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From the Department of Anesthesiology, Centre Hospitalier de lUniversité de Montréal, Université de Montréal, Montréal, Québec, Canada.
Address correspondence to: Dr. François Donati, Centre Hospitalier de lUniversité de Montréal, Hôtel Dieu, Pavillon de Bullion, 3840 rue Saint Urbain, Montréal, Québec H2W 1T8, Canada. Phone: 514-890-8000 ext: 14636; Fax: 514-412-7222; E-mail: francois.donati{at}umontreal.ca
| Abstract |
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Methods: After informed consent, anesthesia was induced in 48 ASA physical status III adults with propofol, fentanyl and mivacurium 0.25 mg·kg1 and maintained with N2O (60%) and one of the three agents chosen at random: sevoflurane 1.9%; isoflurane 1.2%; or propofol 100150 µg·kg1·min1. Train-of-four stimulation was applied every 15 sec to the ulnar nerve. Neuromuscular blockade was monitored with accelerometry. At 5% recovery of the first twitch (T1), a mivacurium infusion was started and adjusted every five minutes to maintain 9095% T1 depression.
Results: The time to 5% T1 recovery after the initial dose was similar in all groups (1315 min). Fifteen minutes after the start of the infusion mivacurium requirements were greater (P < 0.05) in the propofol group (7.5 ± 1.7 µg·kg1·min1; mean ± SD) than in either isoflurane (4.7 ± 1.6 µg·kg1·min1) or sevoflurane (4.5 ± 1.5 µg·kg1·min1) group. Then, the rate remained stable for propofol (6.2 ± 1.4 µg·kg1·min1 after 90 min of infusion) while it decreased with isoflurane to 2.9 ± 1.6 µg·kg1·min1 at 90 min (P < 0.05 vs propofol) and to 1.4 ± 1.0 µg·kg1·min1 in the sevoflurane group (P < 0.05 vs propofol and isoflurane).
Conclusion: Sevoflurane and isoflurane do not prolong the effect of a bolus dose of mivacurium, but potentiation increases with time from 30105 min of exposure. This interaction is greater with sevoflurane than isoflurane.
| Introduction |
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Sevoflurane has been reported to potentiate blockade produced by mivacurium,11 rocuronium,5,7 vecuronium,12 and cisatracurium,4 but the effect is not always consistent. For example, only a slight decrease was found for the ED50 of mivacurium, and no change in its duration of action.8 As is the case for isoflurane, the degree of potentiation provided by sevoflurane might be time-dependent.12
The purpose of this study was to determine the time course of potentiation of neuromuscular blockade by isoflurane and sevoflurane in the clinical situation, when a bolus dose of neuromuscular blocking agent is administered at induction, followed by a maintenance dose when recovery started. Mivacurium was chosen because its two active isomers have a short half-life and infusion rate can be assumed to follow the changing requirements with time.
| Patients and methods |
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For maintenance of anesthesia, patients were randomized by a computer-generated assignment to receive sevoflurane (end-tidal concentration of 1.9%), isoflurane (1.2% end-tidal), or propofol, 100150 µg·kg1·min1 intravenously. Hypotension was treated with ephedrine, 510 mg, and hypertension with fentanyl, 50100 µg. In the propofol group, the infusion rate of propofol was adjusted according to changes in blood pressure. When first twitch (T1) recovered to 5% of pre-mivacurium control after the bolus dose, an infusion was started at a rate of 10 µg·kg1·min1, and adjusted manually every five minutes by 0.52 µg·kg1·min1 steps to maintain T1 between 5 and 10% (9095% blockade). At the end of the procedure, the infusion was stopped, but the anesthetic was maintained stable until full recovery (> 95% T1) was observed. No reversal agent was given. During the whole procedure, skin temperature of the monitored thumb was maintained above 32°C.
Only the data obtained in patients who required an infusion of mivacurium for 90 min or more were retained for further analyses. For all these subjects, time from injection of the bolus dose of mivacurium and T1 recovery of 5% was noted. After the start of the infusion, the rate required to keep 9095% T1 block was noted every five minutes for 90 min. The time to achieve 25%, 75% and 95% recovery of T1 after the infusion was stopped was noted.
