Volatile Agent Identification — FRCA Final MCQ

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HardAnaesthetic Equipment & SafetyVolatile Agent IdentificationFRCA Final

During an anaesthetic equipment viva, a volatile anaesthetic is described as having a normal boiling point of 23.5°C, a saturated vapour pressure of approximately 681 mmHg at 20°C and a blood:gas partition coefficient of 0.42 at 37°C. Its physical properties preclude safe administration from a conventional variable-bypass vaporiser. Which agent is being described?

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Correct answer: A — Desflurane

Desflurane is correct. It has exceptionally low blood solubility, conventionally represented by a blood:gas partition coefficient of 0.42, producing rapid changes in alveolar concentration and relatively rapid emergence. Its boiling point is approximately 23.5°C and its saturated vapour pressure at normal theatre temperature is close to atmospheric pressure. It therefore cannot be delivered safely by a conventional plenum variable-bypass vaporiser and requires a desflurane-specific system. Sevoflurane has a higher boiling point (58.6°C) and blood:gas coefficient of about 0.65. Isoflurane, halothane and enflurane are substantially more blood-soluble; isoflurane has a coefficient around 1.4 and halothane is higher still. The original 50.2°C value was incompatible with the desflurane coefficient and has therefore been corrected.

Reference: Andrews JJ, Johnston RV, Kramer GC. Consequences of misfilling contemporary vaporizers with desflurane. Canadian Journal of Anaesthesia. 1993;40:71–76. https://pubmed.ncbi.nlm.nih.gov/8425247/