Injection Of Mivacurium Research Articles

Onset time, duration of action, and intubating conditions after mivacurium in elderly and younger patients.

The neuromuscular blocking agent mivacurium can be used during anesthesia to facilitate tracheal intubation. Data on onset time, duration of action, and effect on intubating conditions in patients 80 years and older are however limited. We hypothesized that onset time and duration of action of mivacurium would be longer in elderly patients than in younger adults. This prospective observational study included 35 elderly (≥80 years) and 35 younger (18-40 years) patients. Induction of anesthesia comprised fentanyl 1-3 μg kg-1 and propofol 1.5-2.5 mg kg-1 and propofol and remifentanil for maintenance. Acceleromyography was used for monitoring neuromuscular blockade. The primary outcome was onset time defined as time from injection of mivacurium 0.2 mg kg-1 to a train-of-four (TOF) count of zero. Other outcomes included duration of action (time to TOF ratio ≥0.9), intubating conditions using the Fuchs-Buder scale and the intubating difficulty scale (IDS), and occurrence of hoarseness and sore throat postoperatively. No difference was found in onset time comparing elderly with younger patients; 219 s (SD 45) versus 203 s (SD 74) (difference: 16 s (95% CI: -45 to 14), p = .30). Duration of action was significantly longer in elderly patients compared with younger patients; 52 min (SD 17) versus 30 min (SD 8) (difference: 22 min [95% CI: 15 to 28], p < .001). No difference was found in the proportion of excellent intubating conditions (Fuchs-Buder); 31/35 (89%) versus 26/35 (74%) (p = .12) or IDS score (p = .13). A larger proportion of younger patients reported sore throat 24 h postoperatively; 34% versus 0%, p = .0002. No difference was found in hoarseness. No difference in onset time of mivacurium 0.2 mg kg-1 was found comparing elderly and younger patients. However, elderly patients had significantly longer duration of action. No difference was found in intubating conditions.

The efficacy and safety of mivacurium in pediatric patients

BackgroundMivacurium is the shortest acting nondepolarizing muscle relaxant currently available; however, the effect of different dosages and injection times of intravenous mivacurium administration in children of different ages has rarely been reported. This study was aimed to evaluate the muscle relaxant effects and safety of different mivacurium dosages administered over different injection times in pediatric patients.MethodsSix hundred forty cases of pediatric patients, aged 2 m-14 years, ASA I or II, were divided into four groups (Groups A, B, C, D) according to the age class (2–12 m, 13–35 m, 3–6 years and 7–14 years) respectively, also each group were divided into four subgroups by induction dose (0.15, 0.2 mg/kg in 2–12 m age class; 0.2, 0.25 mg/kg in other three age classes), and mivacurium injection time (20 s, 40 s), totally 16 subgroups. Neuromuscular transmission was monitored with supramaximal train-of-four stimulation of the ulnar nerve. Radial artery blood (1 ml) was sampled to quantify plasma histamine concentrations before and 1, 4, and 7 min after mivacurium injection (P0, P1, P2 and P3).ResultsFive hundred sixty-two cases completed the study. There were no demographic differences within the four groups. The onset time of 0.2 mg/kg groups in 2–12 m aged patients were shorter than those of 0.15 mg/kg groups (189 ± 64 s vs. 220 ± 73 s, 181 ± 60 s vs. 213 ± 71 s, P <0.05), and the recovery times were no statistical differences. The T1 25% recovery time of 0.2 mg/kg in 3–6 years aged patients was shorter than that of 0.25 mg/kg group (693 ± 188 s vs. 800 ± 206 s, P <0.05). The onset and recovery times of mivacurium were not different in 13–35 m and 7–14 years aged patients. The plasma concentrations of histamine at P0, P1, P2 and P3 were not different within four groups.ConclusionsThe induction dose and injection time of mivacurium had mostly insignificant effects on onset and recovery times. The main exception to this was that in 2–12 m aged patients, increasing the dose of mivacurium from 0.15 to 0.2 mg/kg accelerated the onset time by about 30 s. Mivacurium produced no significant release of histamine in any age group at the doses studied.Trial registrationClinicalTrials.gov Identifier-NCT02117401, July 14, 2014. (Retrospectively registered)

