Anesthesia and Pharmacology Affecting Monitoring
Where the drugs act
Volatile agents (isoflurane, sevoflurane, desflurane) depress synaptic transmission: they reduce SSEP amplitude and prolong latency in a dose-dependent way, and suppress MEP more profoundly, with cortical effects preceding spinal effects. Propofol behaves similarly and produces burst suppression at high doses. Nitrous oxide reduces amplitude and is often a hidden contributor when MEPs fade. Ketamine, etomidate, and methohexital tend to preserve or enhance amplitude. Opioids and dexmedetomidine have minimal effect on evoked potentials. Neuromuscular blockade abolishes MEP and EMG but leaves SSEP, BAEP, and EEG largely intact - a point tested repeatedly.
Practical implications
When MEPs are required, the team generally uses total intravenous anesthesia (propofol plus remifentanil) with volatile agent kept low and neuromuscular blockade minimized or reversed; even partial blockade (one or two twitches on train-of-four) degrades MEP reliability. Physiologic confounders travel with pharmacology: hypothermia prolongs latency and lowers amplitude, hypotension reduces perfusion, and positioning can produce limb ischemia.
Traps: attributing an SSEP loss to anesthesia when only one limb changed; forgetting that nitrous oxide is a suppressant; assuming a fixed volatile concentration is "safe" without knowing MAC; ignoring the train-of-four when MEPs disappear.
Rule of thumb: cortical MEPs are usually obtainable below roughly 0.5 MAC of volatile agent on a propofol background and typically disappear near 0.75–1.0 MAC. If MEPs vanish symmetrically with stable physiology, check the anesthetic record and twitch count before calling the surgeon.