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Some ventilators may impose higher steady back pressures
of perhaps 100 mm Hg producing more significant
depression of the v/v percentage. The increased patient
uptake of agent with improved ventilation can often
mitigate these effects so as to obviate the need to
compensate for increased back pressure at the vaporizer.
High Back Pressures:
Pressures in excess of 400 mm Hg could conceivably occur
during procedures similar to bronchoscopy or because of
occlusion of downstream tubing and piping or for other
reasons. These effects on v/v percentage cannot be
precisely predicted but the most likely effects will be
reductions in concentration (or small increases).
Back Pressure Fluctuating
Fluctuating back pressures may be imposed on the
vaporizer by downstream components, and assisted or
controlled ventilation to the patient. This can affect the
vaporizer and increase the concentration by intermittently
altering the pressures and hence the flow distribution
within the vaporizer. The greatest effects are observed at
combinations of very low flowrates and low dial setting
with large and rapid pressure fluctuations and become
progressively less important as the dial setting and
flowrate increase and the magnitude and rate of cycling of
the pressure fluctuations decrease.
In clinical use the vaporizers are considered unaffected by
all fluctuating back pressures which would occur under all
normeil clinically encountered conditions appertaining to
human anaesthesia.
Carrier Gas Composition
Small effects can occur when the carrier gas composition is
changed from oxygen to air or nitrous oxide/oxygen
mixture. As a general rule variation of output with carrier
gas composition can be considered of negligible clinical
significance since the effects, if any, are normally less than
10% of setting. Where changes occur the usual effect is
that the output is slightly depressed when nitrous oxide is
employed compared to the output when oxygen is the
carrier gas.
The presence of nitrous oxide reduces the required inspired
concentration of volatile agent and this mitigates this small
depression in output from the vaporizer.
Time Out of Service
If the anaesthetic machine on which the vaporizer is fitted
is left for a period of time with no gases flowing a
concentration of agent may be observable at the machine
outlet when the gas flow is turned 'on' and the vaporizers
turned 'off. This concentration can be expected to faU
rapidly to zero (e.g. within about 15 seconds at 5 litre/min.).
This phenomenon is a normal characteristic of anaesthetic
vaporizers and anaesthetic machines.
Clinically this is normally considered to be insignificant
because of the small volume of vapour involved.
Other Variables
Ambient temperature, input flowrate and duration often
can affect delivered concentrations, particularly when the
vaporizers are used at extremes of the usual clinical range.
The valve design and temperature compensation system of
the Tec Vaporizers reduces the effects to levels when under
most clinical conditions their effect on vaporizing
performance is clinically not significant. The nominal
performance characteristics should be consulted for further
details.
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