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From the Neuromuscular Research Group (NRG), Department of Anesthesiology, Centre Hospitalier de l’Université de Montréal (CHUM) Hôtel-Dieu, Université de Montréal, Montréal, Québec, Canada.
Address correspondence to: Dr. T. M. Hemmerling, Department of Anesthesiology, Université de Montréal, Hôtel-Dieu, 3840, rue St-Urbain, Montréal, Québec H2W 1T8, Canada. Phone: 514-890-8000, ext. 14570; Fax: 514-412-7222; E-mail: thomashemmerling_2000{at}yahoo.com
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Abstract |
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Methods: Fifteen patients were enrolled in the study. Anesthesia was induced with remifentanil 0.25 to 0.5 µg·kg–1·min–, followed by propofol 2 to 2.5 mg–1·kg–1 iv. Analgesia was provided by remifentanil 0.05 to 0.25 µg·kg–1·min–1 iv throughout surgery. A small piezo-electric microphone was attached to the middle of the thenar mass of the right hand to record acoustic signals produced by the contraction of the adductor pollicis muscle. A second microphone was fixed to the medial part of the thigh, 10 cm over the patella, to record the response from the vastus medialis muscle. The ulnar nerve and the im branches of the femoral nerve were stimulated using train-of-four stimulation every 12 sec. Onset, maximum effect, and offset of neuromuscular block were measured after mivacurium 0.2 mg·kg–1 iv and compared.
Results: At the vastus medialis muscle, the onset of NMB was significantly shorter at 1.9 ± 0.6 min vs 2.8 ± 0.7 min, the maximum effect less pronounced at 85 ± 11% vs 96 ± 2% and recovery of NMB to 25%, 75%, 90% of twitch control height more rapid than at the adductor pollicis muscle at 17 ± 2.2 min vs 21.6 ± 4.2 min, 26.7 ± 6.5 vs 21 ± 4.1 min and 30.7 ± 6.6 vs 35.9 ± 7.1 min, respectively.
Conclusions: PMG can be used to measure NMB at the vastus medialis muscle. We found a shorter onset time, less pronounced maximum effect and more rapid recovery of NMB at the vastus medialis muscle than at the adductor pollicis muscle.
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Introduction |
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Methods |
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A small piezo-electric microphone (1.6 cm diameter, Model 1010, Grass Instruments, Astro-Med, Inc., West Warwick, USA; frequency response: 2.5 Hz to 5 kHz, signal output: 20–40 mV into 1 MW) was attached to the middle of thenar mass of the right hand using tape to record the acoustic signals produced by the contraction of the adductor pollicis muscle. Two electrodes were positioned to the medial part of the distal forearm, over the ulnar nerve, as shown in Figure 1
. The arm was fixed to a routine arm board with the thumb able to move uninhibited. A second microphone was fixed to the medial part of the thigh, over the vastus medialis muscle. This microphone was fixed about 10 cm above the upper edge of the patella,8,9 medial to the rectus femoris muscle (Figure 2). The microphone was positioned between two electrodes fixed over the skin to stimulate im branches of the femoral nerve.8 The acoustic signals were amplified and band pass filtered between 0.5 Hz and 1000 Hz using an AC/DC amplifier. The phonomyographic signals were continuously sampled at 100 Hz using the Polyview® software package (Astro-Med Inc., Longueuil, QC, Canada), digitized and stored on a portable microcomputer. The twitch amplitude from PMG signals was measured maximum-to-maximum.
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After the start of the stimulation, mivacurium 0.2 mg·kg–1 was injected within five seconds into a fast flowing solution of Ringer’s lactate. Onset, maximum effect, and offset of NMB were determined. Recordings of signals were continued until TOF ratios were greater than 0.9 in all patients.
The first twitch response of the TOF stimulation was used to analyze the onset time (time to reach maximum decrease of twitch response) and the time to reach 25%, 50%, 75% and 90% of control twitch response (T1/T0). The maximum effect was determined as the maximum decrease of the twitch response and was also recorded. TOF ratios of 0.5, 0.7, 0.8 and 0.9 were also calculated.
Results are expressed as mean ± standard deviation. Repeated-measures of ANOVA followed by post-hoc t test was performed to compare all values (onset, maximum effect and recovery of T1/T0 and TOF) whenever significant differences were found. P < 0.05 was regarded as showing a significant difference. Mean amplitude of first recorded twitch was com-pared between both signals using a standard paired t test. Sample size was calculated using an expected difference of mean of three minutes at T 25% for a Power of 0.8 and a = 0.05. The means of the difference of all pharmacodynamic parameters and 95% confidence intervals were also calculated.
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Results |
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Pharmacodynamic data are presented in the Table; at the vastus medialis muscle, the maximum effect was significantly smaller, onset and offset of NMB faster than at the adductor pollicis muscle. Mean onset, peak effect and recovery are presented in Figure 3a, the means of the differences of all pharmacodynamic parameters with confidence intervals are presented in Figure 3b, respectively.
