Inhaled Anaesthetics - Properties, N2O and Halothane MCQs

Malignant hyperthermia produced by halothane and isoflurane in rare (genetically predisposed) individuals is more common in patients receiving SCh as well.

Desflurane It has minimum blood gas partition coefficient and therefore is the fastest inducing agent. It has very high vapour pressure . Its boiling point is 23°C; therefore it boils at room temperature. It requires special vaporizers due to this property. It produces cardiovascular effects similar to isoflurane except coronary steal phenomenon. Induction with desflurane is unpleasant. It causes maximum irritation to airways . As it can lead to coughing, breath holding and laryngospasm. It can also be used in day care surgery.

Ans. A. 50 : 50 Entonox contain equal amount (50/50) of N2O and O2.

Ans. D. -36.5°C Critical temperature of N2O - 36.5 Critical pressure of N2O - 214.7 atm

In the presence of N 2 O, air bubbles and air filed spaces will increase in size as N 2 O is 35 times more soluble compared to molecular nitrogen (the major component of air), allowing it to diffuse into an air bubble more rapidly than nitrogen is absorbed out of the bubble. If the bubble expands after the incision is closed, intraocular pressure will rise. This complication can be avoided by discontinuing N 2 O prior to the bubble injection. Option a, c and e: increasing the depth of anesthesia, hyperventilation and muscle relaxation have no role in this scenario. They do help in reduction in intraocular pressures in some scenarios. Reference: Drugs in Anesthesia and Intensive Care, Edward Scarth, Susan Smith 5th edition pg 277.

Among the given options, isoflurane is best suited for neurosurgery All the inhalational anaesthetics are cerebral vasodilators The order of vasodilating potency is halothane →enflurane → desflurane → isoflurane → sevoflurane Though isoflurane increases cerebral blood flow and intracranial pressure these effects are less pronounced than with halothane, nitrous oxide, and ether Though inhalational anaesthetics are used in neuro anaesthesia, if the intracranial pressure is persistently raised →they should be replaced with intravenous agents Earlier, it had been believed that isoflurane is cerebro-protective in the presence of ischemia, but now no such evidence of cerebral protection has been documented and there is no difference among the inhaled anaesthetics in this respect Reference : Drugs in Anaesthesia and Intensive Care, Edward Scarth, Susan Smith 5th edition

Minimum alveolar concentration (MAC) of an inhaled anaesthetic is the lowest concentration which is required to prevent movement in response to a defined noxious stimulus in 50% of subjects It is a measure of potency of the inhalational anaesthetics So, lower the MAC value, lower is the concentration of anaesthetic drug required which would imply the drug is more potent. Example: Nitrous oxide with a MAC value of 104 is the →least potent inhaled anaesthetic Methoxy flurane - MAC- 0.2 →most potent Other drugs: Desflurane: 6.0 Sevoflurane- 2.05 Isoflurane- 1.28 Halothane - 0.75 (since methoxyflurane is no longer in use, among the commonly used inhalational agents, halothane is the most potent ) Reference : Drugs in Anaesthesia and Intensive Care, Edward Scarth, Susan Smith 5th edition; Stoelting's pharmacology and physiology in anaesthetic practice 3th edition

Meyer Overton Rule states that the potency of inhalation agents correlates directly with their lipid solubility - the fat/ water partition coefficient Anaesthesia results from dissolving of the drug molecules at specific lipophilic sites but not all lipid soluble molecules are anaesthetics Exception to Meyer Overton Rule: Non- immobilizers (Non- Anaesthetics) Nonimmobilizers are compounds whose properties are like that of inhaled anaesthetics but do not induce immobility at their expected concentrations Reference : Stoelting's pharmacology and physiology in anaesthetic practice 3th edition