NITROUS OXIDE

Structure/Class
  1. Gaseous anaesthetic agent
Pharmacokinetics
  1. Poor solubility in both blood and tissues (blood-gas partition coefficient of 0.47).
    • Therefore arterial tension is rapidly reached and there is rapid equilibrium with brain tissues. Also, rapid elimination by the lungs after discontinuation.
    • Hence, rapid onset of action and rapid offset + recovery.
  2. Probably not metabolized by the human body.
  3. Low potency (MAC >100%) and cannot be used as a single agent for induction.
Pharmacodynamics
  1. Acts on numerous receptors in CNS and may interact with endogenous opioid receptors to provide pain relief.
  2. Organ system effects
    • CVS
      • Has myocardial depressant effect in dose dependent fashion.
      • Reduced CO, SV and MAP but increased TPR (helps compensate for the reduction in CO)
      • Effect most marked in patients with limited CVS reserve.
    • Respiratory
      • Increased pulmonary vascular resistance
      • No significant respiratory depression/loss of airway reflexes.
    • CNS
      • In addition to analgesic properties, also has amnestic properties.
Indications
  1. Procedural sedation in ED as single agent (performing reduction/manipulation/suturing)
  2. Adjunct to other anaesthetic agents (e.g. lignocaine)
Contraindications
  1. Severe pulmonary hypertension
    • Avoid in pulmonary oedema/severe mitral stenosis
  2. Pressure effect/trapped gas syndromes
    • Pneumothorax/bowel obstruction/air embolus/severe head injury (increased risk of pneumocranium)
  3. Acute asthma and severe COAD
  4. Patients that require >50% FiO2 to maintain adequate SpO2
Special precautions
Interactions
Adverse events
  1. Prolonged exposure (>2 to 4 hours) will lead to inactivation of vitamin B12
    • Increased risk of megaloblastosis and bone marrow suppression.
  2. Diffusion hypoxia
    • Occurs due to large amounts of N2O diffuse rapidly back to the alveoli from the blood compartment at the end of the case.
    • Usually present after long cases and if high concentration of N2O used.

 
Dosing/administration
  1. Mask inhalation from blue cylinder.
    • Use 50:50 mix of O2 and N2O (no diffusion hypoxia). Can go up to 30:70 mix of O2 and N2O in short cases.
Toxicology
  1. If diffusion hypoxia is present à use 100% oxygen for 20-30 minutes to wash N2O out.

 
Withdrawal syndrome
Special notes