More commonly we use vapours. And vapours are drugs
that are liquid at room temperature but which is
partially diffused into the atmosphere. Exactly the
same principals that are associated with water
vapour which is all around us all the time.
Anesthetic vapours are calibrated to be released in
a known concentration into the patients circuit.
The first widely used vapour was ether. And ether
was very safe. It was very soluble in fat.
And it lasted a very long time. The only anesthetic
I've ever had was an ether anesthetic. When I was
about 5 years old and I had my tonsils out as everybody
did in that era. And they had two big men in the room
that were there just to hold the patients down while
they were breathing ether. I was 5 years old.
I didn't think I had a lot of mass to hold down but
they still needed to have these men in the room
to do it. Very slow onset. Very slow recovery. Often
severe nausea and vomiting. No anesthetic properties
at all. Excuse me, no analgesic properties
at all. No pain killing property whatsoever.
But despite that, it was quite safe. And it's
still used in some developing countries.
Modern vapours are designed to have much higher fat
solubility and have much more rapid onset of action
than ether. And also a much more rapid recovery
and less hangover effect than ether used to cause.
The most modern drugs, desflurane and sevoflurane
are the best in this respect. Those drugs that came
after ether such as halothane, methoxyflurane, enflurane,
and isoflurane have largely disappeared from practice.
Although isoflurane is still occasionally used.
With methoxyflurane it was possible to measure
methoxyflurane in fat stores up to a year after an
anesthetic had been given. With desflurane and
sevoflurane, measuring for those drugs in
fat stores, they are absent within 24 hours.
So sevoflurane is a commonly used modern vapour
anesthetic. It has a sweet smell which is well
tolerated by patients. So it can be used instead
of intraveneous induction drugs in patients with
needle phobias or with very poor venous access.
It's also very popular in pediatric anesthesia
because children are not as terrified by a mask as
they are by a needle. Even when used by inhalation
it has a rapid onset and recovery is also quite
quick but not as fast as you'll see with desflurane.
It has no analgesic properties whatsoever. And it
can like all vapours cause nausea and vomiting.
Desflurane has very rapid onset and recovery and
little hangover. But unfortunately it's unsuitable
for inhalation induction because it has a very
pungent smell. And people who breath it tend to gag
and are very uncomfortable. So we don't use it that
way. It's very fat insoluble. In fact it basically
is as fat insoluble as nitrous oxide. It's the
closest thing to nitrous oxide in terms of its
fat solubility of any of the vapours. Patients
usually wake up clear headed but it can still cause
nausea and vomiting. And again, it has no analgesic
properties. So modern vapours such as sevoflurane
and desflurane have rapid onset and rapid recovery
and less hangover than older vapours. But they do
produce profound amnesia even in very low doses.
And amnesia is critical to anesthesia,
to general anesthesia. They do have respiratory and
cardiovascular effects that we've already mentioned.
And this must be monitored. They can produce nausea
and vomiting. And they can trigger malignant hyperthermia
which we will talk about in another slide.
They have little or not analgesic effects.
So all of these drugs require augmentation by analgesics.