Inhalational Anesthetic Agents

Nitrous Oxide, N₂O ("Lauging Gas")

Physical Properties

• colorless
• odorless
• nonflammable but, like O₂, supports combustion
• gas in room temp and ambient pressure; liquid under pressure

Organ System Effects

• direct myocardial depressant effect balanced by
• sympathetic nervous system stimulation
• so BP and P stable
• myocardial depression may be unmasked by
  • CAD
  • hypovolemia
• pulmonary vasoconstriction -> increased PVR

• tachypnea + decreased V_T = stable V_E and p_aCO₂
• BEWARE: inhibits carotid body hypoxic drive

• mildly increases CBF, CBV and ICP
• increases CMRO₂
• analgesia

Biotransformation and Toxicity

• eliminated by exhalation
• irreversibly oxidizes cobalt atom of vitamin B₁₂, inhibiting B₁₂-dependent enzymes:
  • methionine synthetase (myelin formation)
  • thymidylate synthetase (DNA synthesis)
• so prolonged exposure can lead to
  • bone marrow depression (megaloblastic anemia)
  • peripheral neuropathy
  • pernicious anemia
• controversial:
  • emetogenic? (not especially)
  • teratogenic (maybe not, but not necessary in early pregnancy, so usually avoided)

Contraindications

• 35 times more soluble in blood than nitrogen, N₂
• so fills and expands any air-containing cavities:
  • air embolism
  • pneumothorax
  • intracranial air
  • lung cysts
  • intraocular air bubbles
  • tympanoplasty
  • endotracheal tube cuff (monitor and reduce pressure periodically)
• may exacerbate pulmonary hypertension

<-Back Next->

Front Page Residency Program Top