Effect of low-dose ketamine regimen with or without magnesium sulphate adjunct in tramadol patient controlled analgesia in a major abdominal surgery ICU population

Introduction

We evaluated the potential benefits of adding magnesium sulphate (MgSO₄) to ketamine (K) in a patient controlled analgesia (PCA) model with tramadol (T) on postoperative pain and cognitive function in major abdominal surgery.

Methods

Twenty-four abdominal open surgery patients were included in a double-blind, randomized study. After extubation a PCA pump with T was placed to promote optimal analgesia. In the first group K was added (0.5 mg/ml) to the pump (Gk, n = 6), in the second group it was MgSO₄ (0.03 mg/ml) (Gm, n = 6), in the third group MgSO₄ was adjunct to K (Gm-k, n = 6) and in the last group NaCl 0.9% was added and served as the control group (Gc, n = 6). Consumption of T and the cognition (using the Mini-Mental State of Examination (MMSE) [1]) were evaluated at the end of hospitalisation in the ICU. For statistical analysis, a Shapiro–Wilk test, Wilcox text and Student t test were used.

Results

K and MgSO₄, separately added to T, did not improve significantly its global consumption, demonstrating comparable postoperative pain scales between groups (P = 0.08). MgSO₄ coupled with K (Gm-k) improved postoperative pain compared with Gc (P < 0.05). This combination reduced the mean global T consumption by 15% (± 5% SD) during the first 24 hours and by 25% (± 8% SD) during the second day of pump infusion compared with the other groups (P < 0.02). K and MgSO₄ did not modify cognitive functioning compared with the control group (mean MMSE scoring: >28 ± 2, SD) (P = 0.1).

Conclusion

Adding a low dose of K or MgSO₄ to T in post-operative major abdominal surgery did not improve analgesia but the combination of both had a statistical sparing effect on T consumption. Magnesium plays a determinant role, by interacting in the glutaminergic pathways, on the K effect. Cognitive determinants in ICU patients are not modified by a low dose of K.