Seroflurane Versus Isoflurane in Single-Breath Induction

Abstract

In Response: Doctors Abramson and Pivalizza raise two important points. First, we recognize that because of equipment limitations, we were unable to use equipotent inhaled concentrations of sevoflurane and isoflurane for our single-breath inductions [1]. However, sevoflurane's low blood-gas solubility and minimal pungency allowed single-breath induction to occur as quickly as it did with isoflurane, despite the use of a lower relative concentration. When a vaporizer capable of delivering 8% sevoflurane is used, anesthesia is induced even more rapidly; there is also a much lower incidence of partial awakening and crowing after redistribution of the initial inhaled dose of sevoflurane. Our second concern relates to the calculation of "minimum alveolar anesthetic concentration (MAC) equivalents" when an anesthetic is administered with N2 O. Katoh and Ikeda found that the concentration of sevoflurane that must be added to 63% N2 O (approximate equals 2/3 MAC) to prevent patient movement is approximately 0.66% (approximate equals 1/3 MAC) [2]. These authors concluded that the MAC fraction of N2 O must be added to the MAC fraction of sevoflurane to compute the total anesthetic effect. Thus, to account for the effect of 50% N2 O, computed MAC equivalents for sevoflurane and isoflurane must each be increased by approximately 0.5 MAC, resulting in effective concentrations of 3.4 and 4.8 MAC for 5% sevoflurane and isoflurane, respectively. In contrast, Abramson and Pivalizza have incorrectly assumed that, since 0.66% sevoflurane added to 63% N2 O yields an anesthetic depth of 1 MAC, each additional 0.66% of sevoflurane adds an additional MAC to the effective depth of anesthesia. This line of reasoning results in the following inconsistency: 5% sevoflurane in O2 corresponds to 2.9 MAC, while 5% sevoflurane in 63% N2 O is equivalent to 7.5 MAC! In conclusion, single-breath inhaled induction of anesthesia with sevoflurane offers a suitable alternative to an intravenous induction in selected patients, especially when an 8% initial concentration is administered. Mark H. Sloan, MD Peter K. Karsunky, MD Pattilyn F. Conard, CRNA, MA Jeffrey B. Gross, MD* *JBG is a lecturer and consultant for Abbott Laboratories. Department of Anesthesiology, University of Connecticut School of Medicine, Farmington, CT 06030-2015