28. Vaporisers Flashcards

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1
Q

Classify the types of vaporiser in use.

A
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:
  1. 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)

  1. 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

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2
Q

How do variable bypass plenum vaporisers work?

A

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’.
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3
Q

What are the potential problems
with this device and how are they
overcome?

A

1.
high fresh gas flow

2.
temperature

3.
pumping effect

4.
Incorrect anaesthetic

5.
Over-filling

6.
Tipping.

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4
Q

A high fresh gas flow through the vaporiser

A

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.

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5
Q

Temperature

A

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.

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6
Q

The ‘pumping effect’

A

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.

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7
Q

Incorrect anaesthetic liquid introduced

A

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.

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8
Q

Over-filling

A

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.

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9
Q

Tipping.

A

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.

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10
Q

Describe the plenum vaporisers with electronic control.

A

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.

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11
Q

Why is it necessary to have
a special vaporiser to deliver
desflurane?

The problem with des vs other agents

A

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.
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12
Q

Overcoming the problem with desflurane

A

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.
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13
Q

DIVA

A

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.

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14
Q

What considerations should be

taken when building a vaporiser?

A

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

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15
Q

How are vaporisers affected by
altitude?

What is dialed up

what determines whether patient anaesthetised

A

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).

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16
Q

How are vaporisers affected by

altitude?

A

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).
We can work out the partial pressure of a gas using Dalton’s law of partial
pressures, which states:
1 E ach gas in a mixture exerts a pressure, known as its ‘partial pressure’,
that is equal to the pressure the gas would exert if it were the only gas
present. And for completeness, Dalton goes on to say:
2 T he total pressure of the mixture is the sum of the partial pressures of all
the gases present.

17
Q

How do we work out the partial pressure - the law

what does it state

A

We can work out the partial pressure
of a gas using Dalton’s law of partial
pressures, which states:

1 Each gas in a mixture exerts a
pressure, known as its ‘partial pressure’,
that is equal to the pressure the
gas would exert if it were the only gas
present.
And for completeness, Dalton goes on to say:

2 The total pressure of the mixture
is the sum of the partial pressures of all
the gases present.

18
Q

How much sevo delivered at sea level

A

At sea level

So, the partial pressure of sevoflurane delivered
as 4% of a gas mixture at sea level is:

• P sevoflurane =

4/100 × 1 atm =

0.04 atm (i.e. 4% of 1 atmosphere)

19
Q

How much sevo delivered at altitude

A

At altitude

At 5.5 km or 3.5 miles high,
ambient partial pressure reduces to 0.5 atm.

If we take our vaporiser up to this
altitude and set it again to deliver
sevoflurane as 4% of the total gas mixture,
we can use Dalton’s law once
more to work out the partial pressure of sevoflurane:

• Psevoflurane =

4/100 × 0.5 atm =

0.02 atm (i.e. 2% of 1 atmosphere)

We can see that the partial pressure
of the sevoflurane has dropped by half.

20
Q

3 situations

A

Now consider the following three situations.

In each, the only condition
that changes is the altitude and
consequently, the ambient air pressure.

The temperature is kept at 20 °C.

The SVP of sevoflurane at 20 °C is 100
mmHg

(we will use mmHg in these calculations
to make the numbers more
manageable).

21
Q

1 At sea level at 20 °C

A

1 At sea level at 20 °C

  • Atmospheric pressure is 760 mmHg
  • SVP of sevoflurane is 100 mmHg

This means that, per unit volume
of gas leaving the vaporising chamber,
100 ‘parts’ of the 760 will
be sevoflurane and 660 ‘parts’ will be air.

This is true because Dalton’s law states:

• Ptotal = Psevoflurane + Pair

and so

• 760 mmHg = 100 mmHg + 660 mmHg

Therefore, the concentration
of sevoflurane in the gas
leaving the vaporiser
is 100/750 = 13%.

