Pharmacokinetic studies of neuromuscular blocking agents: good clinical research practice (GCRP)

Creation of an inventory of quality markers used to evaluate pharmacokinetic literature: A systematic review

WHAT IS KNOWN AND OBJECTIVE

Robust critical appraisal tools for clinical pharmacokinetic studies are limited. Before development of such a tool is possible, quality markers (items deemed important for credibility of study results) must be identified. We aim to create an inventory of quality markers intended for the appraisal of clinical pharmacokinetic studies and to categorize identified markers into associated domains of study quality.

METHODS

Medline via ProQuest central (1946-Sep 2020, EMBASE (1974-Sep 2020), Cochrane database of systematic reviews, Google and Google Scholar were searched using the following search categories: pharmacokinetics, reporting guidelines and quality markers. Reference lists of the identified articles were searched manually. Any article (review, study or guideline) reporting quality markers related to the appraisal of pharmacokinetic literature was eligible for inclusion. Articles were further screened and limited to those reported in English on human subjects only. Cell-based and animal-based pharmacokinetic studies were excluded. Extracted data from included articles included identified or perceived markers of quality and baseline article data. Identified quality markers were then categorized according to manuscript reporting domains (abstract, introduction/background, methodology, results, discussion and conclusion).

RESULTS AND DISCUSSION

Of 789 studies identified, 17 articles were included for extraction of quality markers. A total of 35 quality markers were identified across eight categories. The most frequently reported quality markers were related to method (13/35) and result sections (6/35). Quality markers encompassed all aspects of study design and reporting and were both similar and different to established reporting checklists for clinical pharmacokinetic studies.

WHAT IS NEW AND CONCLUSION

The inventory of quality markers is now suitable to undergo further testing for inclusion in a tool designed for the appraisal of clinical pharmacokinetic studies.

Pharmacodynamic comparison of rocuronium bromide between patients from the plateau area and from the plain area

OBJECTIVES

We aimed to conduct a pharmacodynamic comparison of rocuronium bromide between patients from the plateau area and from the plain area.

METHODS

A total of 104 patients who received laparoscopic cholecystectomy in Sichuan Provincial People's Hospital and Aba Autonomous Prefecture People's Hospital from October 2015 to December 2015 were included in this study. Among them, 46 patients were from the plateau area and 58 were from the plain area. Both groups received total intravenous anesthesia (TIVA) with a dose of 0.6 mg/kg rocuronium bromide during induction. In the meantime, neuromuscular block was monitored using a train-of-four (TOF) stimulation mode. The onset time (time to achieve the lowest TOF value after the injection of rocuronium bromide), duration of maximal neuromuscular block (duration of lowest T1 value), time to 25% recovery, time to 75% recovery, recovery index (time from 25% recovery to 75% recovery), time to extubation, length of stay in Post Anesthesia Care Unit (PACU) and muscle strength upon PACU discharge were all recorded.

RESULTS

The onset time, time to 25% recovery, time to 75% recovery and time to extubation were all significantly prolonged in patients from the plateau area after receiving one single dose of rocuronium bromide (P < 0.05). However, both groups didn't show any significant difference in maximal neuromuscular block, recovery index (time from 25% recovery to 75% recovery), length of stay in PACU, or muscle strength upon PACU discharge (P > 0.05).

CONCLUSIONS

Compared to patients from the plain area, patients from the plateau area showed prolonged onset time of rocuronium bromide, reduced metabolic capabilities, and longer duration of muscular relaxation.

Rocuronium pharmacokinetics and pharmacodynamics in the adductor pollicis and masseter muscles

BACKGROUND

The aim of this study was to characterize the dose-effect relationship of rocuronium at the adductor pollicis and masseter muscles.

METHODS

Ten, ASA I, adult patients, received a bolus dose of rocuronium 0.3 mg/kg during propofol based anesthesia. Train-of-four (TOF) was simultaneously monitored at the masseter and the adductor pollicis muscles until recovery. Rocuronium arterial serum concentrations were measured during 120 min. The first twitch of the TOF response was used to characterize the time-effect profile of both muscles using pharmacokinetic-pharmacodynamic analysis in NONMEM. A decrease in NONMEM objective function (∆OFV) of 3.84 points for an added parameter was considered significant at the 0.05 level.

