Morphine sparing effect of low dose ketamine during patient controlled analgesia

Abstract

Objective: To compare the quality of intravenous patient controlled analgesia (PCIA) of low dose morphine plus ketamine with morphine. Design: Double blind case control study. Setting: Academic hospital. Patients: Thirty-six patients scheduled for elective abdominal hysterectomy were randomly divided into two groups to receive patient controlled intravenous analgesia (PCIA). Interventions: Group M received morphine 21 μg/kg as a bolus, Group MK received morphine 7 μg/kg plus ketamine 14 μg/kg as a bolus. The lockout period in both groups was 7 minutes. Measurements: Morphine consumption, visual analogue pain score (VAPS), pulse oximetry oxygen saturation (SpO₂), respiratory rate (RR), verbal descriptive sedation score (VDSS), nausea, pruritis, dreaming, and hallucinations were recorded at 1, 4, 24 and 48 hours. Equivalence of the two groups was assessed by comparing the 95% confidence interval (CI) for the effect with the equivalence delta (10%). Results: Morphine consumption was significantly lower in Group MK after 24 and after 48 hours (p < 0,05). VAPS was significantly higher in Group MK at 4 hours (p < 0,05), but VAPS was always clinically lower than in Group MK at all times (Equivalence delta > 10%). SpO₂ at 4 hours was marginally higher in Group MK (p = 0,0809). Conclusion: Morphine-ketamine PCIA, in doses used in this study, provided analgesia inferior to that of morphine PCIA, but may improve the respiratory side effect profile of morphine. The analgesia of morphine and ketamine are additive rather than synergistic.