Inhalational Anesthesia - Nitrous Oxide

Inhalational Anesthetic Agents

Nitrous Oxide, N₂O ("Lauging Gas")

Nitrous Oxide

Physical Properties

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

Organ System Effects

Cardiovascular

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

Respiratory

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

CNS

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

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Greg Gordon MD
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