438
P A R T 4
Drugs acting on the central and peripheral nervous systems
in the cell membrane close. