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Correspondence  |   November 2011
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Author Affiliations & Notes
  • Andreas W. Loepke, M.D., Ph.D.
    *
  • *Cincinnati Children's Hospital Medical Center, University of Cincinnati College of Medicine, Cincinnati, Ohio.
Article Information
Correspondence
Correspondence   |   November 2011
In Reply
Anesthesiology 11 2011, Vol.115, 1133-1135. doi:10.1097/ALN.0b013e3182303eea
Anesthesiology 11 2011, Vol.115, 1133-1135. doi:10.1097/ALN.0b013e3182303eea
We would like to thank Drs. Stratmann and Alvi for their interest in our study.1 In the study, we tried to determine the comparative neurotoxic effects of the volatile anesthetics desflurane, isoflurane, and sevoflurane on the developing mouse brain. Accordingly, we first established comparative anesthetic potencies for the three volatile agents and then found neuroapoptosis in neocortical neurons to be similar when animals were exposed to equipotent concentrations of 0.6 minimum alveolar concentration (MAC) for 6 h. Stratmann and Alvi now raise two important issues: first, how are the neurotoxic effects of the different anesthetic agents best compared in small animal model systems, and second, how does one ensure similar anesthetic potency throughout the exposure.
For their first comment, Stratmann and Alvi point out that several studies have observed differential effects of sevoflurane on neuronal cells compared with isoflurane or propofol.2  5 However, none of the studies comparing isoflurane with other anesthetics have established a similar degree of anesthetic potency among the experimental groups in their particular model systems26; instead, they have used concentrations extrapolated from adult studies. Moreover, three of the quoted studies used in vitro  models with dissociated cell cultures,2  4 which may not represent an adequate model system to quantify the degree of neurotoxicity observed in live animals. Accordingly, several in vitro  studies have been unable to detect any neurotoxic anesthetic effects, even when using isoflurane.7,8 Although the mechanism of anesthetic neurotoxicity is just beginning to be understood,9,10 it has been suggested to be associated with anesthetic interference with neuronal networks, which cannot be modeled well in dissociated neuronal cultures.11 
Another important question in comparing different studies of anesthetic neurotoxicity is how brain cell death was quantified. Neuronal cell death after anesthetic exposure is not homogeneously distributed throughout the brain but varies by brain region and even within a particular brain region, and may differ depending on which animal species or anesthetic agents are studied. Accordingly, we selected a region that in our model repeatedly has shown significant anesthesia-induced neuroapoptosis (i.e.  , layer II/III of the neocortex; see figure 3 from the original article). An observer unaware of group assignment repeatedly was able to distinguish sections from previously anesthetized animals from those of unanesthetized littermates but was not able to differentiate among the three anesthetic regimens. Two quantitative methods, cell counts of apoptotic neurons and a colorimetric assay of the activity of the apoptotic enzyme caspase 3, could not be differentiated among the three anesthetic regimens. Thus, we are confident in our findings that a 6-h exposure to 7.4% desflurane, 1.5% isoflurane, or 2.9% sevoflurane, respectively, produced similar degrees of neuroapoptosis in the superficial neocortex of newborn CD1/C57BL/6J hybrid mice. Our qualitative assessment makes significant differences in neuroapoptosis in other brain regions unlikely; however, we cannot rule out that quantitative cell counts may result in differences and that comparative results in other species may differ.
The second point raised by Stratmann and Alvi relates to the important issue of establishing and maintaining equivalence between different anesthetic agents when comparing their neurotoxic potency. To accomplish this, one can either determine the true MAC value for each anesthetic while ensuring stable environmental factors12 (as we did in our study) or use the approach taken by Stratmann et al.  and repeatedly measure MAC in a cohort of subjects while environmental factors change dramatically.13 In the former approach, MAC has been found to remain constant during anesthetic exposure,14 whereas in the latter approach it is not surprising that MAC will change over time because of the changes in environmental factors, such as alterations in sodium concentrations, temperature, oxygenation, carbon dioxide concentrations, glucose, acid base status, or hemoglobin concentrations, which all can lead to significant changes in MAC.15,16 Moreover, MAC differs by age, species, and even between strains of the same species.17 This would suggest that Stratmann and Alvi's extensive comparisons of their data obtained in rats may have only limited applicability to our study in mice. Contrary to the substantial hypercapnia and acidosis observed by Stratmann et al.  in their rat studies,13,18 acid–base derangements were far less severe in our study and did not differ among the desflurane, isoflurane, and sevoflurane groups, suggesting that any effects on MAC would be expected to be less dramatic and similar among the three anesthetic groups.
However, Stratmann and Alvi's comment raises another interesting question: whether the absolute concentration of a drug, which differed by a factor of almost 5 between isoflurane and desflurane, or its MAC concentration determines its neurotoxic effects. Our finding of similar toxicity using equipotent concentrations of the three anesthetic agents seems to suggest that anesthetic potency is the determining factor. If these findings in animals applied to the clinical setting as well, where anesthetics are dosed according to their potency to provide amnesia and immobility during surgery, no benefit would be gained by choosing one anesthetic agent over another.
