Effect site equilibration time is a determinant of induction dose requirement

Relating Observed Psychoactive Effects to the Plasma Concentrations of Delta-9-Tetrahydrocannabinol and Its Active Metabolite: An Effect-Compartment Modeling Approach

The medical use of marijuana is increasing, yet little is known about the exposure-response relationship for its psychoactive effects. It is well known that the plasma concentrations of the principal psychoactive component of marijuana, Δ⁹-tetrahydrocannabinol (THC), do not directly correlate to the observed psychoactive effects. The purpose of this research was to use an effect-compartment modeling approach to predict and relate the concentrations of the psychoactive components (THC and its active metabolite) in the "hypothetical" effect-site compartment to the observed psychoactive effects. A "hypothetical" effect-compartment model was developed using literature data to characterize the observed delay in peak "highness" ratings compared with plasma concentrations of the psychoactive agents following intravenous administration of THC. A direct relationship was established between the reported psychoactive effects ("highness" or intoxication) and the predicted effect-site concentrations of THC. The differences between estimated equilibration half-lives for THC and THC-OH in the effect-compartment model indicated the differential equilibration of parent drug and the active metabolite between plasma and the effect-site. These models contribute to the understanding of the pharmacokinetic-pharmacodynamic relationships associated with marijuana use and are important steps in the prediction of pharmacodynamic effects related to the psychoactive components in marijuana.

Cardiorespiratory and anaesthetic effects of two continuous rate infusions of dexmedetomidine in alfaxalone anaesthetized dogs

Six Beagles were used in this prospective randomised crossover experimental study. Dexmedetomidine was administered at 0, 1 or 2 μg/kg IV for group C, LDA and HDA, respectively. Animals were induced and maintained with alfaxalone at 0.07 mg/kg/min with a CRI dexmedetomidine dose of 0, 0.5 or 1 μg/kg/h for group C, LDA and HDA, respectively. Cardiorespiratory variables, arterial blood gases and depth of anaesthesia were recorded. The recovery times and quality of recovery were scored. Group HDA produced a greater increase in the depth of anaesthesia than LDA. However, with both protocols, CI was halved compared to normal values in dogs. The use of oxygen before and during the anaesthetic maintenance is advisable, mainly if dexmedetomidine is going to be use as a pre-medicant and maintenance agent. The quality of recovery was better in groups receiving dexmedetomidine, without causing an increase in recovery time.

Population pharmacokinetics and pharmacodynamics of brief etomidate infusion in healthy volunteers

This study established the pharmacokinetic and pharmacodynamic relationships of the bispectral index (BIS) and Observer's Assessment of Alertness/Sedation (OAA/S) scale with effect site drug concentrations during and after brief etomidate infusion. Eighteen American Society of Anesthesiologists status I or II volunteers received etomidate (0.2%) infusion at 5 mg/min until the loss of eyelash reflexes, and spontaneous recovery was allowed. Data for plasma etomidate concentrations, BIS, and OAA/S were collected every minute and analyzed by NONMEM. A 2-compartment pharmacokinetic model and a pharmacodynamic sigmoid E(max) model fit the data best, with volumes of distribution at central and peripheral compartments of 4.45 and 74.90 L, respectively, and systemic and intercompartmental clearances of 0.63 and 3.16 L/min, respectively. t(1/2)k(e0) was 1.550 min. EC(50) values were 0.526 and 0.554 µg/mL, and gamma values were 2.25 and 6.24 for BIS and OAA/S, respectively. The prediction probability between OAA/S and BIS was 0.8. The slopes of the curves suggest that BIS is a better monitor of depth of sedation and hypnosis, whereas OAA/S may be more useful for monitoring sleep versus wakefulness. These results should be interpreted within the context of short-term etomidate infusion of less than 10 minutes.

EEG frequency progression during induction of anesthesia: from start of infusion to onset of burst suppression pattern

The anesthetic infusion with propofol influences EEG activity rather smoothly by changing the amplitude activity in different frequency bands. This results in a frequency progression pattern (FPP) which can be related to the depth of anesthesia. An iterative algorithm is proposed for the estimation of the shape of this pattern. The presented method is applied to the data recorded from the start of the propofol anesthetic infusion to the onset of the burst suppression pattern (BSP) with nine patients. The results reveal the underlying FPP and how the onset of the BSP is related to it. The proposed method offers potential for the development of automatic assessment systems for the depth of anesthesia.

