 degrees C. The binding of ketamine and its metabolites was independent of drug concentration.

Concurrent ketamine and alfentanil administration: pharmacokinetic considerations

BACKGROUND

A ketamine-alfentanil combination has been suggested for total i.v. anaesthesia. We determined the pharmacokinetics of ketamine and alfentanil, alone and together, in three groups of adult male rats, to assess any pharmacokinetic interaction.

METHODS

Group 1 animals were infused with i.v. ketamine for 5 min; in group 2, constant low plasma concentrations of alfentanil were maintained by computer-controlled infusion; in group 3, the treatments were combined. Serial plasma and terminal tissue concentrations were measured by high performance liquid chromatography or gas chromatography-mass spectrometry.

RESULTS

In the presence of alfentanil, the mean plasma ketamine concentration-time area under the curve (AUC) value was significantly lower (by 13%, P<0.05), while clearance (CIT) and volume of distribution (Vss) were significantly higher (by 16 and 28%, respectively, both P<0.05). Tissue:plasma distribution coefficients for ketamine in the presence of alfentanil were significantly higher in forebrain (by 128%, P<0.005), hindbrain (by 207%, P<0.01), gut (by 254%, P<0.005), and fat (by 344%, P<0.0001). Mean AUC values for alfentanil did not differ significantly in the presence of ketamine, but alfentanil tissue concentrations were significantly lower in forebrain (by 77%, P<0.0001), hindbrain (by 28%, P<0.01), heart (by 33%, P<0.01), lung (30%, P<0.05), and gut (by 21%, P<0.05). Corresponding tissue:plasma distribution coefficients were significantly lower for forebrain (by 69%, P<0.0001) alone.

CONCLUSIONS

The finding that the distribution of ketamine into the brain was increased by low plasma concentrations of alfentanil could have important clinical applications for pain management.

[Ketamine pharmacokinetics and metabolism after bolus injection of X-ray contrast agents in roentgeno-endovascular interventions in children]

The study was carried out on 13 children (2-12 years) subjected to abdominal aortography. The children were divided into 2 groups. Changes in plasma concentrations of ketamine and its metabolism were evaluated during anesthesia after bolus injection of ionic highly osmolar and nonionic low-osmolar x-ray contrast agents (RCA). Injection of an RCA bolus was associated with a 2-fold more rapid drop of the anesthetic concentration in the blood, increase of renal clearance of ketamine and its metabolites; the osmotic effect of ionic highly osmolar and nonionic low-osmolar RCA on ketamine pharmacokinetics virtually did not differ.

Pharmacokinetics and tissue residues of Telazol in free-ranging polar bears

A pharmacokinetic and tissue residue study was conducted to assess the risks associated with human consumption of polar bears in arctic Canada that have been exposed to the immobilizing drug Telazol, a mixture of tiletamine hydrochloride and zolazepam hydrochloride. Twenty-two bears were remotely injected with about 10 mg/kg of Telazol. Following immobilization, serum samples were collected serially at regular intervals until the bears awakened. Sixteen of the bears were relocated and killed under permit by local hunters at various times from 0.5 to 11 days after dosing. Serum, kidney, muscle and adipose tissue samples were collected immediately after death. All samples were stored at -70 C until analysis by HPLC. The concentration-time data of tiletamine and zolazepam in serum during the immobilization period were fitted to curves by computer and the pharmacokinetic parameters assessed. In addition, the serum and tissue samples collected at the time of death were analyzed for both parent drugs, for one metabolite of tiletamine (CI-398), and for three metabolites of zolazepam (metabolites 1, 2 and 4). A one-compartment model with first-order absorption and elimination best fit the time-series data for the drugs in serum during the immobilization period. This model gave half-lives (mean +/- SE) for tiletamine and zolazepam of 1.8+/-0.2 h and 1.2+/-0.08 h, respectively, clearance values of 2.1+/-0.3 l x h(-1) x kg(-1) and 1.1+/-0.1 l x h(-1) x kg(-1), and volumes of distribution of 5.2+/-0.6 l/kg and 1.8+/-0.2 l/kg. The concentrations of both drugs and their metabolites declined rapidly to trace levels by 24 h post-dosing, although extremely low concentrations of some metabolites were encountered sporadically over the entire sampling period. In particular, zolazepam metabolite 2, remained detectable in fat and muscle tissue at the end of the study, 11 days after dosing. It was concluded that during immobilization, both tiletamine and zolazepam levels decline rapidly in a monoexponential fashion, and their pharmacokinetic parameters in polar bears are similar to those observed in other species. Tissue levels of the drugs and their metabolites declined sufficiently rapidly that individuals eating meat from exposed bears would be unlikely to experience pharmacological effects from the drugs. Nevertheless, slight exposure to the drugs and/or their metabolites might be possible for an indeterminate time after dosing.