RESULTS

Onset time at the masseter (mean ± SD, 1.5 ± 0.9 min) was faster than at the adductor pollicis (2.7 ± 1.4 min, P < 0.05). Recovery, measured as the time to TOF ratio = 0.9 was similar between muscles 29.9 ± 6.7 (adductor pollicis) vs. 29.3 ± 8.1 (masseter). (P = 0.77). The estimated pharmacodynamic parameters [mean (95% CI)] of the adductor pollicis muscle and the masseter muscle were; plasma effect-site equilibration half-time (teq) 3.25 (2.34, 3.69) min vs. 2.86 (1.83, 3.29) min, (∆OFV 383.665); Ce50 of 1.24 (1.13, 1.56) mg/l vs. 1.19 (1.00, 1.21) mg/l, (∆OFV 184.284); Hill coefficient of 3.97 (3.82, 5.62) vs. 4.68 (3.83, 5.71), (∆OFV 78.906).

CONCLUSIONS

We found that the masseter muscle has faster onset of blockade and similar recovery profile than adductor pollicis muscle. These findings were best, explained by a faster plasma effect-site equilibration of the masseter muscle to rocuronium.

Highlights in muscle relaxants

The aim of this article is to provide an overview on neuromuscular blocking agents and a rational selection of the most appropriate agents, along with pharmacological and pharmacoeconomic considerations on neuromuscular blockers and their antagonists. Neuromuscular blocking agents are used during anesthesia to facilitate endotracheal intubation and provide surgically required paralysis. There is continuing development in the field of neuromuscular blocking agents, with new products appearing at regular intervals. All new agents come at increased costs. The proportion of anesthesia-related drug costs on a per-patient basis are small and vary from country to country, but account for no more than 12% of hospital drug budgets. However, given the large number of anesthetics performed over time, the total cost is significant. Moreover, such costs should be put into the perspective of the operating theater and patient admission costs. Appropriate selection of neuromuscular blocking agents can help not only to reduce biological costs secondary to complications, but also to make operating lists proceed smoothly and without incident. The paucity of outcome studies in relation to anesthetic drugs is not surprising given that anesthesia is used to facilitate the provision of therapy rather than being therapeutic in its own right. Accordingly, the assessment of anesthetic drugs has a different priority to therapeutic drugs. As anesthetic drugs are nontherapeutic, it is also difficult to determine the best choice of agent. However, new neuromuscular blocking agents are marketed on the basis of improvements in the frequency of side effects, safety, reliability, duration, reversibility and undesirable hemodynamic effects.

Effect of Epidural Levobupivacaine on Recovery from Vecuronium-Induced Neuromuscular Block in Patients Undergoing Lower Abdominal Surgery

The aim of this study was to evaluate the effect of epidural levobupivacaine on recovery from vecuronium-induced neuromuscular block. Ninety patients undergoing lower abdominal surgery were randomised into two groups after an epidural test dose: the epidural group (n=45) received a bolus of 15 ml of 0.5% levobupivacaine whereas the control group (n=45) did not. Anaesthesia was induced and maintained with propofol, fentanyl, vecuronium and nitrous oxide. Neuromuscular block was induced with vecuronium 0.1 mg/kg and monitored with acceleromyographic train-of-four at the adductor pollicis. Patients in each group received neostigmine at 25% recovery of the first twitch of train-of-four during recovery from anaesthesia. The effect of epidural levobupivacaine on the speed of recovery of neuromuscular function was evaluated. The lag time, onset time and time from vecuronium administration until 25% T1 recovery did not differ between the groups. The times of the recovery index (the time from 25% to 75% recovery of T1) and of the DUR 25-train-of-four 90 (time from 25% T1 to train-of-four ratio of 0.9) in the epidural group were significantly longer than those for the control group (5.2 [2.1] vs 3.04 [1.02] minutes and 10.8 [3.3] vs 8.2 [2.3] minutes, P <0.001). This study shows that epidural levobupivacaine significantly delays the train-of-four recovery from vecuronium-induced block. Although the interaction is small in the clinical setting, anaesthetists should take this interaction into consideration when combining general and epidural anaesthesia during surgery.

[Tracheal intubation quality under remifentanil-propofol with sevoflurane compared with remifentanil-propofol with rocuronium: a randomized double-blind clinical trial]

OBJECTIVE

To determine whether intubation conditions under remifentanil-propofol plus sevoflurane rather than a nondepolarizing neuromuscular blocker are similar to those obtained when a neuromuscular blocker is used.

MATERIAL AND METHODS

In this double-blind controlled trial, 100 patients undergoing outpatient surgery were randomized to 2 groups. Intubation in one group was performed under remifentanil, propofol and sevoflurane. In the other, intubation was performed under remifentanil, propofol, and the nondepolarizing neuromuscular blocker rocuronium. We recorded dysphonia at 24 hours, Cormack-Lehane classification at laryngoscopy, mandibular relaxation, vocal cord position and mobility, and cough or movement during laryngoscopy, on intubation and on cuff inflation. Blood pressure and heart rate before and after tracheal intubation were also recorded.

