Use of inhalational anaesthetic agents during surgeries with motor evoked potential (MEP) monitoring remains restricted due to its suppressive effect on MEP signals. Objective of our study was to evaluate possibility of MEP recording undersevoflurane anaesthesia with high frequency multi-pulse trans-cranial stimulation and its comparison in upper and lower extremities.

In total 17 patients undergoing elective spinal surgery, transcranial electrical stimulation with 200-400V, 500-1000Hz, and 4-6 pulse stimulation were delivered to scalp while anaesthesia being maintained with sevoflurane in oxygen and air. Partial neuromuscular blockade (TOF-3) was maintained with infusion of atracurium. MEPs were recorded at Abductor PollicisBravis and Tibialis Anterior in upper and Lower Limbs respectively on both right and left sides. MEP recording was attempted at a baseline level and then at 0.7, 0.5 and 0.3 Minimun Alveolar Concentration (MAC) of Sevoflurane.

In upper limbs, MEPs could be recorded in 22 (64.7%) limbs at all three (0.3, 0.5 and 0.7) MAC level, as against only 4 (11.7%) lower limbs. When compared to baseline, at 0.3 and 0.5 MAC of sevoflurane, MEP could be recorded in 30 (96.77%) upper limbs each, while only 13 (38%) and 17 (50%) in lower limbs respectively. At 0.7 MAC, MEP recording was possible in 22 (71%) upper limbs and only 5 (21%) in lower limbs. Median amplitude of upper limb at baseline was 411μV which reduced to 48% at 0.3 MAC and to 40% at both 0.5 and 0.7 MAC. Lower limb median amplitude at baseline was 205μV which was reduced to 44% at 0.3 and 0.5 MAC, but, was significantly reduced to 24% at 0.7 MAC.

These results indicate that success rate of MEP recording under sevoflurane anaesthesia was greater in upper limbs as compared to lower limbs. Thus, clinically used MAC of sevoflurane can be utilised in MEP monitoring of upper limbs in combination with high frequency and multi-pulse stimulation. However, same pattern of stimulation is not sufficient to overcome depressant effects of same MAC level of Sevoflurane on Lower Limb MEP recording. Hence, sevoflurane may not be advisable for MEP monitoring from lower limbs in patients undergoing spine surgery.