Influence of fluctuations in pressure
during ventilation
When ventilation is being carried out without fresh gas
de-coupling, pressure fluctuations on the anaesthetic
vaporiser can cause a higher concentration to be
delivered than is shown on the handwheel setting.
The vapour in the vaporising chamber is compressed
when pressure raises, and it expands when pressure
falls. When this effect is strong enough it will pump small
quantities of saturated vapour backwards through the
inlet of the vaporising chamber into the fresh gas.
This pumping effect becomes greater,
– the higher the ventilation pressure and ventilation
frequency,
– the more rapid the fall in pressure during expiration,
– the lower the fresh gas flow,
– the lower the concentration set,
– the smaller the quantity of anaesthetic agent in the
vaporiser.
The ability of the Vapor to compensate for these effects
will reduce them in practise so that the requirements of
DIN 13252 and other Standards can easily be met.
When anaesthetic ventilators are being used, which
provide a continuous supply of fresh gas to the breathing
system (without a buffer for fresh gas de-coupling), and
ventilation pressures are greater than 20 mbar and
concentration set at <1 vol.% and/or a fresh gas flow at
<1 L/min, the Vapor should be filled completely, so that
deviations due to fluctuations in pressure can be kept as
low as possible.
Influence of positive/negative pressure
relative to ambient and dynamic pressure
Vapor's application range is limited to between –100 and
200 hPa relative to the ambient atmospheric pressure at
the Vapor outlet.
Pressure in the Vapor is a little higher than ambient
atmospheric pressure, as the fresh gas flow builds up
dynamic pressures of between 0.5 and 150 mbar in the
flow control system.
When O
2
flushing is activated on Dräger anaesthetic
machines a negative pressure is produced at the Vapor
outlet which may be up to 100 mbar, according to ISO
5358. 100 mbar negative pressure has the same effect
as an increased altitude of 1000 m or a drop in boiling
point of about 3.5
°
C (see "Influence of temperature",
page 33).
37
Description
Influence of fluctuations in pressure
Influence of positive/negative pressure
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