British Journal of Anaesthesia, Vol 82, Issue 3 323-328, Copyright © 1999 by The Board of Management and Trustees of the British Journal of Anaesthesia
E. P. van Dongen, H. T. ter Beek, M. A. Schepens, W. J. Morshuis, A. de Boer, L. P. Aarts and E. H. Boezeman
Intraoperative recording of myogenic motor potentials evoked by
transcranial electrical stimulation (tcMEP) is a method of monitoring the
integrity of the vulnerable motor pathways during thoracoabdominal aortic
aneurysm (TAAA) surgery. Deflation of the left lung during TAAA surgery may
result in impairment of arterial oxygenation. Ventilation with nitrous
oxide may cause further desaturation. We studied the effects of 20%, 40%
and 60% nitrous oxide in oxygen on within-patient variability and magnitude
of tcMEP in response to six pulse transcranial electrical stimulation
during fentanyl-low-dose propofol anaesthesia with partial neuromuscular
block. Ten patients (two females; aged 63-74 yr) were studied. After
achieving a stable anaesthetic state and before surgery, 10 tcMEP were
recorded from the right tibialis anterior muscle during addition of 20%,
40% and 60% nitrous oxide in oxygen in random order. When ventilation with
40% or 60% nitrous oxide in oxygen was performed, there was 50-70%
depression of tcMEP amplitude (P < 0.05) and 40-60% reduction in tcMEP
area under the curve (P < 0.05) compared with 20% nitrous oxide in
oxygen. There was no significant difference in the coefficients of
variation for tcMEP between the three nitrous oxide anaesthetic regimens.
Our results suggest that increasing doses of nitrous oxide reduce the MEP
waveform to six pulse transcranial electrical stimulation, but even with
60% nitrous oxide in oxygen, the tcMEP were recordable and as reproducible
as with 20% and 40% nitrous oxide regimens. The method is sufficiently
robust for use in aortic surgery.
CLINICAL INVESTIGATIONS
Effect of nitrous oxide on myogenic motor potentials evoked by a six pulse train of transcranial electrical stimuli: a possible monitor for aortic surgery
Department of Anaesthesiology and Intensive Care, Department of Cardiothoracic Surgery and Department of Clinical Neurophysiology, St Antonius Hospital, Koekoekslaan 1, 3435 CM Nieuwegein, The Netherlands; Department of Pharmaco-epidemiology and Pharmacotherapy, Faculty of Pharmacy, University of Utrecht, The Netherlands
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