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Re: Nitrous oxide and cognitive function in the elderly
- Jacqueline M. Leung, M.D., M.P.H., Laura P. Sands, Ph.D. (Purdue University) (15 December 2006)
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Nitrous oxide and cognitive function in the elderly
- Mohammad R. Abdul Rahim (7 December 2006)
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Jacqueline M. Leung, M.D., M.P.H., Professor of Anesthesia & Perioperative Care University of Califiornia, San Francisco, Laura P. Sands, Ph.D. (Purdue University)
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We thank Mohammad R Abdul Rahim for his interest and comments on our work. We would like to take the opportunity to respond. First, the title of an article typically does not describe all the results or conclusions about a study. One needs to read the entire article to examine all the co -variates that have been measured. Second, duration of exposure to nitrous oxide (same as duration of surgery) was examined, please see page 3 of the article when N2O measurement was described. Third, the surgical risks of patients exposed to N2O vs. those with no exposure were compared and there was no difference (see table 1 in paper). We did not perform a factorial design in the surgical risk randomization because the definitions for surgical risk such as the amount of blood loss and duration of surgery were factors that could not have been predicted preoperatively. Fourth, intraoperative use of opioids was standardized and there was no difference between the N2O vs. the oxygen groups. Fifth, all patients were mechanically ventilated during the operation (please see page 2 of paper). Finally, with clinical trials, it is standard practice to compare the two groups with respect to demographic factors and other relevant characteristics to ensure that the two groups were balanced. If there was any factor found to be significantly different between the two groups, then further analysis is performed to determine whether the factor has any association with the outcome of interest. Sincerely, Jacqueline M. Leung, MD, MPH, Laura P. Sands, PhD Conflict of Interest:None declared |
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Mohammad R. Abdul Rahim, Anaesthetist
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Editor: I read with great interest the article by Leung and colleagues"Nitrous Oxide does not change the incidence of postoperatice delirium or cognitive decline in elderly surgical patients". In spite of the fact that this was a well conducted, randomised controlled trial, giving level III evidence and applicable to clinical practice, I feel several points need to be clarified by the authors. Nitrous oxide was included in the article title, but the title failed to mention other factors that were examined as well. The duration of surgery and hence the exposure to nitrous oxide should have been examined as an independant factor, not included within the surgical risk. Surgery was of low, intermediate and high risk; were these factors randomised to eliminate bias? Although the results of postoperative delirium occuring with or without nitrous oxide were mentioned in the article, there was not mention of it in the results. Intraoperative and postoperative opiates were administered either intravenously or neuroaxially. Although there was an emphasis on the effect of postoperative opiates, no emphasis was given to intra-operative opiates and their effect. In addition the paper did not make it clear how many patients were mechanically ventilated throughout the procedure, and how many were having their respiratory rate controlled by incremental doses of intra-operative IV opiates. Neither was it clear why there were two comparisions of post-operative delirium; firstly between patients receiving desflurane and those not receiving desflurane, and secondly between desflurane and those receiving isoflurane. Finally, it would have been helpful if reference had been made to any in vitro studies carried out in relation to the use of nitrous oxide in clinially relevant concentrations in a similar manner to the studies carried out by Jevtovic-Todorovic et al in 1998 and Beals et al in 2003 which examined the effects of suprapharmacological concentrations of nitrous oxide on the brain. Conflict of Interest:None declared |
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