Departments of 1Anaesthesiology and 2Surgery, University of Erlangen, Nuremberg, Germany
Accepted for publication: March 13, 2000
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Abstract |
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Br J Anaesth 2000; 85: 2515
Keywords: neuromuscular block, succinylcholine; neuromuscular block, rocuronium; larynx
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Introduction |
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Rocuronium has been studied as an alternative to succinylcholine; in most studies, however, the onset and offset of the neuromuscular block have been measured at peripheral, easily accessible muscles. Neuromuscular blockade at the larynx should be compared with peripheral muscles, such as the adductor pollicis muscle, using similar monitoring methods, such as surface electromyography.
For more than two decades, intraoperative monitoring of the recurrent laryngeal nerve has been the focus of extensive research in otolaryngological surgery of the thyroid gland. Initially, needle electrodes were used, placed translaryngeally through the operating site;4 later, needle electrodes were applied endoscopically.5 Although electromyographic signals obtained via needle electrodes are regarded as the gold standard in monitoring, special expertise is required during insertion and electrodes are needed which are not displaced during surgery.5
Recently, non-invasive forms of monitoring the laryngeal response to stimulation of the recurrent laryngeal nerve have been tested. Special tracheal tubes with integrated electrodes proved reliable but expensive,6 and a postcricoid electrode inserted into the upper pharynx lacked reliability.6
We used a new disposable surface electrode,7 attached to a tracheal tube and placed between the vocal cords at intubation, to measure the onset of neuromuscular block obtained with two doses of rocuronium compared with succinylcholine.
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Materials and methods |
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Anaesthesia was induced using an infusion of remifentanil 0.5 µg kg1 min1. Two minutes later, target-controlled infusion of propofol (target concentration 4 µg ml1) was started, programmed to reach the target concentration within 30 s. After induction of anaesthesia, the patients were ventilated using a face mask for 3 min and intubated using an oral Woodbridge tube (Mallinckrodt, Bicester, UK) (size 7.0 for females and 8.0 for males) with the surface laryngeal electrode (Magstim company, Whitland, UK) attached 2 cm above the beginning of the cuff (Fig. 1). The electrode was placed between the vocal cords for optimal electromyography (EMG) recording. The recurrent laryngeal nerve function of all patients was checked 3 days after operation using indirect laryngoscopy by the otolaryngologist. Any lesion or damage to the vocal cords due to the surface laryngeal electrode was noted.
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After induction and fixation of the tracheal tube, the recurrent laryngeal nerve was stimulated transcutaneously with an external nerve stimulator (Multiliner®; Tönnies, Würzburg, Germany) at the notch of the thyroid cartilage, to produce maximum response of the adductor laryngeal muscles. The external nerve stimulator has a probe that is attached at the neck with an elastic band. It delivers a current between 0 and 70 mA. Single twitch stimuli (0.1 Hz; pulse width 0.2 ms) were applied to the left recurrent nerve to determine supramaximal stimulation, and were recorded using Multiliner® software. The current was increased to that with the maximal EMG response (<70 mA) and then increased by 10 mA to ensure supramaximal stimulation. The amplitude of the compound action potentials was measured and recorded (Fig. 2). After stimulation of the left ulnar nerve with Ag/AgCl electrodes, evoked EMG single-twitch responses (0.1 Hz; pulse width 0.2 ms) from the adductor pollicis muscle via Ag/AgCl electrodes placed over the base of the thenar area were recorded; automatic calibration of the Datex Relaxograph® NMT 100 (Datex Instrumentarium Corporation, Helsinki, Finland) was used to determine supramaximal stimulation (070 mA). The right thumb was equipped with an acceleromyographic (AMG) probe to record the neuromuscular response of the adductor pollicis muscle. The automatic calibration setup of the TOF-guardNMT (Organon, Helsinki, Finland) was used to determine supramaximal stimulation (single twitch, 0.1 Hz).
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The times from the end of injection of the neuromuscular blocking drug to the first twitch depression, to 90% block and to the maximum twitch depression (lag time, 90% depression T1/T0 and onset time, respectively) and the maximum block were measured. The clinical duration of neuromuscular block at the adductor pollicis muscle was measured using the AMG; after reaching the initial maximum block, train-of-four stimulation was started at the right adductor pollicis muscle every 15 s; times for the first twitch response (T1/T0) to return to 25, 75 and 90% and for the TOF to reach 0.7 were measured.
The results are expressed as mean (SD) and range. The physical characteristics of the three groups were compared using analysis of variance (ANOVA) and corrected according to the number of comparisons (Bonferroni), P<0.05 was regarded as significant. The pharmacodynamic variables were compared within the groups between the different monitoring sites using the paired-sample t-test, and analysis of variance (ANOVA) was used between groups, corrected for the number of comparisons (Bonferroni); P<0.05 was regarded as significant.
The correlation between lag time, 90% block, onset time and maximum effect at the two muscle sites within groups was analysed using Pearsons test; P<0.05 was regarded as significant.
