Plasma concentrations of nociceptin/orphanin FQ: comparison of levels after general and neuraxial anesthesia for arthroscopic knee surgery

BACKGROUND Nociceptin/orphanin FQ (N/OFQ) is an endogenous opioid heptadecapeptide. Preclinically, the pharmacologic action of N/OFQ has been characterized for the treatment of pain in non-human primates. Clinically, the pharmacologic action of N/OFQ is unclear, and concentrations have only been measured under certain clinical conditions. The aims of this study were to measure the plasma concentrations of N/OFQ in different postoperative pain states and to identify the potential relationship between postoperative pain states and N/OFQ plasma concentrations. METHODS Two groups of 14 patients scheduled for knee arthroscopy were included in this study. Postoperative pain in the first group (IV group) was controlled by intravenous patient-controlled analgesia (IV-PCA). Postoperative pain in the second group (ES group) was controlled by epidural patient-controlled analgesia (E-PCA) or the remnant analgesic effects of spinal anesthesia. Plasma concentrations of N/OFQ were measured by enzyme-linked immunosorbent assay. Numerical rating scale (NRS) scores were recorded for all patients. Differences between the two groups with regards to plasma concentrations of N/OFQ and NRS scores were evaluated by the Mann-Whitney U-test. RESULTS Plasma concentrations of N/OFQ (mean ± SD) were 70.4 ± 128.0 pg/ml in the IV group and 19.2 ± 43.4 pg/ml in the ES group. NRS scores (mean ± SD) were 3.1 ± 1.9 in the IV group and 0.5 ± 1.1 in the ES group. The differences in plasma N/OFQ concentrations between groups were not significant (P = 0.06). NRS scores were significantly lower in the ES group as compared with the IV group (P = 0.0019). CONCLUSIONS Plasma concentrations of N/OFQ increase in acute postoperative pain states, but are not correlated with the level of postoperative pain.