THERE are subtle differences
between sevoflurane and
isoflurane. Once you factor in
some financial considerations,
these differences lead to a number
of clearly defined practicalities in
their usage.
The basic
difference is one of
solubility. Isoflurane
is more soluble in
tissues than
sevoflurane. The
consequence of this
is that, under the
sorts of time scales
we work over in
general practice,
patients become
saturated with sevoflurane much
more rapidly than they become
saturated with isoflurane.
Why does this matter?
When a patient is saturated with an
inhalation agent, the amount it
breathes out is almost the same as
the amount it breathes in. It is the
expired concentration which most
closely represents what is present in
the brain and which actually keeps the patient asleep.
If you run a circle circuit with very high fresh gas flow, it behaves very
much like a parallel Lack. With
sevoflurane you can dial 2.5% on the
vaporiser, the dog breathes in 2.5%, it
breathes out 2.3% and you dump that 2.3% into the
atmosphere to
augment global
warming and deplete
your finances.
With a more
soluble agent, such as
isoflurane, under high
fresh gas flow
conditions, you would
perhaps dial 2% on the vaporiser for your patient
to breathe in. Because isoflurane is somewhat more soluble than
sevoflurane, the expired breath might
only contain 1.5%. Ignoring concerns
about man-made global warming, the
cost of isoflurane allows you to waste
this to scavenge with reasonable
equanimity.
The situation is quite different when
using low flow rates into the same circle
circuit. Under high flow there is very
little recirculation. As the fresh gas flow is reduced, there is
progressively more
mixing of the
expired gas with the
lower fresh gas flow
and so less wastage
through the spill
valve of the circuit.
In this example
using isoflurane,
most recirculated
gas will be 1.5%.
This may well not
keep your patient
asleep. Adding a
low flow of 2%
isoflurane to this
cannot get the
inspired
concentration
remotely high
enough to do the
job. So a much
higher isoflurane setting would be needed to increase the
inspiratory mixture to the 2% needed
for anaesthesia.
You can certainly do this. With low
flow you can easily have 3% on the
vaporiser mixing with recycled 1.5% to
get you the 2% needed in the
inspiratory gas to keep your patient
asleep.
So you might have a labrador on
500ml/min fresh gas flow, the vaporiser
set to 3% (or even 4%), the dog
breathing in 2% and breathing out
1.5%, which is re-converted to the
desired 2% by mixing with the 3% in
the fresh gas flow.
All very fascinating, if you like this
sort of thing. But not what most
people have going through their head
when they are doing a bitch spay.
Lower solubility
What happens with a lower solubility
agent like sevoflurane? As you turn
down the fresh gas flow once your
patient is stable (this is only a matter
of minutes with a low solubility
agent), it breathes in the 2.5% from
the vaporiser, and breathes out
somewhere around 2.3% which is
largely recirculated. Adding a little
2.5% from a small fresh gas flow will
still give you a
concentration
which is likely to
keep your patient
asleep.
So the rule is
that under low
flow conditions,
with a low
solubility agent the
setting on the
vaporiser is still
very close to what
the patient will be
breathing, both in
and out. With
sevoflurane you
can markedly
reduce the fresh
gas flow without
having to change
the vaporiser from whatever setting you want the patient
to breathe, usually 2.5-3% depending
on which other drugs you have on
board.
How low is low fresh gas flow?
How low do you want to go? You
absolutely must supply at the basal
metabolic requirement for oxygen to
your patient. A generous estimate of
this is 5ml/kg/min. That’s
150ml/min for a 30kg labrador.
At 150ml/min, your concerns are
about the accuracy of your rotameters and vaporiser. Rotameter
accuracy depends on whether your
anaesthetic machine is designed to
work at this low gas flow.
If in doubt you can always just run
10-20% higher than this. Accuracy
becomes less important if you are using
even a little above the basal oxygen
requirement.
The second mechanical problem is
accuracy of your vaporiser at these
flows. This should not be an issue as
sevoflurane vaporisers are designed to
run at low flow rates.
So, is sevoflurane as simple as “get
your labrador stable at reasonable flow
rate and then drop the fresh gas flow to
300ml/kg”, less for a smaller patient?
Almost.
What are the problems?
No nitrous oxide. Just don’t.
Response time is slow. If you need to change the circuit concentration you
cannot do it quickly with low fresh gas
flow. Changing the vaporiser to 4% with
a fresh gas flow of 300ml/min will not
stop a labrador kicking the surgeon if
you have underestimated its need for
sevoflurane.
To rapidly adjust depth you really
need to increase fresh gas flow to
deliver that higher concentration rapidly.
Then turn the fresh gas flow back down
again.
Intrinsic to a low solubility agent is
the ability to overdose very rapidly.
Putting the vaporiser to 4% and FGF to
2.0l/min will put your patient’s blood
pressure through the floor within
minutes if you leave the settings here by
accident.
The converse of this is that hitting
the oxygen flush button might well
result in your patient waking up.
So, is sevoflurane worth using? With
a low flow circle approach it is
undoubtedly economical for even quite
large sized dogs. It’s a nice agent to use.
Once you are set up to use it effectively
you are in a situation to decide if you
would like it to be your main volatile
agent.
That’s a whole different decision-making process.
