Sequence-Dependent Analgesic Efficacy of Ketamine and Magnesium Sulfate After Radical Nephrectomy

Background and Objectives: Ketamine and magnesium sulfate (MgSO₄) are NMDA receptor antagonists that act through distinct mechanisms. Preclinical data indicate that their analgesic interaction is sequence-dependent: ketamine administered before MgSO₄ produces synergistic antinociception, whereas the reversed sequence is antagonistic. The primary outcomes were postoperative pain intensity (Numerical Rating Scale, NRS 0-10, at rest and on movement) and cumulative intravenous morphine consumption over 48 h, evaluated in patients undergoing open radical nephrectomy to test the hypothesis of sequence-dependent analgesic interaction. Materials and Methods: In this randomized, double-blind, placebo-controlled trial, 208 patients scheduled for elective open radical nephrectomy received two sequential intravenous boluses intraoperatively: Drug A immediately after induction, Drug B 10 min later. Agents were ketamine 0.2 mg/kg, MgSO₄ 15 mg/kg, or placebo (0.9% NaCl) in all nine possible combinations. Primary outcomes were postoperative pain intensity (NRS 0-10, at rest and on movement) and cumulative intravenous morphine consumption, assessed at 14 time points over 48 h. Secondary outcomes included sedation, nausea, vomiting, and the presence of hallucinations. Results: The ketamine → MgSO₄ (K → Mg) sequence significantly reduced NRS pain scores compared to placebo at multiple time points, including 30 min, 1 h, 3 h, 6 h, and 32 h postoperatively, with differences exceeding the minimum clinically important difference of 2 NRS points at the earliest assessments. The MgSO₄ → ketamine (Mg → K) sequence did not differ from placebo at any time point. Cumulative morphine consumption was comparable across groups. No hallucinations or psychomimetic events were observed. Conclusions: Intraoperative ketamine followed by MgSO₄ (K → Mg) provides clinically meaningful postoperative analgesia after open radical nephrectomy; the reversed sequence (Mg → K) offers no benefit over placebo. These findings provide the first clinical confirmation of sequence-dependent NMDA receptor antagonism and support the K → Mg protocol as a safe, simple addition to multimodal perioperative analgesia. Trial registration: ISRCTN registry, ISRCTN83633282.