Written questions
Model answers available
- A 50-year-old patient undergoes general anaesthesia maintained with volatile inhalational agents.
- (a)Define Minimum Alveolar Concentration (MAC) and its variant, explain its relationship Meyer-Overton hypothesis.3 marks
- (b)Outline the factors that effecting MAC.3 marks
- (c)Describe the factors that dictate the speed of onset of volatile agents.4 marks
- (a)Describe the physical properties, manufacture and storage of nitrous oxide including.3 marks
- (b)Explain the phenomena associated with nitrous oxide administration:
- (i)Concentration Effect1 mark
- (ii)Second Gas Effect1 mark
- (iii)Diffusion Hypoxia1 mark
- (d)Describe about the adverse side effect of nitrous oxide4 marks
- (a)Explain the use of nitrous oxide as an adjuvant to volatile anaesthesia.5 marks
- (b)Discuss the factors determining the safety and efficacy of volatile agents.5 marks
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Questions without a written answer yet
- (a)Using a graph, describe the alveolar elimination (FA/FA0) of desflurane at the conclusion of general anaesthesia compared to other volatile agents.4 marks
- (b)Discuss factors that determine the rate of elimination of desflurane.5 marks
- (c)Outline two (2) specific physicochemical or environmental properties of desflurane that contribute to its potent greenhouse effect.1 mark
Single best answer
8 SBAs on inhalational agents
Viva
4 viva questions
How does nitrous oxide speed up sevoflurane?
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By the second gas effect. Nitrous oxide is given in high concentration and taken up rapidly, which shrinks the alveolar gas volume. That concentrates the sevoflurane remaining in the alveolus and draws additional gas in to replace the lost volume, so the sevoflurane alveolar partial pressure rises faster than its own uptake characteristics would predict.
Why is nitrous oxide contraindicated in a patient with a pneumothorax?
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Because it is about 34 times more soluble in blood than nitrogen. It therefore diffuses into a closed air-filled space faster than nitrogen can diffuse out, so the space expands. In a pneumothorax that risks conversion to a tension pneumothorax. The same reasoning applies to bowel obstruction, intracranial air and middle-ear surgery.
Desflurane is the fastest agent but the least potent. Explain.
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Two different coefficients govern the two properties. Speed depends on blood:gas solubility — desflurane’s 0.42 is the lowest, so alveolar partial pressure rises and falls fastest. Potency depends on oil:gas solubility — desflurane’s 19 is the lowest, so MAC is highest at 6.6%. Low solubility in blood makes it quick; low solubility in lipid makes it weak.
Which volatile would you avoid in a patient with raised intracranial pressure, and why?
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All volatiles cause cerebral vasodilatation and uncouple flow from metabolism in a dose-dependent way, with the effect becoming significant above about 1 MAC — so the principle is to stay below that rather than to pick one agent. Isoflurane is conventionally preferred at equipotent doses, and nitrous oxide is best avoided because it raises cerebral blood flow and would expand any intracranial air.