2001B10 Briefly describe the adverse effects of nitrous oxide.

Impurities

·   Manufactured by superheating NH₄NO₃ to 250°C

·   Products: N₂O plus NH₃, N₂, NO, NO₂

·   Removal of impurities by scrubbers may be incomplete

o e.g. NO₂: airway damage, headache, heart failure

Storage

·   Need to reduce filling ratio from 0.75 to 0.67 in hot climates

Entonox lamination

·   Occurs below pseudocritical temperature (~-6°C)

·   Results in delivery of hyperoxic then hypoxic mixture

Supports combustion

·   Like O₂

·   Not itself flammable

Absorption

·   N/A, concentration effect beneficial

Distribution

·   N₂O 30x more soluble, BGPC 0.47; diffuses rapidly into air spaces

·   N₂ 30x less soluble, BGPC 0.014; diffuses slowly from air spaces

·   Hence expansion of existing closed air spaces

o ↑size pneumothorax

o ↑Bowel gas -> difficult laparoscopy

o ↑Ocular gas (of ocular surgery with SF6?)

o ↑Pneumocephalus (if brain surgery)

Metabolism

·   Minimal, 0.004%

Excretion

Diffusion hypoxia:

·   Reverse of the concentration effect

·   Switch from N₂O/O₂ to N₂/O₂

·   Rapid output N₂O into alveolus, but very slow uptake of N₂

·   Dilution of alveolar O₂ -> hypoxia

·   Avoided by high %O₂ during washout

Cardiovascular

·   Direct: ↓heart rate, ↓contractility, ↓SVR

·   Via ↑SNS output: ↑HR, ↑contractility, ↑SVR

Respiratory

·   ↑RR, ↓TV

·   ↑SNS output -> ↑PVR -> ↑RV strain

CNS

·   MAC 105%, MAC-awake 67%; cannot be used alone as anaesthetic

·   Pro-emetic

·   ↑CBF, ↑ICP if susceptible

·   Minimal effect on EEG, can’t be monitored with BIS

·   Euphoria, abuse potential

Mutagenesis

·   Oxidises the cobalt atom in Vitamin B₁₂, a cofactor for methionine synthetase

·   Impaired DNA synthesis

·   Teratogenesis

·   Subacute combined degeneration of the cord (chronic abuse)

·   ? ↑Risk abortion in PACU staff