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Nitrous oxide: Closed spaces

Basic pharmacokinetic principles

  • NO has a blood:gas partition ratio of 0.47 and is thus 30 times more soluble in blood than nitrogen, which has a blood:gas partition ratio of 0.015
  • Thus, NO accumulates in closed gas spaces that contain N2 faster than the N2 can diffuse out. In other words, the highly blood soluble NO is brought to the space faster than the poorly soluble N2 can be carried away from the space.
  • Volume of distensible space will increase until the NO concentration in the space is equal to the concentration in the alveoli and blood concentration in volume percent
  • As a consequence, an alveolar concentration of 50% might double the gas space volume and 75% might quadruple the volume
  • This phenomenon is time-dependent based on the location of the air space

Closed gas spaces

  • Compliant spacesPneumothorax – increased volume from NO diffusion into the air space can impair cardiorespiratory function, use of NO is thus contraindicated in presence of pneumothorax
    • Air embolism – expansion in blood occurs rapidly (seconds vs. minutes), increased risk in procedures at risk for embolism (posterior fossa craniotomies, laparoscopy), if suspect air embolism must discontinue NO immediately
    • Equipment cuffs – increased ETT cuff volume can lead to increased pressure on tracheal mucosa, increased volume in catheters (Swan-Ganz) can cause issues if in the pulmonary artery etc., increased volume in LMA will increase volume and exert pressure on surrounding tissue
    • Other compliant spaces include bowel gas (especially in setting of obstruction), pneumoperitoneum
  • Noncompliant spaces
    • Middle ear – can cause adverse effects on hearing after tympanoplasty
    • Pneumocephalus – following dural closure or pneumoencephalography, leads to increased ICP