Anaes · Vaporisers
Vaporisers
Also known as Vaporiser · Plenum vaporiser · Variable-bypass · Draw-over vaporiser · Desflurane vaporiser · Temperature compensation
The vaporiser vaporises the liquid volatile anaesthetic agent and adds a precise concentration to the fresh gas flow. The framework rests on four exam-critical ideas: the variable-bypass plenum vaporiser (the standard for sevoflurane and isoflurane — a fraction of the gas is saturated in the chamber and diluted by the bypass); the temperature compensation (the bimetallic strip or the bellows compensates for the cooling of the vaporisation); the desflurane exception (its near-atmospheric saturated vapour pressure demands a heated, pressurised vaporiser); and the safety systems (the agent-specific filling, the interlock, the flow-resistance). Built on the AAGBI checking guidelines (2012), the anaesthetic machine assessment (Tiviraj 2016), and the closed-circuit review (Parthasarathy 2013).
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Red flags
- Desflurane boils at 22.8 degrees Celsius — its saturated vapour pressure (669 mmHg at 20 degrees) is near atmospheric. A standard variable-bypass vaporiser could let it boil and deliver a dangerously high concentration. Desflurane MUST use a heated, pressurised vaporiser.
Meet the patient
You are checking the anaesthetic machine at the start of a list when a new registrar asks why the desflurane vaporiser lives in its own heated box and needs a power lead. The answer is one physical fact — and that fact is the difference between a safe induction and a lethal one.[1]
Hold two questions and every vaporiser viva falls into place: how does the dial control the concentration? (the splitting ratio) and why does desflurane need a different machine? (it boils at room temperature). Everything below is a refinement of those two.[1]
References3ShowHide
- [1]Hartle A, Anderson E, Bythell V, et al. Checking anaesthetic equipment 2012: association of anaesthetists of Great Britain and Ireland Anaesthesia, 2012.PMID 22563957
- [2]Tiviraj S, et al. Development of An Assessment Test for An Anesthetic Machine J Med Assoc Thai, 2016.PMID 27501622
- [3]Parthasarathy S The closed circuit and the low flow systems Indian J Anaesth, 2013.PMID 24249885