2019B Question 05

Outline the safety features of currently used mechanical variable bypass vapourisers.

Examiner Report

62.6 % of candidates achieved a pass in this question.

Two important domains needed to be addressed in the answer. Both of these relate to ensuring that a known, reliable concentration of vapour is delivered from the vaporiser. It was difficult to pass without addressing both of these key areas.

  1. Vaporisers are only compatible with a specific volatile agent. This is because each agent has a different saturated vapour pressure. Safety measures to achieve this include: calibration of the vaporiser and splitting ratio for the specific SVP; geometrical shape of filling ports and bottles; labelling; and colour coding.
  2. Vaporiser output remains constant across a range of operating conditions. This requires temperature, flow, and pressure compensation. Each one of these needed to be explained in terms of how it works and why it is a safety feature.

Other safety features that gained marks included: only one vaporiser can be turned on at once; the maximum setting on the control dial is well below SVP; the control dial has to be pushed in before it can be turned on; anti-spill mechanism; a fluid level indicator window; leak test using suction bulb on common gas outlet as part of machine check; MRI compatibility. Beginning the answer with a simple definition of “variable bypass” was sensible, and scored some marks.

The following were areas of difficulty:

  • Lengthy explanations of how a vaporiser works, complete with diagrams, but without connecting any of this information with safety
  • Temperature compensation was often poorly described – many candidates could not explain the concept of a heat sink, and the metal vaporiser casing was often described as “insulation”
  • Referring to volatile agents as “gas”
  • Referring to the different geometrical shape of filling ports as the “pin index” system
  • Desflurane vaporisers were often included – these are not variable bypass
  • Vaporisers were described as being part of the anti-hypoxia safety mechanism
  • A variety of (non-existent) high and low pressure alarms were variably described
  • It was common for candidates to list multiple other safety features of anaesthetic machines generally – only those related to vaporisers could earn marks

Model Answer


  • Intro: Definitions and how it works
  • Diagram
  • Table: Problem and solution


Factor Details
Key features

- Pressure gradient provided by fresh gas supply (cf. inspiratory effort)

- High resistance

- Unidirectional

- Agent-specific

Reason for use - Most volatile agents are lethal at SVP
e.g. Sevoflurane 160mmHg = 22% ≈ 11 MAC
Tec 5

- Vapouriser chamber producing volatile-saturated gas

- Bypass chamber

- Splitting ratio controlled by a user-controlled dial.

Tec 6

- Vapourising chamber at 39°C → 2atm pressure

- Computer-controlled injection into the bypass stream

Variable Bypass Vaporiser

Desflurane Vaporiser

Problems and Solutions

Problem Prevention
Spilling → Liquid in bypass chamber → Supersaturated - Specifically designed channels prevent spill
Wrong drug

- Specific shape and size of filling hole and plug

- Colour coding of vapouriser and bottle

Two vapourisers running at once - Interlock
Disconnection, leak

- Same interlock device

- Machine self-testing

Empty → No volatile delivery

- Alarms on machine (low MAC)

- Alarms on vapouriser: Desflurane

Inaccuracy at extremes of flow rate

- Very high: Wicks, baffles, bubbling ensures proper saturation

- Very low: Machine limits low flow rate e.g. 0.2L/min

↓ Temp → ↓ SVP → ↓ %

- Bimetallic strip: Different coeff of thermal expansion

- Copper heat sink: High SHC and high thermal conductivity

- Tec 6: Heat to 39°C → 2 atmospheres → Injection (BP 24°C, close to room temp, would be dangerous if unheated and uncontrolled)

Pumping effect

- Adequate length of tubing between vapouriser and circuit

- Unidirectional valve between vapouriser and circuit

- Inspiratory valve prevents back pressure from patient

MRI incompatibility - ?
Transport - ?

Last updated 2021-08-23

results matching ""

    No results matching ""