Tutorial: Context-Sensitive Decrement Times for Inhaled Anesthetics

Abstract

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.

The complexity of recovery from anesthesia

“…We recently simulated the time course of recovery for sevoflurane, desflurane, and isoflurane, assuming an individual of normal weight [10]. Our findings confirm those of Bailey and explain the results of Arain et al We found that if the concentration in the vessel-rich group needs to decrease by 80% or less at the end of anesthesia, then the differences in time course between sevoflurane and desflurane are minimal.…”

Meta-Analysis of Average and Variability of Time to Extubation Comparing Isoflurane with Desflurane or Isoflurane with Sevoflurane

“…From the context-sensitive decrement times for inhalation anesthetics, the percentage reduction in extubation time with desflurane (vertical axis) is likely larger for patients with long extubation times at baseline (horizontal axis). 70,71 The linear weighted rank regression slope shown equals 1.3% per minute (P ϭ 0.029) (SYSTAT 12, Systat Software, San Jose, CA). The corresponding value without weighting equals 1.1% per minute (P ϭ 0.014).…”

Awakening arterial blood and end-tidal concentrations of isoflurane in female surgical patients

“…[8] The total body uptake and elimination period of isoflurane are supposed to be proportional to the administered concentrations and duration of general anesthesia according to computer simulation. [21] In this study, the awakening time was 18.5 minutes after discontinuing 1.29 MAC isoflurane, which was longer than 15.5 minutes in pediatric neurosurgical patients receiving maintained 0.9 to 1.0 MAC isoflurane. [5] Furthermore, the actual arterial blood concentration of isoflurane was 1.18% at the end of mean 330-minute cardiac surgery in our previous 20-minute pharmacokinetic study, [10] which was higher than 0.84% in this mean 108-minute study.…”