RESULTS

No significant between-group differences were observed in dysphonia 24 hours after surgery, Cormack-Lehane classification at laryngoscopy, mandibular relaxation, the position or mobility of vocal cords, or cough or movement during laryngoscopy, intubation or cuff inflation. After intubation the mean (SD) systolic blood pressure was 119.7 (75.4) mm Hg in the rocuronium group and 97.5 (54.5) mm Hg in the sevoflurane group. Mean heart rate was 80.7 beats/min in the rocuronium group and 66.7 beats/min in the sevoflurane group. The differences were significant (P < .05).

CONCLUSIONS

Adequate doses of remifentanil, propofol, and sevoflurane provide intubation conditions that are similar to those achieved by using a nondepolarizing neuromuscular blocker, without exposing patients to additional risk. Avoiding use of a neuromuscular blocker would circumvent the development of complications associated with use of these agents or their antagonists and costs would be lower.

Pharmacokinetic-pharmacodynamic modeling of the influence of chronic phenytoin therapy on the rocuronium bromide response in patients undergoing brain surgery

BACKGROUND

Antiepileptic drugs decrease the intensity of the effect of neuromuscular blocking agents. The objective of this study was to evaluate the influence of chronic phenytoin therapy (CPT) on the pharmacokinetics (PK) and pharmacodynamics (PD) of rocuronium.

METHODS

A total of 21 patients undergoing intracranial surgery were enrolled in the study. Ten of these were under CPT. Rocuronium was administered intravenously. Arterial blood samples were drawn, and the T1% (percentage change from the response to the supramaximal stimulus) derived from electromyogram was continuously recorded. NONMEM: software was used to construct, evaluate and validate the PKPD models.

RESULTS

The PKPD of rocuronium was described using a three-compartment PK model and effect compartment model. The CPT therapy was found to increase the total plasma clearance from 0.26 to 0.75 L min(-1). The PD model parameter estimates were k(e0)= 0.073 min(-1), IC(50) (the steady-state plasma concentration eliciting half of the maximum response) = 836 ng mL(-1) and gamma = 3.13.

CONCLUSIONS

Chronic phenytoin therapy increases the clearance of rocuronium from 0.26 to 0.75 L min(-1) but has no effect on the k(e0), IC(50) or gamma parameters.

Influence of gender on the course of neuromuscular block following a single bolus dose of cisatracurium or rocuronium

BACKGROUND AND OBJECTIVE

There is increasing evidence of gender differences in the pharmacokinetics and pharmacodynamics of aminosteroid neuromuscular blocking agents. Compared to males, females are more susceptible, requiring approximately 30% less rocuronium to achieve the same degree of neuromuscular block. However, little information is available whether this difference is applicable to modern benzylisoquinolines (cisatracurium).

METHODS

In all, 848 patients (423 males, 425 females) undergoing general surgery under total intravenous anaesthesia with muscle relaxation, tracheal intubation and mechanical ventilation were studied. Patients were randomized to receive a single bolus dose of cisatracurium (0.1 mg kg-1, 221 males and 199 females) or rocuronium (0.6 mg kg-1, 202 males and 226 females). The onset time for 95% depression of T1, clinical duration until 25% recovery and recovery index (T1 from 25% to 75%) were determined with an NMT electromyographic module of the Datex-Ohmeda S/5 Anaesthesia Monitor. The data for male and female groups were compared with appropriate statistical tests (unpaired t-test, Mann-Whitney rank sum test and Fisher's exact test).

RESULTS

In both groups (cisatracurium and rocuronium), males were significantly taller (P < 0.001) and heavier (P < 0.001) than females, but the body mass index was comparable. For rocuronium, the onset time was shorter 91.7 +/- 14.3 s vs. 108.0 +/- 14.6 s (P < 0.001) and the clinical duration was increased in females 43.3 +/- 7.8 min vs. 31.3 +/- 5.5 min (P < 0.001). In the cisatracurium group, both onset times (248.9 +/- 60.7 s for males vs. 253.4 +/- 70.9 s for females) and clinical duration (42.6 +/- 6.9 min for males vs. 43.1 +/- 6.9 min for females) were similar. The recovery index was identical for males and females in both groups.

CONCLUSIONS

Females were more sensitive than males to a single bolus dose of rocuronium. Under the study conditions described, the onset time was shorter and the clinical duration was increased in female patients. This suggests that the routine dose of rocuronium should be reduced in females compared to males. On the contrary, we could demonstrate no gender differences in the onset time or clinical duration of cisatracurium.

