0 N2O

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Nitrous Oxide (N2O) (Laughing gas)

This is the only inhalational anaesthetic that causes Pulmonary Vasoconstriction.


  • Made by heating NH4 Nitrate at 245 - 270 degree C
  • Blue Cylinder
  • Colorless and odorless
  • Non inflammable but like O2 supports combustion.
  • It is Gas in room temperature and ambient pressure , but is liquid under pressure.
  • 35 times more soluble in blood than N2
  • 1.5 times heavier than air.
  1. BP and Pulse Stable
  2. Inhibits Carotid body hypoxic drive.
  3. Analgesia (equivalent to 50mg Pethidine)
  4. Elimination by exhalation

Irreversibly oxidizes Cobalt atom of Vit B12 , inhibiting
  • methionine synthetase (myelin formation) 
  • thymidylate synthetase (DNA synthesis)
Therefore prolong :
  1. Bone marrow depression
  2. Peripheral neuropathy
  3. Pernicious Anaemia 


Contraindications : (because fills and expands any air cavity)

  1. Air Embolism
  2. Intracranial Air
  3. Pneumothorax
  4. Lung Bulae/cysts
  5. Intraocular air bubbles
  6. Tympanoplasty
  7. ET tube cuff.

May exacerbate pulmonary hypertention.

Concentration effect : 

Rapid uptake of N2O into blood allows more N2O to enter the blood, thus shortening induction time.
It also Increases uptake of other inhalational agents, this is "Second gas effect".


Diffusion Hypoxia = Fink's effect = 3rd gas effect
  • in old and debilitated patients.
  • at time of reversal of anaesthesia.
When a patient is recovering from N2O anaesthesia, large quantities of this gas cross from the blood into the alveolus (down its concentration gradient) and so for a short period of time, the O2 in the alveolus is diluted by this gas. This could potentially cause the partial pressure of oxygen to decrease and could temporarily lead to hypoxia.

Avoid it by giving 100% O2 at the end of anaesthesia.





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