Effect of nitrous oxide on myogenic motor potentials evoked by a six pulsetrain of transcranial electrical stimuli: a possible monitor for aortic surgery

Intraoperative recording of myogenic motor potentials evoked by transcrania l electrical stimulation (tcMEP) is a method of monitoring the integrity of the vulnerable motor pathways during thoracoabdominal aortic aneurysm (TAA A) surgery. Deflation of the left lung during TAAA surgery may result in im pairment of arterial oxygenation. Ventilation with nitrous oxide may cause further desaturation. We studied the effects of 20%, 40% and 60% nitrous ox ide in oxygen on within-patient variability and magnitude of tcMEP in respo nse to six pulse transcranial electrical stimulation during fentanyl-low-do se 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 a nterior 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 w as performed, there was 50-70% depression of tcMEP amplitude (P<0.05) and 4 0-60% reduction in tcMEP area under the curve (P<0.05) compared with 20% ni trous oxide in oxygen. There was no significant difference in the coefficie nts of variation for tcMEP between the three nitrous oxide anaesthetic regi mens. Our results suggest that increasing; doses of nitrous oxide reduce th e MEP waveform to six pulse transcranial electrical stimulation, but even w ith 60% nitrous oxide in oxygen, the tcMEP were recordable and as reproduci ble as with 20% and 40% nitrous oxide regimens. The method is sufficiently robust for use in aortic surgery.