... FFARCS, Professor Department of Anaesthesia, Research and Teaching. Block, University Hospital of South Manchester, Withington, Manchester M20 8LR.
Anaesthesia. 1984, Volume 39, pages 574-577 APPARATUS
Electromyography in anaesthesia A comparison between two methods
N. D. P U G H , B. K A Y
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T. E. J. H E A L Y
Summary Tno instruments measuring evoked compound muscle action potentials (EMG)produced by train of four stimulation of the ulnar nerve were compared. The neuromuscular transmission section of a Datex Anaesthesia and Brain Monitor (ABM), which utilises an integration technique to measure the EMG. and the Medelec MS6, by which amplitude of the EMG was recorded and measured were attached to the same electrodes placed over adductor pollicis. Eight patients scheduled for surgery requiring non-depolarising neuromuscular blockade were studied. The changes in neuromuscular transmission measured by the two methods correlated well, with no statistically significant difference in results. The ABM provides a simple and accurate automatic measurement of evoked EMG for use in the study of neuromuscular transmission. Key words Measurement techniques; electromyography. The action of neuromuscular blocking agents is being monitored increasingly during anaesthesia. The options available to the anaesthetist for assessment of neuromuscular block require electrical stimulation of a motor nerve with the measurement of either the evoked muscle tension using a force transducer, or analysis of the muscle compound action potential (EMG). The principal techniques for examining the EMG are measurement of the EMG peak height or integration of the EMG signal. Both approaches to the analysis of the evoked muscle response following administration of muscle relaxants have been published. 'JIt would, therefore, be instructive to examine the relation between the values obtained from two instru_
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ments measuring EMG, one by peak height measurement, the other by integration of EMG signal.
Method Informed consent to a protocol approved by the hospital ethical committee was given by eight patients scheduled for surgery requiring the use of a non-depolarising muscle relaxant. Induction of anaesthesia was by thiopentone 4 mg/kg, followed by inhalation of 70% nitrous oxide in oxygen. Two monitoring systems were compared, a Datex Anaesthesia and Brain Monitor (ABM) and a Medelec MS6. The ABM incorporates a neuromuscular transmission (NMT) monitor
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N.D. Pugh, BSc, Physicist, Regional Department of Medical Physics and Bioengineering, B. Kay, MB ChB, FFARCS, Reader, T.E.J. Healy, BSC, MD, FFARCS, Professor Department of Anaesthesia, Research and Teaching Block, University Hospital of South Manchester, Withington, Manchester M20 8LR. 0003-2409/84/060574 + 04 %03.00/0 @ 1984 The Association of Anaesthetists of Gt Britain and Ireland 574
Electromyography in anaesthesia
Fig. 1. Schematic diagram of muscle compound action potential with methods of measurement. The muscle response to stimulation was assessed by measuring maximum deflection of the action potential in the case of the MS6 and by automatic integration of the areas between baseline and action potential from Stimulus 4 to 24 as in the case of the ABM. artifact; E d area integrated by ABM. A , < 10 mV; T , = 4 ms; T2= 20 ms.
which integrates the area enclosed within the muscle compound action potential. The MS6 records muscle compound action potential, the maximum deflection is then measured (Fig. I). Nerve stimulation was by the NMT section of the ABM. using transcutaneous ECG silver/silver chloride electrodes situated over the ulnar nerve at the wrist. The stimulator produces a train-offour (TOF) stimuli, of pulse duration 0.2 ms and frequency 2 Hz, every 20 seconds. Stimulus current is adjustable between 0-70 mA, with supramaximal stimulus level being established automatically. Muscle response was measured using surface electrodes placed over the adductor pollicis, to which both the ABM and MS6 were attached via their isolation preamplifiers. The stimulus from the ABM automatically triggers the EMG response processor and was also used to trigger the MS6 via an optoisolator. In the ABM stimulus artifact is removed by gating the signal, each muscle response is then rectified and integrated by an integrating circuit having a time constant of 36 ms. A peak hold facility keeps this integrated value for 70 ms to allow a central processing unit to sample the response. The muscle response is displayed on a visual display unit in the form of a bar graph. There is also a digital display of the relation between the responses. Tois the height of the initial muscle response after calibration. This is assigned the value of l000/,. TI is the height of the first
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response of each subsequent train-of-four stimuli. It is displayed as a percentage of To. TRis the height of the last response to each train-of-four stimuli. It is displayed as a percentage of T I . The integrated muscle response was also recorded as a bar graph on a Marko H327-1 single channel pen recorder. On the MS6 system, muscle response was displayed on an oscillograph and recorded on ultra-violet sensitive paper, providing a permanent written record. Peak amplitube of response was measured and T I and TR calculated. After a stable level of anaesthesia was achieved and the response to TOF nerve stimulation was constant, the ABM was recalibrated so that the next muscle response, and the corresponding response on the MS6, became the standard control ( To). A non-depolarising neuromuscular blocking agent was then administered intravenously, and the evoked EMG was recorded until a new stable level of response was again obtained. The outputs of the two machines measuring the recorded muscle responses were then compared i.e., the amplitude of each muscle response recorded on the bar graph from the NMT section was compared with the amplitude of the maximum deflection recorded at each muscle response by the MS6. Following administration of the drug, the per cent depression was calculated for T , and TRin response to each TOF stimuli.
