The effects of propofol, small-dose isoflurane, and nitrous oxide on cortical somatosensory evoked potential and bispectral index monitoring in adolescents undergoing spinal fusion

Abstract

In this study we compared the effects of propofol, small-dose isoflurane, and nitrous oxide (N(2)O) on cortical somatosensory evoked potentials (SSEP) and bispectral index (BIS) monitoring in adolescents undergoing spinal fusion. Twelve patients received the following anesthetic maintenance combinations in a randomly determined order: treatment #1: isoflurane 0.4% + N(2)O 70% + O(2) 30%; treatment #2: isoflurane 0.6% + N(2)O 70% + O(2) 30%; treatment #3: isoflurane 0.6% + air + O(2) 30%; treatment #4: propofol 120 microg . kg(-1) . min(-1) + air + O(2) 30%. Cortical SSEP amplitudes measured during anesthesia maintenance with treatment #3 (isoflurane 0.6%/air) were more than those measured during maintenance with treatment #1 (isoflurane 0.4%/N(2)O 70%) (P < 0.0001) and treatment #2 (isoflurane 0.6%/N(2)O 70%) (P < 0.0052). Cortical SSEP amplitudes measured during treatment #4 (propofol 120 microg . kg(-1) . min(-1)/air) were more than treatment #1 (isoflurane 0.4%/N(2)O 70%) (P < 0.0001), treatment #2 (Iso 0.6%/N(2)O 70%) (P < 0.0007), and treatment #3 (isoflurane 0.6%/air) (P < 0.0191). In addition, average BIS values measured during treatments 1, 2, 3 and 4 were 62, 62, 61, and 44 respectively. Only treatment #4 (propofol 120 microg . kg(-1) . min(-1)/air) uniformly maintained BIS values less than 60. Our study demonstrates that propofol better preserves cortical SSEP amplitude measurement and provides a deeper level of hypnosis as measured by BIS values than combinations of small-dose isoflurane/N(2)O or small-dose isoflurane alone.

Figure 1

A, right posterior tibial cortical SSEPs recorded under anesthetic conditions of treatment #1 (isoflurane 0.4% + N₂O 70% + O₂ 30%). B, right posterior tibial cortical SSEPs recorded on the same patient under anesthetic conditions of treatment #4 (propofol 120 μg · kg^-1 · min^-1 + air + O₂ 30%). Measurements of N34 amplitude (a) and P38 amplitude (b) are illustrated.

Figure 2

Mean cortical SSEP amplitude measurements for each anesthetic combination. Treatment #4 (propofol 120 μg · kg^-1 · min^-1 + air + O₂ 30%) allowed for significantly more robust cortical SSEP amplitude measurement than treatments 1, 2, and 3. The x-axis represents the four different anesthetic maintenance combinations studied: treatment #1, isoflurane 0.4% + N₂O 70% + O₂ 30%; treatment #2, isoflurane 0.6% + N₂O 70% + O₂ 30%; treatment #3, isoflurane 0.6% + air + O₂ 30%; treatment #4, propofol 120 μg · kg^-1 · min^-1 + air + O₂ 30%. The y-axis represents cortical SSEP amplitude size measured in microvolts. Each box plot represents the mean cortical SSEP amplitude value observed for each anesthetic maintenance combination. The lines that extend from the top and bottom of the box-plot represent the standard deviation in cortical SSEP amplitude values observed for each anesthetic maintenance combination.

Figure 3

Mean bispectral index (BIS) values observed for each anesthetic combination. Only treatment 4 consistently maintained BIS values below 60. The x-axis represents the four different anesthetic maintenance combinations studied: treatment #1, isoflurane 0.4% + N₂O 70% + O₂ 30%; treatment #2, isoflurane 0.6% + N₂O 70% + O₂ 30%; treatment #3, isoflurane 0.6% + air + O₂ 30%; treatment #4, propofol 120 μg · kg^-1 · min^-1 + air + O₂ 30%. The y-axis represents the BIS monitor values recorded for each of the four treatment groups. Each box plot represents the mean BIS value observed for each anesthetic maintenance combination. The lines that extend from the top and bottom of the box plot represent the standard deviation in BIS values observed for each anesthetic maintenance combination.