1. Vapor pressure, partial pressure, and concentration of inhaled anesthetics: The impact of atmospheric pressure on vaporizer output. Top. Variable bypass vaporizers are calibrated at 20°C and 1 atm (standard temperature and pressure). The partial pressure of isoflurane in the vaporization chamber is its vapor pressure, 238 mm Hg. Isoflurane vapor adds to the carrier gases (O2 and nitrous oxide [N2O]), and the sum of the partial pressures equals atmospheric pressure (∼760 mm Hg at sea level) at all points in the flow path. With fresh gas flow of 3 L/min and the vaporizer set at 2.0%, the carrier flow through the vaporization chamber is 134 mL. About 61 mL/min of isoflurane vapor (0.34 mL/min liquid) is added to the carrier gases so that the output of the vaporizer after dilution is 3061 mL/min at 2.0% isoflurane (Piso = 15.2 mm Hg). Bottom. With the same flow and vaporizer settings in Denver (5280 ft elevation; ambient pressure, 630 mm Hg), isoflurane comprises a larger portion of the carrier gas mixture, and after dilution, the output of the vaporizer is 3081 mL/min at about 2.6% isoflurane. Thus, in Denver, the delivered concentration of isoflurane is 30% higher than that at sea level. However, the delivered partial pressure of isoflurane in Denver is 16.7 mm Hg, only 10% higher than that at sea level. Similarly, liquid isoflurane is vaporized at 0.37 mL/min in Denver, about 10% more than that at sea level. Because its partial pressure directly determines the uptake and effect of isoflurane on patients, only minor changes in vaporizer settings are required at high altitudes. Partial pressures of N2O and O2 carrier gases in Denver are about 17% lower than those at sea level. Thus, at similar carrier gas flows, N2O has a significantly reduced anesthetic action in Denver versus sea level. Moreover, the inspired O2 concentration in Denver should be increased to reduce the risk of hypoxia, so increasing the N2O concentration may not be possible.
Source: Chapter 38. Pharmacology of Inhalational Anesthetics, Anesthesiology, 2e
Citation: Longnecker DE, Brown DL, Newman MF, Zapol WM. Anesthesiology, 2e; 2012 Available at: Accessed: October 17, 2017
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