Midazolam attenuates ketamine-induced abnormal perception and thought process but not mood changes

PURPOSE

To determine the effects of midazolam, 30 ngxmL(-1), on altered perception, mood, and cognition induced by ketamine.

METHODS

After ketamine was administered to achieve target concentrations of 50, 100, or 150 ngxmL in 11 volunteers, perception, mood, and thought process were assessed by a visual analog scale. Mini-Mental State examination (MMSE) assessed cognition. Boluses of midazolam, 30, 14.5, and 12 microgxkg(-1), were injected every 30 min to maintain the plasma concentration at 30 ngxmL(-1), which was reached 30 min after each injection.

RESULTS

Ketamine produced changes in perception about the body (P < 0.01, 0.001, and 0.0001 at 30, 60, and 90 min), surroundings (P < 0.01 and 0.0001 at 60 and 90 min), time (P < 0.002 and 0.0001 at 60 and 90 min), reality (P < 0.001 and 0.0001 at 60 and 90 min), sounds (P < 0.002 at 90 min), and meaning (P < 0.05 at 90 min). Subjects felt less energetic and clearheaded (P < 0.02 and 0.05) during ketamine, midazolam, and their co-administration. Ketamine impaired thought process (P < 0.003 and 0.0001 at 60 and 90 min). Ketamine and midazolam decreased mean total MMSE and recall scores (P < 0.001 for both). Co-administration reduced the number of subjects with perceptual (body, P < 0.01 and 0.001 at 30 and 60 min) and thought process abnormalities. Within the range of observation, co-administration did not affect the changes in mood or recall.

CONCLUSION

Midazolam attenuates ketamine-induced changes in perception and thought process.

A fatality due to injection of tiletamine and zolazepam

A 22-year-old male with more than 28 needle marks on his right arm was found dead. First, he was suspected as a drug abuser. Blood, urine, spleen, and injection-site tissue was collected during autopsy. The blood and urine specimens were screened for drugs. Immunoassay studies did not show any illegal drugs. However, two unidentified peaks were isolated in both of these biological fluids by routine gas chromatography-flame-ionization detection (GC-FID) and thermionic specific detection. Additional gas chromatography-mass spectrometry analysis determined these two peaks to be tiletamine and zolazepam. These two agents are used in combination as veterinary anesthesia. The concentrations of these drugs in blood were quantitated by GC-FID and found to be 0.85 mg/L of tiletamine and 3.3 mg/L of zolazepam. In urine, tiletamine and its metabolite, 2-(ethylamino)-2-(2-thionyl) cyclohexanol, were identified to be present along with zolazepam. The concentrations of tiletamine and zolazepam in spleen were revealed to be 0.92 and 3.5 mg/kg, respectively. Injection-site tissue concentrations were determined to be 25.1 mg/kg tiletamine and 23.3 mg/kg for zolazepam. The cause of death in this case was determined to be due to the multiple drug intoxication of tiletamine and zolazepam.

Target controlled infusion of ketamine as analgesia for TIVA with propofol

PURPOSE

To determine the accuracy of a target controlled infusion system for ketamine and to assess its suitability for the provision of analgesia when used in conjunction with a propofol infusion in spontaneously breathing patients.

METHODS

Nineteen, adult, ASA I-III patients scheduled for elective surgery were studied. After premedication with 20 mg temazepam an appropriate plasma concentration of ketamine was selected and, when the target controlled infusion (TCI) system indicated that this had been achieved, anesthesia was induced and maintained using a propofol infusion. The plasma ketamine concentration was measured at predetermined intervals and cardiovascular and respiratory parameters recorded at 10 min intervals. Patients were reviewed in recovery and 24 hr postoperatively to assess the adequacy of their recovery and the presence of any undesirable side effects.

RESULTS

The TCI system had a median performance error against predicted plasma concentrations of 18.9% (SE 2.5%) and a median absolute performance error of 23.3% (SE 2.3%). Divergence was 20.3% (SE 30.1%) and wobble was 12.9% (SE 2.1%). There was a mean decrease in arterial pressure of 6.4% (SD 19.7%) and a mean increase in heart rate of 4.3% (SD 17.4%). Little respiratory depression occurred and all patients made a rapid postoperative recovery with none describing unpleasant dreams or hallucinations.

