Describe the effects of anesthetics on somatosensory and motor evoked potentials

Describe the effects of anesthetics on somatosensory and motor evoked potentials.

The EEG is sensitive to all anesthetics and becomes markedly depressed at the upper end of the clinically relevant dosage range, hence its use in monitoring depth of anesthesia. The anesthetic technique that provides for hemodynamic stability and effective recording of SSEPs and/or MEPs is obviously preferred. Inhalational agents and bolus intravenous drugs can affect SSEPs (Figure 21.2). 

The inhalation response is generally dose-related; therefore a low concentration with or without nitrous oxide and a continuous opioid infusion is frequently utilized with success. Muscle relaxants do not interfere with SSEP recording and may facilitate anesthetic management. If the potentials remain difficult to obtain, changing to a total intravenous anesthesia (TIVA) technique may be neces-sary. It is important to note that SSEPs are also affected by ischemia, hypothermia, hypoxia, hypotension, and anemia.

While SSEPs are easily obtained using a low concentra-tion of volatile agents and muscle relaxants, MEPs are more challenging and often require the use of TIVA with minimal or no muscle relaxants. The inhalational agents, including nitrous oxide, have been shown to depress MEPs in a variety of circumstances. Intravenous anesthetic agents, such as propofol, midazolam, droperidol, and sodium thiopental by infusion or bolus, may cause a significant decrease in the amplitude or latency of MEPs. Ketamine and opioid analgesics, such as fentanyl, produce less promi-nent changes. An opioid infusion with low concentrations of propofol is a frequently successful technique for moni-toring MEPs. Remifentanil is particularly useful because of its potency and titratability. An intraoperative “wake-up test” may be requested if monitoring is unavailable or inadequate.