Contact heat evoked potentials as a measure of the analgesic component of anesthesia - a patient study

To assess the applicability of contact heat evoked potentials (CHEPs) as a direct measure of analgesia, we analyzed the following endpoints. (1) Correlation between remifentanil-induced changes in VAS and CHEP amplitudes. (2) Correlation between CHEP amplitude reduction and remifentanil infusion rate. Furthermore, we investigated the sedative effects of remifentanil and the effect of propofol-induced loss of responsiveness (PI-LOR) on CHEPs. After determination of the individual pain threshold (visual analog scale (VAS) of 10), 120 adult ASA physical status I or II patients randomly received remifentanil in one of four predefined infusion rates (0.05, 0.1, 0.2, or 0.4 [Formula: see text]g/kg/min). After equilibration, a heat stimulus 20% above the individual pain threshold was applied to the volar forearm. Eventually, propofol was infused until LOR, and the heat stimulus was applied again. CHEPs, VAS, and the Observer's Assessment of Alertness/Sedation (OAA/S) scale, as well as middle latency auditory evoked potentials (MLAEPs), were assessed. A total of 119 patients were included in the analysis. CHEP amplitudes could not be detected for all patients at all times. Remifentanil dosage and VAS showed a strong inverse correlation (r=-0.46; p<0.001). We also detected a correlation between remifentanil dosage and CHEP amplitudes (r=0.29; p=0.003) but not between VAS and CHEPs (r=-0.17; p=0.868). Both the OAA/S score and MLAEPs indicated a correlation between remifentanil dosage and degree of sedation. After PI-LOR, CHEPs were no longer detectable. The exact mechanism for the detected CHEP reduction remains to be determined. CHEPs monitoring appears unsuitable for direct assessment of the analgesic component of general anesthesia.Trial registration: German Clinical Trials Register ID DRKS00003300, Oct 24, 2011.