Muscle relaxation effect and safety of mivacurium in patients with early hepatocirrhosis

Objective To observe muscle relaxation effect and safety of mivacurium in hepatocirrhosis patients, and provide a guidance in its clinical application. Methods Candidates for elective laparoscopic surgery were divided into a hepatocirrhosis group(group C, n=32) and a non hepatocirrhosis group (group N, n=25). Muscular relaxation monitor was applied with a train of four stimulation(TOF). The intubation dosage was 0.2 mg/kg. Tracheal intubation was performed when train of four stimulation ratio(TOFr, T4∶T1) was 0, and pump infusion of mivacurium was started when muscle relaxation began to recover (TOFr>0) with a velocity of 0.6 mg·kg-1·h-1, which was switched to 0.3 mg·kg-1·h-1 and then stopped in 30 min and 20 min prior to end of suturing in group C and group N respectively. The key parameter included plasma cholinesterase(PchE) activity, histamine concentration, variation of MAP, HR, onset time of mivacurium, acting duration of the first dose, time of TOFr recovery, time from drug withdrawal to extubation, and recovery index. Results The PchE activity were(4 304±901) U/L and (6 227±1 289) U/L in two groups (P 0.05). The mean histamine concentrations of all patients were (0.196±0.020), (0.238±0.021), (0.278±0.024) μg/L prior to intravenous injection of mivacurium and in 5 min and 10 min post to injection, there were significant differences between the three times points(P 0.05). The duration of the first dose were(20±4) min and (16±3) min respectively, the recovery of TOFr to 75% were (35±10) min and (24±5) min, time from drug withdrawal to extubation were (35±9) min and (24±7) min, and there were significant differences between the two groups (P<0.05). Conclusions The plasma histamine concentration increase slightly without significant influence on MAP and HR, with satisfactory intubation conditions, good muscle relaxation and fast recovery from muscle relaxation when mivacurium was applied in patients with early hepatic cirrhosis. Compared with non hepatic cirrhosis patients, the time of muscle relaxation recovery induced by mivacurium was longer in hepatic cirrhosis patients, but compared with other muscle relaxant, it still had obvious advantages. Because mivacurium didn't directly depend on the liver to metabolize, so it could reduce the burden of liver on drug metabolism. It is recommended to be applied in patients with early hepatic cirrhosis. Key words: Cirrhosis of liver; Mivacurium chloride; Histamine; Cholinesterase; Monitoring of muscle relaxation

Comparison of clinical applications of single-dose intravenous injection of mivacurium and cisatracurium in adults’ vocalcordpolyps resection under self-retaining laryngoscope

Objective: Through comparing the clinical observation of mivacurium and cisatracurium in vocal polyp extraction, to study the advantage of mivacurium in vocal polyp extraction. Methods: Forty American Society of Anesthesiologists(ASA) physical status I~II patients for vocal polyp extraction, aged 18~60 years old, were randomly divided into two groups as Mivacurium injection group(Group M) and Cis-atracurium injection group(Group C), each group includes 20 subjects. None of the patients are allergic, has serious diseases of cardiovascular system, liver or kidney.None of them has asthma, airway high response, difficult airway or neuromuscular diseases.Those patients who use beta-blockers or calcium channel blockers for a long time were excluded .All the subjects had the same premedication, fasting and fluid fobidden time. All the subjects who get into the operating room get the routine monitoring of electrocardiogram(EEG), blood pressure(BP), heart rate(HR), pulse oxygen saturation(SpO2) and the TOF WATCH SX. Each group gives the same medicine other than the muscle relaxant during induction of general anesthesia to do the vocal polyp extraction by the same experienced operator. Two groups were recorded in each index of anesthesia induction and tracheal intubation conditions, operation conditions, anesthesia, muscle relaxation monitoring. Results: There are no statistically significant in Cormack-Lehane grading system,Cooper’s grading system and operation satifaction(p>0.05). 2. Group A have shorter intubation time than Group B(p 0.05). Conclusion: 1. A single intubating dose of mivacurium can provide similar intubation and surgeon satisfaction for the vocal polyp extraction. 2. Compared with cis-atracurium, mivacurium can shorten the intubation time and the recovery time of anesthesia. The adverse reactions of mivacurium is mild, and it has less Residual muscle relaxation. Therefore mivacurium is more suitable for the vocal polyp extraction than cis-atracurium.