Analysis Two methods of analysis were used. First, a direct comparison was made of values of T I and TR measured simultaneously by the two methods. Second, an analysis was conducted for each individual patient. Four time response curves were plotted using the T, and TRmeasurements from the ABM and MS6. Using these graphs the mean times for the group of patients to reach the following degrees of blockade were than calculated: TI, 90-10% in 10% increments; TR, 90-25% fade in 10% or 5% increments. The mean times to reach a given percentage muscle response determined using the ABM and MS6 were then compared using the paired 1-test.
Results A scattergram of T, values obtained by both machines is shown in Fig. 2. Correlation is ex-
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N.D. Pugh, B. Kay and T.E.J. Healy
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Fig. 2. Scattergram of values for the first muscle response of a train of four measured by the ABM and MS6. The correlation coefficient for TI measured using the ABM and MS6 was found to be 0.976 with a slope of 0.985 and intercept of 0.180.
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Fig. 4. Relationship between TI depression and mean time measured with ABM and MS6. The mean time for TI to reach the given levels of block are shown for the ABM ( x ) and MS6 (0).
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Fig. 3. Scattergram of values for the fourth muscle response of a train of four as a percentage of the first response measured by the ABM and MS6. A high correlation was found for the values of TR obtained ( r = 0.981). The intercept was found to be 8.74 and the slope of the best fit line was 0.91.
r, w0) Fig. 5. Relationship between TRdepression and mean time measured with ABM and MS6. The mean time for TR to reach the above levels of block are shown for the ABM ( x ) MS6 (0).
Discussion tremely close (r = 0.976, p c 0.0001). The scattergram Fig. 3 shows the TRvalues, also with a close correlation (r = 0.984, p < 0.0001). Figures 4 and 5 show the mean times to different levels of T , and TR,for both the ABM and MS6. There was no statistically significant difference between the ABM and MS6 results (f-test for paired data, p < 0.01). The means, standard error of the measurements and the number of patients are given in Tables 1 and 2.
There is, as yet, no recognised standard method of measurement of evoked muscle response. The effects of neuromuscular blocking agents are usually measured by changes in evoked muscle action potential or evoked muscle tension caused by various patterns of nerve stimulation, usually single stimulation, tetanic stimulation or TOF. While different methods of measuring muscle response may not be directly comparable, the correlation of two methods of measuring a re-
Electromyography in anaesthesia
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Mean (SEM) and number of patients for T , response Number of ABM mean (SEM) time MS6 mean (SEM) time Response (%) patients (mins) (mins) Table 1.
7 7 7 6 5 5 5 5 3
90 80 70 60 50 40 30 20 10
1.53 (0.32) 2.73 (0.42) 3.52 (0.70) 3.50 (0.50) 3.75 (0.60) 4.26 (0.62) 4.75 (0.68) 5.60 (0.72) 7.65 ( I .35)
1.60 (0.32) 2.93 (0.25) 3.63 (0.53) 3.36 (0.48) 3.50 (0.63) 3.84 (0.72) 4.47 (0.75) 5.37 (0.77) 7.40 ( I .67)
Mean (SEM) and number of patients for TRresponse Number of ABM mean (SEM) time MS6 mean (SEM) time Response (%) patients (mins) (mins) Table 2.
90 80 70 60 50 40 30 25
8 8 8 6 6 4 3 3
2.90 (0.42) 3.52 (0.57) 4.00 (0.70) 3.28 (0.57) 3.25 (0.47) 4.17 (0.87) 4.90 (1.20) 5.53 ( I .22)
sponse provides an opportunity to interpret results reported using either method. The ABM is designed for use as a clinical monitor of certain aspects of patient responses to anaesthesia. The NMT element has been shown to produce results equating to those obtained using the MS system, which has been used to investigate changes in muscle responses,' indicating that the ABM N M T would be a suitable instrument for studies of actions of neuromuscular blocking drugs. Compared to the MS6, the ABM is compact and simple to use although
2.74 (0.43) 3.47 (0.55) 4.10 (0.67) 3.63 (0.55) 4.18 (0.60) 4.33 (0.84) 5.30 (1.34) 6.33 (1.29)
less versatile. The digital display of T, and TR give a simple and accurate automatic measurement of response, which can also be recorded.
References 1. ALI HH, UTTING JE, GRAY TC. Quantitative assess-
ment of residual anti-depolarising block. British Journal of Anaesthesia 1971; 43: 473-7. 2. LAM HS, CASSNM, NG KC. Electromyographic monitoring of neuromuscular block. British Journal of Anaesthesia 1981; 5 3 1351-8.
