Ans. is 'b' i.e., N2O Nitrous oxide N2Oo It is colourless, odourless, heavier than air, nonirritating and noninflammable gas.o It is also called laughing gas.o MAC is 105% (least potent) - even pure N2O at 1 atmospheric pressure can not produce adequate anaesthesia.o It has good analgesic but poor muscle relaxant activity.o It is supplied under pressure in blue coloured steel cyclinders.o It has very low blood solubility - induction is quick and smooth with rapid recovery.o Second gas effect and diffusion hypoxia occur with N2O only.o N2O is generally used as a carrier and adjuvant to other anaesthetics - A mixture of 70% N2O + 25 - 30% O2 + 0[?]2 - 2% another potent anaesthetic is employed for most of the surgical procedures.o Entonox is a mixture of 50% N2O and 50% O2.o It has little effect on respiration, heart and BP.o It can cause bone marrow depression and vit B12 deficiency.o N2O is the only anaesthetic reported to produce hematologic toxicity and neurotoxicity with long term administration.o Both toxicides are the result of the interaction of N2O with vit B12.o Complete bone marrow failure can be expected after several days of continuous exposure.o Bone marrow changes are preventable by pretreating patients with large doses of folinic acid.o Other manifestations of vit B12 deficiency eg. megaloblastic anemia, subacute combined degeneration of cord may also occur.o Methemoglobinemia and laryngospasm may occur.o It is contraindicated in patients with air cavities like pneumothorax, pneumoperitoneum and volvulus - expansion and increase in pressure occur.