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Additive contribution of nitrous oxide to halothane MAC in infants and children
D J Murray1, M P Mehta, R B Forbes
1Department of Anesthesia, University of Iowa College of Medicine, Iowa City 52242.
Insights
Nitrous oxide (N2O) significantly reduces the minimum alveolar concentration (MAC) of halothane in infants and children. This additive effect of N2O on halothane MAC is similar to findings in adults, suggesting N2O can be used to lower anesthetic requirements.
Area of Science:
- Anesthesiology
- Pediatric Anesthesia
- Pharmacology
Background:
- Minimum alveolar concentration (MAC) is a standard measure of anesthetic potency.
- Nitrous oxide (N2O) is known to reduce the MAC of volatile anesthetics.
- Understanding these interactions is crucial for safe and effective pediatric anesthesia.
Purpose of the Study:
- To determine the MAC of halothane in oxygen (O2) alone and in combination with varying concentrations of N2O in infants and small children.
- To assess the additive effects of N2O on halothane MAC in this pediatric population.
Main Methods:
- Fifty-one infants and children were studied.
- Anesthesia was induced with halothane and N2O (25%, 50%, or 75%).
- End-tidal concentrations were adjusted to determine MAC, with responses to surgical incision monitored.
Main Results:
- The MAC of halothane in O2 was 0.94 vol %.
- MAC values decreased significantly with increasing N2O concentrations: 0.78 vol % (25% N2O), 0.44 vol % (50% N2O), and 0.29 vol % (75% N2O).
- A linear relationship was observed, with a predicted MAC for N2O of 105 vol %, similar to adult values.
Conclusions:
- All tested concentrations of N2O significantly reduced the MAC of halothane in pediatric patients.
- The contribution of N2O to halothane MAC in children is additive, mirroring findings in adults.
- These results support the use of N2O as an adjunct to halothane to reduce anesthetic requirements in pediatric patients.
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.