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*Department of Anesthesiology and Pain Management, University of Texas Southwestern Medical Center, Dallas, Texas, and
Department of Anesthesiology, Cedars-Sinai Medical Center, Los Angeles, California
Address correspondence and reprints to Paul F. White, MD, Department of Anesthesiology and Pain Management, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., F2.208, Dallas, TX 75235-9068. Address e-mail to pwhite{at}mednet.swmed.edu
| Abstract |
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Implications: We studied the reversal of rapacuronium-induced block with edrophonium and found that the residual rapacuronium block can be readily antagonized during propofol-based anesthesia. However, reversal of rapacuronium appears to be less predictable during sevoflurane-based anesthesia.
| Introduction |
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| Methods |
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Routine intraoperative monitoring was used. After premedication with midazolam 2 mg IV, anesthesia was induced with propofol 22.5 mg/kg IV and fentanyl 12 µg/kg IV. Neuromuscular monitoring was performed by using electromyography (EMG) (Datex RelaxographTM, Helsinki, Finland). The EMG response of the adductor pollicis was obtained by stimulating the ulnar nerve at the wrist with supramaximal square-wave TOF stimulation of 0.2-ms duration every 10 s by using surface electrodes. The EMG recording apparatus was connected to the patient before the induction of anesthesia, and baseline calibration sequence was performed after the patient lost consciousness. After obtaining the baseline recording, rapacuronium 1.5 mg/kg was injected over 5 s into a fast-flowing IV line in the forearm to facilitate tracheal intubation.
Then, patients were randomly allocated to receive propofol infusion (100 µg · kg-1 · min-1) or sevoflurane (1.0%, end-tidal), in combination with nitrous oxide (N2O) 66% in oxygen for maintenance of anesthesia. Incremental doses of fentanyl (50 µg IV) were given to treat clinical signs of inadequate analgesia. Ventilation was controlled to maintain the end-tidal carbon dioxide (ETCO2) concentration between 35 and 40 mm Hg. Central and peripheral temperatures were maintained >36.5°C and >32.5°C, respectively, by using an external warming device. The end-tidal concentrations of sevoflurane and N2O were monitored continuously by using a respiratory gas analyzer (Datex Capnomac UltimaTM, Helsinki, Finland).
Neuromuscular block was allowed to recover spontaneously until the T1 height of the TOF stimulation returned to 25% of its baseline value, and then, edrophonium 1.0 mg/kg IV, in combination with atropine 0.015 mg/kg was administered over 30 s. Anesthetic maintenance with either propofol or sevoflurane in combination with N2O was continued after reversal until a TOF ratio of 0.8 was achieved. The following neuromuscular variables were determined from the EMG recording: 1) degree of maximum block; 2) time from injection of rapacuronium to 25% T1 recovery (i.e., clinical duration); and 3) times from 25% T1 recovery to TOF ratio of 0.7 and 0.8.
An a priori power analysis was performed based on detecting a difference of 30% or more in the time interval from 25% T1 recovery to TOF ratio of 0.8 after reversal between the two study groups. A group size of 30 was considered adequate (
= 0.05, power = 0.8, and SD = 1.0 min). For statistical analysis, the
2 test and unpaired Students t-test were used for analyzing demographic data, anesthetic requirements and clinical duration of action between the two treatment groups and between the two study sites. The time intervals from 25% T1 recovery to TOF ratio of 0.7 and 0.8 were analyzed by using Mann-Whitney U-test. P < 0.05 was considered statistically significant.
| Results |
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| Discussion |
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Although neostigmine may be a more reliable antagonist in the presence of profound neuromuscular block, the faster onset and reduced muscarinic side effects associated with edrophonium may be more advantageous for reversal of residual neuromuscular block when a greater degree of spontaneous recovery has occurred (i.e., 25% vs 10% of T1 recovery) (14). In this study, use of edrophonium provided a clinically acceptable recovery, with average times of 3.4 and 5.9 minutes to achieve a TOF ratio of 0.8 after reversal of rapacuronium-induced neuromuscular block during propofol and sevoflurane anesthesia, respectively.
Enhancement of nondepolarizing muscle relaxants by volatile anesthetics usually results in either a prolongation of the duration of relaxant action or a decrease in the dosage requirement of the muscle relaxant (47). Several mechanisms have been proposed for the enhancement of neuromuscular block by volatile anesthetics, including increased availability of the muscle relaxant at the neuromuscular junction because of increased muscle blood flow caused by the volatile anesthetics (15), increased muscle relaxation resulting from the central depressant effects, as well as decreased availability of acetylcholine at the neuromuscular junction (16).
