To the Editor:
The recent paper by Loftus et al. ,1“Intraoperative ketamine reduces perioperative opiate consumption in opiate-dependent patients with chronic back pain undergoing back surgery,” was of interest to me because I occasionally use the technique described. This group of patients is very complex and I congratulate the authors for undertaking this study.
There are four points that warrant clarification. First, the primary outcome of the study was based on data derived from a review of medical records (i.e. , mean [SD] 48-h postoperative oral morphine equivalent consumption of 500  mg). However, the placebo group consumed only 309 (341) mg of this substance. Can the authors comment on this large difference and its possible relevance?
Second, the term “morphine equivalents” requires further explication. This terminology was confusing because it was applied to oral and intravenous formulations. Which formulation was used was not always immediately clear. For example, in their table 1,1were “median preoperative morphine equivalents” delivered orally or intravenously? The text implies that these equivalents are intravenous morphine. If this supposition is correct, then it appears to me that both groups of patients may be consuming more morphine equivalents at 6-week follow-up (data presented as mean [SD]) than they did preoperatively (data presented as median [interquartile range]). In fact, the placebo group appears to have much higher rates of morphine consumption at 6 weeks compared with their own preoperative consumption levels and the treatment group's consumption at 6-week follow-up. There is a possibility that the treatment group's 6-week follow-up consumption has also increased from the preoperative period, which is concerning. Can the authors clarify and comment on these points?
Third, the treatment group had more spinal levels operated on than did the control group. In fact, this difference reached statistical significance. However, this feature of the study was not addressed by the authors. Do the authors believe this difference was clinically significant?
Finally, with regard to these observations, specifically with respect to possible increases in morphine consumption among both groups at 6-week follow-up and the chronic nature of back pain, I believe that a more extended follow-up period is warranted. Do the authors plan to do this?
Christchurch Hospital, Christchurch, New Zealand. firstname.lastname@example.org