Objective: To evaluate the effects of increasing end-tidal concentrations of sevoflurane and increasing stimulation voltage on motor evoked potentials, so as to provide evidence in making anesthesia plan for intraspinal tumor surgery.
Methods: In the study, 48 patients scheduled to undergo intraspinal tumor surgery [American Society of Anesthesiology,(ASA) I-II, 18-65 years old] were enrolled. After general anesthesia induction, the patients were assigned to receive sevoflurane anesthesia of increasing end-tidal concentration in the sequence of 0.0%, 0.5%, 1.0% and 1.5% respectively, under a background of propofol and remifentanil. All the observations were done before the important steps of surgery. Remifentanil infusion rate was 0.2 μg /(kg×min), while the propofol infusion rate was adjusted to maintain the bispectral index values within the range of 30-50. At each concentration, 4 stimulation voltages of 300 V, 400 V, 500 V and 600 V were employed to elicit motor evoked potentials (MEPs). The amplitude and latency of each MEP were compared. The success ratio was also recorded.
Results: The concentration of sevoflurane and the stimulation voltage had impacts on the amplitude and latency of MEPs. Under each stimulation voltage, the MEPs amplitude decreased following increasing end-tidal sevoflurane concentrations, and significant differences were found in comparing 1.5% sevoflurane (left 20.50 μV, 70.71 μV, 135.97 μV, 190.00 μV , right 14.29 μV, 50.71 μV, 73.10 μV, 77.50 μV) with 0.0% sevoflurane (left 143.00 μV, 388.10 μV, 484.53 μV, 500.00 μV, right 176.00 μV, 407.60 μV, 384.35 μV, 451.00 μV) and 0.5% sevoflurane (left 100.00 μV, 362.57 μV, 444.05 μV, 435.00 μV, right 115.00 μV, 207.15 μV, 258.34 μV, 358.50 μV), left χ(2)= 27.46,P<0.01, right χ(2)= 60.49,P<0.01; left χ(2)= 20.73,P<0.01, right χ(2)= 55.05,P<0.01;left χ(2)= 34.25,P<0.01,right χ(2)=33.58,P<0.01;left χ(2)= 28.61,P<0.01 ,right χ(2)= 49.04,P<0.01; while there were no statistical differences in the latency changes (P=0.26). Under each end-tidal sevoflurane concentration, the MEPs amplitude increased following increasing stimulation voltages, and significant differences were found in comparing 300 V (left 143.00 μV, 100.00 μV, 61.50 μV, 20.50 μV , right 176.00 μV, 115.00 μV, 41.07 μV, 14.29 μV) with 400 V (left 388.10 μV, 362.57 μV, 198.81 μV, 70.71 μV, right 407.60 μV, 207.15 μV, 89.00 μV, 50.71 μV) and 500 V (left 484.53 μ V, 444.05 μV, 216.24 μV, 135.97 μV, right 384.35 μV, 258.34 μV, 187.50 μV, 73.10 μV) and 600 V (left 500.00 μV, 435.00 μV, 344.00 μV, 190.00 μV, right 451.00 μV, 385.50 μV, 156.00 μV, 77.50 μV), left χ(2)= 45.55,P<0.01, right χ(2)= 25.73,P<0.01; left χ(2)= 46.67,P<0.01, right χ(2)= 55.30,P<0.01;left χ(2)= 47.36,P<0.01,right χ(2)= 47.82,P<0.01; left χ(2)= 38.67,P<0.01, right χ(2)= 45.87,P<0.01; while the latencies were decreased, and significant differences were found in comparing 300 V with 400 V and 500 V and 600V(left F=7.50,P=0.01 , right F=13.33,P<0.01), but the differences had little clinical significance. The success ratio decreased by increasing end-tidal sevoflurane concentration, and significant differences were found in comparing 1.5% sevoflurane (left 43.8%,70.8%, 77.1%,81.3%, right 37.5%,60.4%,75.0%,66.7%) with 0.0% sevoflurane (left 79.2%,87.5%,95.8%,93.8%, right 75.0%,95.8%,95.8%, 95.8%) and 0.5% sevoflurane (left 72.9%,89.6%,95.8%,95.8%, right 66.7%,89.6%,95.8%, 97.9%); the success ratio increased by increasing stimulation voltage, and significant differences were found in comparing 300 V(left 79.2%,72.9%,62.5%,43.8%, right 75.0%,66.7%,60.4%, 37.5%)with 400 V(left 87.5% ,89.6%,77.1%,70.8% , right 95.8%,89.6%,79.2%,60.4%)and 500 V(left 95.8%,95.8%,91.7%,77.1%, right 95.8%,95.8%,81.3%,75.0%)and 600 V (left 93.8%, 95.8%,89.6%,81.3%, right 95.8%,97.9%,89.6%,66.7%), but there were no statistical differences in the success ratio of MEPs between the group with stimulation voltage of 600 V , end tidal sevoflurane concentration of 1.5% and the group with stimulation voltage of 300 V, end tidal sevoflurane concentration of 0.0% (P=0.22).
Conclusion: Sevoflurane inhibited MEPs in a dose-dependent manner. It can decrease the amplitudes and prolong the latencies. But increasing stimulation voltage will facilitate MEPs monitoring and increase the success ratio. Sevoflurane can be used in larger parts of MEPs monitoring surgery by increasing the stimulation voltage.