A systematic review and meta-regression analysis of mivacurium for tracheal intubation

We systematically reviewed factors associated with intubation conditions in randomised controlled trials of mivacurium, using random-effects meta-regression analysis. We included 29 studies of 1050 healthy participants. Four factors explained 72.9% of the variation in the probability of excellent intubation conditions: mivacurium dose, 24.4%; opioid use, 29.9%; time to intubation and age together, 18.6%. The odds ratio (95% CI) for excellent intubation was 3.14 (1.65-5.73) for doubling the mivacurium dose, 5.99 (2.14-15.18) for adding opioids to the intubation sequence, and 6.55 (6.01-7.74) for increasing the delay between mivacurium injection and airway insertion from 1 to 2 min in subjects aged 25 years and 2.17 (2.01-2.69) for subjects aged 70 years, p < 0.001 for all. We conclude that good conditions for tracheal intubation are more likely by delaying laryngoscopy after injecting a higher dose of mivacurium with an opioid, particularly in older people.

Role of endogenous histamine in altered lung mechanics in rabbits.

Unlike the effects of exogenous histamine, those of endogenous histamine on the lung mechanics have not yet been characterized. The site of endogenous histamine liberation by mivacurium was determined, as were the effects of this histamine on the airway and parenchymal mechanics in control rabbits (group C) and rabbits pretreated with H1 and H2 receptor blockers (group AH). The effectiveness of the receptor blockade was ensured by challenges with exogenous histamine. Pulmonary input impedance at low frequencies (ZL) was measured in anesthetized mechanically ventilated open-chest rabbits under control conditions and every minute after administration of an intravenous bolus of mivacurium (2 mg/kg) and exogenous histamine (10 microg/kg). Histamine levels were determined in serum samples taken from the carotid artery and jugular vein before and 1, 3, and 6 min after mivacurium injection. Parameters of airway resistance (Raw) and inertance and parenchymal damping (G) and elastance (H) were extracted from ZL spectra. Mivacurium induced significant increases in plasma histamine levels, with the venous concentrations being significantly higher than those in the artery. The mivacurium-induced increase in Raw (28.7 +/- 2.3%; mean +/- SD) in group C was significantly higher than that in group AH (6.6 +/- 3.4%), whereas the responses in G were not inhibited significantly (23.9 +/- 6.9% vs. 15.5 +/- 3.0%). The significant increases in Raw (70.6 +/- 12.6%) and G (21.0 +/- 4.9%) after exogenous histamine administration were virtually completely abolished by antihistamine pretreatment (3.6 +/- 3.7% and 0.3 +/- 2.6%). After mivacurium administration, endogenous histamine is liberated at least partly in the systemic circulation, and it induces primarily a heterogeneous airway constriction with minor changes in the parenchymal properties. This response was considerably reduced but not abolished by antihistamine pretreatment, a circumstance suggesting that mivacurium may liberate other constrictor mediators that might also contribute to the airway and parenchymal constriction.