For statistical analyses, Jandel Sigmastat statistical software was used. One-way ANOVA and Kruskall Wallis one-way ANOVA on ranks was used to compare patient characteristics, dose of anesthetics, duration to 5% T1 recovery and recovery to 25%, 75% and 100% after the infusion was stopped. Two-way repeated measures of ANOVA with Fischers least significant difference test for post-hoc analysis was used to compare data between groups for the infusion requirements. A P value less than 0.05 was considered to indicate statistically significant differences.
| Results |
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No statistically significant differences were found between groups with respect to age, weight, gender distribution, dose of induction agents, and duration of anesthesia or surgery (Table I
). The time from injection of mivacurium to 5% T1 recovery was similar in all three groups (Table II
).
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| Discussion |
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Mivacurium was chosen for this study because it is the non-depolarizing neuromuscular agent with the shortest half-life. The active isomers of the drug have an elimination half-life of one to two minutes.13 Thus, at any point in time, past history of drug administration has little influence on neuromuscular blockade and infusion rates can be thought of as indirect measures of concentration required at the neuromuscular junction. The design of the study parallelled the clinical situation. A bolus dose of mivacurium was given shortly after induction of anesthesia, before the introduction of the volatile agent, and stable muscle relaxation was maintained. The infusion regimen was preferred to the intermittent bolus method because it allowed neuromuscular blockade to be constant throughout surgery. The set minimum of 90 min for the duration of the infusion was somewhat arbitrary, but we wanted to consider all patients for the same duration to avoid swings in the mean infusion rates as either resistant or sensitive patients happen to drop out. Because the degree of potentiation of neuromuscular blockade depends on the concentration of volatile agent given,14 equipotent doses of isoflurane (1.2%) or sevoflurane (1.9%), equivalent to 1 MAC, were given. In addition, all groups received nitrous oxide.
The infusion rate of mivacurium required to maintain neuromuscular blockade decreased during the first 2025 min after starting the infusion. The same phenomenon has been reported in other infusion studies with mivacurium,11,15 cisatracurium,16 rocuronium,16 atracurium17 and vecuronium.17 Infusion rates are relatively high when the patient recovers from a bolus dose, as occurred in all the above studies, because enough drug had to be provided not only to maintain blockade, but also to prevent recovery from the bolus dose.18 During stable infusion, only the drug to maintain blockade is needed. After this early phase, the mivacurium requirements during propofol-N2O anesthesia remained stable with time, in accordance with previous studies,15,19 which suggests that the long acting cis-cis isomer of mivacurium has no clinically detectable effects.
Potentiation of muscle relaxants by volatile anesthetics has been investigated in many studies,112 and the results vary because the time of exposure to the volatile agent varied markedly from one study to the next. For example, Miller et al.1 demonstrated that one hour of exposure to isoflurane reduced the requirement of d-tubocurarine by 70% compared with halothane anesthesia. Presumably, the difference would have been greater if the control group had received no volatile agent. Yet, a shorter (2030 min) exposure to isoflurane had less impact on the ED90 of rocuronium (a 30% decrease),20 and the duration of action of atracurium and vecuronium administered was not modified by isoflurane introduced at the same time as the neuromuscular blocking agents.6
Although the data from the above studies suggest an increase in the isoflurane-induced potentiation of neuromuscular blockade, its exact time dependence remains unclear. Withington et al.10 found a significant reduction of plasma concentration of atracurium required to maintain 90% twitch depression under enflurane anesthesia after 45 min of exposure, and this concentration decreased for the next 75 min. This indicates that the process is not complete two hours after introduction of enflurane. A similar design has not been adopted for isoflurane or sevoflurane. Cannon et al.3 infused vecuronium during 1.2% isoflurane anesthesia, and found that steady state infusion rates were reduced by 67% and corresponding concentrations were decreased by 54%, after at least 1.5 hr of stable anesthesia.
