00:00
And when we buy an anesthetic machine, we nearly
always buy new monitors at the same time.
00:00
The principle of the modern
anesthetic machine is that, it is
a partial rebreathing system. And what that means
is that, when the patient inspires
oxygen, air, and anesthetic
vapour, they don't use
all of it. And in fact, as far as oxygen is concerned,
it said that the average 70 kilogram individual
burns about 200 milliliters of oxygen
a minute. So all we, in theory,
have to do, is deliver 200 milliliters of oxygen
to the patient every minute. The only thing
the patient does in response to this,
is produces carbon dioxide. We can't allow
the patient to rebreathe the carbon dioxide, because
that would result in the respiratory acidosis we talked about
earlier. Would cause tremendous
stimulation to ventilation, and in fact,
would make it almost impossible to safely
anesthetize the patient. So we have to scrub
the carbon dioxide out of the expired gases.
And we do this by using soda-lime
canisters. And you can see on this machine,
the two devices that are piled one on top of the other,
are soda-lime canisters. And fresh
gas flow comes in from the bottom.
01:18
Expired gas comes from the bag
in the lower part of the diagram.
01:24
And then, as the bag is compressed,
they're a combination of
fresh gas and expired gas,
is forced up through the
exchange material and carbon dioxide
is scrubbed out. So the material going
to the patient is free of carbon dioxide,
it has vapour in it, it has
additional oxygen, because most of us try to compensate
for that 200 ml of oxygen that the average
patient burns every minute, by giving them
something more than 200 ml. I, on average,
give 400 ml at least per minute, in a patient
who's not otherwise requiring extra oxygen.
02:06
So there's a safety margin
that we all work with.
02:12
The benefit of this kind of a system is that
we don't waste a lot of material. So patients
expire unused vapour and inspire it again,
which allows them to continue
to remain anesthetized even rebreathing
the same anesthetic vapour repeatedly.
02:30
They get rid of carbon dioxide with every
breath, and they get new oxygen
with every breath. The overall
waste of gas is very low, so
we're not venting gas to the vapour, or to the atmosphere,
particularly seeing that vapours
do have some environmental risks associated
with them, and we're also maintaining cost
effectiveness by reducing the amount
of vapour that we use overall. So,
the soda-lime canisters I mentioned,
scrubs the carbon dioxide out of the expired
gas. So the expired gas on this slide is blue
and the fresh gas flow is red.
So, that fresh gas flow
will consist of new vapour and oxygen,
and perhaps some air. The expired gas will
consist of unused vapour,
some unused oxygen and carbon
dioxide. All the gases go through the soda-lime
canisters. The carbon dioxide
is removed and everything else goes back
to the patient. So, soda-lime consists
of Calcium hydroxide, about 75%,
and water, about
20%, Sodium hydroxide, about 3%
and Potassium hydroxide, about
1%. It's an exothermic reaction
so that, when the
gas is passed through the soda-lime,
heat is generated, plus water is created.
04:06
So that in time, the soda-lime becomes
wet and warm. And, as it becomes wetter,
and it absorbs more carbon dioxide,
it becomes exhausted. And it has
a colour dye in it that changes colour and warns
the anesthesiologist that it's time
to change the soda-lime. So the overall
reaction, as I mentioned, is carbon
dioxide, plus calcium hydroxide, leads to the formation
of calcium carbonate, and water,
and heat. Water, calcium carbonate
and heat collect in the soda-lime.
04:42
The colour dye in the soda-lime, which changes colour
when the material is exhausted, and indicates it's time
to replace the soda-lime shows up.
The anesthesiologist hopefully notices it. And,
in addition to the soda-lime changing
colour, the expired gases
are being monitored, so it's possible to see,
and inspired gases, it's possible to see
if carbon dioxide is building up in the circuit, and the patient
is rebreathing carbon dioxide, which is a situation
we simply can't allow to happen.
Baralyme is the same substance
basically as soda-lime, except it uses barium
hydroxide instead of calcium hydroxide
in the granules. So, similar systems
exist in submarines, diving bells
and decompression chambers,
but in nowhere near
the finickitiness that is
apparent in the anesthetic machine. In those systems,
they're just big blocks of Baralyme or
soda-lime, and they're periodically checked to see if they're
getting exhausted, but there's no effort to control
flow through them, or to control the amount
of gas they're engaged with.
06:00
So, the overall reaction is, heat produced
by the scrubbing of expired gases may be sufficient
to cause partial breakdown of Sevoflurane,
with the release of a substance called Compound A,
which can cause kidney damage in rats.
We actually don't know if this is a problem
in humans, but the recommended flow through
an anesthetic machine when using Sevoflurane, is 2 - 3
liters of fresh gas per minute to maintain
lower temperatures in the soda-lime. With
Desflurane, we can turn the total flow
way down. And this actually results in
Desflurane being a very economic drug to use,
because we use so little of it per minute.
06:42
Excess gases in the circuit are released
through an adjustable valve mechanism
and are scavenged to the outside atmosphere.
As I already mentioned, there is some risk
of environmental damage from vapours being scavenged
in this way. So we try to keep the total amount
being released into the atmosphere at a lower level.
Anesthesiology has become a very safe area