What is the effect of sevoflurane (Sevoflurane) on somatosensory evoked potentials (SSEPs) in patients undergoing surgical procedures, particularly those requiring neurological monitoring?

Effect of Sevoflurane on Somatosensory Evoked Potentials

Sevoflurane produces dose-dependent suppression of SSEP amplitude and prolongation of latency, but remains compatible with successful intraoperative neuromonitoring when used at concentrations ≤0.6 MAC, particularly when combined with intravenous agents.

Dose-Dependent Effects on SSEP Parameters

Sevoflurane consistently demonstrates concentration-dependent depression of SSEPs across multiple studies:

• Amplitude suppression occurs progressively as sevoflurane concentration increases, with reductions ranging from 18-29% variability at surgical concentrations 1
• Latency prolongation increases in a dose-dependent manner, with mean increases of approximately 2 ms at moderate concentrations (BIS 45-55) 2
• At end-tidal concentrations of 0.5%, 1.0%, and 1.5%, both amplitude decreases and latency prolongations are statistically significant and progressive 3

Comparative Anesthetic Effects

When compared to propofol-based anesthesia, sevoflurane shows distinct characteristics:

• Propofol produces superior SSEP signals with less within-patient variability in amplitude and latency, and minimal dose-dependent effects on recordings 1, 2
• Sevoflurane demonstrates faster onset and offset of SSEP suppression compared to propofol, allowing more rapid adjustments and faster recovery of signals 1, 4
• Desflurane produces even stronger inhibitory effects than sevoflurane at equivalent MAC concentrations, with significantly lower amplitudes and longer latencies 5

Clinical Feasibility for Neuromonitoring

Despite its suppressant effects, sevoflurane remains clinically viable for surgeries requiring SSEP monitoring:

• Successful monitoring is achievable in all patients when sevoflurane is used judiciously, as demonstrated in 100% of cases across multiple studies 3
• Optimal concentration range appears to be ≤0.6 MAC when combined with intravenous agents (propofol/remifentanil background), maintaining adequate signal quality while preserving monitoring reliability 3, 5
• Individual baseline establishment is critical—measuring baseline SSEP amplitude under total intravenous anesthesia before introducing sevoflurane helps account for significant individual variability 3

Recovery and Emergence Advantages

Sevoflurane offers distinct benefits during the critical emergence period:

• Faster neurological assessment is possible, with significantly shorter times to eye-opening (5.1-5.2 minutes vs 16.5-20.6 minutes with propofol) and toe movement (5.4-7.9 minutes vs 15.7-17.4 minutes) 1, 4
• Superior conscious state on emergence, with patients demonstrating better lucidity and cooperation compared to propofol 4
• Rapid signal recovery when concentration is decreased, allowing dynamic adjustment during critical surgical moments 1, 4

Critical Pitfalls to Avoid

Several important caveats must be recognized when using sevoflurane with SSEP monitoring:

• Avoid concentrations >0.6 MAC as the sole anesthetic, as higher concentrations produce excessive amplitude suppression that may compromise monitoring reliability 3, 5
• Recognize "anesthetic fade" phenomenon—voltage thresholds increase proportionally with duration of anesthetic exposure, which can lead to false-positive interpretations 6, 7
• SSEP monitoring has inherent limitations regardless of anesthetic choice—it monitors only posterior and lateral spinal cord columns, missing anterior motor column injuries that are more vulnerable to ischemia 6
• Motor evoked potentials (MEPs) are more sensitive to anesthetic suppression than SSEPs and should be the primary monitoring modality when motor pathway integrity is the concern 6, 7

Practical Anesthetic Strategy

For surgeries requiring SSEP monitoring where sevoflurane is preferred:

• Use balanced technique: Maintain sevoflurane at ≤0.6 MAC with continuous infusion of propofol and remifentanil to minimize volatile agent concentration while preserving its emergence benefits 1, 3
• Establish baseline early: Record SSEP parameters under total intravenous anesthesia before introducing sevoflurane to define individual patient baseline 3
• Maintain stable concentrations: Once monitoring is established, avoid frequent concentration changes during critical surgical periods to minimize signal variability 1
• Consider propofol-based technique if baseline SSEP amplitudes are already marginal or if MEP monitoring is also required, as propofol produces less signal suppression 2, 5