**Cincinnati Children's Hospital Medical Center, University of Cincinnati College of Medicine, Cincinnati, Ohio. pedsanesthesia@gmail.com
References
Istaphanous GK, Howard J, Nan X, Hughes EA, McCann JC, McAuliffe JJ, Danzer SC, Loepke AW: Comparison of the neuroapoptotic properties of equipotent anesthetic concentrations of desflurane, isoflurane, or sevoflurane in neonatal mice. ANESTHESIOLOGY 2011; 114:578–87
Wang QJ, Li KZ, Yao SL, Li ZH, Liu SS: Different effects of isoflurane and sevoflurane on cytotoxicity. Chin Med J 2008; 121:341–6
Wei H, Kang B, Wei W, Liang G, Meng QC, Li Y, Eckenhoff RG: Isoflurane and sevoflurane affect cell survival and BCL-2/BAX ratio differently. Brain Res 2005; 1037:139–47
Yang H, Liang G, Hawkins BJ, Madesh M, Pierwola A, Wei H: Inhalational anesthetics induce cell damage by disruption of intracellular calcium homeostasis with different potencies. ANESTHESIOLOGY 2008; 109:243–50
Bercker S, Bert B, Bittigau P, Felderhoff-Müser U, Bührer C, Ikonomidou C, Weise M, Kaisers UX, Kerner T: Neurodegeneration in newborn rats following propofol and sevoflurane anesthesia. Neurotox Res 2009; 16:140–7
Liang G, Ward C, Peng J, Zhao Y, Huang B, Wei H: Isoflurane causes greater neurodegeneration than an equivalent exposure of sevoflurane in the developing brain of neonatal mice. ANESTHESIOLOGY 2010; 112:1325–34
Spahr-Schopfer I, Vutskits L, Toni N, Buchs PA, Parisi L, Muller D: Differential neurotoxic effects of propofol on dissociated cortical cells and organotypic hippocampal cultures. ANESTHESIOLOGY 2000; 92:1408–17
Lin D, Feng C, Cao M, Zuo Z: Volatile anesthetics may not induce significant toxicity to human neuron-like cells. Anesth Analg 2011; 112:1194–8
Yon JH, Daniel-Johnson J, Carter LB, Jevtovic-Todorovic V: Anesthesia induces neuronal cell death in the developing rat brain via  the intrinsic and extrinsic apoptotic pathways. Neuroscience 2005; 135:815–27
Head BP, Patel HH, Niesman IR, Drummond JC, Roth DM, Patel PM: Inhibition of p75 neurotrophin receptor attenuates isoflurane-mediated neuronal apoptosis in the neonatal central nervous system. ANESTHESIOLOGY 2009; 110:813–25
Olney JW, Young C, Wozniak DF, Ikonomidou C, Jevtovic-Todorovic V: Anesthesia-induced developmental neuroapoptosis. Does it happen in humans? ANESTHESIOLOGY 2004; 101:273–5
Sonner JM: Issues in the design and interpretation of minimum alveolar anesthetic concentration (MAC) studies. Anesth Analg 2002; 95:609–14
Stratmann G, Sall JW, Eger EI 2nd, Laster MJ, Bell JS, May LD, Eilers H, Krause M, Heusen F, Gonzalez HE: Increasing the duration of isoflurane anesthesia decreases the minimum alveolar anesthetic concentration in 7-day-old but not in 60-day-old rats. Anesth Analg 2009; 109:801–6
Eger EI 2nd, Johnson BH: MAC of I-653 in rats, including a test of the effect of body temperature and anesthetic duration. Anesth Analg 1987; 66:974–6
Eger EI 2nd, Saidman LJ, Brandstater B: Minimum alveolar anesthetic concentration: A standard of anesthetic potency. ANESTHESIOLOGY 1965; 26:756–63
Stoelting RK, Hillier SC: Pharmacokinetics and pharmacodynamics of injected and inhaled drugs, Pharmacology and Physiology in Anesthetic Practice, 4th edition. Edited by Stoelting RK. Philadelphia, Lippincott Williams & Wilkins, 2005, pp 3–35
Sonner JM, Gong D, Eger EI 2nd: Naturally occurring variability in anesthetic potency among inbred mouse strains. Anesth Analg 2000;91:720–6
Stratmann G, May LD, Sall JW, Alvi RS, Bell JS, Ormerod BK, Rau V, Hilton JF, Dai R, Lee MT, Visrodia KH, Ku B, Zusmer EJ, Guggenheim J, Firouzian A: Effect of hypercarbia and isoflurane on brain cell death and neurocognitive dysfunction in 7-day-old rats. ANESTHESIOLOGY 2009; 110:849–61