Tutorial: Context-Sensitive Decrement Times for Inhaled Anesthetics

Abstract

In Brief Context-sensitive decrement times for inhaled anesthetics connect two values: a) the duration of anesthesia (nominally at a constant alveolar concentration)—the “context” and b) the time to decrease the alveolar or vital tissue (e.g., brain, heart, kidney, and liver, collectively called the vessel-rich group of tissues) concentration by some fractional “decrement” of the starting concentration. Increasing duration of anesthesia increases the time to a given decrement in a nonlinear manner that may considerably delay recovery. In the present report we use a commercially available simulation program (Gas Man®) to confirm and enlarge on these concepts. In this simulation, increasing duration of anesthesia can markedly delay complete awakening for isoflurane. Increasing anesthesia duration imposes considerably less delay in awakening from sevoflurane compared with isoflurane. For desflurane, only prolonged anesthesia or decrements of 95% and more should delay awakening from anesthesia. These changes are shown to be the result of the relative solubility of each anesthetic in blood and tissue. An increase in cardiac output is also shown to delay awakening. IMPLICATIONS: Increasing duration of anesthesia (the “context”) delays recovery from an inhaled anesthetic. Duration, anesthetic solubility, and cardiac output each can materially influence recovery by affecting the time to achieve a particular decrement in concentration (the context-sensitive decrement time). The relationship of these factors is nonlinear.