Somatosensory evoked potentials are used to identify neurological injury during surgical procedures such as resection of spinal cord tumors, instrumentation of the spine, CEA and aortic surgery. SSEPs primarily monitor the integrity of the dorsal spinal columns, but are unable to test motor pathways.
Evoked potentials are represented by waveforms on a graph of voltage vs. time. Peak amplitude and poststimulus latency are analyzed. Latency is the time between the stimulus and the detection. Latencies are long, intermediate and short. Short-latency occurs from the nerve stimulated or the brain stem are least affected by anesthetic agents. Intermediate or long-latency EPs generally arise from the sensory cortex. Short and intermediate are the only EPs we monitor intraoperatively.
Volatile agents have the most dramatic effect on EPs, decreasing amplitude and increasing latency. These should ideally be minimized to 0.5 MAC or less; however, successful monitoring can be done up to 1.0 MAC. Nitrous oxide decreases amplitude, but does not affect latency. At high doses IV anesthetics can also decrease amplitude and increase latency. Even though barbiturates can cause an isoelectric EEG, EPs are preserved. At very high doses opioids can decrease latency and amplitude, otherwise they have minimally effects.
Agents are summarized below: