A "vaporiser", as
it name suggests, has something to do with producing "vapour". So before going
onto discussing vaporisers, we need to have some idea as to what a vapour is. And, unfortunately, to understand what a vapour is, we need to know about
something called critical temperature.
If you take a
gas, and compress it really hard, the particles that compose it are brought ever
so close to each other. As you keep compressing , the particles will at some
point coalesce and convert the gas into liquid. However, if the gas is above a
certain temperature, called a "critical temperature", whatever amount of
pressure you apply, that gas will not become a liquid. This temperature is
called "critical temperature" and every gas has its particular critical
temperature. For Isoflurane, it is about 200 degrees Celsius. Now to what a
vapour is:
A
gas that is currently below its critical temperature is called a “vapour”. If
compressed with enough pressure, it will condense into a liquid.
A gas that is
currently above its critical temperature remains a gas. However hard you
compress it, it will not condense into a liquid.
Click on button to get a way of remembering the difference between a gas and
vapour
Click on button to find out what happens to Isoflurane on Venus
The purpose of a
vaporiser is to add anaesthetic vapor into the fresh gas flow in a way
that the output of the vaporiser delivers the set concentration of anaesthetic
agent accurately.
Fresh gas enters
the inlet of the vaporiser and is divided into two flow pathways. The splitting
valve, depending on the setting of the control dial, adjusts how much goes
through each of the pathways. The fresh gas that is sent along the "by pass"
pathway doesn't get into contact with any vapor. On the other hand, the fresh
gas that is sent to the vaporising chamber becomes fully saturated with vapor. At the exit end of the vaporiser, the by pass gas (vaporless) meets the chamber
gas (fully saturated with vapor) and the two mix. The resultant output depends
on how much of fresh gas went though each of the pathways.
The image below
shows you that if you dial a high anaesthetic concentration requirement, the
splitting valve sends more fresh gas via the vaporising chamber and less through
the by pass pathway.
Similarly, the
image below shows you that if you dial a low anaesthetic concentration
requirement, the splitting valve sends less fresh gas via the vaporising chamber
and more through the by pass pathway.
Click on button to learn about splitting ratios
The image below
shows that if you set the dial to zero to make vaporiser deliver no anaesthetic
vapor, the splitting valve sends all the fresh gas via the by pass pathway and
nothing through the vaporising chamber.
The basic
vaporiser discussed above has a very simple design. Unfortunately, this simple
design has some problems and these will be explained in more detail in later
sections.
Problems of the Basic Design
The basic
vaporiser discussed previously has a very simple design. Unfortunately,
this simple design has some problems:
The flow rate
of fresh gas going through the vaporiser can affect its output.
The
temperature of the vaporiser drops with use and this can affect its output.
Some
ventilators transmit a "positive pressure" back into vaporiser which can
affect its output.
As discussed
before, part of the fresh gas flow enters the vaporisation chamber and picks up
vapor.
However, this vaporisation is not very efficient. If one uses a high fresh gas
flow, the vaporisation process can't keep up with so much gas arriving into the
vaporisation chamber. The result is that, relative to the high flow of fresh gas
flow, the amount of anaesthetic vaporised is inadequate. So this means that at
high flows, the vaporiser delivers less anaesthetic concentration than is set on
the dial.
The solution employed by modern vaporisers to solve this problem is to make the
vaporisation much more efficient by increasing the surface area of contact
between the fresh gas and anesthetic agent. So even when there are high flows,
the efficient vaporisation means that all gas going through the vaporisation
chamber is fully saturated. Because of this ability to saturate fresh gas at all
flow rates, the output concentration remains accurate to the setting on the dial
over a wide range of flows. I.e. The output concentration is independent of
flow.
One method that vaporisers use to increase the efficiency of vaporisation is to
dip wicks into the anaesthetic agent. Due to capillary action, the anaesthetic
agent rises into the wicks. This dramatically increases the surface area of
anaesthetic agent exposed to the fresh gas entering the vaporisation chamber and
thereby improves the efficiency of vaporisation.
Certain vaporisers (e.g. "Copper Kettle") use bubbles to increase the surface
area for vaporisation. In these, some of the fresh gas flow is bubbled through a
disk made out of a special material (sintered disk) that is very porous. The
disk is submerged into the anaesthetic agent and when fresh gas is sent through
it, a large number of tiny bubbles form. The tiny bubbles of fresh gas have a
very large total surface and thus become fully saturated with vapor efficiently.
For vaporisation
to occur, the anaesthetic molecules have to "escape" from the liquid state and
become vapor. To escape, the molecules need energy and they take it from the
liquid that they escape from. As more and more molecules escape, more and more
energy is lost from the liquid. It is the "energy" in a liquid that causes it to
have a temperature. Hot liquids have more energy than cold fluids. Therefore, as
the escaping molecules take energy, the temperature of the liquid falls (see latent
heat of vaporisation). The lower temperature of liquid means that it has
less energy and this makes it more difficult for the remaining molecules to
escape and become vapor. Thus at lower temperatures, there is less vaporisation.
The less
vaporisation then will decrease the concentration of anaesthetic delivered by
the vaporiser. I.e. It will deliver an anaesthetic concentration below the
setting you dialled.
There are two
common solutions to this problem. One is that we can give heat to the liquid to
minimise the temperature drop. The other is to increase the flow of fresh gas
into the vaporising chamber to compensate for the reduced vaporisation
efficiency of the cold fluid.
GIVING HEAT
In most
vaporisers, we don't actually give heat "actively". That is, we don't
electrically heat it (complicated and needs a power supply) and we don't light a
fire under it (absolutely dangerous).
Instead, we make
it easy for the vaporiser to use heat from the surrounding air. We do this by
making the vaporiser out of metal which is a good conductor of heat and will
help transfer heat from the surrounds into the liquid.
In addition, the metal casing
is made very thick. Metal has a high specific heat capacity. That is, metal has a high ability to "store" energy and give it up when
necessary. When the liquid anaesthetic vaporises, its temperature drops below
that of the thick surrounding metal casing. The metal casing then donates the
heat it has to the liquid, minimising the temperature drop of the liquid. The
thicker the metal, the more energy that it can store. However, the metal casing
cannot give up heat indefinitely and after sometime, it also has a
significant temperature drop. But because it has a high ability to store heat,
the drop is slow. In between your anaesthetic, when you turn the vaporiser off
and have coffee before your next case, the metal will continue to "absorb" heat
from the surroundings and its temperature will rise, ready to donate heat when
you turn the vaporiser on again. To summarise, the metal casing is a good
conductor, so allows heat to be easily transferred from the surroundings to the
fluid, and is also a good "heat reservoir", so is able to donate heat and help
resist the temperature drop.
GIVING MORE FLOW
When the
temperature of the liquid drops, we have seen that the output concentration of
the vaporiser drops. A way of compensating for that problem is to increase the
flow of gas via the vaporising chamber (altering the splitting ratio). One could
manually do this by measuring the temperature of the liquid with a thermometer
and increasing the dial setting according to some kind of reference chart. This would be quite tedious as you would have to do it all the time. Modern
vaporisers have removed the hard work. When the liquid drops its temperature,
the flow of gas through the vaporising chamber is automatically increased
without you having to turn the dial.
This is
accomplished by an automatic temperature compensating valve that influences how
much flow occurs into the vaporising chamber. So, the splitting of gas between
the by pass pathway and the vaporising chamber is controlled by two valves: the
dial you set (splitting valve) and the temperature compensating valve which
operates automatically according to the liquid temperature.
The automatic
temperature compensating valve uses the physical property that substances (e.g.
metals and liquids ) become smaller when the temperature lowers. A metal rod
(shown in black below) shortens as the temperature drops. Similarly, a liquid
filled in collapsing bellows (shown in green below) becomes smaller in volume
when cooled to a lower temperature.
This property is
used in the design of temperature compensating valves in vaporisers. In the
design that uses a metal rod, the rod offers some resistance to flow into the
vaporising chamber. As the vaporiser cools, the rod becomes shorter, making the
valve move away from the opening. This reduces the resistance to flow and thus
more flow occurs into the vaporising chamber.
Some vaporisers
use liquid instead of the metal rod. The liquid is filled inside collapsible
bellows. As the temperature falls, the liquid in the bellows contracts into a
smaller volume. This makes the bellows shrink, pulling the valve away and
thereby increase flow.
Another method
uses a "bi metallic" strip. Different metals expand and contract to differing
extents when exposed to temperature changes. In the example below, the "green"
metal expands and contracts less than the "red" metal.
In a bimetallic
strip, two metals with very different degrees of thermal expansion ( "different
coefficients of thermal expansion" ) are fixed together. In the example below,
when the temperature drops, the "green" metal contracts much more than the "red"
metal. Because they are fixed together, they cannot contract independently,
like in the diagram above. Instead, the "green" metal "tries" to drag the "red"
metal and causes the bimetallic strip to bend.
In the vaporiser,
the bimetallic strip is fixed in such a way that it offers a resistance to flow
entering the vaporising chamber. When the temperature of the vaporising chamber
drops, the bimetallic bends and moves away. This reduces the resistance to flow
and thus more flow occurs into the vaporising chamber.
Positive pressure
ventilation result in intermittent pressure changes. During the positive
pressure, there is a pressure rise and during expiration, there is a sharp
drop in pressure. These pressure changes can be transmitted back into the
vaporiser and can affect the concentration of anaesthetic agent delivered.
The effect of changing pressure affecting the output of the vaporiser is called
the "pumping effect". In this section, this effect and the methods used by
vaporiser designers to prevent it from happening are explained. Below is
shown a basic vaporiser and beyond it a bag to represent positive pressure
ventilation.
When the bag is squeezed (positive pressure ventilation), pressure is
transmitted back into the vaporiser as shown below. This "back pressure"
is transmitted to both, the "by pass" channel and also to the vaporising
chamber. This "back pressure" opposes the flow of the fresh gas in both
the "by pass" channel and the vaporising chamber. The fresh gas tries to
move forward and gets compressed both in the 'by pass' channel and the
vaporising chamber. However, the vaporising chamber volume is much larger
than the 'by pass' channel volume, and thus, more fresh gas gets compressed into
it than into the 'by pass' channel.
This extra fresh
gas that enters the vaporising chamber collects anaesthetic vapor as shown
below.
Now see what
happens when the positive pressure is suddenly released (expiration). The
previously compressed gases now suddenly expands in all directions.
Some of the
rapidly expanding gas (containing vapor) enter the inlet of the vaporiser and
cross over into the 'by pass' channel as shown below.
Normally, a
vaporiser 'by pass' channel does not have vapor. So this vapor due the
'pumping effect' is additional. When this 'by pass' vapor flows across to
the exit of the vaporiser, it meets the vapor from the vaporising chamber.
The addition of the 'by pass' vapor to the vapor from the vaporising chamber
raises the final concentration of anaesthetic delivered. i.e. The 'pumping
effect' increases the delivered concentration of anaesthetic agent.
Vaporiser
designers have various tricks to reduce the 'pumping effect' and some of these
are discussed below:
LARGE 'BY
PASS CHANNEL
The 'by pass'
channel can be made larger, ideally equal to the volume of the empty space of
the vaporising chamber. Therefore, when there is 'back pressure',
the effects will be equal in both; in the vaporising chamber and the 'by pass'
channel. Therefore, when there is 'back pressure', no longer will extra
fresh gas go into the vaporising chamber.
LONG INLET TUBE
The vaporiser
inlet tube can be made longer. When the 'back pressure' is suddenly
released during expiration, as discussed before, the extra gas in the vaporising
chamber will suddenly expand. However, thanks to the long inlet tubing,
the extra gas containing vapor expands into the long inlet tube and doesn't
reach the 'by pass' channel.
INCREASED
RESISTANCE
The vaporiser can
be designed to have a high internal resistance to flow. This high
resistance "resists" changes to flow caused by the intermittent 'back pressure'
of positive pressure ventilation.
ONE WAY VALVE
A 'one way' valve
(also called unidirectional valve) can be put between the vaporiser outlet and
the ventilator / breathing system. On way valves allow flow in one
direction, but not in the other. In the diagram below, the one way valve
is allowing gases to flow forwards.
However, this
valve prevents flow from occurring in the reverse direction. This prevents
the transmission of 'back pressure' to the vaporiser.
Desflurane has a
very low boiling point (about 23 degrees Centigrade) and even at room
temperature, has an high vapor pressure.
Also, for small
changes in temperature, the vapor pressure of desflurane changes quite
dramatically. I.e. desflurane is said to have a very steep "Vapor Pressure
versus Temperature curve".
These physical
properties of desflurane created a big headache for vaporiser designers.
An operating room
temperature is not perfectly constant. It keeps changing slightly
depending on various factors including the number of medical students (young
body heat) watching the surgery. These changes in operating room
temperature then change the temperature of vaporisers present in that room.
As discussed elsewhere, the standard vaporisers try to resist changes in
temperature (e.g. by having thick metal construction). However, these
mechanisms are not perfect and in practice small changes in vaporiser
temperature still occur. This is not a big problem with anaesthetic agents
such as Isoflurane or Sevoflurane which have a relatively less steep
"Vapor Pressure versus Temperature curves". In them, small temperature
changes will lead to only small changes in vapor pressure and this can be
compensated by mechanisms such a the bimetallic strip. With Desflurane,
with its steep "Vapor Pressure versus Temperature curve", even these small
temperature changes can cause large changes in vapor pressure which cannot be
compensated for with simple devices such a bimetallic strip. So a whole
new vaporiser design had to be made.
The solution
chosen for the problem is to have a vaporiser that heats the Desflurane to a
very precisely controlled temperature that is not affected by changes in room
temperature. The heated vapor is then "injected" into the fresh gas flow.
Before
discussing the desflurane vaporiser in detail, let us first understand the
concept of vaporisers "injecting anaesthetic" ( No, I am not referring to you "injecting"
propofol into a patient! ).
You will recall
that "standard" vaporisers work by splitting the fresh gas flow into two
pathways, one going through the vaporising chamber and picking up anaesthetic
agent and the other "by passes" the chamber and thus has no anaesthetic.
The two streams then mix at the end of the vaporiser to give the final
concentration of anaesthetic.
Another option is
to "inject" the anesthetic agent directly into the fresh gas flow. In this
method, the fresh gas flow coming from the flow meters does not split into two
streams. There is only one stream for the fresh gas flow, and into this
stream, the anaesthetic agent is directly injected.
The desflurane
vaporiser works in a similar way. There is a tank (sump) which contains
desflurane which is electrically heated to a highly controlled constant
temperature (approximately 40 degrees C). Because of the heat, above the
liquid Desflurane is gaseous Desflurane at a pressure of about two atmospheres
(about 1500 mmHg or 200 kPa). This Desflurane gas is injected into the
fresh gas flow.
The amount of
Desflurane concentration in the fresh gas is controlled by the dial setting set
by you. The dial moves a valve which varies the resistance to Desflurane flow
from the tank to the fresh gas.
If you want a
higher concentration of desflurane, the valve attached to the dial reduces the
resistance to flow of desflurane and more of it gets injected into the fresh
gas.
Conversely, if
you want a lower concentration of desflurane, the valve attached to the dial increases the resistance to flow of desflurane and less of it gets injected into
the fresh gas.
THE PROBLEM of
FLOW
You may recall
that the "standard" vaporiser is flow dependant. That is, at higher flows,
the vaporisation is inadequate and the concentration of anaesthetic delivered
will fall unless special modifications are made to the basic design.
The Desflurane vaporiser discussed so far will also be affected by the flow rate
of fresh gas going through it. However, it is very important to note that
the mechanism of how flow can affect a "normal" vaporiser and a Desflurane
vaporiser are very different.
In an "ordinary vaporiser", the high flow rates cause the vaporiser output
concentration to drop because of inadequate vaporisation. In the Desflurane
vaporiser, this is not the reason for drop in concentration at high flows, as there is no problem with vaporisation. In fact, because of its low
boiling point, Desflurane very easily becomes vapor.
In the desflurane vaporiser, the reason for a drop in concentration of
anaesthetic at high flows is that the anaesthetic becomes more diluted by that
high flow.
If you keep the rate of injection of Desflurane constant, and increase the fresh
gas flow, the injected Desflurane will be diluted more and the delivered
concentration will drop.
Conversely, if you keep the rate of injection of Desflurane constant, and
decrease the fresh gas flow, the injected Desflurane will be less diluted
and the delivered concentration will increase.
One solution would be for you to manually adjust the dial setting to match the
fresh gas flow. For low flows, you will have to reduce the dial setting to
reduce the rate of Desflurane injection, and for high fresh gas flows, you will
need to do the opposite. This would be really tedious in our modern times.
Fortunately, the Desflurane vaporiser automatically adjusts the rate of
injection of desflurane to match the flow rate, and thus keeps the delivered concentration
constant.
We are now ready to discuss the workings of the Desflurane vaporiser.
You will need to refer to the numbers on the diagram below:
Your flow meters deliver the fresh gas flow [1]. The fresh gas travels
through pipe [2]. Note that, unlike other vaporisers, none of the fresh
gas goes to the vaporising chamber [4]. The vaporising chamber is
electrically heated [3]. Using sensors for feedback, the temperature is
kept very constant. The heating causes the Desflurane to become a gas
under pressure [4] and this travels down pipe [5]. The dial you control is
fixed to a valve [6] that changes the resistance to Desflurane flow.
When you increase the concentration setting, the valve opens a bit and lowers
the resistance, allowing more Desflurane to flow through. The Desflurane
then goes via pipe [7] and meets the fresh gas at [8]. The Desflurane mixes
with the fresh gas and a final concentration emerges from the exit of the
vaporiser [9].
Now we can discuss how the vaporiser, to keep the output concentration accurate,
adjusts the Desflurane flow when the fresh gas flow changes. Here
is the same diagram again:
The fresh gas flows in pipe [2]. This pipe has a fixed resistance [10] in
its path. For the fresh gas flow to overcome this resistance [10], the
pressure in pipe [2] rises. Higher the fresh gas flow in pipe [2], higher will
be the pressure rise in pipe [2] since more flow has to occur through the same
fixed resistance [10]. Similarly, when the fresh gas flow is decreased, the
lesser flow will find it easier to go through the fixed resistance and the
pressure in pipe [2] drops. It is important to remember that the
pressure in pipe [2] is proportional to the fresh gas flow going through it.
Higher the flow, higher is the pressure.
Pipe [5] carries desflurane under pressure from the vaporising chamber [4] to
the fresh gas flow at [8]. The flow of Desflurane is resisted by two
valves [6,13]. Valve [6] is the valve that you control when you set the
dial to a particular concentration. Valve [13] is an electronically
controlled valve. Computer [12], the vaporiser's "brain", is able to alter the
flow of Desflurane by controlling valve [13]. Device [11] is called a "differential
pressure transducer". It has a diaphragm that on one side is exposed to the
pressure in pipe [2] carrying fresh gas and the other side of the diaphragm is
exposed to the pressure in pipe [5] carrying Desflurane. When the pressure is
equal on both sides of the diaphragm, it lies in a neutral position. If one side
of the diaphragm is at a higher pressure than the other side, the pressure
difference makes the diaphragm move. In this way, the differential pressure
transducer [11] is able to measure the pressure difference between the fresh gas
pipe [2] and the Desflurane pipe [5]. It continuously keeps computer [12]
informed about pressure difference information.
