2017B02 Using a labelled diagramme, describe how a mechanical (non-cassette) variable-bypass vapouriser
achieves the concentration set on the dial. Describe the mechanisms that compensate for temperature
and downstream pressure changes.

Diagramme

Vapouriser stream

·  Gas passes through volatile liquid; evaporation till SVP reached

·  Ensure saturation: wicks, baffles, formerly bubbling gas through liquid

·  SVP sevoflurane ~22%, lethal if not diluted

Bypass stream

·  Does not pass through volatile liquid

Control of splitting ratio

·  User-controlled dial alters resistance in vapouriser channel, corresponds to desired % or partial pressure

·  e.g. splitting ratio 12:1 -> ~2% sevoflurane

·  A bimetallic strip controls resistance to flow through the bypass channel

·  Note device is less precise at very high >15L/min or very low <0.5L/min FGF rate

Need for controls

·  SVP of many drugs is deadly, e.g. desflurane 660mmHg

Why?

·  Latent heat of vapourisation -> ↓ temp -> ↓SVP -> ↓volatile partial pressure

·  (Gay-Lussac’s law: ↓temp -> ↓SVP, non-linear relationship)

Bi-metallic strip

·  Controls resistance to flow through bypass channel

·  Two metal strips with different coefficients of thermal expansion

·  ↓temp in device -> ↑resistance to flow in bypass channel -> restored partial pressure volatile

Heat sink (copper)

·  Provides latent heat of vapourisation

·  Has high SHC: i.e. supply lots of heat -> minimal ↓temp

·  Has high thermal conductivity -> ↓minimal lag

Why

Pumping effect:

·  ↑downstream pressure -> gas forced back into vapouriser ± fresh gas supply -> resaturation

·  ↑↑Volatile concentration -> CVS depression, death

Prevention

·  Check valve between vapouriser and circuit (↓ back flow)

·  Reasonable length of tubing between vapouriser and circle

·  Inspiratory valve in circuit (↓ backpressure during expiration)

·  Pressure-limiting valves in circuit (APL valve if spont vent, APL bypass if PPV)