Extensively Long Recovery after Mivacurium Caused by Rare “Silent” Gene Mutations

Extensively Long Recovery after Mivacurium Caused by Rare “Silent” Gene Mutations. Gätke et al. (page 600)In patients who have the butyrylcholinesterase gene (BChE ), mivacurium is rapidly hydrolyzed in plasma, and duration of action is short. An estimated 24% of the white population carries a genetic variant allele of the BChE enzyme, which can result in slow hydrolysis of mivacurium and prolonged neuromuscular block. In this article, Gätke et al. describe the anesthetic course of a healthy 30-yr-old man undergoing a correction of the inner nose who received a single dose of 10 mg mivacurium to facilitate tracheal intubation.Ninety minutes after administration of mivacurium, at completion of surgery, the patient had no response either to train-of-four or to tetanic stimulation. The patient was transferred to the intensive care unit, where neuromuscular blockade was continuously monitored until a train-of-four ratio of 0.75 was obtained—nearly 8 h (469 min) after injection of mivacurium. While the patient was in the intensive care unit, venous blood samples were collected at 12 regular intervals (from 134 to 494 min after mivacurium administration). A blood sample drawn when the patient arrived in the intensive care unit was analyzed to determine BChE activity, phenotype, and genotype. The patient’s BChE activity was zero, indicating that he was homozygous for silent mutations. Subsequent to the patient’s discharge from the hospital, the researchers obtained permission to perform complete nucleotide sequencing of the BchE gene of not only the patient, but also of his parents and siblings.DNA analysis revealed two point mutations, the known S7 variant and an undescribed mutation, which introduced a stop codon at amino acid residue 172 of the 574 amino acid residues of normal BChE. Biochemical data supported the finding that the patient was compound heterozygous for two silent mutations. Pedigree analysis showed that S7 was inherited from the patient’s mother and the novel mutation was inherited from his father, proving that both alleles are affected in the patient. Screening for abnormal BChE genotypes is not practical or cost-effective in daily clinical practice, but it could be used, according to the authors, to supplement equivocal results obtained by biochemical methods and in clinical situations such as the one described in this article.

L’efficacité du mivacurium baisse si son injection est retardée par une prise de pression non-invasive sur le bras homolatéral

Objective: We assessed the neuromuscular characteristics of 0.2 mg·kg –1 of mivacurium while its injection was concomitant to a non invasive blood pressure measurement in the ipsilateral arm. Patients: Thirty-one patients ASA I-II were randomized into two groups. Group cuff ( n = 15) and Group control ( n = 16). Methods: General anaesthesia was induced with fentanyl, thiopentone and mivacurium in all patients, however in the cuff group, measurement of non invasive blood pressure was performed immediately after the injection of mivacurium. Comparison was made on neuromuscular blockade of the adductor pollicis (AP) by mechanomyography, and intubating conditions which were guided by the visual estimation of the orbicularis oculi’s (OO) response. Results: In the cuff group, six out of 15 patients did not have complete blockade at the OO against one out of 16 in the control group, (Fisher exact test p < 0.05). Intubation time was significantly delayed in the cuff group, 201 ± 66 s versus 123 ± 32 s in the control group, ( t test p < 0.001). The maximum neuromuscular blocking effect at the AP was significantly greater in the control group 99 ± 2 % against 89 ± 7 % in the cuff group, ( t test p < 0.01). The onset of maximum blockade at the AP was longer in the cuff group 294 ± 40 s versus 179 ± 92 s, ( t test p < 0.001] in the control group. Time to 25 % recovery was shorter in the cuff group 16 ± 3 min versus 20 ± 5 min, in the control group ( t test p < 0.05). Conclusion: This study suggests that non invasive blood pressure measurement of the ipsilateral arm, concomitant to the injection of mivacurium decreases the potency of mivacurium. This finding is mostly explained by the early hydrolysis of mivacurium in the plasma of the excluded arm.