Relation of EEG spectrum progression to loss of responsiveness during induction of anesthesia with propofol

OBJECTIVE

Our purpose is to find out whether the Loss of obeying a Verbal Command (LVC) and the preceding progression of the EEG frequency patterns are mutually related during an anesthetic induction.

METHODS

EEG was analyzed from sixteen patients, anesthetized with a fixed rate infusion of propofol (30 mg/kg/h). An artifact-purified EEG from electrode Fz referenced to the average of mastoid signals was filtered to consecutive 4 Hz frequency passbands with a filter bank. Signal envelope time-series were computed for all the passbands and studied as a function of the elapsed induction time t and as a function of the relative time r, which is t divided by the time of the LVC.

RESULTS

A frequency band specific biphasic activity pattern progressed from high towards low frequencies systematically and uniformly in the population studied when presented on the relative time scale r, irrespective of individual responses to the dosage.

CONCLUSIONS

The grouping of the individual progression patterns on the r scale indicates a strong relation between the EEG spectral behavior and the LVC.

SIGNIFICANCE

EEG may provide phenomenological grounds for a continuous control variable of a sedative drug effect, even in between the discrete clinical end-points.

Gender differences in drug effects: implications for anesthesiologists

BACKGROUND

The gender aspect in pharmacokinetics and pharmacodynamics of anesthetics has attracted little attention. Knowledge of previous work is required to decide if gender-based differences in clinical practice is justified, and to determine the need for research.

METHODS

Basis for this paper was obtained by Medline searches using the key words 'human' and 'gender' or 'sex,' combined with individual drug names. The reference lists of these papers were further checked for other relevant studies.

RESULTS

Females have 20-30% greater sensitivity to the muscle relaxant effects of vecuronium, pancuronium and rocuronium. When rapid onset of or short duration of action is very important, gender-modified dosing may be considered. Males are more sensitive than females to propofol. It may therefore be necessary to decrease the propofol dose by 30-40% in males compared with females in order to achieve similar recovery times. Females are more sensitive than males to opioid receptor agonists, as shown for morphine as well as for a number of kappa (OP2) receptor agonists. On this basis, males will be expected to require 30-40% higher doses of opioid analgesics than females to achieve similar pain relief. On the other hand, females may experience respiratory depression and other adverse effects more easily if they are given the same doses as males.

CONCLUSION

These examples illustrate that gender should be taken into account as a factor that may be predictive for the dosage of several anesthetic drugs. Moreover, there is an obvious need for more research in this area in order to further optimize drug treatment in anesthesia.

Target-controlled infusion of propofol induction with or without plasma concentration constraint in high-risk adult patients undergoing cardiac surgery

BACKGROUND

Calculated plasma (Cp) and calculated effect site concentrations (Ce) of propofol associated with loss of consciousness (LOC) have been studied in young healthy patients. The aim of the study was to evaluate the calculated propofol concentrations required to induce LOC in ASA III adult patients undergoing cardiac surgery using a smooth target controlled infusion of propofol.

METHODS

After informed consent, 44 patients were premedicated with 0.5 mg alprazolam orally. Propofol TCI using the pharmacokinetic set of Marsh et al. incorporated in the Diprifusor (ThalfKeo of 2.6 min) was used. Propofol Ce was progressively increased by 0.5 micro g/ml until LOC was obtained. The constraint on the maximum gradient between Cp and Ce was either 1 micro g/ml in group 1 or not limited in group 2. Hemodynamic variations were assessed.

RESULTS

Mean preoperative left ventricular ejection fractions were 44 +/- 15.4% and 56 +/- 11.4% in groups 1 and 2, respectively (P < 0.01). At LOC, mean Cp was 1.9 micro g/ml in both groups but mean Ce was 1.08 +/- 0.31 and 1.43 +/- 0.42 micro g/ml in groups 1 and 2, respectively (P < 0.01). The mean induction time was 12.8 +/- 7.1 min in group 1 and 8.5 +/- 2.7 min in group 2 (P < 0.05). No episode of hypotension has been observed in either group.

