2019B05 Outline the safety features of currently used mechanical variable bypass vapourisers.



·      Intro: definitions and how it works

·      Diagramme

·      Table: problem and solution



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




Problems and solutions:



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

·  ?


·  ?



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