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.