With 1.7% sevoflurane the infusion rate and concentration of vecuronium was the same as with 1.2% isoflurane, after two hours of continuous administration.21 However, comparison with a non-volatile anesthetic was not made. The relationship between dose and response of rocuronium is affected only modestly by a ten-minute exposure to sevoflurane or isoflurane.7 The same applies to mivacurium, and in this case, only late indices of recovery, such as time taken for the return to a train-of-four ratio of 80% were prolonged by sevoflurane.8 The duration of action to 25% recovery of T1 was not affected.8 This is a finding analogous to ours, where duration of the intubating dose was not modified by the presence of the volatile anesthetic. Bevan et al.11 compared mivacurium infusion requirements between patients given propofol or sevoflurane in adults and children, in a design very similar to ours. However, they did not analyze the changes in infusion rate with time, and their patients received mivacurium infusions for only 50 min, compared with 90 min in our study. Their mean infusion rates for mivacurium were 5.9 µg·kg1·min1 with propofol-N2O anesthesia, and 2.9 µg·kg1·min1 with sevoflurane 1 MAC-N2O. This is very similar to our findings at comparable infusion times. At 30 min, the mean infusion rates in the present study were similar: 6.3 and 4.0 µg·kg1·min1, for propofol and sevoflurane, respectively. Exposure to sevoflurane for an additional hour is associated with a marked decrease in mivacurium requirements (1.4 µg·kg1·min1).
Sevoflurane was given in this study at 1.9% end-tidal, approximately 1 MAC value at age 40 yr, which is evaluated to be between 1.7 and 2.05%.22 An equipotent concentration of isoflurane (1.2%) was also administered. It is likely that the infusion rates required for both anesthetics would have converged to the same value if more time had been allowed. A previous study with vecuronium comparing isoflurane and sevoflurane21 reported similar infusion rates and concentrations after more than two hours of stable anesthetic. Thus, it appears that both isoflurane and sevoflurane can potentiate neuromuscular blockade to the same degree. The differences between isoflurane and sevoflurane found in the present study are probably the result of a faster equilibrium time with the neuromuscular junction. Sevoflurane is less lipid-soluble than isoflurane,22 but pharmacokinetic studies have failed to identify a faster time constant for access to muscle (approximately 60 min for both agents).23 Nevertheless, the time constant is of the same magnitude as the time course of blockade in our study, suggesting that the site of action of potentiation is in muscle. The kinetic study23 provided a huge variability on the estimation of the time constants, the standard deviation being approximately half the mean value. Thus, time constant of sevoflurane in muscle could be faster than that of isoflurane.
The present study suggests that a short exposure (30 min or less) to either sevoflurane or isoflurane does not potentiate mivacurium neuromuscular blockade to any significant extent. The same could be true of other short-to intermediate-acting neuromuscular blocking agents.
Potentiation between the volatile agents and neuromuscular blocking agents becomes apparent only after 45 min or so, and the intensity of the phenomenon increases for the next hour. This accentuation is greater with sevoflurane than isoflurane. Thus, these volatile agents can lead to a reduction of requirements for neuromuscular blocking agents only for relatively long procedures.
Revision received August 14, 2002. Accepted for publication May 23, 2002.
| References |
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2 Miller RD, Way WL, Dolan WM, Stevens WC, Eger EI II. Comparative neuromuscular effects of pancuronium, gallamine, and succinylcholine during forane and halothane anesthesia in man. Anesthesiology 1971; 35: 50914.[Medline]
3 Cannon JE, Fahey MR, Castagnoli KP, et al. Continuous infusion of vecuronium: the effect of anesthetic agents. Anesthesiology 1987; 67: 5036.[Medline]
4 Wulf H, Kahl M, Ledowski T. Augmentation of the neuromuscular blocking effects of cisatracurium during desflurane, sevoflurane, isoflurane or total i.v. anaesthesia. Br J Anaesth 1998; 80: 30812.
5 Bock M, Klippel K, Nitsche B, Bach A, Martin E, Motsch J. Rocuronium potency and recovery characteristics during steady-state desflurane, sevoflurane, isoflurane or propofol anaesthesia. Br J Anaesth 2000; 84: 437.
