Medical Safety Principles

The partial pressure of most anesthetic vapors is dependent on temperature, not barometric pressure, and in an extremely cold environment more anesthetic vapor might be required. In one study, the delivered concentration of halothane increased with altitude, but its alveolar partial pressure remained constant. Although the concentration of the inspired volatile anesthetic was increased, the anesthetic effect remained unchanged at the given vaporizer setting. In other words, halogenated hydrocarbon vapors are delivered at a constant potency at constant temperature irrespective of altitude. Desflurane is the only exception to this rule. Unlike variable-bypass vaporizers, the Datex-Ohmeda (Steeton, UK) Tec 6 and Tec 6 plus vaporizers require manual adjustments of the concentration control dial at altitudes other than sea level to maintain a constant partial pressure of anesthetic.