and repolarization, no further contractions
can occur. So Succinylcholine,
the only commonly used depolarizing
muscle relaxant, irreversibly blocks access
to the muscle end-plate by Acetylcholine.
This causes a short phase of depolarization
of muscle fibers, characterized by muscle
twitching or fasciculations.
Succinylcholine has a very rapid onset
of action and produces very profound
paralysis in all voluntary muscles.
Under normal circumstances, it's metabolised
by pseudo-cholinesterase in under 18 minutes.
Unfortunately, a common side effect of this drug
is Myalgia, pain in the muscles. And this is
similar to the kind of pain one gets from exercising
very vigorously when you're not fit
and the next day you can barely move. Well,
Succinylcholine produces exactly the same kind of feeling
in the muscles. And patients often will phone
the doctor, the surgeon and complain,
they feel like they've been hit by a truck. So it's very
unpleasant. The other issue with Succinylcholine,
there are a number of issues with Succinylcholine, but another
issue is, it releases histamine, and this can induce asthma
attacks in patients who are susceptible.
In patients with spinal cord injuries,
massive trauma, burns, and renal
failure, Succinylcholine can
cause the release of huge amounts of potassium
into the bloodstream. And Hyperkalemia
causes cardiac arrest. And this is,
obviously, an extremely serious
complication. It also increases intraocular
pressure. So in patients who have
eye injuries, it can actually cause significant
damage at the time of induction.
It causes increased intracranial pressure. So in somebody
who has an intracranial bleed, or a brain tumor, this
can be a problem. It causes increased
intragastric pressure, and this can lead
to vomiting, particularly at the time of induction,
which isn't really vomiting, this is
regurgitation. And aspiration can occur
with damage to the lungs. And,
like the vapours, this drug triggers malignant hyperthermia.
And we will talk more about this very rare,
but very serious genetic problem in another
lecture. So it does trigger hyperthermia,
malignant hyperthermia in susceptible patients.
About 1:1000 patients are genetically pseudo-cholinesterase
deficient and cannot metabolize Succinylcholine,
and will have a markedly prolonged block from the drug.
These patients should not be given Succinylcholine.
And about 1:500 patients have an
which metabolises Succinylcholine slowly. And
the only way you can actually determine whether this
is happening, is if you're using a neuromuscular
monitor and watching neuromuscular
recovery constantly. So,
it's not unusual for that patient who doesn't metabolise
Succinylcholine properly to get a second
dose of another muscle relaxant, which can confuse
the whole issue. This drug is losing favor
with anesthesiologists and is largely saved for trauma
patients or unstable patients who require
profound and rapid paralysis to allow
airway management. I've only used
this drug about 3 times in the last year.
The non-depolarizing muscle relaxants are all