Transcranial magnetic-evoked potentials under total intravenous anaesthesia and nitrous oxide

M. Sihle-Wissel1, M. Scholz2 and G. Cunitz1

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


    Abstract
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 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Magnetic stimulation of the cortex and recording of the motor-evoked potentials (MEPs) by electromyography (EMG) is a well proven method to assess the descending pathways of the spinal cord and detect neurological impairment. We have assessed, in 33 adult patients undergoing spinal surgery, the influence of four total i.v. anaesthesia regimens (TIVA) on this recording technique. In 20 patients, the effect of 50% nitrous oxide was also studied. MEP amplitudes, latencies and success rates of stimulation were obtained in the steady-state after induction of anaesthesia. Combinations of midazolam and ketamine, and alfentanil and etomidate had the least effect on MEPs. Propofol (in combination with alfentanil or ketamine) showed marked depression of the MEP amplitude and the lowest success rates of stimulation. The latencies did not change at all. The addition of nitrous oxide significantly depressed the registered MEPs and lowered the success rates.

Br J Anaesth 2000; 85: 465–7

Keywords: brain, electromyography; brain, evoked potentials; brain, cortex; anaesthesia, i.v.; anaesthetic gases, nitrous oxide


    Introduction
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Recording motor-evoked potentials (MEPs) is regarded as a sensitive method of monitoring the integrity of descending motor tracts of the spinal cord. Muscle action potentials of the upper or lower limbs are the responses to the central stimulation. Good correlation between neurological deficits and MEP changes have been shown in animal models and in humans.

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.


    Methods and results
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
The study was approved by the Ethics Commitee of the Ruhr University Bochum. It included 33 adult patients of both sexes, ASA class I–II, mean age 51 yr, mean body weight 77 kg, who were due to undergo surgery for lumbar disc hernia. Patients with a history of seizure, with implants or neuropathy were excluded. The patients were allocated randomly to one of four groups, according to the combination of i.v. anaesthetic drugs to be used: alfentanil–etomidate (AE); alfentanil–propofol (AP); propofol– ketamine (PK); midazolam–ketamine (MK).

Prior to the administration of any drug, baseline MEP recordings were obtained. All patients then received midazolam 0.05 mg kg–1 i.v. as premedication. Anaesthesia was induced by i.v. injection (lasting 30 s) of: in group AE, alfentanil 20 µg kg–1 and etomidate 0.3 mg kg–1; in group AP, alfentanil 20 µg kg–1 and propofol 2 mg kg–1; in group PK, propofol 2 mg kg–1 and ketamine 2 mg kg–1; in group MK, midazolam 0.1 mg kg–1 and ketamine 2 mg kg–1. 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 kg–1 h–1 and etomidate 0.6 mg kg–1 h–1; in group AP, alfentanil 100 µg kg–1 h–1 and propofol 10 mg kg–1 h–1; in group PK, propofol 10 mg kg–1 h–1 and ketamine 2 mg kg–1 h–1; in group MK, midazolam 0.1 mg kg–1 h–1 and ketamine 2–4 mg kg–1 h–1. Tracheal intubation was performed, facilitated by suxamethonium 1.5 mg kg–1 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 oxide–oxygen 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 Kruskall–Wallis 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 midazolam–ketamine and alfentanil–etomidate 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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Table 1 Changes in motor-evoked potential amplitude, latency, threshold, and arterial blood pressure and heart rate, after application of four i.v. anaesthesia regimens and nitrous oxide. Amplitude, latency and threshold: mean (SEM); mean arterial pressure (MAP) and heart rate (HR): mean (SD)
 

    Comment
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Serious neurological deficits with postoperative loss of motor function in the limbs can complicate neurosurgical, orthopaedic or abdominal vascular surgery procedures. Animal studies have suggested that magnetic motor-evoked potential (MEP) monitoring is more sensitive than somatosensory-evoked potential (SSEP) monitoring in detecting spinal cord injury.1 Combined neuromonitoring, consisting of SSEP and MEP may be useful.

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 alfentanil–propofol group is compared with the propofol–ketamine 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.


    References
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 Abstract
 Introduction
 Methods and results
 Comment
 References
 
1 Levy W, McCaffrey M, York D. Motor evoked potential in cats with acute spinal cord injury. Neurosurgery 1986; 19: 9–19[ISI][Medline]

2 Herdmann J, Lumenta CB, Huse KO. Magnetic stimulation for monitoring of motor pathways in spinal procedures. Spine 1993; 18: 551–9[ISI][Medline]

3 Taniguchi M, Nadstawek J, Langenbach U, Bremer F, Schramm J. Effects of four intravenous anesthetic agents on motor evoked potentials elicited by magnetic transcranial stimulation. Neurosurgery 1993; 33: 407–15[ISI][Medline]

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: 969–74[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: 1066–71[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: 558–62[ISI][Medline]