CONCLUSION

The TCI system provided a clinically acceptable degree of control of the plasma ketamine concentration although some further improvement should be possible by amending the pharmacokinetic model. Clinically the combination with a propofol infusion proved to be a satisfactory anesthetic technique.

Ketamine antagonises alfentanil-induced hypoventilation in healthy male volunteers

BACKGROUND

The effects of ketamine on respiration, alone, or in combination with opioids, have not been completely clarified. Both stimulant and depressant effects have been reported, as well as attenuation of opioid-induced hypoventilation at the expense of increased oxygen consumption. These conflicting results might partly be due to dose-dependent mechanisms. We have, therefore, determined the ventilatory effects of ketamine, in combination with alfentanil, using infusions to different pseudo steady-state concentrations.

METHODS

On two separate days, eight healthy male volunteers were given alfentanil as a continuous computer-controlled infusion, aiming at a plasma concentration of 50 ng x mL(-1). After reaching apparent steady-state for alfentanil, racemic ketamine or placebo was administered in a protocol randomised for the two days. On the ketamine days a computer-controlled infusion, aiming for escalating ketamine plasma concentrations of 50, 100 and 200 ng x mL(-1), was added to the alfentanil infusion. On the placebo days saline was added. Using a face-mask with an occlusion valve, respiratory parameters were measured during air-breathing and after 6 repetitive 30-s CO2 challenges.

RESULTS

The alfentanil infusion induced hypoventilation by decreasing respiratory rate, while tidal volume and respiratory drive were unaffected. This hypoventilation was antagonised by ketamine in a concentration-dependent manner mainly through an increase in respiratory rate. The CO2 response was not affected by alfentanil or ketamine.

CONCLUSION

In the dose range of interest for postoperative, intensive-care and pain-clinic settings, ketamine antagonises the resting hypoventilation induced by alfentanil.

Middle latency auditory evoked potentials during total intravenous anesthesia with droperidol, ketamine and fentanyl

We investigated whether total intravenous anesthesia with ketamine, fentanyl and droperidol would affect middle latency auditory evoked potentials and explicit memory, and whether dreams during the anesthesia are related to plasma concentrations of fentanyl and the infusion technique. A total number of 40 patients were the subjects for this study. Twenty patients (group A) were maintained with intravenous ketamine 2 mg kg-1 hr-1 and fentanyl 5 micrograms kg-1 hr-1 for the first 60 min and 3 micrograms kg-1 hr-1 for the next 90 min, and droperidol 0.1 mg kg-1. The remaining 20 patients (group B) were maintained with intravenous ketamine 2 mg kg-1 hr-1, droperidol 0.1 mg kg-1 and fentanyl 50-100 micrograms in a bolus intermittently as needed by vital signs such as increases in heart rate and arterial blood pressure. Middle latency auditory evoked potentials, plasma fentanyl and ketamine levels were measured; explicit memory and dreams were also estimated. There were no patients who recollected explicit memories of intraoperative events in both groups. The middle latency auditory evoked potentials were not significantly changed during the anesthesia in both groups. We could find no significant differences in latencies and amplitudes of the middle latency auditory evoked potentials between the both groups. Plasma fentanyl levels of group B patients were significantly lower than those of group A patients and the incidence of the dreams was significantly higher in group B patients. We conclude that the anesthesia with ketamine, fentanyl and droperidol is not associated with the explicit memories, though the middle latency auditory evoked potentials were not significantly changed as compared with those in the waking state. In addition, dreams during the anesthesia may correlate with plasma fentanyl concentrations or the infusion technique.

A pharmacodynamic study of propofol or propofol and ketamine infusions in ponies undergoing surgery

The pharmacodynamics of infusions of propofol alone (group 1) were compared with the pharmacodynamics of infusions of propofol and ketamine together (group 2) in eight ponies undergoing castration. Anaesthesia was induced with detomidine, 20 micrograms kg-1, followed by ketamine, 2.2 mg kg-1. Subsequently, a bolus dose of propofol, 0.5 mg kg-1, was administered intravenously to both groups, and an infusion of propofol was given for an average of 74 minutes to group 1, and an infusion of propofol and ketamine was given for 60 minutes to group 2. The mean (SD) infusion rates of propofol were 0.330 (0.050) mg kg-1 min-1 in group 1, and 0.124 (0.009) mg kg-1 in group 2, and the ketamine infusion rate was maintained constant at 40 micrograms kg-1 min-1. Arterial hypotension and marked respiratory depression were evident in some of the ponies receiving propofol alone, whereas in the ponies anaesthetised with propofol and ketamine, respiratory and cardiovascular parameters were well maintained. All the ponies in both groups recovered quickly from anaesthesia, with mean times to sternal recumbency and standing of 19.8 (8.0) minutes and 27.2 (7.4) minutes respectively for group 1 and 8.4 (3.2) min and 14.9 (10.1) minutes for group 2.