6 Swen J, Rashkovsky OM, Ket JM, Koot HWJ, Hermans J, Agoston S. Interaction between nondepolarizing neuromuscular blocking agents and inhalational anesthetics. Anesth Analg 1989; 69: 7525.
7 Lowry DW, Mirakhur RK, McCarthy GJ, Carroll MT, McCourt KC. Neuromuscular effects of rocuronium during sevoflurane, isoflurane, and intravenous anesthesia. Anesth Analg 1998; 87: 93640.
8 Lowry DW, Mirakhur RK, Carroll MT, McCarthy GJ, Hughes DA, OHare RA. Potency and time course of mivacurium block during sevoflurane, isoflurane and intravenous anesthesia. Can J Anesth 1999; 46: 2933.
9 De Mey JC, Fonck K, Mareels K, Rolly G. The influence of isoflurane on a continuous infusion of mivacurium. Anaesthesia 1995; 50: 9479.[Medline]
10 Withington DE, Donati F, Bevan DR, Varin F. Potentiation of atracurium neuromuscular blockade by enflurane: time-course of effect. Anesth Analg 1991; 72: 46973.
11 Bevan JC, Reimer EJ, Smith MF, et al. Decreased mivacurium requirements and delayed neuromuscular recovery during sevoflurane anesthesia in children and adults. Anesth Analg 1998; 87: 7728.
12 Ahmed AAK, Kumagai M, Otake T, Kurata Y, Amaki Y. Sevoflurane exposure time and the neuromuscular blocking effect of vecuronium. Can J Anesth 1999; 46: 42932.
13 Lacroix M, Donati F, Varin F. Pharmacokinetics of mivacurium isomers and their metabolites in healthy volunteers after intravenous bolus administration. Anesthesiology 1997; 86: 32230.[Medline]
14 Miller RD, Way WL, Dolan WM, Stevens WC, Eger EI II. The dependence of pancuronium- and d-tubocurarine-induced neuromuscular blockades on alveolar concentrations of halothane and forane. Anesthesiology 1972; 37: 57381.[Medline]
15 Diefenbach C, Mellinghoff H, Lynch J, Buzello W. Mivacurium: dose-response relationship and administration by repeated injection or infusion. Anesth Analg 1992; 74: 4203.
16 Miller DR, Wherrett C, Hull K, Watson J, Legault S. Cumulation characteristics of cisatracurium and rocuronium during continuous infusion. Can J Anesth 2000; 47: 9439.
17 Diefenbach C, Mellinghoff H, Grond S, Buzello W. Atracurium and vecuronium: repeated bolus injection versus infusion. Anesth Analg 1992; 74: 51922.
18 Donati F. Cumulation and flexibility with infusions of neuromuscular blocking drugs (Editorial). Can J Anesth 2000; 47: 93642.
19 Savarese JJ, Ali HH, Basta SJ, et al. The clinical neuromuscular pharmacology of mivacurium chloride (BW B1090U). A short-acting nondepolarizing ester neuromuscular blocking drug. Anesthesiology 1988; 68: 72332.[Medline]
20 Oris B, Crul JF, Vandermeersch E, Van Aken H, Van Egmond J, Sabbe MB. Muscle paralysis by rocuronium during halothane, enflurane, isoflurane, and total intravenous anesthesia. Anesth Analg 1993; 77: 5703.
21 Kurahashi K, Maruta H. The effect of sevoflurane and isoflurane on the neuromuscular block produced by vecuronium continuous infusion. Anesth Analg 1996; 82: 9427.[Abstract]
22 Lowry DW, Mirakhur RK, McCarthy GJ, Carroll MT, McCourt KC. Neuromuscular effects of rocuronium during sevoflurane, isoflurane, and intravenous anesthesia. Anesth Analg 1998; 87: 93640.
23 Smith I, Nathanson M, White PF. Sevofluranea long awaited volatile anaesthetic. Br J Anaesth 1996; 76: 43545.
24 Yasuda N, Lockhart SH, Eger EI II, et al. Comparison of kinetics of sevoflurane and isoflurane in humans. Anesth Analg 1991; 72: 31624.
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