Now let us see how the vaporiser copes when the fresh gas flow is increased.
The fresh gas flow has been increased by you [1]. Increased flow fresh gas
flows through pipe [2] and meets fixed resistance[10]. The increased flow
through the fixed resistance [10] makes the pressure in pipe [2] to rise and
this pressure is experienced by differential pressure transducer [11].
Since the desflurane pressure in pipe [5] is now lower than the fresh gas
pressure in pipe [2], the diaphragm in the differential pressure transducer [11]
moves and a signal about the pressure difference is sent to the computer [12].
The computer [12], acts on the information
provided by the differential pressure transducer. It proceeds to increase
the flow of desflurane to inject into the increased fresh gas flow. It
commands the electronically controlled valve [13] to reduce the resistance to
flow. As the valve [13] opens up and lowers the resistance, the Desflurane
flow increases.
The increased flow of Desflurane causes the pressure in pipe [5] to rise and
this rise pushes the diaphragm of the differential transducer back to its
neutral position and it sends a signal to the computer [12]
that the desflurane pressure has increased sufficiently and the computer stops
telling the valve to open further.
The increased pressure [5] increases the Desflurane flow which mixes with the
increased fresh gas flow and maintains the output concentration [9]. If
the fresh gas flow changes the system will again alter the rate of injection of
Desflurane.
Click on button for more details about the Desflurane vaporiser
Many anesthetic
machines have more than one vaporiser attached so that one has a choice of
inhalational agents to use. However, it is important that only one
vaporiser be used at a given time to avoid overdose with different vapors going
into the patient simultaneously. There are many different safety
mechanisms available which prevents more than one vaporiser to be used
simultaneously. I describe one such system below. Please note that
your anesthesia machine may use a different system.
In this system,
each vaporiser has two pins protruding out. When the vaporiser is in use,
the pins protrude outwards. When the vaporiser is turned off, the pins
retract back to where they were.
On the other
hand, if any of the pins are pushed in (i.e. by another vaporiser) this locks
the vaporiser dial in the OFF position. When the pin is no longer pushed
in, the dial once again becomes unlocked and can be turned.
When you put two
vaporisers together, their pins touch.
When one
vaporiser is turned on, it protrudes its pins which then pushes in the pins of
adjacent vaporisers and locks them. When this vaporiser is turned off, its
pins retract and releases the pins on the adjacent vaporisers and thereby
unlocks them. In this way, only one dial can be turned on at a given time.
Click on button to see photos of an interlock mechanism
It is important
to fill the correct agent into the correct vaporiser. If a wrong agent is
filled into a vaporiser, you will be giving the wrong drug, and worse, since
vaporiser designs for different agents vary, you may seriously overdose your
patient.
Early vaporisers
had simply a funnel into which you could pour virtually anything by mistake
(including coffee).
Modern vaporisers
have special filling systems specific for each anaesthetic agent to prevent
inadvertent filling with an wrong agent. Think of it as a "lock and key" system,
i.e. a particular key will fit only a specific lock.
The joke about the three beautiful women at The Pearly Gates .....
There
are various systems in use. In the system below, the Isoflurane filler
(key) has a notch in a corner. This fits perfectly with the filling hole
in the Isoflurane vaporiser. The filling hole has pin at the corner over which
the notch of the Isoflurane filler key can pass over.
A different
anaesthetic agent such as Halothane (not commonly used anymore) has a
different filling key. In this case, the key has a notch at the side instead of
at the corner. So the Halothane filler key will not fit into the
Isoflurane vaporiser filling hole.
The
system described above is only one type of agent specific filling system. There
are others that are there and depend on the manufacturers and the country you
work in.
In addition to
the physical shapes being different, the key fillers are also color coded
(purple for Isoflurane, yellow for Sevoflurane, blue for desflurane).
Click on button to see photos of some filling systems
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