Is time to peak effect of neuromuscular blocking agents dependent on dose? Testing the concept of buffered diffusion

BACKGROUND AND OBJECTIVES

For neuromuscular blocking agents, an inverse relationship between potency and time to peak effect has been observed. To test the hypothesis that this relationship is due to buffered diffusion, we investigated the influence of dose on time to peak effect. Pharmacokinetic-pharmacodynamic simulations were performed to support the expected relationships between potency, dose, peak effect and time to peak effect.

METHODS

Pigs (20-28 kg body weight) were anaesthetized with ketamine and midazolam, followed by pentobarbital and fentanyl intravenously. Neuromuscular block was measured by stimulating the peroneal nerve supramaximally at 0.1 Hz and measuring the response of the tibialis anterior muscle mechanomyographically. After an initial dose to establish the individual ED90 of a neuromuscular blocking agent (rocuronium, vecuronium, pipecuronium or d-tubocurarine), five different doses of the same compound were administered to each animal, aiming at 20%, 40%, 60%, 75% or 90% block, in a random order. Doses were given 45 min after complete recovery of the twitch response.

RESULTS

For rocuronium and pipecuronium, time to peak effect increased with dose, whereas dose did not affect time to peak effect of vecuronium and d-tubocurarine. Simulations predict that time to peak effect decreases with dose if buffered diffusion is taken into account.

CONCLUSIONS

The results suggest that buffered diffusion does not play a dominant role in the time to peak effect of neuromuscular blocking agents. Therefore it is unlikely that the observed inverse relationship between potency and time to peak effect of neuromuscular blocking agents in the clinical range is due to buffered diffusion.

Good clinical research practice in pharmacodynamic studies of neuromuscular blocking agents II: the Stockholm revision

The set of guidelines for good clinical research practice (GCRP) in pharmacodynamic studies of neuromuscular blocking agents, which was developed following an international consensus conference in Copenhagen, has been revised and updated following the second consensus conference in Stockholm in 2005. It is hoped that these guidelines will continue to help researchers in the field and assist the pharmaceutical industry and equipment manufacturers in enhancing the standards of the studies they sponsor.

Priming with rocuronium accelerates neuromus-cular block in children: a prospective randomized study

PURPOSE

To determine the effects of a priming technique with respect to onset time and duration of action of rocuronium (1.5 x ED(95), 2.0 x ED(95)) in a pediatric patient population.

METHODS

Eighty-four children, age one to seven years undergoing elective pediatric surgery, were studied in a randomized controlled trial. Neuromuscular function was assessed by accelerometry in response to single-twitch stimulation to assess onset of neuromuscular block, followed by train-of-four (TOF) stimulation at the wrist every 15 sec. Children were randomly allocated to one of four groups: Groups 1 and 3 received saline placebo, followed one minute later by a single bolus dose of rocuronium 0.45 mg.kg(-1) iv (1.5 x ED(95)) and 0.6 mg kg(-1) iv (2.0 x ED(95)), respectively. Patients in Groups 2 and 4 received an initial dose of rocuronium 0.045 mg.kg(-1) iv and 0.06 mg.kg(-1) iv, respectively, followed one minute later by rocuronium 0.405 mg.kg(-1) and 0.54 mg.kg(-1)iv, respectively.

RESULTS

Rocuronium priming significantly accelerated the time to maximum rocuronium-induced neuromuscular block when compared to placebo [median (95% confidence interval)]: 122.5 (98-186) vs 92.5 (68-116) sec (1.5 x ED(95), Group 1 vs Group 2, P < 0.05) and 85 (60-142) vs 55 (48-72) sec (2.0 x ED(95), Group 3 vs Group 4, P < 0.05), respectively. Spontaneous recovery to a TOF-ratio >or= 0.9 was dose-dependent and not influenced by priming.

CONCLUSION

Priming accelerated the onset of rocuronium in children. A priming interval of one minute and a cumulative dose of rocuronium 1.5 x ED(95) resulted in an onset of neuromuscular block comparable to a single dose of rocuronium (2.0 x ED(95)).

