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N2O (Nitrous Oxide)

  1. Also called as ‘laughing gas’ (name given by Humphry Davy). It was first prepared by Joseph Priestley in 1774.

Nitrous oxide is not use in any surgery working in cavity e.g pleural cavity, middle ear cavity, emphysematous blebs, Due to properties of nitrous to expand in cavity, it will not allow to fix any implant to stay because of presence of gas throughout the surgery. (AIIMS Nov 08)

  1. Two ‘L’
    1. Laughing gas
    2. Lighter than air
  2. High solubility in blood
  3. Used as Entonox - 50% N2O + 50% O2
  4. Pin index 3;5
  5. Five 'P'
    1. Priestly discovered in 1846
    2. Pointing effect
    3. In practice safest anaesthesia
    4. Poor muscle relaxant relieves pain only
    5. Not used in Pneumo conditions as pneumothorax etc.
  6. S/E:
    1. Bone marrow depression
    2. Second Gas effect or Diffusion hypoxia
    3. Megaloblastic anemia
  7. Preparation
    1. Prepared by heating ammon
    2. ium nitrate between 245 to 270oC.
  8. Physical properties
    1. It is colourless, non irritating and sweet smelling.
    2. Critical temperature is 36.5oC which is above room temperature, therefore it can be kept in liquefied state.
    3. Stored as liquid in blue colour cyclinder (cylinder pressure 760 psi.)
    4. 1.5 times heavier than air.
    5. 35 times more soluble than nitrogen.
    6. It is not a complete anaesthetic. It is used as a carrier gas to other inhalational agents. 
    7. MAC is 104% which is not possible to deliver so it is not a complete anaesthetic.
    8. Maximum concentration of nitrous oxide which can be given is 66% (or a minimum of 33% oxygen is to be given other wise severe hypoxia can occur).
    9. Blood gas coefficient is 0.47 making it agent with faster induction and recovery. 
    10. It is non inflammable, non explosive.
    11. Good analgesic
    12. Not a muscle relaxant
  9. Systemic Actions
    1. Cardiovascular System:
      In vitro it depresses myocardium but in vivo this effect is not seen because of stimulation of sympathetic system, so it can be used safely for cardiac patients and patients in shock.
    2. Cerebral: Increases cerebral metabolic rate and raises the intracranial tension:
      Respiratory System: It causes tachypnea and decrease in tidal volume, maintaining the minute volume. Ventilatory response to hypoxia is blunted.
    3. Liver: Does not effect hepatic system significantly.
    4. Immunologic System: Effects chemotaxis and motility of leukocytes.
  10. Side Effects
    1. Hematological system: it inactivates Vitamin B12 so impaired methionine and deoxythymidine synthesis which leads to defect in folate metabolism and therefore bone marrow depression causing aplastic and megaloblastic anemia. Vitamin B12 inactivation is seen in nitrous oxide is used for more than 6 hours.
    2. Neurological System: Vitamin B12 deficiency can cause demyelinating lesion of spinal cord (subacute degeneration of spinal cord) especially posterior columns and lateral spinothalamic tracts.
  11. Contraindicated in:
    1. Pneumothorax: Nitrous oxide doubles the pneumothorax volume in 10 minutes and triples in 30 minutes producing severe cardiorespiratory embarrassment.
    2. Pneumoperitoneum
    3. Pneumoencephalum: Entry of nitrous oxide can significantly increase the intracranial tension and once the pneumoencephalum has developed nitrous oxide should not be used for 1 week.
    4. Middle ear surgery and tympanoplasties: Pressure in middle ear cavity may rise to 20 to 50 mmHg which can displace the graft
    5. Laparoscopic surgeries: Chances of air embolism are high in laparoscopies.
    6. Acute intestinal obstruction and volvulus of gut: Increases gut distension.
    7. Eye surgeries: where sulphur hexafluoride is to be used.

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