28. Vaporisers Flashcards
Classify the types of vaporiser in use.
A vaporiser is a device used during inhalational anaesthesia to administer a given concentration . of a volatile anaesthetic agent. There are various types on the market and they can be classified as follows:
- Variable Bypass Vaporisers
a Plenum Vaporisers
Halothane Enflurane Isoflurane Sevoflurane e.g. Tec 5® & Tec 7® - (GE) Vapour 2000® - (Drager) SIGMA DELTA® - (Penlon)
b
Plenum Vaporisers
with
Electronic Control
Halothane
Enflurane
Isoflturane
Sevoflurane
Aladin Cassette Vaporiser® - (GE)
- Measured Flow Vaporisers
a Desflurane Vaporiser
Tec 6® - (GE)
D Vapour 2000® -
SIGMA ALPHA® - (Penlon)
b Direct Injection of Volatile Anaesthetic
Vaporiser (DIVA
Halothane Enflurane Isoflurane Sevoflurane Desflurane
The Drager DIVA®
The Maquet 950® Series
How do variable bypass plenum vaporisers work?
Variable bypass vaporisers work
as the name suggests.
There are two possible paths for
fresh gas to flow through the vaporiser:
via the vaporising chamber itself or
via the bypass pathway.
Gas, which enters the vaporising chamber,
becomes fully saturated with vapour.
As it exits the vaporiser it is
reintroduced to the vapour-free
bypass gas and the two mix.
This mixture is then delivered to the patient.
The resulting concentration of
volatile agent present in the
mixture depends on
how much fresh gas
went through each
of the pathways.
The path of the fresh gas flow is
determined by the
‘splitting valve’,
which is attached to the control dial
on the outside of the vaporiser.
This dial is calibrated from
0 to 5% for isoflurane and
0 to 8% for sevoflurane.
When it is turned to zero,
the valve is closed and
no fresh gas flows through the
vaporising chamber.
As the anaesthetist turns the
control dial to deliver a
higher concentration of volatile,
the splitting valve opens wider,
allowing a greater proportion of the
fresh gas flow to travel
though the volatile chamber.
The ratio of fresh gas flowing through the chamber to that flowing via the bypass pathway is called the ‘splitting ratio’.
What are the potential problems
with this device and how are they
overcome?
1.
high fresh gas flow
2.
temperature
3.
pumping effect
4.
Incorrect anaesthetic
5.
Over-filling
6.
Tipping.
A high fresh gas flow through the vaporiser
A high fresh gas flow through the vaporiser
could affect its output
because it may result in insufficient vapour
being available to fully saturate
the fresh gas passing through the chamber.
Inside the vaporising chamber a series of wicks and baffles are dipped into the volatile liquid. . This greatly increases the surface area of volatile anaesthetic exposed to fresh gas flow, ensuring that the gas leaves the chamber fully saturated.
In this way, the output concentration
is independent of flow.
Temperature
As an anaesthetic liquid turns to
vapour it absorbs energy
Consequently, there is a fall in
the temperature of the
liquid in the chamber,
which leads to a decrease in
the rate of vaporisation because
fewer molecules will
have sufficient energy to evaporate.
This leads to a fall in the SVP
of the volatile and
so to a fall in the concentration of
anaesthetic agent delivered to the patient.
This effect is more marked at
high flow rates when the
rate of vaporisation increases.
Plenum vaporisers are
not electrically heated however,
their casing contains copper,
which is a very good conductor of heat
from the environment and so
conducts energy to the liquid
as it cools, helping to mitigate this effect.
The addition of a ‘bimetallic strip’ helps
to compensate for fluctuations in
output due to temperature.
As the chamber cools, the two different metals comprising the strip contract to different degrees and cause the strip to bend.
This increases the splitting ratio
of the free gas flow
as the temperature drops
and vice versa.
The ‘pumping effect’
The ‘pumping effect’.
Positive pressure ventilation of the patient will cause intermittent pressure changes, both upstream to the patient (desirable) and downstream to the vaporiser (undesirable).
If positive pressure is transmitted to the vaporiser chamber, it can result in gas saturated with vapour being displaced ‘backwards’ and into the bypass channel.
As the positive pressure is
released, there will be an expansion
of gas forward towards the patient.
When the vapour from the usually vapour-free bypass channel mixes with the fully saturated gas from the vaporiser chamber it will result in an increase in the concentration of anaesthetic agent delivered to the patient.
A non-return valve is inserted at the
outlet of the vaporiser.
The vaporiser is designed to have a high internal resistance, to resist the
changes in flow caused by
positive pressure ventilation.
Incorrect anaesthetic liquid introduced
Incorrect anaesthetic liquid introduced
to vaporiser.
