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Inhalational anesthetics

Last updated: November 18, 2020

Summary

Inhalational anesthetics are used for the induction and maintenance of general anesthesia as well as sedation. The exact mechanisms by which they act are still unknown. The most common inhalational anesthetics are sevoflurane, desflurane, and nitrous oxide. Of these, sevoflurane is the most common because of its rapid onset of action and the fact that patients recover quickly from it. Inhalational anesthetics cause respiratory depression, a decrease in arterial blood pressure and cerebral metabolic demand, and an increase in cerebral blood flow. While side effects differ based on the substance (e.g., halothane can cause hepatotoxicity), the most common side effect is nausea.

Overview

  • Inhalational anesthetics provide both analgesia and narcosis and can be used for induction and maintenance of general anesthesia.
  • The exact mechanism of action of inhalational anesthetics is still unknown.
  • The most widely used inhalational anesthetics are:
    • Nitrous oxide
    • Desflurane
    • Sevoflurane
    • Isoflurane
    • Enflurane (no longer marketed in the US)
    • Halothane (no longer marketed in the US)
    • Methoxyflurane

References:[1]

Pharmacokinetics and pharmacodynamics

Pharmacokinetic principles

Pharmacodynamic principles

  • Measure of potency of inhalational anesthetics: minimal alveolar concentration (MAC)
    • MAC is the fraction of volume of the anesthetic present in the inspired air that provides sufficient analgesia in 50% of patients, meaning that patients will not respond to an extremely painful stimulus such as surgical skin incision.
    • MAC is inversely related to anesthetic potency (potency = 1/MAC) and represents the ED50 value.
    • The lower the MAC value, the more fat soluble the anesthetic. For example:
      • Halothane has a slow induction and high potency because of its high lipid and blood solubility.
      • Nitrous oxide (N2O), in contrast, has a fast induction and low potency due to low lipid and blood solubility.

Pharmacokinetics and pharmacodynamics of common inhalational anesthetics

References:[1][2][3][4]

Pharmacodynamics

General effects

Specific characteristics of common inhalational anesthetics

Specific characteristics
Nitrous oxide
  • Can cause expansion of gas trapped in a cavity
  • Usually insufficient if used alone → often combined with a more potent inhalational anesthetic to achieve the “second gas effect”
  • Rapid onset and recovery
Desflurane
  • Very rapid onset and recovery
  • Pungent odor; irritates airways → not suitable for induction of anesthesia
Sevoflurane
  • Most commonly used inhalational anesthetic
  • Rapid onset and recovery
  • Nonpungent → suitable for induction of anesthesia
Isoflurane
  • Most potent of the fluranes
  • Relatively slow onset and recovery
  • Pungent odor → not suitable for induction of anesthesia
Methoxyflurane
Enflurane
  • Seizures (proconvulsive)
  • Medium speed of onset and recovery
Halothane
  • Hepatotoxicity
  • Medium speed of onset and recovery

References:[1][2][3]

Adverse effects

References:[1][3][5][7][8]

We list the most important adverse effects. The selection is not exhaustive.

References

  1. King A, Weavind LM, Joshi GP, Nussmeier NA. General Anesthesia: Induction. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/general-anesthesia-induction.Last updated: September 7, 2017. Accessed: September 25, 2017.
  2. Le T, Bhushan V. First Aid for the USMLE Step 1 2015. McGraw-Hill Education ; 2014
  3. Katzung B,Trevor A. Basic and Clinical Pharmacology. McGraw-Hill Education ; 2014
  4. Drug record - Halothane. https://livertox.nih.gov/Halothane.htm. . Accessed: October 6, 2017.
  5. Halothane Hepatotoxicity. http://emedicine.medscape.com/article/166232-overview. Updated: October 17, 2016. Accessed: October 12, 2017.
  6. ASA. Practice guidelines for preoperative fasting and the use of pharmacologic agents to reduce the risk of pulmonary aspiration: Application to healthy patients undergoing elective procedures. Anesthesiology. 2011; 114 (3): p.495-511. doi: 10.1097/aln.0b013e3181fcbfd9 . | Open in Read by QxMD
  7. Eger EI II . Characteristics of anesthetic agents used for induction and maintenance of general anesthesia. Am J Health Syst Pharm. 2004; 61 (20).
  8. Wenker OC. Review of Currently Used Inhalation Anesthetics; Part I. The Internet Journal of Anesthesiology. 1999; 2 (3).