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 to the end of the patient’s 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 that 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 patient’s 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 and fat stores up to a year after an anesthetic had been given.
With desflurane and sevoflurane, measuring for those drugs and fat stores they're 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 a very rapid onset, recovery and little hangover but unfortunately it’s unsuitable formulation 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 wakeup 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.