The Mechanism of Pancuronium Potentiation of Mivacurium Block: Use of the Isolated-Arm Technique

The neuromuscular blocking effects of mivacurium are greatly enhanced when mivacurium is preceded by a subparalyzing dose of pancuronium. The mechanism of this potentiation has not been elucidated. This study investigated the effects of the anticholinesterase activity of a small dose of pancuronium on the neuromuscular blocking effects of mivacurium. Forty patients were enrolled in the study. The neuromuscular effects of 7.5 and 15 μg/kg pancuronium, followed by 50 and 100 μg/kg mivacurium, were assessed in Groups PM1 and PM2 (n = 20), respectively. The neuromuscular effects of 65 and 130 μg/kg mivacurium were assessed in Groups M1 and M2 (n = 20), respectively. One arm was excluded from circulation with a tourniquet, which was inflated before the injection of pancuronium and deflated 3 min after the injection of mivacurium. The plasma cholinesterase activity was measured before induction for all patients and 3 min after the injection of pancuronium for Groups PM1 and PM2. The plasma cholinesterase activity was decreased by 16% and 33% after pancuronium administration in Groups PM1 and PM2, respectively. In the nonexcluded arm, pancuronium significantly potentiated the effects of mivacurium. In the excluded arm, no significant block was detected for Groups M1 and M2, whereas the maximal degree of neuromuscular block was 79% and 100% for Groups PM1 and PM2, respectively. Using the isolated-arm technique, we suggest that pancuronium potentiation of the neuromuscular blocking effects of mivacurium is more likely attributable to an increase in the effective plasma concentration of mivacurium than to occupancy of postsynaptic acetylcholine receptors. Implications Using the isolated-arm technique, we suggest that pancuronium potentiation of the neuromuscular blocking effects of mivacurium is more likely attributable to an increase in the effective plasma concentration of mivacurium than to occupancy of postsynaptic acetylcholine receptors.

Can final EMG baseline be used as a reference to calculate neuromuscular recovery?

Investigations recording recovery times of muscle relaxants have used initial or final baseline of a neuromuscular trace, or both, as a reference for data analysis. We evaluated the use of final baseline of EMG traces as a reliable reference to calculate recovery times. We analyzed EMG traces from 82 children who had full spontaneous neuromuscular recovery following a single dose of mivacurium. Times from administration of mivacurium to 25, 50, 75, and 90% EMG recoveries were measured using both initial and final baselines as a reference. EMG traces with final baseline of 100 +/- 10% of the initial baseline were regarded as optimal. Recovery times from all other traces were compared to the times obtained from these optimal traces. Poor final baseline was defined as that of < 80% of initial baseline. Inter-group comparisons were made using Kruskal-Wallis test followed by Mann-Whitney U tests. EMG recovery times were similar for optimal traces whether the reference was the initial or the final baseline of the EMG trace. If the final baseline was used as the reference, then traces with poor final EMG baseline also showed similar neuromuscular recovery times. If the initial baseline was used as the reference for EMG traces with poor final baseline, then neuromuscular recovery times became 24-55% longer (P < 0.05). We conclude that the final baseline of an EMG trace can be used as a reference for calculations of neuromuscular recovery times following a bolus injection of mivacurium.

Pharmacokinetics of mivacurium isomers and their metabolites in healthy volunteers after intravenous bolus administration.