In this study, the clinical duration of action after a single bolus dose of rapacuronium was similar during propofol- and sevoflurane-based anesthetic techniques. Our inability to demonstrate augmentation of the relaxant effect may be related to the time-dependent nature of the potentiating effect of volatile anesthetics. Jalkanen and Meretoja (17) showed that a 10-minute exposure to isoflurane (1.5% end-tidal) was insufficient to affect either the onset or the recovery times after a bolus dose of mivacurium. However, a 30-minute exposure to the volatile anesthetic significantly shortened the onset and prolonged the recovery times. Similar time-dependent potentiation was reported by Withington et al. (18). The time-dependent potentiation of neuromuscular block may be influenced by uptake of volatile anesthetics into skeletal muscle (17). With prolonged exposure to the volatile anesthetic, the muscle tissue achieves equilibrium with partial pressure in blood, thereby potentiating the relaxant effect. Thus, it is possible that the clinical duration of rapacuronium would have been significantly prolonged (versus propofol) had the exposure time to sevoflurane been longer.
Volatile anesthetics can also impair reversal of nondepolarizing muscle relaxant-induced neuromuscular block (811). Morita et al. (9) demonstrated that the 50% effective dose value of edrophonium required to obtain a TOF ratio of 0.5 was significantly increased when the reversal drug was administered during 1.0 MAC sevoflurane or isoflurane anesthesia compared with a fentanyl-diazepam-nitrous oxide anesthetic technique. These authors also found that when the volatile anesthetic was discontinued at the time neostigmine was administered, the impairment of reversal was reduced, but not eliminated (11). In our study, the recovery times after reversal of rapacuronium-induced block with edrophonium were similar during either propofol or sevoflurane-based anesthesia. This finding probably relates to the time-dependent potentiation of neuromuscular block by volatile anesthetics. In the presence of volatile anesthetics, the neuromuscular block at the time of reversal drug administration is a summation of the muscle relaxant-induced paralysis and the direct effect of the volatile anesthetic on neuromuscular transmission. It is likely that the residual neuromuscular block at the time of reversal (i.e., at 25% T1 recovery) in our study was predominantly produced by rapacuronium rather than a direct effect of sevoflurane. Of interest, there were two patients in the sevoflurane group in whom prolonged recovery occurred after reversal. This finding suggests that reversal of rapacuronium-induced block may be more difficult when it is attempted during sevoflurane anesthesia. In addition to the time-dependent nature of the enhancement of volatile anesthetics on muscle relaxant activity, the failure to find a significant difference in recovery times during propofol- or sevoflurane-based anesthesia may relate to: 1) inadequate group sizes; 2) the higher degree of variability in the sevoflurane (versus propofol) group; and 3) the TOF ratio endpoints chosen to assess the reversibility of the residual block. A greater difference between the two groups may have been observed whether it had been possible to monitor the patients neuromuscular function until a TOF ratio of 0.9 was achieved.
The study can also be criticized because the anesthetics were continued after the administration of the reversal drugs. Although many practitioners discontinue the maintenance anesthetics when the reversal drugs are administered, such a practice was not possible in this study because of the clinical circumstances (i.e., the operation had not been completed). Moreover, discontinuing the anesthetics would have further confounded the interpretation of these reversal data. Another shortcoming of this study relates to the use of EMG for monitoring neuromuscular function. Because of the well known downward drift of the EMG amplitude with time, the first twitch height (T1) of a TOF stimulation may only recover to 7080% of the baseline value despite TOF ratios exceeding 0.9 (19). In our study, the final T1 values recovered to 77 ± 8% of baseline values. Thus, a T1 value of 25%, which correlated with the reappearance of the fourth palpable response to TOF stimulation in 75% of the patients in this study, would be calculated to correspond to an average of 32% recovery based on the final baseline values. It is generally accepted that TOF ratios measured with EMG and those recorded with the gold standard mechanomyography provide similar information (18,20). Our results are consistent with those observed by Beemer et al. (21) who reported that the maximum depth of block effectively antagonized by edrophonium corresponds to the reappearance of the fourth response to TOF stimulation. It is also possible that the two cases of prolonged recovery after reversal in the sevoflurane group would have been prevented if neostigmine was used for reversal rather than edrophonium.
In conclusion, the clinical duration of action after a single dose of rapacuronium 1.5 mg/kg was similar during both sevoflurane and propofol-based anesthesia. In most cases, the neuromuscular block produced by rapacuronium was rapidly antagonized (<five minutes) by using edrophonium 1.0 mg/kg and atropine 0.015 mg/kg at 25% T1 recovery. Although the recovery times after reversal of rapacuronium-induced block did not differ statistically between the propofol and sevoflurane groups, greater variability was noted during sevoflurane-based anesthesia.
| Acknowledgments |
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