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Results |
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The mean amplitude of the EMG response to placement of the laryngeal electrode at the adductor laryngeal muscles was 0.9 (0.5) mV (0.42 mV) after supramaximal stimulation. Comparison of the onset of neuromuscular blockade at the two monitoring sites showed that, for both drugs, the lag time, 90% block and onset time were significantly shorter at the larynx than at the adductor pollicis, with no significant differences between EMG and AMG measurements (P<0.01, Table 2); the mean maximum effect reached at the three sites in the three groups was >93% and did not differ statistically between groups.
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At the adductor pollicis muscle, 90% block and the onset time were significantly shorter in the succinylcholine group (P<0.01) than in both rocuronium groups (Table 2). The 90% block and onset time of succinylcholine at the larynx were significantly shorter than for both doses of rocuronium, and the times for rocuronium 0.6 mg kg1 were significantly longer than for rocuronium 0.9 mg kg1 (Table 2). There was no correlation of the lag time, 90% block or onset time in the three groups between the larynx and the adductor pollicis muscle.
The clinical duration using AMG at the adductor pollicis muscle is shown in Table 3; the clinical duration was shortest for succinylcholine, followed by rocuronium 0.6 and 0.9 mg kg1.
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Discussion |
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Evoked electromyographic responses monitored using a disposable surface electrode attached to a tracheal tube were used to measure the onset of block at the larynx. The technique proved reliable in 90 patients and was easy to use without any major technical problems. It was connected to a standard EMG-monitoring device (Multiliner®); recording was performed using modified nerve conduction software, providing not only information about the amplitude of the compound action potential but also about the latency and duration of the signal. It could be recorded in all patients. Since patients were undergoing thyroid surgery, only the onset of neuromuscular block could be measured in this way; monitoring of the duration of the neuromuscular block at the larynx would have meant delaying the start of surgery for almost 1 h in the rocuronium 0.9 mg kg1 group and was therefore omitted. We did, however, monitor nerve integrity in all patients, because the surface laryngeal electrode was used to monitor and identify the recurrent laryngeal nerve intraoperatively. In all patients, a compound action potential could still be traced and recorded after a mean of 2.5 h of surgery. We showed in another study8 that the surface laryngeal electrode was able to deliver reliable evoked EMG signals for up to 6 h.
All patients were routinely checked 3 days after operation by an otolaryngologist who was not aware of the study, or by the use of the surface electrode, for any lesion or damage to the vocal cords or impairment of vocal cord function; none was found. Transcutaneous stimulation of the laryngeal recurrent nerve was possible in all patients in the same manner as that demonstrated by Donati and colleagues9 to produce contraction of the adductor laryngeal muscles. No harm from the stimulation, such as burns, skin irritation and cardiac arrhythmia, was noted. The comparison of the two drugs was undertaken for several reasons: rocuronium 0.6 mg kg1 is widely used for intubation in our clinical setting, and provides a peak effect sufficient to ensure good intubation conditions with a reasonable onset time, and rocuronium 0.9 mg kg1 is used in clinical practice to produce onset times equivalent to those given by succinylcholine 1 mg kg1.
Meistelman and colleagues10 investigated the onset and duration for rocuronium 0.5 mg kg1 in 14 patients. They found an onset time of 1.4 (0.1) min at the vocal cords with only 77 (5)% maximum blockade; recovery time to 90% T1/T0 was 22±3 min using a cuff pressure method in which the pressure change in the cuff of a tracheal tube placed between the vocal cords was evaluated.
Wright and colleagues11 used the same method and compared succinylcholine 1 mg kg1 with rocuronium at, among other doses, 0.8 mg kg1; succinylcholine had a much faster laryngeal onset than rocuronium (34 (12) vs 96 (45) s) and a much narrower range of effect (1858 vs 44183 s), with peak effects similar to those found in the present study.
In a recent study, DHonneur and colleagues12 used a specially designed tracheal tube to measure onset and clinical duration for succinylcholine 1 mg kg1 and rocuronium 0.6 mg kg1 at the larynx. There was a much faster onset and shorter duration for succinylcholine than for rocuronium, onset time being 58 (10) s (4575) vs 124 (39) s (70175) and recovery to 90% T1/T0 8.3 (3.2) min (5.714.5) vs 34.9 (7.6) min (26.346.2). This is in concordance with our findings.
Whether a neuromuscular blocking drug can be used to replace succinylcholine for RSI is also related to its clinical duration. It is questionable whether it is important that complete relaxation of the larynx occurs after 47 s, as with succinylcholine 1 mg kg1, or after 64 s, as with rocuronium 0.9 mg kg1. However, the return of neuromuscular transmission at the larynx to control levels after 510 min or 3045 min might define whether a patient survives the cant intubatecant ventilate situation. Another important difference in the onset time of rocuronium in comparison with succinylcholine is the larger interindividual variability. Reliable and sufficient relaxation after a given onset time is more predictable with succinylcholine.
Because of its much longer duration of action and greater interindividual variability, we would not recommend rocuronium 0.9 mg kg1 as a substitute for succinylcholine 1 mg kg1 for RSI, unless there are contraindications to the use of succinylcholine.
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Footnotes |
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References |
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