Previous studies report the pharmacokinetics of mivacurium isomers after an infusion using venous blood sampling. Although the extent of the mivacurium arterial-venous gradient is not known, the sampling site is likely to influence mivacurium pharmacokinetic parameters because the drug is rapidly metabolized as it traverses the circulation. The objectives of this study were (1) to determine the pharmacokinetics of mivacurium isomers in healthy persons after intravenous bolus administration using intensive arterial blood sampling, and (2) to characterize the formation and elimination of mivacurium metabolites in human plasma. Eight persons classified as American Society of Anesthesiologists physical status 1 or 2 who were scheduled to undergo elective surgery under balanced anesthesia received 0.15 mg/kg mivacurium chloride as an intravenous bolus. Arterial blood samples were collected every 10 s during the first 2 min and at frequent intervals for 4 h thereafter. Plasma concentrations of mivacurium isomers and their metabolites were determined by two stereoselective high-performance liquid chromatographic methods coupled with fluorometric detection and noncompartmental pharmacokinetic parameters. Mean elimination half-lives of the trans-trans, cis-trans, and cis-cis isomers were 2.4, 2, and 28.5 min, respectively, with corresponding mean plasma clearances of 29.2, 45.7, and 6.7 ml.min 1.kg-1. The volumes of distribution at steady state of the trans-trans, cis-trans, and cis-cis isomers were 0.047, 0.054, and 0.189 l/kg, respectively. Plasma concentrations of monoester and alcohol metabolites peaked 25 s (median) after mivacurium injection, with half-lives in the range of 90 min, except for the cis alcohol metabolite, which was only negligibly and transiently formed. Substantial hydrolysis of mivacurium isomers by cholinesterases was confirmed by the rapid appearance of mivacurium metabolites in plasma. The intensive arterial sampling proved to be critical for the trans-trans and cis-trans isomers because the area under the curve between 0 and 2 min accounted for 75% and 86% of the total, respectively.

Dose—response and minimum time to satisfactory intubation conditions after mivacurium in children

We assessed neuromuscular blocking effects and tracheal intubation conditions following mivacurium in 121 anaesthetised children aged 1-10 years. The study was conducted in three parts. Parts 1 and 2 were undertaken during thiopentone-alfentanil-nitrous oxide anaesthesia; neuromuscular blockade was evaluated by recording the force of contraction of the adductor pollicis in response to train-of-four stimulation at 0.1 Hz. In part 1 the potency of mivacurium was determined in 15 children using a single dose-response technique; in part 2 onset and recovery times were determined in six children following a dose of mivacurium 0.2 mg.kg-1. In part 3 of the study, clinical intubation conditions were assessed in two groups of 50 children whose tracheas were intubated 60 or 90 s after injection of mivacurium 0.2 mg.kg-1 during thiopentone-nitrous oxide anaesthesia. The ED50 and ED95 of mivacurium were 54 and 105 micrograms.kg-1 respectively. The times to 90% and 100% depression of control twitch were 1.3 (0.3) and 1.9 (0.5) min; times to 5%, 25%, 75% and 90% recovery were 6.4 (1.0), 8.4 (1.1), 12.5 (1.1) and 14.4 (1.9) min, respectively. Intubation conditions were rated satisfactory in 33/50 children (0.66, 95% confidence interval 0.51-0.79) at 60 s and in 49/50 children (0.98, 95% confidence interval 0.89-1.0) at 90 s (p = 0.0001). Intubation conditions 90 s after mivacurium 0.2 mg.kg-1 were significantly better than those obtained in 10 patients given anaesthetic drugs alone (p = 0.002).

Neuromuscular and hemodynamic effects of mivacurium and succinylcholine in adult patients during nitrous oxide-propofol-fentanyl anesthesia.

The neuromuscular and hem+odynamic effects of mivacurium 0.15 mg/kg and succinylcholine 1 mg/kg were compared in 26 adult patients (ASA I and II) during nitrous oxide-oxygen-propofol-fentanyl anesthesia. Neuromuscular block was monitored by recording the compound electromyogram of the hypothenar muscle resulting from supramaximal train-of-four stimuli applied to the ulnar nerve. Time to onset of over 95% block and duration to 25% recovery of control twitch after injection of mivacurium were significantly longer than for succinylcholine (201 +/- 37.6 vs 54 +/- 5.2 sec and 13.0 +/- 2.2 vs 8.4 +/- 2.1 min; mean +/- SD). Onset of mivacurium with priming technique was shortened (125 +/- 20.7 sec), but was also slower than that of succinylcholine. Although the recovery index during spontaneous recovery was significantly longer for mivacurium than for succinylcholine (6.9 +/- 1.3 vs 5.1 +/- 0.9 min), antagonism with neostigmine at 25% recovery of twitch height sufficiently facilitated the recovery index of mivacurium (4.5 +/- 1.0 min) to a level similar to that of succinylcholine with no statistical difference. The hemodynamic effects of mivacurium were few as compared to those of succinylcholine. In conclusion, mivacurium is considered to have additional advantages for short procedures when succinylcholine is undesirable.