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Discussion |
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The differences between the pharmacodynamic profile of the adductor pollicis muscle and the vastus medialis muscle could be explained by multiple factors: the dimension of muscular fibres,10 differences in blood perfusion11,12 and the density of acetylcholine receptors. One previous study found a significant difference in proportion of type I and type II fibres between the adductor pollicis muscle and the vastus medialis muscle (superficial and deeper parts).13 The proportion of type I fibres in the adductor pollicis muscle was found as 80.4 ± 8.7% and the proportion in the superficial and the deeper part of the vastus medialis muscle as 43.7 ± 7.0% and 61.5 ± 9.5%, respectively. This observation could explain in part the differences in pharmacodynamic profile found between both muscles.
Recently, the corrugator supercilii muscle has been advocated as a muscle which more closely reflects onset and offset of NMB at the larynx.14 Although we did not study the corrugator supercilii muscle in the present study, it is interesting to note that NMB after mivacurium 0.2 mg·kg–1 resembles the NMB found at the vastus medialis muscle in the present study.15 The fact that the proportion of type I fibres in corrugator supercilii found to be at 51%,16 is similar to the proportion of type I fibres in the vastus medialis muscle (44%) might explain the similar pharmacodynamic profiles of the two muscles. Whether the vastus medialis muscle reflects well NMB at the larynx or the diaphragm, should be the focus of future studies.
In a previous study, the comparison of NMB at the vastus medialis muscle (via acceleromyography) and the adductor pollicis (via mechanomyography) was performed.8 It was found that onset time after vecuronium 0.1 mg·kg–1 was faster at the vastus medialis than at the adductor pollicis muscle. Although the complete pharmacodynamic profile was not presented or recorded, those authors presumed a faster recovery of NMB at the vastus medialis muscle than at the adductor pollicis muscle. This hypothesis has been confirmed by our findings.
We used the same technique of nerve stimulation as was proposed in the previous study.8 This form of nerve stimulation differs from the usual stimulation of an afferent motor nerve, as it is a direct stimulation of im branches of the nerve. The motor nerve supplying the vastus medialis muscle origins from the posterior divisions of the femoral nerve and is entering the vastus medialis from its deep surface,17–19 in a bi-layered fascial envelope next to the Hunter’s canal (adductor canal), where direct stimulation is not possible. This was pointed out in a commentary20 in response to Saitoh’s study. However, the technique of stimulating im nerve branches has been used at other muscles, such as the larynx.21,22 Whether this form of stimulation changes the monitoring results, cannot be answered. However, this form of stimulation is less selective and might explain why in some patients, the control amplitude was too weak for any neuromuscular monitoring, hence the replacement of three patients. In general, our experience with stimulation and recording of signals of the vastus medialis muscle showed that the more ‘muscular’ the thigh of a given patient, the higher the signal amplitude and the easier the signal recording. The more fat tissue was moved at the moment of stimulation, the more artefact interferences are noticed, the lower the signal amplitude. Due to the fact that we recorded the original signals, we were able to exclude these low-quality signals. Monitoring the muscle response simply by recording digitized data, as done previously,8 most probably does not record these artefacts since acceleromyographic probe movement will still occur. Similar problems of signal recording can be noticed in recording of other, more profoundly, located muscles with fat tissue in between the muscle and the skin, such as the diaphragm. In our experience, independent from the monitoring method used to measure neuromuscular transmission, monitoring becomes difficult when larger fat layers lay between muscle and skin with the monitoring device attached. Obviously these problems do rarely occur when hand muscles are used for monitoring.
In general, signals from the vastus medialis muscle were less powerful than signals from the adductor pollicis muscle. This could be explained by the efficacy of stimulation or by quantity of fat tissue between the muscle and the microphone. The amount of fat located between the muscle and the skin also seems to cause a change in the waveform of the acoustic signal from the vastus medialis muscle. Examples of acoustic signals from patients with different fat distribution are presented in Figure 4. Signals from the thigh with a high proportion of fat are characterized by one or two repetitions of the signal, in a rebound pattern. We hypothesize that the acoustic response depends on the focus of stimulation and physical characteristics of the leg and vastus medialis (muscle formation, fat distribution).
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Footnotes |
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Assessed October 4, 2004. Accepted for publication March 3, 2005. Final revision accepted April 8, 2005.
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References |
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2 Hemmerling TM, Michaud G, Trager G, Donati F. Simultaneous determination of neuromuscular blockade at the adducting and abducting laryngeal muscles using phonomyography. Anesth Analg 2004; 98: 1729–33.
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4 Hemmerling TM, Michaud G, Trager G, Deschamps S, Babin D, Donati F. Phonomyography and mechanomyography can be used interchangeably to measure neuromuscular block at the adductor pollicis muscle. Anesth Analg 2004; 98: 377–81.
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