CONCLUSION

In ASA III patients undergoing cardiac surgery, smooth propofol TCI induction, using the pharmacokinetic set of Marsh et al. incorporated in the Diprifusor, is associated with LOC at a low mean calculated plasma concentration of 1.9 micro g/ml and good hemodynamic stability.

Midazolam premedication reduces propofol dose requirements for multiple anesthetic endpoints

PURPOSE

This study investigates the interactions between midazolam premedication and propofol infusion induction of anesthesia for multiple anesthetic endpoints including: loss of verbal contact (LVC; hypnotic), dropping an infusion flex (DF; motor), loss of reaction to painful stimulation (LRP; antinociceptive) and attainment of electroencephalographic burst suppression (BUR; EEG).

METHODS

In a double blind, controlled, randomized and prospective study, 24 ASA I-II patients received either midazolam 0.05 mg x kg(-1) (PM; n = 13) or saline placebo (PO; n = 11) i.v. as premedication. Twenty minutes later, anesthesia was induced by propofol infusion at 30 mg x kg(-1) x hr(-1). ED50, ED95 and group medians for times and doses were determined and compared at multiple anesthetic endpoints.

RESULTS

At the hypnotic, motor and EEG endpoints, midazolam premedication significantly and similarly reduced propofol ED50 (reduction: 18%, 13% and 20% respectively; P <0.05 vs unpremedicated patients) and ED95 (reduction: 20%, 11% and 20% respectively; P <0.05 vs unpremedicated patients). For antinociception (LRP), dose reduction by premedication was greater for propofol ED95 (reduction: 41%; P <0.05 vs unpremedicated patients) than ED50 (reduction: 18%; P <0.05 vs unpremedicated patients). Hemodynamic values were similar in both groups at the various endpoints.

CONCLUSIONS

Midazolam premedication 20 min prior to induction of anesthesia reduces the propofol doses necessary to attain the multiple anesthetic endpoints studied without affecting hemodynamics in this otherwise healthy population. The interaction differs for different anesthetic endpoints (e.g., antinociception vs hypnosis) and propofol doses (e.g., ED50 vs ED95).

In vivo cerebral pharmacokinetics and pharmacodynamics of diazepam and midazolam after short intravenous infusion administration in sheep

The cerebral pharmacokinetics and pharmacodynamics of midazolam and diazepam were examined in chronically instrumented sheep via measurements of their arterio-venous concentration difference across the brain during and after 2-min i.v. infusions. Diazepam (30 mg) or midazolam (10 mg) were administered on 5 separate occasions to 4 sheep. For both drugs, rapid cerebral uptake occurred during the infusion, which quickly turned to elution in the postinfusion period. However, this process was more rapid for midazolam than diazepam. The cerebral pharmacokinetics of both was better described by a kinetic model with slight membrane limitation rather than flow limitation. For diazepam, the estimated brain:plasma partition coefficient was 2.67, and the first and second compartments filled with half-lives of 2.2 and 0.5 min, respectively. For midazolam, these values were 0.27, 0.26 and 1.34 min, respectively. In a subset of sheep, pulmonary arterial-arterial gradients were too small to measure suggesting limited metabolism and small distribution volumes for both drugs in the lungs. Simultaneous dynamic measurements of cerebral blood flow and algesimetry lagged behind both the arterial and sagittal sinus blood concentrations. The changes in cerebral blood flow were best described by a previously published a dynamic model that incorporated long half-lives for drug dissociation from the benzodiazepine receptor (13.3 and 5.5 min for midazolam and diazepam, respectively). Effect compartment modeling of the cerebral blood flow data showed apparent effect compartment half-lives (t1/2,keo) that were longer than the half-lives of cerebral equilibration.

