1Department of Anaesthesia and Intensive Care, and 2Department of Neurosurgery,Ruhr University, Knappschaftskrankenhaus, D-44892 Bochum, Germany*Corresponding author
Accepted for publication: April 17, 2000
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Abstract |
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Br J Anaesth 2000; 85: 4657
Keywords: brain, electromyography; brain, evoked potentials; brain, cortex; anaesthesia, i.v.; anaesthetic gases, nitrous oxide
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Introduction |
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Electrical stimulation may be used, but we prefer single-pulsed magnetic stimulation which is not painful. Previous studies have shown that the sensivity of MEPs is compromised by the suppressive action of many commonly used anaesthetics, but the results of human and animal studies are often conflicting. This study was designed to determine the effects of four i.v. anaesthesia techniques on MEPs recorded in patients undergoing spinal surgery.
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Methods and results |
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Prior to the administration of any drug, baseline MEP recordings were obtained. All patients then received midazolam 0.05 mg kg1 i.v. as premedication. Anaesthesia was induced by i.v. injection (lasting 30 s) of: in group AE, alfentanil 20 µg kg1 and etomidate 0.3 mg kg1; in group AP, alfentanil 20 µg kg1 and propofol 2 mg kg1; in group PK, propofol 2 mg kg1 and ketamine 2 mg kg1; in group MK, midazolam 0.1 mg kg1 and ketamine 2 mg kg1. Mask ventilation was started by a second anaesthetist at the first sign of unconsciousness or respiratory depression. Thereafter, a continuous i.v. infusion was started: in group AE, alfentanil 100 µg kg1 h1 and etomidate 0.6 mg kg1 h1; in group AP, alfentanil 100 µg kg1 h1 and propofol 10 mg kg1 h1; in group PK, propofol 10 mg kg1 h1 and ketamine 2 mg kg1 h1; in group MK, midazolam 0.1 mg kg1 h1 and ketamine 24 mg kg1 h1. Tracheal intubation was performed, facilitated by suxamethonium 1.5 mg kg1 after precurarization with atracurium 5 mg. If necessary, additional doses of etomidate, propofol or ketamine were injected before intubation. Ventilation of the lungs was maintained with 40% oxygen in air for 15 min. At this point, anaesthesia was regarded as being in a steady-state and MEP measurements were again taken. There was no muscle relaxation. Oscillometric blood pressure, heart rate, oxygen saturation and end-tidal carbon dioxide were controlled for each series of measurements.
After the steady-state measurements, 50% nitrous oxide was added and maintained for 10 min after reaching the desired end-expiratory concentration (five patients from each group) and MEP recordings obtained. This completed the study and the patients underwent the operation with enflurane in nitrous oxideoxygen for maintenance of anaesthesia.
For the recording of MEPs, central stimulation was performed with a magnetic stimulator (Magstim 200, The Magstim Co., Wales, UK), which produces a maximum magnetic field of 1.5 Tesla. A single coil (diameter 14 cm) was placed over the vertex. Its best position was determined in the awake state and marked on the scalp. The stimulations were started at 30% of maximum power. Thereafter, the power was increased in steps of 5% until a response was observed (the brain stimulation threshold). Electromyo graphic responses (EMG) to stimulation were recorded in duplicate from the contralateral thenar muscles using subdermal paired needle electrodes. A Neuropack Four (Nihon Kohden, Tokio, Japan) was used to record MEPs. Stimulation threshold, success rates, latency and amplitude of the magnetic motor-evoked potentials were determined.
For statistical evaluation, the Wilcoxon test for matched pairs and the KruskallWallis test were used. A P-value of <0.05 was considered statistically significant.
The results are presented in Table 1. Each drug combination produced a significant increase in threshold stimulation, but the MEP amplitude was not depressed by midazolam and ketamine (MK), or by alfentanil and etomidate (AE). The observed increase in amplitude in the midazolam/ketamine group was not significant. Combinations using propofol led to a considerable and significant reduction in the MEP amplitude. The MEP latencies were unaffected by each combination. When considering the success rates of obtaining a response to stimulation, the combinations midazolamketamine and alfentaniletomidate were superior to both combinations using propofol (AP and PK). The addition of nitrous oxide had a pronounced depressive effect on MEPs. All patients recovered well after surgery.
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Comment |
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The results of our study demonstrate that MEPs were least affected in the AE and MK groups. Etomidate, together with fentanyl, has been shown to allow MEP recording even in patients with pre-existing spinal neuropathy with a success rate similar to the 77.7% of our AE group.2 Induction doses of etomidate, however, can lead to significant depression of the MEPs.3 In contrast to previous investigations on the effect of propofol on magnetically induced potentials in rats,4 our studies clearly indicate that propofol significantly affects MEP recordings. A similar result was obtained by Taniguchi et al.3 Ketamine seems to diminish the depressant effect of propofol when the alfentanilpropofol group is compared with the propofolketamine group, where propofol was applied in the same dosage. This might be due to a central excitatory effect, because ketamine, in a small dosage, was found to increase the MEP amplitude up to 120% from baseline.5 It seems possible that in our studies, ketamine, combined with smaller doses of propofol, might have achieved better results. We could also show a strong depressant effect of nitrous oxide on MEPs. The success rate of obtaining a response was low (55%). Nitrous oxide, in a concentration of less than 50%, had previously been recommended, although a depressant action had also been reported.6 None of our techniques affected the MEP latencies. In this study, MEP responses were recorded from intact motor pathways of the upper limbs. In the case of pre-existing neurological deficits and in the case of also monitoring the lower limbs, the results might have been slightly modified (e.g. with regard to the success rates), but certainly not completely different.
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References |
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2 Herdmann J, Lumenta CB, Huse KO. Magnetic stimulation for monitoring of motor pathways in spinal procedures. Spine 1993; 18: 5519[ISI][Medline]
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4 Fishback AS, Shields CB, Linden RD, Zhang YP, Burke D. The effects of propofol on rat transcranial magnetic motor evoked potentials. Neurosurgery 1995; 37: 96974[ISI][Medline]
5 Kalkman CJ, Drummond JC, Patel PM, Sano T, Chesnut RM. Effects of droperidol, pentobarbital, and ketamine on myogenic transcranial magnetic motor-evoked responses in humans. Neurosurgery 1994, 35: 106671[ISI][Medline]
6 Jellinek D, Platt M, Jewkes D, Symon L. Effects of nitrous oxide on motor evoked potentials recorded from skeletal muscle in patients under total anesthesia with intravenously administered propofol. Neurosurgery 1991; 29: 55862[ISI][Medline]