Perioperative considerations for patients with morbid obesity

The anesthetic management of the MO patient requires an important focus on a number of issues beginning with a careful preoperative evaluation and synthesizing pre-existing disease processes with the anesthetic management plan. The common misperception that all MO patients are "full stomach" has been challenged and may be a nonissue. New approaches to pre-oxygenation to lessen the likelihood of desaturation during apnea may be a valuable tool if difficulty is encountered in tracheal intubation. In addition, promising results have been demonstrated with the use of the ILMA for ventilation and for blindly establishing tracheal tube placement. Proper patient positioning is essential to aid in successful intubation when a laryngoscope is employed. Intraoperative anesthetic management can be guided with a processed electroencephalogram monitor to help improve emergence and to enhance wakefulness in the PACU. Careful consideration must be given to postoperative analgesic needs by minimizing the use of opioids and employing nonopioid analgesics including NSAIDs, alpha2-adrenergic agonists, and low doses of ketamine.

Simultaneous versus sequential pharmacokinetic-pharmacodynamic population analysis using an Iterative Two-Stage Bayesian technique

A method for simultaneous pharmacokinetic-pharmacodynamic (PK-PD) population analysis using an Iterative Two-Stage Bayesian (ITSB) algorithm was developed. The method was evaluated using clinical data and Monte Carlo simulations. Data from a clinical study with rocuronium in nine anesthetized patients and data generated by Monte Carlo simulation using a similar study design were analysed by sequential PK-PD analysis, PD analysis with nonparametric PK data and simultaneous PK-PD analysis. Both PK and PD data sets were 'rich' with respect to the number of measurements per individual. The accuracy and precision of the estimated population parameters were evaluated by comparing their mean error (ME) and root mean squared error (RMSE), respectively. The influence of PD model misspecification on the results was also investigated. The simultaneous PK-PD analysis resulted in slightly more precise population parameter estimates than the sequential PK-PD analysis and the nonparametric PK method. In the presence of PD model misspecification, however, simultaneous analysis resulted in poor PK parameter estimates, while sequential PK-PD analysis performed well. In conclusion, ITSB is a valuable technique for PK-PD population analysis of rich data sets. The sequential PK-PD method is better suited for the analysis of rich data than the simultaneous analysis.

CISATRACURIUM VS. ROCURONIUM: A PROSPECTIVE, COMPARATIVE, RANDOMIZED STUDY IN ADULT PATIENTS UNDER TOTAL INTRAVENOUS ANAESTHESIA

AIMS

To compare the pharmacodynamics of cisatracurium and rocuronium-induced neuromuscular block following single dose, allowing either spontaneous or neostigmine-accelerated complete recovery.

METHODS

Following the ethics committee approval and informed consent, 120 patients scheduled for elective surgery under TIVA with tracheal intubation were randomized into 4 groups with different cisatracurium (CIS, 0.10 or 0.15 mg.kg(-1)) or rocuronium (ROC, 0.60 or 0.90 mg.kg(-1)) doses administered. For each patient, the onset time for 95 % depression of T1, clinical duration until 25 % recovery, recovery index (T1 from 25 to 75 %) and time from T1 25 % to TOF-ratio 0.9 were determined allowing either spontaneous or induced recovery.

RESULTS

The onset times were 277 (SD 58), 220 (46), 91 (16) and 77 (16) s for the CIS 0.10, CIS 0.15, ROC 0.60 and ROC 0.90 groups (p < 0.05), respectively, with lower variability in both ROC groups (p < 0.05). The clinical durations were 42 (7), 52 (7), 35 (11) and 52 (12) min, respectively (p < 0.05 for lower doses). Recovery index was identical in all groups allowing either spontaneous recovery - 15.9 (1.8), 15.5 (1.7), 16.1 (3.7) and 16.1 (4.0) min, or following neostigmine administration - 4.4 (0.9), 4.5 (0.8), 4.3 (0.8) and 4.7 (0.7) min for respective groups. During spontaneous recovery, the variability of DUR25-TOF90 was twice as great for ROC than CIS groups (p < 0.05), while after neostigmine administration it was uniform in all groups.

CONCLUSIONS

For equipotent doses, the onset times for CIS were approximately three times longer than for ROC. The average clinical duration for both relaxants ranged from 35 to 52 min with acceptable variability. Neostigmine administration accelerated the recovery and reduced its variability. When allowing for spontaneous recovery, less scatter was demonstrated for both CIS groups than for ROC ones.

Model-based control of neuromuscular block using mivacurium

BACKGROUND

Short-acting agents for neuromuscular block (NMB) require frequent dosing adjustments for individual patient's needs. In this study, we verified a new closed-loop controller for mivacurium dosing in clinical trials.

METHODS

Fifteen patients were studied. T1% measured with electromyography was used as input signal for the model-based controller. After induction of propofol/opiate anaesthesia, stabilization of baseline electromyography signal was awaited and a bolus of 0.3 mg kg-1 mivacurium was then administered to facilitate endotracheal intubation. Closed-loop infusion was started thereafter, targeting a neuromuscular block of 90%. Setpoint deviation, the number of manual interventions and surgeon's complaints were recorded. Drug use and its variability between and within patients were evaluated.