The Cardiovascular Effects of Mivacurium Chloride (BW B1090U) in Patients Receiving Nitrous Oxide—Opiate—Barbiturate Anesthesia

The dose-effect relationship of mivacurium chloride on arterial blood pressure, heart rate, and plasma histamine was determined in 97 consenting ASA physical status I-II patients receiving nitrous oxide-oxygen-opiate-barbiturate anesthesia. In the absence of surgical stimulation during steady state anesthetic conditions with controlled ventilation, average maximum change in tachograph-counted heart rate was 7% or less after 10-15-s injection of mivacurium at all doses from 0.03 to 0.30 mg/kg. Average peak change in mean arterial pressure measured via radial arterial catheter was 7% or less after all doses from 0.03 to 0.15 mg/kg. Transient (0.2-4.5 min) decreases in arterial blood pressure were noted after 10-15-s injection in some patients at 0.20, 0.25, and 0.30 mg/kg. When they occurred, these changes were usually accompanied by facial erythema lasting 2-5 min and were correlated with increases in plasma histamine level (P less than 0.001). Facial erythema, decrease in blood pressure, and elevation of histamine level were all accentuated by increasing the dose of mivacurium and by more rapid injection of the drug. For example, mean blood pressure decreased an average of 13% after injection of mivacurium 0.25 mg/kg over 10-15 s. In contrast, during administration over 30 and 60 s of this dose, arterial pressure decreased 7.6 and 1.5%, respectively (P less than 0.001, 10-15 s vs. 60-s injection). Average peak histamine level, which increased to 132% of control after administration of 0.25 mg/kg over 10-15 s, did not change after injection over 60 s.(ABSTRACT TRUNCATED AT 250 WORDS)

The Dose-Response Relationship of Mivacurium Chloride in Humans during Nitrous Oxide-Fentanyl or Nitrous Oxide-Enflurane Anesthesia

The dose-response relationships of mivacurium chloride during N2O/fentanyl or N2O/enflurane anesthesia were compared in 70 patients intraoperatively. Responses were defined in terms of percentage changes in the evoked twitch tension of the adductor pollicis muscle, and dose-response curves were constructed following probit transformation of the responses. End-tidal concentrations of enflurane during the were study were 0.9-1.2%. When compared with the dose-response curve determined during N2O/fentanyl anesthesia the curve determined during N2O/enflurane anesthesia was displaced significantly to the left (P less than 0.05). As a result, the doses of mivacurium that depressed twitch tension by 50% (ED50) and 95% (ED95) were 39 and 67 micrograms/kg, respectively, during N2O/fentanyl anesthesia, and 27 and 52 micrograms/kg during N2O/enflurane anesthesia. Regression lines describing the relationship between the maximum depression of twitch tension (response) and the time interval between the injection of mivacurium and the return of twitch tension to 90% of the control value (duration) were constructed. The response-duration line for N2O/enflurane anesthesia was displaced significantly to the left of the line for N2O/fentanyl (P less than 0.05), indicating that enflurane anesthesia was associated with a prolongation of mivacurium-induced neuromuscular blockade. The neuromuscular blocking effect of mivacurium is both enhanced by and prolonged during N2O/enflurane compared with that during N2O/fentanyl anesthesia.