Ketamine and norketamine plasma concentrations after i.v., nasal and rectal administration in children

It has been suggested that nasal administration of ketamine may be used to induce anaesthesia in paediatric patients. We have examined the pharmacokinetics of ketamine and norketamine after nasal administration compared with rectal and i.v. administration in young children. During halothane anaesthesia, 32 children, aged 2-9 yr, weight 10-30 kg, were allocated randomly to receive ketamine 3 mg kg-1 nasally (group IN3) or ketamine 9 mg kg-1 nasally (group IN9); ketamine 9 mg kg-1 rectally (group IR9); or ketamine 3 mg kg-1 i.v. (group IV3). Venous blood samples were obtained before and up to 360 min after administration of ketamine. Plasma concentrations of ketamine and norketamine were measured by gas liquid chromatography. Statistical comparisons were performed using ANOVA and the Kruskall-Wallis test, with P < 0.05 as significant. Mean plasma concentrations of ketamine peaked at 496 ng ml-1 in group IN3 within 20 min, 2104 ng ml-1 in group IN9 within 21 min, and 632 ng ml-1 in group IR9 within 42 min. Plasma concentrations of norketamine peaked at approximately 120 min after nasal ketamine, but appeared more rapidly after rectal administration of ketamine and were always higher than ketamine concentrations in the same situation. Calculated bioavailability was 0.50 in groups IN3 and IN9 and 0.25 in group IR9. We conclude that nasal administration of low doses of ketamine produced plasma concentrations associated with analgesia, but using high doses via the nasal route produced high plasma concentrations of ketamine similar to those that induce anaesthesia. However, the large volume of ketamine required was partly swallowed and led to an unacceptable variability of effect that precludes this route for induction of anaesthesia.

Ketamine concentrations during cardiopulmonary bypass

PURPOSE

To describe the serum concentrations of ketamine following a clinically relevant dosing schedule during cardiopulmonary bypass (CPB).

METHODS

DESIGN

Prospective case series.

SETTING

Tertiary care teaching hospital.

PATIENTS

Six patients undergoing coronary artery bypass grafting and over age 60 yr.

INTERVENTION

Following induction of anaesthesia each patient received a bolus of ketamine 2 mg.kg-1 followed by an infusion of 50 micrograms.kg-1.min-1 which ran continuously until two hours after bypass.

MAIN OUTCOME MEASURES

Ketamine serum concentrations were measured at five minutes after bolus, immediately following aortic cannulation, 10 and 20 min on CPB, termination of CPB, termination of the drug infusion and three and six hours after infusion termination.

RESULTS

At the time of aortic cannulation, ketamine concentrations were 3.11 +/- 0.81 micrograms.ml-1, these levels decreased by one third with the initiation of CPB. By the end of CPB the concentrations had returned to levels roughly equivalent to those observed at the time of aortic cannulation. Following cessation of the infusion, ketamine concentration declined in a log-linear fashion with a half-life averaging 2.12 hr. (range 1.38-3.09 hr).

CONCLUSION

This dosage regimen maintained general anaesthetic concentrations of ketamine throughout the operative period. These levels should result in brain tissue concentrations in excess of those previously shown to be neuroprotective in animals. Thus we conclude that this infusion regimen would be reasonable to be use in order to assess the potential neuroprotective effects of ketamine in humans undergoing CPB.

Determination of ketamine and norketamine enantiomers in plasma by solid-phase extraction and high-performance liquid chromatography

A high-performance liquid chromatographic method is described for determination of sub-anaesthetic concentrations of the enantiomers of ketamine and its metabolite norketamine in plasma. The samples are purified by reversed-phase solid-phase extraction. The enantiomers are separated on a Chiral AGP column with a mobile phase containing 16% methanol and a 10 mM phosphate buffer at pH 7.0, and measured by UV-detection at a wavelength of 220 nm. Linear calibration curves with correlation coefficients better than 0.995 have been obtained in the range 10-320 ng/ml. Minimum detectable concentrations were about 2 ng/ml.