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Title: Can Mathematical Modeling Explain the Measured Magnitude of the Second Gas Effect?
Austin Authors: Korman, Ben;Dash, Ranjan K;Peyton, Philip J 
Affiliation: Department of Anaesthesia and Pain Medicine, Royal Perth Hospital, Perth, Australia
Department of Physiology, Medical College of Wisconsin, Milwaukee, Wisconsin
Department of Biomedical Engineering, Medical College of Wisconsin, Milwaukee, Wisconsin
Anaesthesia, Perioperative and Pain Medicine Unit, Melbourne Medical School, University of Melbourne, Parkville, Australia
Department of Anaesthesia, Austin Health, Heidelberg, Victoria, Australia
Issue Date: Jun-2018
Publication information: Anesthesiology 2018; 128(6): 1075-1083
Abstract: Recent clinical studies suggest that the magnitude of the second gas effect is considerably greater on arterial blood partial pressures of volatile agents than on end-expired partial pressures, and a significant second gas effect on blood partial pressures of oxygen and volatile agents occurs even at relatively low rates of nitrous oxide uptake. We set out to further investigate the mechanism of this phenomenon with the help of mathematical modeling. Log-normal distributions of ventilation and blood flow were generated representing the range of ventilation-perfusion scatter seen in patients during general anesthesia. Mixtures of nominal delivered concentrations of volatile agents (desflurane, isoflurane and diethyl ether) with and without 70% nitrous oxide were mathematically modeled using steady state mass-balance principles, and the magnitude of the second gas effect calculated as an augmentation ratio for the volatile agent, defined as the partial pressure in the presence to that in the absence of nitrous oxide. Increasing the degree of mismatch increased the second gas effect in blood. Simultaneously, the second gas effect decreased in the gas phase. The increase in blood was greatest for the least soluble gas, desflurane, and least for the most soluble gas, diethyl ether, while opposite results applied in the gas phase. Modeling of ventilation-perfusion inhomogeneity confirms that the second gas effect is greater in blood than in expired gas. Gas-based minimum alveolar concentration readings may therefore underestimate the depth of anesthesia during nitrous oxide anesthesia with volatile agents. The effect on minimum alveolar concentration is likely to be most pronounced for the less soluble volatile agents in current use.
DOI: 10.1097/ALN.0000000000002131
PubMed URL: 29481374
Type: Journal Article
Appears in Collections:Journal articles

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