Department of Anaesthesiology, University Erlangen-Nuremberg, Germany*Corresponding author: Centre Hospitalier de lUniversité de Montréal, Hotel-Dieu, Départment de lanesthésie, 3840 Rue Saint-Urbain, Montréal (Quebec) H2W 1T8, Canada
Accepted for publication: July 13, 2000
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Abstract |
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Br J Anaesth 2000; 85: 85660
Keywords: neuromuscular block, mivaccurium; surgery, gynaecological
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Introduction |
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We simultaneously determined the onset and offset of neuromuscular block produced by mivacurium 0.2 mg kg1 at the larynx, diaphragm, adductor pollicis, orbicularis oculi and corrugator supercilii muscle using non-invasive, superficial electrodes. To record the diaphragmatic response, we modified the technique of skin EMG by placing the electrodes at the back of the patient, lateral to vertebrae T12/L1 or L1/L2, just inferior to the 12th rib, thus presenting a novel site for monitoring diaphragmatic neuromuscular block.
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Materials and methods |
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Anaesthesia was induced using alfentanil 20 µg kg1 followed by a target-controlled infusion of propofol (target concentration 4 µg ml1) programmed to reach the target concentration within 30 s. After induction of anaesthesia, the trachea was intubated through the mouth using a Woodbridge tube (Mallinckrodt, UK, size 7.0) with the surface laryngeal electrode (Magstim company, UK) attached 2 cm above the origin of the cuff. The electrode was placed between the vocal cords for optimal EMG tracing.4 Anaesthesia was maintained with a target-controlled infusion of propofol (target concentration 3 µg ml1) and alfentanil in increments of 10 µg kg1 given at the discretion of the anaesthetist; mechanical ventilation was adjusted to achieve an end-tidal carbon dioxide-pressure of 3.54.5 kPa.
Two Ag/AgCl-skin-electrodes were attached lateral to vertebrae T12/L1 or L1/L2 (wherever the maximum response was better) on the right paravertebral side of the back, inferior to the 12th rib, and placed 2 cm apart for monitoring of the response of the right diaphragmatic muscular crux to phrenic nerve stimulation5 (Fig. 1). One Ag/AgCl-electrode was used as a ground electrode and placed on the hip bone distant to the recording site.
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The time from the end of injection of mivacurium to the first twitch depression (lag time), the maximum twitch depression (onset time) and the maximum block (%-reduction of the maximal neuromuscular response) were measured. To determine clinical duration of neuromuscular block, single-twitch responses every 15 s were used; time for the first twitch response (T1/T0) to return to 25, 75 and 90% was measured.
Statistical analysis
The results are expressed as mean (standard deviation (SD)) and range. The pharmacodynamic parameters were compared between the different monitoring sites using analysis of variance (ANOVA), followed by Fishers least significant difference strategy for multiple comparisons; P<0.05 was regarded as showing a significant difference.
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Results |
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Mean maximum block was more than 98% at all sites. It was 99 (SD 4)% (range 88100%) and 99 (4)% (range 82100%) at the larynx and the diaphragm, respectively. The mean maximum block at the adductor pollicis, the corrugator supercilii and the orbicularis oculi muscle was 99 (2)% (93100%), 99 (3)% (84100%) and 98 (4)% (86100%), respectively. There was no statistically significant difference between the muscles.
Onset times and clinical duration of neuromuscular block are presented in Figure 3A and Figure 3B. Lag time and onset time of neuromuscular block were significantly shorter at the larynx and the diaphragm than at the other muscles (P<0.005), without being significantly different between the diaphragm and the larynx or between the orbicularis oculi and the corrugator supercilii muscle.
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The clinical duration of neuromuscular block at the corrugator supercilii muscle was significantly shorter than at the adductor pollicis muscle (P<0.05), but not statistically different from the clinical duration at the orbicularis oculi muscle.
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Discussion |
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The onset times determined in this study correspond well with times determined by judging the intubating conditions at the larynx and the diaphragm, but the responses are in contrast to visual observation of the orbicularis oculi response to stimulation.
Le Corre and colleagues7 estimated the onset of several neuromuscular blocking drugs at the orbicularis oculi muscle and found that visual estimation was a good predictor of excellent or good intubation conditions. They concluded that loss of orbicularis oculi responses are acceptable predictors of good intubation conditions. They estimated an onset time of 99 s at the orbicularis oculi muscle after mivacurium 0.2 mg kg1 and performed tracheal intubation with good to excellent intubation conditions. This would be in concordance with the diaphragmatic and laryngeal onset times measured in this study (78 and 89 s) but not with another study which compared the effects of mivacurium 0.2 mg kg1 at the diaphragm and the geniohyoid muscles. DHonneur and colleagues8 determined the onset time at the diaphragm, using surface EMG at 158 s longer than it was in our study. The major difference between their study and this one is the different calibration time: whereas DHonneur and colleagues only waited 3 min before measurements were started, the current study set-up used a 10-min calibration time of supramaximal stimulation. McCoy and colleagues9 found that the longer the duration of control stimulation the shorter the onset measured.
