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Additive Contribution of Nitrous Oxide to Halothane MAC in Infants and Children

Department of Anesthesia, University of Iowa College of Medicine, Iowa City, Iowa.

Abstract

Fifty-one infants and small children (14.7 ± 7.2 mo) were studied to determine the MAC of halothane in O₂ (n = 11) and in the presence of three different nitrous oxide (N₂O) concentrations (25% [n = 13], 50% [n = 13], and 75% [n = 14]).

In the three N₂O groups, after randomly assigning patients to an N₂O group, anesthesia was induced with halothane and N₂O using a pediatric circle system. After endotracheal intubation, halothane and N₂O end-expired concentrations were adjusted to predetermined concentrations. The initial halothane concentrations in each group were based on the assumption that each percent N₂O reduced halothane concentrations by 0.01 vol % (assumed halothane MAC = 1.0 vol %). Based on the response of the preceding subject in each group, halothane concentrations were increased or decreased depending on whether the response was to move or not to move, respectively, in response to the surgical incision. The mean duration of constant end-tidal concentrations before skin incision was 10 min. End-tidal gases were sampled and measured from a separate distal sampling port of an endotracheal tube during controlled ventilation (Perkin-Elmer Mass Spectrometer).

The MAC value for halothane in O₂ was 0.94 ± 0.08 vol % (mean ± SD). The MAC values of halothane in the presence of 25%, 50%, and 75% N₂O were 0.78 ± 0.12 vol %, 0.44 ± 0.10 vol %, and 0.29 ± 0.06 vol %, respectively. All concentrations of N₂O significantly reduced the MAC of halothane. A regression analysis through all four data points yielded a linear relationship (r² = 0.87) with a predicted MAC for N₂O of 105 vol %.

Unlike halothane and isoflurane, the predicted MAC of N₂O in infants and children is similar to that reported by others in adults. Similar to the results of clinical studies in adults, the contribution of N₂O to halothane MAC in children is additive.

Key Words: ANESTHETICS, GASES-nitrous oxide • ANESTHESIA, PEDIATRIC • POTENCY, ANESTHETIC-nitrous oxide • INTERACTIONS (DRUG), NITROUS OXIDE, HALOTHANE

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