The combined effect of age and premedication on the propofol requirements for induction by target-controlled infusion

In this prospective study, we evaluated the combined influence of age and premedication on propofol requirements for the induction of anesthesia and their hemodynamic effects using a target-controlled infusion. We studied 180 patients separated into three age groups: 20-39 yr, 40-59 yr, and more than 59 yr. In each age group, patients were randomly allocated to receive either no premedication (n = 20), fentanyl (2 microg/kg) (n = 20), or midazolam (0.03 mg/kg) plus fentanyl (2 microg/kg) (n = 20). The concentration of propofol targeted for the induction was 5 microg/mL, to be reached in 2 min. The dose, time, and predicted plasma concentration of propofol at hypnosis were measured. Baseline and postinduction heart rate and arterial blood pressure were registered. Computer simulation was used to calculate the effect site propofol concentration at hypnosis. The concentration of propofol, effect site propofol concentration, time, and induction dose and their hemodynamic effect were significantly different among groups with respect to age and premedication. The combined effect of the two factors was additive, but without significant interaction. The propofol requirements were significantly less in the midazolam-fentanyl groups, regardless of age, and among the premedicated patients older than 60 yr compared with the other age groups. We conclude that the combined effect of age and premedication on the requirements of propofol for the induction of anesthesia should be considered when the concentration is targeted with a target-controlled infusion system.

Closed-loop controlled administration of propofol using bispectral analysis

Ten patients, undergoing elective orthopaedic surgery under spinal anaesthesia, were sedated with propofol using a closed-loop feedback control system. The bispectral index (BIS), a new processed EEG parameter, was used as control variable. Propofol administration was controlled by a patient individualised adaptive model-based controller incorporating target-controlled infusion technology combined with a pharmacokinetic-dynamic model. This feedback control system for propofol administration proved to be adequate and safe. BIS was found to be well suited as control variable.

Comparison of spontaneous frontal EMG, EEG power spectrum and bispectral index to monitor propofol drug effect and emergence

BACKGROUND

The aim of this study was to investigate the accuracy of frontal spontaneous electromyography (SEMG) and EEG spectral edge frequency (SEF 95%), median frequency (MF), relative delta power (RDELTA) and bispectral index (BIS) in monitoring loss of and return of consciousness and hypnotic drug effect during propofol administration at different calculated plasma target concentrations.

METHODS

Propofol was administered by using a target-controlled infusion at different propofol steady-state concentrations. All variables were measured simultaneously at specific calculated concentrations and endpoints.

RESULTS

Loss of consciousness was accurately monitored by BIS, SEMG and SEF 95%, and propofol drug effect by BIS only. Return of consciousness was predicted by BIS, MF and SEF 95%. Due to the biphasic EEG pattern of propofol and the lack of reproducible data at specific propofol concentrations, the clinical usefulness of SEF 95%, MF and RDELTA was very limited. SEMG was useful to detect loss and return of consciousness, but without predictive value.

CONCLUSIONS

The BIS might be an accurate measure to monitor depth of anaesthesia and hypnotic drug effect. Other neurophysiologic measures have limited value to monitor depth of anaesthesia and hypnotic drug effect.

Clinical usefulness of the bispectral index for titrating propofol target effect-site concentration

The bispectral index, a new processed electroencephalographic parameter which may give information on depth of anaesthesia, was used in 58 patients undergoing outpatient gynaecological surgery in order to study if the addition of bispectral index monitoring to standard clinical monitoring could improve the titration of target propofol concentration when using effect-site target-controlled propofol infusion for sedation. In Group 1 (n = 30), the bispectral index was recorded but the anaesthetist was unaware of the readings and therefore only classical signs of depth of anaesthesia were used to guide the anaesthetist in controlling the effect-site concentration. In Group 2 (n = 28), bispectral index readings were available to the anaesthetist and effect-site concentration was adjusted to ensure that bispectral index was maintained between 40 and 60. Similar propofol induction and maintenance doses, blood and effect-site concentrations and mean bispectral index were found in the two groups. A greater percentage of bispectral index readings lying outside the target range (i.e. < 40 or > 60) and more movement at incision and during maintenance were found in Group 1. There was a trend towards more implicit awareness in patients in Group 1. Bispectral index was found to be useful for measuring depth of sedation when using propofol target-controlled infusion. Propofol dosage could not be decreased but a more consistent level of sedation could be maintained due to a more satisfactory titration of target effect-site concentration.