Rimaniol and colleagues6 studied the effect of mivacurium 0.15 and 0.25 mg kg1 at the orbicularis oculi muscle using AMG. They found that for mivacurium 0.15 mg kg1 the onset of neuromuscular block was not different between the orbicularis oculi and the adductor pollicis muscle (235 (SD 76) versus 232 (67) s), but the offset was significantly shorter at the orbicularis oculi muscle. For mivacurium 0.25 mg kg1, the onset was significantly shorter at the orbicularis oculi muscle than at the adductor pollicis muscle (90 (25) versus 160 (30) s). In the current study, the onset of block after mivacurium 0.2 mg kg1 was shorter at the orbicularis oculi muscle than at the adductor pollicis muscle (194 (40) versus 202 (45) s) but failed to reach statistical significance.
We stimulated the phrenic and the recurrent laryngeal nerve transcutaneously without difficulty. It could be that in patients with large necks or an enlargened thyroid gland, transcutaneous stimulation of the phrenic nerve using a surface probe might be difficult; in those patients, we prefer percutaneous stimulation via needle electrodes. Concomitant stimulation of the brachial plexus was absent or minimal and did not affect the monitoring of the diaphragmatic response; monitoring the diaphragm on the patients back is certainly advantageous to rule out artefacts from concomitant stimulation of the brachial plexus.
Donati and colleagues2 introduced surface EMG of the diaphragm to monitor neuromuscular block into research practice; it was an easy and objective way to measure onset and offset of block at this muscle. Skin electrodes attached at the 7th or 8th intercostal space between the mid-clavicular and the anterior axillary line, however, cannot be used during open and laparoscopic abdominal surgery as they are within the surgical fields.
Because the muscular crura of the lumbar diaphragm are inserted into the first two to three lumbar vertebrae, it seemed possible to monitor the response to phrenic stimulation on the patients back, lateral to vertebrae T12/L1 or L1/L2 (Fig. 2). Future studies will show whether this location can be used for routine intra-operative monitoring of the diaphragm.
The corrugator supercilii muscle, a small muscle of the superciliary arch, located medially under the eyebrow, is inserted into the forehead skin and responsible for vertical frowning of the forehead. It can be monitored using an AMG probe attached to the medial part of the superciliary arch at a 90° angle to the muscle contraction direction, in a similar way to the AMG measurement of the orbicularis oculi muscle.6 10
The only study of neuromuscular block at the corrugator supercilii muscle was recently presented as an abstract by Plaud and Donati.11 They measured maximum block and recovery to T1/T0=25% after rocuronium 0.6 mg kg 1 at the corrugator supercilii muscle and the larynx. They concluded that the neuromuscular response measured at the corrugator supercilii muscle was a good reflection of the laryngeal muscle response. We found significant differences in onset time and clinical duration between the larynx and the corrugator supercilii muscle but the maximum block was not significantly different.
The morphological difference between the orbicularis oculi muscle and the corrugator supercilii muscle has been the focus of a recent anatomical study. Goodmurphy and colleagues12 have found that the orbicularis oculi muscle fibres are small, rounded and 89% fast-twitch type-II-muscle fibres, whereas the corrugator supercilii muscle fibres were larger, polygonal, and 49% of fast-twitch type II. The capillary index (capillary area per unit of contractile area) of the corrugator supercilii muscle, however, was 2.4 times the capillary index of the orbicularis oculi muscle. The difference in capillary index and the larger size of the muscle fibres13 might explain the faster onset of neuromuscular blocking drugs at the corrugator supercilii muscle. Further studies of the action of other neuromuscular blocking drugs at the corrugator supercilii muscle are needed to define the role of this muscle as a predictor of the laryngeal muscle response.
In conclusion, we present the first simultaneous determination of neuromuscular block at the larynx, diaphragm, adductor pollicis, orbicularis oculi and corrugator supercilii muscle. Surface electrodes were used to record evoked responses at all sites. The monitoring of the diaphragmatic response at the back of the patient allows intra-operative monitoring during abdominal surgery. In contrast to previous findings with rocuronium,11 onset and clinical duration of neuromuscular block after mivacurium at the corrugator supercilii muscle did not accurately reflect onset and clinical duration of neuromuscular block at the larynx or the diaphragm. More studies are needed to define the role of this muscle for neuromuscular monitoring in clinical practice.
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References |
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