Ans: A (Nitrous oxide) Ref: Ajax Yadav, Short Textbook of Anesthesia, 1st edition, pg 59.Explanation:Nitrous oxide produces Aplastic anemia and Megaloblastic anemiaNITROUS OXIDE (LAUGHING GAS)Stored as liquid in blue color cylinderE5 times more heavier than air35 times more soluble than nitrogenAnesthetic propertiesIt is not a complete anesthetic.It is used as a carrier gas to other anestheticsMAC is 104% (It is not possible to deliver, so not a complete anesthetic).Maximum concentration which can be given is 66%.A minimum of 33% of Oxygen has to be given (Otherwise can cause severe hypoxia)It is usually given as N20 66% and 02 33% mixtureBlood gas coefficient is 0.47 making it agent w ith faster induction and recoveryGood analgesicNot a muscle relaxantWhen given with other inhalational agent, it increases the concentration of that agent (Second gas effect) and also its own concentration (Concentration effect)Sudden stoppage can lead to Diffusion Hypoxia (Should always be tapered slowly by giving 100% oxygen)MetabolismIt is not metabolized in human body.It is excreted unchanged in lungsSmall amount is excreted via skin (Percutaneous excretion)Impurities in N20 cylinder can cause severe laryngospasm, methhemoglobinemia and Pulmonary edemaSystemic ActionsCVS: It does not depress the myocardium in vivo, so it can be safely used in patients with cardiac disease and shockCNS: It increases cerebral metabolic rate and raises intra cranial tensionRS: It causes tachypnea and decrease tidal volume, maintaining minute volumeVentilatory response to hypoxia is bluntedNo significant effect on Liver and KidneysSide EffectsHematological: It inactivates vitamin B12 (If used for >6 hours) - defective folate metabolism - Bone marrow depression - Aplastic anemia and Megaloblastic anemiaCNS: Vitamin B 12 deficiency can cause Demyelination of spinal cord (Subacute degeneration of spinal cord) especially involving posterior columns and lateral spinothalamic tractsClosed Gas Spaces: Compliant spaces like gut. penumoperitoneum, pneumothorax and non- compliant spaces like middle ear cavity, pneumoencephalum can develop very high pressure following nitrous oxide inhalation.This happens because for I mole of nitrogen removed. 35 moles of nitrous oxide enter (35 times more soluble than nitrogen.Therefore, Nitrous oxide is contraindicated inPneumothoraxPneumoperitoneumPneumoencephalumMiddle ear surgeryPosterior fossa surgeryLaproscopic surgeryAcute intestinal obstruction and volvulus of gutEye surgeriesUses in AnesthesiaAs a supplement to anesthesia (not a complete anesthetic)As a carrier gas for inhalational agentsAs analgesic in obstetrics, dental pain, burn dressing and acute trauma