Influence of fentanyl, alfentanil, and sufentanil on motor evoked potentials

The effects of the opioids fentanyl, alfentanil, and sufentanil on motor pa thways were studied in a total of 30 rabbits. Compound muscle action potent ials (CMAP) were recorded from the extensor muscles of the upper extremity as well as evoked:spinal Cord potentials (ESCP) from the thoracic epidural space in response to electrical stimulation of the motor cortex. After esta blishing stable baseline values, an equipotent intravenous bolus of one of the three opioids was applied that abolished reflex motor response to noxio us stimulation. Motor evoked potentials (MEP) were recorded from: the time of bolus administration until recovery of MEP amplitudes and latencies. Aft erwards, the opioids were administered continuously with cumulative dosage up to total absence of motor evoked response. Our results show a dose-depen dent suppression of the CMAP: When reflex movement to noxious stimulation w as extinguished, we found a significant (P < .001) reduction of the amplitu des to 34 +/- 18% (mean +/- SD) in the fentanyl group, to 43 +/- 24% in the alfentanil group, and to 53 +/- 20% of baseline values in the sufentanil g roup, increasing opioid plasma levels were associated with complete extinct ion of the CMAP. We hypothesize that the descending volleys within motor pa thways are mainly inhibited at a spinal level, because ESCP, particularly t he number of spinal I-waves, are not severely affected even when CMAP are c ompletely suppressed. In conclusion, intraoperative monitoring of descendin g pathways by means of MEP during anesthesia with opioids is feasible at an esthetic plasma concentrations maintaining a surgical level of analgesia. E ven with high opioid plasma levels, a valid MEP monitoring could be perform ed evaluating neural activity of spinal MEP.