[Middle-latency auditory evoked potentials during high-dose opioid analgesia]

Neuropsychological and neurophysiological investigations indicate that a 30-40 Hz oscillatory brain mechanism is necessary for the adequate uptake and processing of elementary successive sensory events. An oscillatory component of that particular frequency range can be observed in the mid-latency auditory evoked potential (MLAEP). It is suppressed under non-specific anesthetic agents (agents not acting on specific structures of the brain or receptors). Opioids produce analgesia, sedation, and are also used for induction and maintenance of general anesthesia. The effect of opioids on MLAEP has not yet been examined sufficiently. Because intraoperative wakefulness and awareness is not rarely observed under high-dose opioid analgesia, the effect of general anesthesia with fentanyl on MLAEP and auditory evoked neuronal 30-40 Hz oscillation was studied. Following informed consent in 20 patients scheduled for elective surgery, anesthesia was induced with fentanyl 0.01-0.02 mg/kg i.v. Auditory evoked potentials (AEP) were recorded before and 5 min after the start of general anesthesia over the vertex (positive) and mastoids on both sides (negative). Auditory clicks were presented binaurally at 70 dBnHL with a frequency of 9.3 Hz. Using the electrodiagnostic system Pathfinder I (Nicolet Co.), 1000 successive stimuli were averaged over a 100-ms post-stimulus period and analyzed off-line. Latencies of the peaks V, Na, and Pa were measured. By Fast-Fourier Transform-analysis corresponding power spectra were calculated to analyze energy portions of the AEP frequency components. In the awake state AEP showed an oscillatory component between 20- and 100-ms post-stimulus latency. Corresponding power spectra indicated a dominant 30-40 Hz frequency.(ABSTRACT TRUNCATED AT 250 WORDS)