Evidence Is Lacking for Interventions Proposed to Prevent Unintended Awareness During General Anesthesia for Cesarean Delivery

Abstract

To the Editor: We congratulate Drs. Robins and Lyons on their comprehensive and thought-provoking review of awareness during cesarean delivery under general anesthesia with subsequent explicit recall.1 Historically, patients undergoing cesarean delivery with general anesthesia have been considered to be at greater risk for awareness, but recent research suggests that, with modern anesthetic techniques, the risk may be comparable to that of the general surgical patient.2 We have concerns regarding some of the techniques proposed to decrease the likelihood of awareness in the setting of cesarean delivery1 and would welcome clarification. Because processed electroencephalograph (EEG) monitors have not been sufficiently assessed in pregnant patients, it may be imprudent to assume that they are informative in this patient population. Furthermore, none of the patients in the B-Aware study who had “definite” awareness had a cesarean delivery3; therefore, evidence for the utility of processed EEG monitors in pregnant patients cannot be inferred from this study. In general, there is equipoise regarding the ability of processed EEG monitoring to decrease awareness in any surgical population, particularly when a potent inhaled anesthetic is administered.4,5 For women undergoing cesarean delivery with propofol-based total IV anesthesia, the case for using a processed EEG monitor may be more compelling. The further recommendation to administer a high concentration of nitrous oxide is surprising because current processed EEG monitors measure nitrous oxide's anesthetic effects poorly. The combination of nitrous oxide and processed EEG monitoring may therefore prompt unnecessarily deep anesthesia with consequences for the neonate that might include neurotoxicity.6 Moreover, the suggestion that because “the effect of nitrous oxide on memory is uncertain, prudent advice would be to regard the target MAC as that of the volatile alone” compounds this possibility. Whereas nitrous oxide and volatile anesthetics are additive for producing patient immobility, they may be antagonistic for producing analgesia.7 Accumulating preclinical and clinical evidence suggests that GABAergic general anesthetics attenuate the analgesic effects of nitrous oxide. On the strength of these data, a patient may derive more analgesic benefit from a sole potent volatile anesthetic than from its combination with nitrous oxide.7 This is likely to be even more significant when the administration of opioids is delayed as occurs during cesarean delivery.7 Therefore, it seems sensible to avoid nitrous oxide in this context, especially because any pharmacokinetic benefit over newer volatile anesthetics is minimal, and any hypothetical decrease in the rate of awareness has not been substantiated in clinical trials. Robert D. Sanders, BSc, MBBS, FRCA Departments of Leucocyte Biology and Anaesthetics Intensive Care and Pain Medicine Imperial College London Chelsea and Westminster Hospital London, UK [email protected] Michael S. Avidan, MBBCh, FCASA Department of Anesthesiology Washington University in St. Louis St. Louis, Missouri