Drug exposure during oxycodone-S-ketamine patient-controlled analgesia reflects pain-related dosing behaviour after lumbar fusion surgery

Background: Opioids remain the cornerstone of acute postoperative pain management but are associated with potentially serious adverse effects. We aimed to investigate the effect of oxycodone patient-controlled analgesia with adjunctive S-ketamine on pain after lumbar fusion surgery. Methods: In this secondary analysis, we examined oxycodone and S-ketamine exposure and PCA behaviour during the first 24 h. We assessed current pain versus subsequent PCA activity and whether the association between plasma oxycodone and adequate analgesia changed with postoperative time. Results: Plasma concentrations of administered drugs rose rapidly after PCA initiation, but variation was large. Metabolite concentrations accumulated over time. PCA attempt and delivered-dose rates decreased over postoperative time (RR 0.965 and 0.966 per hour, respectively; both P<0.001). Older age was associated with a lower delivered-dose rate (RR 0.905 per 10 years, 95% CI 0.829-0.987). Each 1-point higher current pain score was associated with more subsequent PCA attempts (RR 1.09, 95% CI 1.04-1.14) and delivered doses (RR 1.07, 95% CI 1.03-1.11). The oxycodone-analgesia association weakened over time [interaction P <0.001; odds ratios (Ors) per 10 ng/mL 1.16, 1.13, and 1.00 at 1, 4, and 24 h, respectively]. In active S-ketamine groups, including S-ketamine, norketamine, and 6-hydroxynorketamine concentrations left the pain-PCA association unchanged (RR 1.09 and 1.07) and preserved the time-dependent oxycodone association (ORs 1.30, 1.26, and 1.02). Conclusions: During PCA, drug exposure develops from patient-generated dosing histories while pain and recovery change. Current pain was associated with subsequent PCA activity, and the oxycodone-analgesia association was time-dependent.