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

Structure:

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

Introduction

Variable Bypass Vaporiser

Desflurane Vaporiser

Problems and Solutions