Ketamine in chronic pain patients

The September edition of Anesthesiology reports that in patient's who are chronically taken opioids for pain, using ketamine intraoperatively may be useful to decrease morphine consumption after spinal surgery. The authors randomized pt who were taking preoperative opioids to either 0.5 mg/kg ketamine preop plus 10 mcg/kg/min infusion until wound closure vs. placebo. Many previous studies looking at ketamine have studied it in opioid naive patients and have run the ketamine infusion into the post operative period for up to 72 hrs. This study stopped the infusion at the end of surgery which I believe is important for the practicing anesthesiologist, since most hospital policies will not allow a ketamine infusion on the floor without having a locked pump which adds to logistical constraints and increases costs. Lavand homme et al. have already shown that adding ketamine to an epidural for abdominal surgery can reduce chronic pain up to 6 months later. This study looked at patients having spinal surgery. The results: decreased morphine consumption at 24 hr by 30%, at 48 hrs a 37% decrease. In the PACU, VAS was decreased by 26.7%. This is admittedly modest. However, once again, in this study, ketamine was able to decrease pain scores at a 6 week follow up visit (by 26.2%). This seems like a predictable reaction as we know that ketamine inhibits NMDA which is important in producing secondary hyperalgesia which is thought to be a physical manifestation of wind up (central sensitization) at the spinal cord. It is thought that this is responsible for the development of persistent post surgical pain (chronic pain). As a side note, the authors also reported that patients who received ketamine also reported a decreased use of antidepressants at their 6 week follow up (10% in placebo vs. 0% in the ketamine group). Previous authors have also reported that ketamine has significant antidepressant effects. Also important to note; ketamine was much more effective in decreasing morphine use in patients who were using 40 mg per day or more of oral morphine (or its equivalent). In these patients, morphine use was decreased by 52% at 48 hours, whereas, there was no difference in patients taking less than this equivalent amount preoperatively.

The authors conclude that ketamine probably has a role in patients undergoing moderately painful surgery in reducing opioid consumption. Unfortunately, morphine related side effects were not different between groups.