Effect of Respiratory Gas Mixture

The mixture of airway gases becomes an important issue when any type of potentially flammable endotracheal tube is used. Combustion is more vigorous when excess oxidizer is present, and most clinicians recognize the need to reduce the FIO₂ to less than 0.40 or to the minimum concentration consistent with patient oxygenation. That nitrous oxide also is a powerful oxidizer is less well recognized, but adding nitrous oxide as a diluent for oxygen is just as dangerous as having a high FIO₂ . ^[64] Use of an air-oxygen mixture appears to be acceptable. Some investigators prefer helium as a diluent to nitrogen because helium has a higher thermal conductivity and may delay the ignition of an endotracheal tube for a few seconds.^{[63] [65]} Helium also has a lower density and allows the use of a smaller endotracheal tube without turbulence and high flow resistance.^{[67] [68] [69]} However, the index of flammability is reduced by only 1% to 2% when oxygen-helium is substituted for oxygen-nitrogen.^[66]

The volatile anesthetics used in clinical practice are nonflammable and nonexplosive in clinically relevant concentrations.^{[70] [71]} During an airway fire, however, they may undergo pyrolysis to potentially toxic compounds.^[72] On this theoretical basis, the ANSI Z136.3 standard recommends not using volatile anesthetics during airway laser surgery. This is an extremely conservative stance given the low concentration of anesthetic gas compared with the products of combustion from an endotracheal tube and the lack of evidence of toxicity due to the pyrolytic products. When Venturi ventilation is used, administration of volatile agents usually is not practical.