Motor and sensory disability has a strong relationship to induction dose of thiopental in patients with the hypertropic variety of Charcot-Marie-Tooth syndrome

In a prospective study, we determined the anesthetic induction dose of thiopental and the clinical variables influencing the appropriate induction dose of thiopental in 20 patients with the hypertrophic variety of Charcot-Marie-Tooth syndrome (CMT). As controls we chose 50 patients without CMT. Motor disturbance was evaluated in terms of muscle weakness of the distal lower and upper extremities. We examined sensory disturbance by evaluating loss of sensation in the index finger and great toe. The preinduction cardiac output was measured by echocardiography. Anesthesia was induced with repeated injections of 50 mg thiopental. The minimum induction dose of thiopental (MID) was confirmed when the eyelash reflex ceased. We maintained anesthesia with enflurane and nitrous oxide. The 95% confidence interval of the MID in patients used as the controls was 2.5-4.9 mg/kg. The MID in 11 patients with CMT was less than 2.5 mg/kg. MIDs in the patients with CMT were significantly smaller than those of the control patients (P < 0.0001). Also we found a strong relationship between the MID and the severity of both motor and sensory disturbances (P = 0.003 and 0.002, respectively). There was no relationship between the MID and other clinical variables, such as age, gender, inherited type, body weight, and preinduction cardiac output. Because delay in the recovery from anesthesia can be caused by an inappropriate dose of thiopental in CMT patients in whom motor and sensory function is seriously impaired, the dose of thiopental probably should be reduced and based on the individual patient's response.

Open loop control of multiple drug effects in anesthesia

Current open-loop computer-controlled infusion pumps do not explicitly control the transient adverse side effects of intravenous drugs during anesthesia. We used optimal control principles to synthesize a single-input multiple-output controller that regulates concentrations at the site of desirable drug effect while penalizing excessive side-effect drug concentrations. The cost function incorporates model-based predictions of future effect-site concentrations, and the capability of the anesthesiologist to anticipate upcoming surgical events. The controller was evaluated and then compared with alternative control strategies through computer simulation of a physiologically based pharmacokinetic model for the intravenous drug alfentanil. Multiple-effect control offers an analytic approach to limit the overshoot in adverse side-effect concentrations at the consequence of increasing the time to achieve the desired drug effect.

A pharmacokinetic-pharmacodynamic model for quantal responses with thiopental

The pharmacokinetic-pharmacodynamic model developed here characterizes the relationship between simulated plasma concentrations of thiopental and two dichotomous endpoints determined at induction of anesthesia: loss of voluntary motor power (clinical endpoint), and burst suppression of the electroencephalogram (EEG endpoint). The model incorporated data from two separate thiopental patient studies: a pharmacokinetic study with 21 males, and a pharmacodynamic study with 30 males. In the pharmacodynamic study, cumulative quantal dose-response curves for the clinical and EEG endpoints were developed from observations made during a constant-rate infusion of thiopental. Population mean parameters, derived from the bolus pharmacokinetic thiopental study, were used to simulate concentration-time data for the 150 mg.min-1 thiopental infusion rate used in the dose-response study. A single biophase model incorporating the two endpoints was generated, combining the pharmacokinetic and pharmacodynamic data from the two groups. Estimates of the mean effective thiopental concentrations affecting 50% of the population (EC50S) for the clinical and EEG endpoints were 11.3 and 33.9 micrograms.ml-1, respectively. The half-time for equilibration between arterial thiopental and the effect compartment was 2.6 min. These results are in reasonable agreement with previously described quantal concentration-response data, and with pharmacodynamic models developed for graded EEG responses. Simulation of bolus doses of thiopental with the new model provided ED50s for the clinical and EEG endpoints of 265 mg and 796 mg, respectively; the dose predicted to produce loss of voluntary motor power in 90% of an adult male population was 403 mg. A model combining population pharmacokinetics with cumulative dose-response relationships could prove useful in predicting dosage regimens for those drugs with responses that are categorical.