RESULTS

Median time of closed-loop control for the 11 patients included in the data processing was 135 [89-336] min (median [range]). Four patients had to be excluded because of sensor problems. Mean absolute deviation from setpoint was 1.8 +/- 0.9 T1%. Neither manual interventions nor complaints from the surgeons were recorded. Mean necessary mivacurium infusion rate was 7.0 +/- 2.2 microg kg-1 min-1. Intrapatient variability of mean infusion rates over 30-min interval showed high differences up to a factor of 1.8 between highest and lowest requirement in the same patient.

CONCLUSIONS

Neuromuscular block can precisely be controlled with mivacurium using our model-based controller. The amount of mivacurium needed to maintain T1% at defined constant levels differed largely between and within patients. Closed-loop control seems therefore advantageous to automatically maintain neuromuscular block at constant levels.

Control of Muscle Relaxation During Anesthesia: A Novel Approach for Clinical Routine

During general anesthesia drugs are administered to provide hypnosis, ensure analgesia, and skeletal muscle relaxation. In this paper, the main components of a newly developed controller for skeletal muscle relaxation are described. Muscle relaxation is controlled by administration of neuromuscular blocking agents. The degree of relaxation is assessed by supramaximal train-of-four stimulation of the ulnar nerve and measuring the electromyogram response of the adductor pollicis muscle. For closed-loop control purposes, a physiologically based pharmacokinetic and pharmacodynamic model of the neuromuscular blocking agent mivacurium is derived. The model is used to design an observer-based state feedback controller. Contrary to similar automatic systems described in the literature this controller makes use of two different measures obtained in the train-of-four measurement to maintain the desired level of relaxation. The controller is validated in a clinical study comparing the performance of the controller to the performance of the anesthesiologist. As presented, the controller was able to maintain a preselected degree of muscle relaxation with excellent precision while minimizing drug administration. The controller performed at least equally well as the anesthesiologist.

Effects of mivacurium on the diaphragm evaluated by cervical magnetic stimulation of the phrenic nerves

BACKGROUND AND OBJECTIVE

Non-depolarizing neuromuscular blocking agents have differential effects on the diaphragm and skeletal muscles. We employed a new method to study the effects of mivacurium on the diaphragm and compared the results obtained with this method with published data.

METHODS

Anaesthesia was induced and maintained with propofol and alfentanil and the trachea was intubated after topical anaesthesia. Contractions of the diaphragm were induced by cervical magnetic stimulation of the phrenic nerves and quantified by measuring airway pressure responses. The neuromuscular effects on skeletal muscles were measured by acceleromyography of the adductor pollicis muscle. Mivacurium (0.15 mg kg(-1)) was injected and neuromuscular responses were recorded until the effects had waned.

RESULTS

Eleven male and 10 female patients (ASA I-II; 57 +/- 16 yr; 78 +/- 13 kg; mean +/- standard deviation) participated. Median maximal reduction of twitch response was less (P < 0.05) for the diaphragm (89%) than for the adductor pollicis (100%). Time to 25% recovery was shorter for the diaphragm than for the adductor pollicis (8.8 +/- 2.2 min vs. 22.6 +/- 5.0 min, P < 0.05). The difference between the recovery index of the diaphragm (7.3 min (3.6-18.4)) and the adductor pollicis (8.2 min (4.4-20.9) (median (range)) just missed our chosen level of statistical significance (P = 0.06). The recovery time to train-of-four 0.8 was shorter for the diaphragm (median and 95% confidence interval 25.1 +/- 10.2 min) than for the adductor pollicis (median and 95% confidence interval 37.5 +/- 9.4 min, P < 0.05).

CONCLUSIONS

The duration of the clinical effect of mivacurium on the diaphragm is markedly shorter than on the adductor pollicis muscles but there was only a small difference in the recovery index of the two muscles. These effects and the time courses determined with the new method closely resemble the results obtained with different methods in other studies.

