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