22
Q

At an altitude of 5.5 km at 20 °C

A

At an altitude of 5.5 km at 20 °C

  • Atmospheric pressure is 380 mmHg
  • SVP of sevoflurane is 100 mmHg (unchanged)

This means that, per unit volume
of gas leaving the vaporising chamber,
100 ‘parts’ of the 380 will be
sevoflurane and 280 ‘parts’ will be air.

Therefore, the concentration of
sevoflurane in the gas
leaving the vaporiser
is 100/380 = 26%.

23
Q

3 At the bottom of a mine at 20 °C

A

3 At the bottom of a mine at 20 °C

  • Atmospheric pressure is say, 1000 mmHg
  • SVP of sevoflurane is 100 mmHg (unchanged)

This means that, per unit volume
of gas leaving the vaporising chamber,

100 ‘parts’ of the 1000
will be sevoflurane and 900

‘parts’ will be air.

Therefore, the concentration of
sevoflurane in the gas leaving
the vaporiser is
100/1000 = 10%.

24
Q

3 At the bottom of a mine at 20 °C

A

3 At the bottom of a mine at 20 °C
• Atmospheric pressure is say, 1000 mmHg
• SVP of sevoflurane is 100 mmHg (unchanged)
This means that, per unit volume of gas leaving the vaporising chamber, 100
‘parts’ of the 1000 will be sevoflurane and 900 ‘parts’ will be air.
Therefore, the concentration of sevoflurane in the gas leaving the vaporiser is
100/1000 = 10%.

25
Q

The measured flow vaporiser (e.g. Tec 6) at 5.5 km altitude

A

Unfortunately, the same is not true of the
Tec 6 vaporiser.

As explained vearlier, the Tec 6 heats 
desflurane to 39 °C to ensure that its 
SVP is constant and so fluctuations 
in ambient temperature do not result 
fluctuations in delivery of the anaesthetic agent. 

As a result, the SVP of desflurane inside
this vaporising chamber at 39 °C is
2 atmospheres, regardless of ambient
pressure.

Turning the dial on the Tec 6 vaporiser to
4% reflects the volume of gas
that will be injected into the fresh gas
flow to result in a gas mixture of 4%
desflurane being delivered to the patient.

However, as the desflurane leaves
the vaporiser at altitude,
this is 4% of a much lower ambient pressure

and so the partial pressure
of desflurane in the alveoli
will be much lower.

Using Dalton’s law again,

we can see why this results
in a drop in the partial pressure of
desflurane being delivered
when compared to sea level:

• Pdesflurane =
4/100 × 0.5 atm =
0.02 atm (i.e. 2% of 1 atmosphere)

Because of the way the vaporiser works,
SVP remains constant at altitude.

This means the anaesthetist will have
to dial in a higher percentage of
desflurane to achieve the same
clinical effect at altitude as at sea level.

26
Q

The measured flow vaporiser (e.g. Tec 6) at 5.5 km altitude

A

Unfortunately, the same is not true of the
Tec 6 vaporiser.

As explained vearlier, the Tec 6 heats 
desflurane to 39 °C to ensure that its 
SVP is constant and so fluctuations 
in ambient temperature do not result 
fluctuations in delivery of the anaesthetic agent. 

As a result, the SVP of desflurane inside
this vaporising chamber at 39 °C is
2 atmospheres, regardless of ambient
pressure.

Turning the dial on the Tec 6 vaporiser to
4% reflects the volume of gas
that will be injected into the fresh gas
flow to result in a gas mixture of 4%
desflurane being delivered to the patient.

However, as the desflurane leaves
the vaporiser at altitude,
this is 4% of a much lower ambient pressure

and so the partial pressure
of desflurane in the alveoli
will be much lower.

Using Dalton’s law again,

we can see why this results in a drop in the partial
pressure of desflurane being delivered when compared to sea level:
• Pdesflurane = 4/100 × 0.5 atm = 0.02 atm (i.e. 2% of 1 atmosphere)
Because of the way the vaporiser works, SVP remains constant at altitude.
This means the anaesthetist will have to dial in a higher percentage of
desflurane to achieve the same clinical effect at altitude as at sea level.