we're not using it as much as we used to.
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
principles 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 patient circuit. The first widely
used vapour was Ether. And ether
was very safe, because it was
not very potent, in reality. It was
very soluble in fat, and it was
relatively low potency, 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 lower 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 and 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 intravenous 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
recovery and little hangover, but
unfortunately it's unsuitable for inhalation induction, because
it has a very pungent smell, and people who breathe
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 these
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 no analgesic effects.
So, all of these drugs require
augmentation by analgesics. Induction drugs
are the drugs that we give