Standardised colour coding of vaporisers and bottles (sevoflurane – yellow, isoflurane – purple, desflurane – blue),
and keyed fillers reduce this risk.
Over-filling
Over-filling can cause overdose
and spillage of anaesthetic liquid
onto the patient circuit is potentially fatal.
Low filling ports help to reduce the
risk of overfilling. Transparent window with a
‘fill line’ is visible on the front of the vaporiser.
Tipping.
Tipping.
If the vaporiser tips past 45°
anaesthetic liquid can obstruct the
valves and result in very high
concentrations of vapour being
delivered to the patient.
Take care when moving vaporisers.
Regularly check the seating
of the vaporiser
on the back bar.
Describe the plenum vaporisers with electronic control.
These vaporisers are manufactured by GE,
who have called them
‘Aladin cassettes’.
Although these cassettes look very different from the standard plenum vaporisers, they function in essentially the same manner and are colour-coded in the standard way.
They can supply desflurane.
Each cassette is a sump for
anaesthetic liquid and the
concentration of anaesthetic delivered
to the patient depends
on the splitting ratio of the free
gas flowing through the cassette,
just as in the ‘ordinary’ plenum vaporisers.
Each different cassette plugs into a single slot in the front of the anaesthetic machine during use (i.e. one cassette is removed and replaced with another to change anaesthetic agent)
and when it is inserted,
it pushes open an inflow
and an outflow valve.
The electronic control mechanism
is situated inside the anaesthetic machine
and the anaesthetist uses a digital display to programme the machine to
deliver a specified concentration
of anaesthetic or
to target an end tidal concentration
of anaesthetic agent.
These vaporisers are portable,
can be tipped and are maintenance free but
they cannot be used without power.
Why is it necessary to have
a special vaporiser to deliver
desflurane?
The problem with des vs other agents
The physical properties of desflurane
made it necessary to design its unique vaporiser.
Desflurane is extremely volatile and
its boiling point is 23 °C at atmospheric pressure,
i.e. around room temperature.
Because of its volatility, small changes in ambient temperature would result in large changes in desflurane’s saturated vapour pressure (SVP) inside the vaporisation chamber
and this would affect the
concentration of anaesthetic agent
delivered to the patient
This is not a problem with other volatile anaesthetic agents, because their boiling points are well above room temperature and so small variations in ambient temperature in theatre do not have a clinically significant effect on the SVP inside the vaporising chamber.
Overcoming the problem with desflurane
To overcome this problem,
the desflurane vaporiser heats
the anaesthetic agent to precisely 39 °C
to ensure a constant SVP.
Rather than free gas flowing
into the vaporiser as in the plenum vaporisers,
the anaesthetic vapour is injected
into the free gas flow downstream
of the vaporisation chamber.
The anaesthetist will control
the concentration of desflurane delivered
to the patient using a
dial calibrated from 0 to 12%.
As the setting of the dial increases,
resistance to the flow of desflurane
into the fresh gas flow decreases and
more is injected, and vice versa.
The rate of injection of desflurane must be adjusted according to the fresh gas flow otherwise turning the gas flow up would result in a dilution of the anaesthetic agent in the final gas mixture. This coupling is achieved by an electronic control unit in the vaporiser.
DIVA
The DIVA is a measured flow meter
that can give all types of anaesthetic
agent, including desflurane.
In simple terms, the anaesthetic is
heated to a specific temperature in an
evaporation chamber before the
vapour is passed into the patient gas circuit.
As in the desflurane vaporiser,
a microprocessor
couples fresh gas flow to the
rate of injection of the anaesthetic agent.
What considerations should be
taken when building a vaporiser?
The properties of the anaesthetic
to be delivered should be taken into
account.
These are explained above, but listed below:
- Saturated vapour pressure at room temperature
- Boiling point at atmospheric pressure
• MAC of anaesthetic agent:
range on dial must increase with MAC
such that:
o Isoflurane MAC = 1.15, range 0–5% on dial
o Sevoflurane MAC = 2.10, range 0–8% on dial
o Desflurane MAC = 6.00, range 0–12% on dial
How are vaporisers affected by
altitude?
What is dialed up
what determines whether patient anaesthetised
Plenum vaporisers (e.g. Tec 5 and 7)
Although we dial up the percentage of anaesthetic agent we want to deliver to the patient, it is not actually the percentage concentration of volatile being inhaled that determines whether a patient is anaesthetised,
but the partial pressure of that volatile.
Because we usually work at sea
level at a pressure of 1 atmosphere,
the values for ‘percentage concentration’
and ‘partial pressure’ delivered are
happily inter-changeable (see proof below).
