In Response

Abstract

We thank Drs. Kopp and Jobson1 for drawing attention to the developing brain’s delicate sensitivity to a wide variety of stressors, drugs, and medical treatments, including oxygen. In this regard they raise 2 questions: (1) whether the combination of isoflurane and 30% oxygen used in our study, rather than isoflurane alone, was the cause for the observed neuronal injury and (2) what implications a 2% cortical neuronal loss, as observed in our mouse study after a prolonged anesthetic exposure, would have for humans. The notion that oxygen toxicity may have contributed to the dramatically increased rate of neuronal apoptotic cell death induced by isoflurane exposure in our study in 7-day-old mice is a legitimate concern, since increasing oxygen content may amplify the rate of oxygen free radicals formed by isoflurane.2 Even without exposure to isoflurane, 80% oxygen has previously been demonstrated to induce neuronal apoptotic cell death in 7-day-old rat pups after only 2 hours of exposure.3 Importantly, however, even after a 12-hour exposure to 40% oxygen, which was still greater than that used in our study and twice the duration, no demonstrable neuronal cell death was observed.3 In fact, studies of anesthetic toxicity that exposed anesthetized and control animals to the same inspired oxygen concentration4,5 have found qualitatively similar results for isoflurane-induced neuroapoptosis as studies using supplemental oxygen.6–8 To closely resemble the clinical setting, mouse pups in our study were exposed to a clinically relevant isoflurane concentration with supplemental oxygen similar to that given children undergoing anesthesia, albeit for an extended exposure time, whereas control animals were exposed to room air, analogous to children who do not undergo anesthesia at all.9 Accordingly, while the slightly disparate oxygen concentrations, 21% in controls and 30% in anesthetized animals, represent a potential confounder, we feel that isoflurane, rather than isoflurane plus supplemental oxygen, was the main culprit for the dramatically increased cell death observed in the exposed group and that supplemental oxygen most certainly did not alter our findings’ potential implications for current clinical practice, where supplemental oxygen is widely used during pediatric anesthesia. Regarding the direct clinical implications, however, we caution against immediately equating our observation of a 2% cortical neuronal loss in mice to children undergoing anesthesia. Apoptotic cell death is a requirement for normal brain development and eliminates, both in small rodents as well as humans, more than 50% of neurons.10 It remains unknown whether anesthetics target neurons that would have been eliminated by this process or whether they remove neurons otherwise needed for proper brain function. Moreover, given the many disparities between clinical anesthesia practice and animal research,11 Drs. Kopp and Jobson will probably agree that it would be an oversimplification to directly equate cell counts from studies in mice to the effects of pediatric anesthesia and that additional research is urgently needed, both in humans to assess the phenomenon’s clinical implications as well as in animals to further elucidate the underlying mechanisms responsible for the findings in our study. George K. Istaphanous, MD Department of Anesthesia Children’s Hospital Los Angeles Los Angeles, California Andreas W. Loepke, MD, PhD, FAAP Department of Anesthesiology Cincinnati Children’s Hospital Medical Center Cincinnati, Ohio [email protected]