Cisatracurium-Induced Neuromuscular Blockade Is Affected by Chronic Phenytoin or Carbamazepine Treatment in Neurosurgical Patients

The effect of chronic anticonvulsant therapy (CAT) on the maintenance and recovery profiles of cisatracurium-induced neuromuscular blockade has not been adequately studied. In this study, we compared the pharmacokinetics and pharmacodynamics of cisatracurium after a prolonged infusion in patients with or without CAT. Thirty patients undergoing intracranial surgery were enrolled in the study: 15 patients under CAT (carbamazepine and phenytoin, Group A) and 15 controls receiving no anticonvulsant therapy (Group C). Anesthesia was standardized and both groups received a bolus of cisatracurium followed by an infusion to maintain a 95% twitch depression. A steady-state was obtained and the infusion was kept constant for 2 additional hours. Neuromuscular blockade was then allowed to spontaneously recover. Blood samples were taken for measurement of cisatracurium plasma concentration during the steady-state period (Cp(ss)95) and at various times during recovery. Demographic and intraoperative data were similar. CAT resulted in faster 25% and 75% recovery of the first twitch. The rate of infusion of cisatracurium needed to maintain a 95% twitch depression at steady-state was 44% faster in Group A (P < 0.001). The clearance of cisatracurium was significantly faster in Group A when compared with Group C (7.12 +/- 1.87 versus 5.72 +/- 0.70 L . kg(-1) . min(-1), P = 0.01). The Cp(ss)95 was also significantly larger in Group A (191 +/- 45 versus 159 +/- 36 ng/mL, P = 0.04). In addition, patients receiving CAT had a 20% increase in the clearance of cisatracurium that, in turn, resulted in a faster recovery of neuromuscular blockade after an infusion of the drug. Also, patients under CAT had a 20% increase in their Cp(ss)95, indicating an increased resistance to the effect of cisatracurium.

Impact of peripheral elimination on the concentration-effect relationship of remifentanil in anaesthetized dogs

BACKGROUND

This study elucidates the impact of sampling site when estimating pharmacokinetic-pharmacodynamic (PK-PD) parameters of drugs such as remifentanil that undergo tissue extraction in the biophase. The interrelationship between the concentrations of remifentanil predicted for the effect compartment and those measured in arterial, venous, and cerebrospinal fluid were investigated under steady-state conditions.

METHODS

Following induction of anaesthesia with pentobarbital, an arterial cannula (femoral) and two venous catheters (jugular and femoral) were inserted. Electrodes were placed for EEG recording of theta wave activity. Each dog received two consecutive 5-min infusions for the PK-PD study and a bolus followed by a 60-min infusion was started for the steady-state study. Cerebrospinal fluid, arterial and venous blood samples were drawn simultaneously after 30, 40, and 50 min. At the end of the infusion, arterial blood samples were collected for pharmacokinetic analysis.

RESULTS

Remifentanil PK-PD parameters based on theta wave activity were as follows: apparent volume of distribution at steady-state (V(ss)) (231+/-37 ml kg(-1)), total body clearance (Cl) (63+/-16 ml min(-1) kg(-1)), terminal elimination half-life (t(1/2 beta)) (7.71 min), effect compartment concentration at 50% of maximal observed effect (EC(50)) (21+/-13 ng ml(-1)), and equilibration rate constant between plasma and effect compartment (k(e0)) (0.48+/-0.24 min). The mean steady-state cerebrospinal fluid concentration of 236 ng ml(-1) represented 52 and 74% of that in arterial and venous blood, respectively.

CONCLUSIONS

Our study re-emphasizes the importance of a sampling site when performing PK-PD modelling for drugs undergoing elimination from the effect compartment. For a drug undergoing tissue elimination such as remifentanil, venous rather than arterial concentrations will reflect more exactly the effect compartment concentrations, under steady-state conditions.

Pharmacokinetics and pharmacodynamics of mivacurium in patients phenotypically heterozygous for the usual and atypical plasma cholinesterase variants (UA)

BACKGROUND

Mivacurium is hydrolyzed by plasma cholinesterase (pChe). The purpose of this study was to evaluate the pharmacodynamics and the pharmacokinetics of the three isomers of mivacurium in patients phenotypically heterozygous for the usual and the atypical pChe variant (UA).

METHODS

Thirty-two patients were included in a dose-response study, in which the patients received one of four doses of mivacurium. An additional bolus dose of mivacurium, to a total of 0.1 mg kg-1, was given followed by a continuous infusion adjusted to maintain 91-99% neuromuscular block. The times to different levels of recovery following the infusion were measured using mechanomyography and train-of-four (TOF) nerve stimulation. Twelve of the patients with an estimated duration of anaesthesia of more than 90 min were (randomly) selected for the pharmacokinetic part of the study. Venous samples were taken for determination of the three isomers of mivacurium. These results were compared with results from a previous study in phenotypically normal patients (UU).

