Volatile Anesthetics and Neuromonitoring

Volatile halogenated inhalation anesthetics, such as isoflurane, sevoflurane, desflurane, and halothane, significantly impact neuromonitoring by causing a dose-dependent decrease in amplitude and an increase in latency of evoked responses, with more pronounced effects on cortical than subcortical responses. For the EEG, these agents typically cause an initial increase in frequency followed by a decrease in frequency and amplitude at higher doses, with burst suppression occurring at around 1.5 MAC and electrical silence at very high doses. Sevoflurane at high doses during mask induction can induce seizure activity. In somatosensory evoked potentials (SSEPs), adequate recordings can generally be obtained at 0.5 to 1 MAC in patients without neurological pathology, but even low levels can abolish potentials in patients with baseline neurological impairment, necessitating careful titration. Motor evoked potentials (MEPs) are highly sensitive to these agents, often necessitating the use of total intravenous anesthesia (TIVA) for reliable monitoring, although concentrations of 0.5 MAC or less may be used in some cases. Brainstem auditory evoked potentials (BAEPs) are relatively resistant to the effects of volatile anesthetics, making them reliable for brainstem function monitoring. Visual evoked potentials (VEPs) are highly sensitive to these agents, but studies indicate that a balanced regimen with 0.5 MAC sevoflurane, propofol, and remifentanil can yield similar VEP amplitudes to TIVA, suggesting that volatile agents can be used without significantly compromising VEP monitoring under certain conditions. In summary, understanding and managing the effects of volatile inhalation agents is crucial for balancing adequate anesthesia with the preservation of neuromonitoring signals during surgery.