Satisfactory intubating conditions can be achieved within 60 s after the injection of rocuronium bromide (Esmeron®) 0.60 mg kg−1, a steroidal non-depolarizing neuromuscular blocking agent . This is not in agreement with the onset time measured at the adductor pollicis (time from the end of injection till achievement of maximum or total relaxation). The duration of the onset time may be longer than 2 min and good intubating conditions are achieved before maximum or complete relaxation is achieved at the adductor pollicis muscle . Because, in comparison with the adductor pollicis muscle, the masseter muscle is more sensitive to relaxants , we investigated the relaxation responses of this muscle to determine whether it could be a more suitable estimate of clinical relaxation.
After approval by the ethics committee of our hospital, 20 patients with ASA physical status I or II, and aged between 18 and 50 years gave their consent to be included in the study. Patients with any evidence of neuromuscular, renal, hepatic or electrolyte disorders or taking medication known to influence the action of muscle relaxants were excluded. Pregnant or lactating women were also excluded. Patients were premedicated with diazepam 10 mg orally 1 h before induction of anaesthesia.
Before anaesthesia was started an ulnar and mandibular nerve with the corresponding monitoring sites (respectively the thumb and the chin) were prepared for simultaneous acceleromyographic measurements. The ulnar nerve was stimulated at the wrist using two Bioflex electrodes (Biometer, Denmark). The piezoelectric transducer of the TOF-Guard (Biometer-Organon Teknika, Belgium) was fixed at the thumb. The ipsilateral mandibular nerve was stimulated with two other electrodes just inferior to the zygomatic arch and anterior to the mandibular condyle. The piezoelectric transducer was fixed at the underside of the chin with the flat side directed cranially (the direction in which we expected the largest extension).
After an intravenous (i.v.) induction with sufentanil (0.25 μg kg−1) and propofol (1.5-2 mg kg−1), a laryngeal mask was inserted and manual ventilation was started with oxygen (F1O2:1.0). With a laryngeal mask in place, the chin was able to move freely after stimulation of the mandibular nerve (without manipulation of the chin with a facemask). Anaesthesia was further maintained with sufentanil (1 μg kg−1 min−1) and propofol (6 mg kg−1 h−1). Both TOF-Guards were set up for an automatic start-up procedure to determine the level of amplitude required for a supramaximal stimulation current. This was achieved in every patient at both recording sites without applying the 5-gainfactor. Stimulation of the mandibular nerve did not cause total jaw closure but jaw movement was not limited by the laryngeal mask.
When a stable response was achieved during at least 1 min using a single twitch stimulation with a frequency of 0.1-Hz, a bolus dose of 0.6 mg kg−1 rocuronium was injected rapidly into a fast running infusion via an elbow vein. To improve the exactness of the measurements, attention was paid first to ensure synchronous stimulation, and secondly to the injection of rocuronium which always took place at the same time instant between two consecutive stimulations. The subsequent reduction in acceleration of the chin and thumb were recorded. Once total or maximum relaxation of the masseter muscle was achieved the laryngeal mask was removed and intubation was performed by the same experienced anaesthesiologist. The patients were not actively warmed but the skin temperature always remained above 32°C at both monitoring sites.
The intubating conditions were rated using the scale of Goldberg et al. as excellent (easy passage of the tube without coughing, vocal cords relaxed), good (passage of the tube with slight cough, vocal cords relaxed), poor (passage of the tube with moderate coughing or bucking, some vocal cord movement), or not possible (vocal cords adducted or not visualized, jaw not relaxed) .
The lag and onset time start from the injection of rocuronium to the first detectable reduction in response and the maximum or total relaxation respectively.
The onset of relaxation was analysed using paired t-tests. Statistical significance was assumed at P<0.05.
The mean age (and standard deviation) of the 20 patients studied was 34.4 years (±8.8), the mean weight 64.6 kg (±14.6), the mean height 164.0 cm (±12.4) and a gender distribution of 13 males and 7 females.
Table 1 summarizes the mean lag and onset time for the masseter and adductor pollicis muscle with the corresponding relaxation. The lag and onset time and the corresponding relaxation at the masseter muscle were significantly shorter and more pronounced, respectively (P<0.05). The mean onset time of 61.0 s (±36.7) at the masseter muscle corresponds with the intubating time of 60 s in clinical practice. At that time the mean twitch height at the adductor pollicis muscle was depressed only 38%. Eighteen out of 20 patients achieved total relaxation at the masseter muscle. Only nine patients achieved total relaxation at the adductor pollicis. In all patients the trachea could be intubated with good (6 patients) or excellent (14 patients) conditions once relaxation was complete or maximum at the masseter muscle.
Our results correspond with the results of other investigators studying the sensitivity of the masseter and adductor pollicis muscle to relaxants. Smith et al. and Plumley et al. observed a greater sensitivity at the masseter muscle with pancuronium and succinylcholine, respectively [3,5]. With atracurium the sensitivity of both muscles was similar . Although the ED50, ED90 and ED95 were almost identical, the authors remarked that this relation was not consistent in all patients, and in some patients the masseter was much more sensitive. A more rapid onset of blockade at the masseter muscle was observed in all patients .
We preferred accelerometry to isometric jaw tension measurements for masseter muscle relaxation measurements [3,6]. The advantages of accelerometry were that it is simple to install and does not require a preload and offers the possibility for patients whose trachea is not intubated to be monitored. Jaw tension measurements may lead to damage of the teeth, lips or tongue . In addition, using this approach the relaxation of the masseter can be linked to the intubating conditions.
In addition to the masseter muscle, contraction of the medial pterygoid and temporalis muscle are also responsible for jaw closure . Although these three muscles are supplied by the mandibular nerve, only the nerve to the masseter muscle which runs across the lateral surface of the mandible can be stimulated easily . The anatomy precludes stimulation of the other two muscles to any extent. Direct stimulation of the masseter is also unlikely because relaxation appeared faster and was more intense in comparison with the adductor pollicis relaxation.
Cantineau et al. end their manuscript with the problem of neuromuscular monitoring at the adductor pollicis muscle after injection of rocuronium. In particular they wrote: 'This muscle might not be a good predictor of concurrent optimal intubating conditions' . We conclude from our results that rocuronium has a rapid onset of action, in particular at the masseter muscle. With a mean onset time of 61.0 s the responses of this muscle may be a better predictor of optimal intubating conditions with rocuronium.
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