RESULTS

The estimated ED50 and ED95 were 24 and 69 microg kg-1, respectively. The median (range) infusion rate was 3.7 microg kg-1 min-1 (1.2-2.9) and the time to a TOF ratio of 0.7 was 29.8 min (16.1-44.8). The median clearances of the cis-cis, cis-trans and trans-trans isomers were 3.7, 29 and 28 ml kg-1 min-1, respectively. The elimination half-lives of the isomers were 45, 6.7 and 6.3 min, respectively.

CONCLUSION

In patients heterozygous for the usual and the atypical variant (UA), the potency of mivacurium is higher, the infusion requirements lower and the rate of spontaneous recovery prolonged, compared with phenotypically normal patients. The clearances of the active isomers are significantly lower and the elimination half-lives longer in heterozygous patients than in phenotypically normal patients (UU). The pharmacokinetics of the inactive cis-cis isomer was not affected.

Selecting neuromuscular-blocking drugs for elderly patients

The physiological changes that occur with increasing age can have significant effects on the pharmacokinetics of neuromuscular-blocking drugs. Changes in cardiac output can affect drug distribution and therefore the speed of onset of neuromuscular block. A decrease in muscle mass and increase in body fat with age can also affect their distribution. The deterioration in renal and hepatic function associated with aging affects the clearance and elimination of many neuromuscular-blocking drugs. The effects of these physiological changes on the pharmacokinetics of neuromuscular-blocking agents may not become apparent clinically in healthy individuals until the age of at least 75 years. There is very little evidence to suggest any alteration in the sensitivity of the neuromuscular junction to neuromuscular-blocking drugs with increasing age. Neuromuscular-blocking drugs that undergo a significant degree of organ-dependent elimination, such as pancuronium bromide, vecuronium bromide, rocuronium bromide and doxacurium chloride, may have a significantly prolonged duration of action in elderly patients. These drugs can be used safely in elderly patients if the anaesthetist is aware of their altered pharmacokinetics in this patient group. Appropriate changes must be made to drug dosage and dose intervals. As the pharmacokinetic changes can be unpredictable, monitoring of neuromuscular block is strongly advised when using these drugs in such patients. The risk of residual block occurring postoperatively after the use of pancuronium bromide increases with age. The duration of action of mivacurium chloride may also be prolonged in the elderly; this change has not been demonstrated to be a result of an alteration in plasma cholinesterase activity. In contrast, there is no evidence of an alteration in the action of suxamethonium chloride (succinylcholine chloride) with increasing age. Atracurium besilate and cisatracurium besilate undergo predominantly organ-independent elimination. Onset of block with these two drugs may be prolonged in the elderly, but their clinical duration of action does not alter significantly with age, making them particularly suitable for use in this patient group. Although atracurium besilate may cause histamine release, there is little evidence of it producing haemodynamic changes in the elderly. Its (1R,1R')-isomer, cisatracurium besilate, has very little direct or indirect cardiovascular effect and is, therefore, the most suitable nondepolarising agent to use in elderly patients.

Pharmacokinetics and pharmacodynamics of mivacurium in young adult and elderly patients

BACKGROUND

Mivacurium is hydrolyzed by plasma cholinesterase, and is therefore less dependent on liver metabolism and renal elimination than other neuromuscular blocking drugs. This might favor the use of mivacurium in elderly patients. The purpose of this study was to compare the pharmacodynamics and the pharmacokinetics of the three isomers of mivacurium and their metabolites in young adult and elderly patients.

METHODS

Sixty-four patients were included in a dose-response study, in which 32 young adults and 32 elderly patients received one of four doses of mivacurium. An additional bolus dose of mivacurium to a total of 0.1 mg/kg was given followed by a continuous infusion adjusted to maintain a 91-99% neuromuscular block. The times to maximum block and different levels of recovery were measured using mechanomyography and train-of-four (TOF) nerve stimulation. Thirty-two patients were randomly selected for the pharmacokinetic study. Venous samples were taken for determination of the three mivacurium isomers and the metabolites.

RESULTS

The estimated ED95 were 0.053 and 0.061 mg/kg in young adults and elderly patients, respectively (NS). The median infusion rate did not differ, but duration to a TOF ratio of 0.7 was significantly longer in elderly patients than in young adult patients (21.0 vs. 16.5 min). No statistically significant difference between the age groups in clearance and elimination half-life of the isomers was seen. The half-lives of the metabolites were significantly prolonged in the elderly patients.

CONCLUSION

There were no significant differences in the potency or infusion requirements between the adult and elderly patients, but the rate of recovery was significantly, though only moderately prolonged, in the elderly patients. No significant difference in clearance was seen but the elimination half-lives of the metabolites was longer in the elderly patients.