Sensory Evoked Potentials (SEPs)
Described in terms of site of origin (stimulus), latency, and amplitude.
Generally originate near the median/ulnar nerves or posterior tibials. Recording electrodes are on the scalp or spinal cord. Note that volatile anesthetics increase SSEP latency and decrease SSEP amplitude – nitrous oxide decreases SSEP amplitude but does not affect latency [Banoub et. al. Anesthesiology 99: 716, 2003]. The threshold for usefulness of SSEPs during volatile anesthesia is at ~ 0.5 – 0.75 MAC. Barbiturates, benzodiazepines, and opiates may interfere with SSEPs but to a much lesser extent than volatile anesthetics. SSEPs are controversial because their sensitivity is unestablished – it is clear that SSEPs showing prolonged increase in latency can be associated with severe neurologic injury, however the actual threshold (both in terms of duration and amount of latency) is not known [Kumar et. al. Anaesthesia 55: 225, 2001]. According to Barash, a 50% reduction in amplitude in response to a surgical maneuver and while the anesthetic regimen is held constant, is significant.
How Changes in Physiology Affect SSEPs
Temperature, SBP, PaO2, and PaCO2 all affect SEPs and must be controlled during surgery [Baoub et. al. Anesthesiology 99: 716, 2003]. Room temperature irrigation fluids can also affect SSEPs, thus body temperature fluids should be used for irrigation in neurosurgical cases
Similar to SSEPs, visual evoked potentials are highly sensitive to the use of anesthetic agents
More resistant to anesthetic influences than SSEPs and VEPs. According to Barash, a 1 ms increase in latency while the anesthetic regimen is held constant is considered significant.
Selection of an Anesthetic Agent
All volatile agents depress evoked potentials. Nitrous oxide added to other volatile agents profoundly depresses the amplitude of both SSEPs and VEPs. Barash recommends using TIVA or at least above average IV opiates because they produce minimal changes in SEP waveforms. Dexmedetomidine can be added to the anesthetic regimen and will reduce MAC requirements while having essentially no effect on SSEP amplitude. That said, sevoflurane and desflurane have less effect on SEPs than earlier anesthetic agents, with Barash stating that desflurane
Motor Evoked Potentials (MEPs)
Not widely practiced – requires placement of a stimulating scalp electrode or magnetic coil (requires lower voltages) and an intramuscular recording electrode. MEPs are both difficult to obtain and have questionable accuracy. Still, they are sometimes used for intramedullary spinal tumors, scoliosis surgery, and intracranial tumors near the motor strip. Subject to the same effects on physiologic derangements as SEPs. Also profoundly affected by volatile anesthetic agents, less-so by nitrous oxide. Opiates have almost no effect. Nitrous oxide/opiate techniques have been successful with MEPs. Full paralysis makes the MEP essentially useless, however a continuous IV infusion titrated to 1-2 twitches will allow accurate MEP use.