2019B Question 05
Outline the safety features of currently used mechanical variable bypass vapourisers.
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.
- 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.
- 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
- Intro: Definitions and how it works
- Table: Problem and solution
- Pressure gradient provided by fresh gas supply (cf. inspiratory effort)
- High resistance
|Reason for use||- Most volatile agents are lethal at SVP
e.g. Sevoflurane 160mmHg = 22% ≈ 11 MAC
- Vapouriser chamber producing volatile-saturated gas
- Bypass chamber
- Splitting ratio controlled by a user-controlled dial.
- Vapourising chamber at 39°C → 2atm pressure
- Computer-controlled injection into the bypass stream
Variable Bypass Vaporiser
Problems and Solutions
|Spilling → Liquid in bypass chamber → Supersaturated||- Specifically designed channels prevent spill|
- Specific shape and size of filling hole and plug
- Colour coding of vapouriser and bottle
|Two vapourisers running at once||- Interlock|
- 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)
- Adequate length of tubing between vapouriser and circuit
- Unidirectional valve between vapouriser and circuit
- Inspiratory valve prevents back pressure from patient
|MRI incompatibility||- ?|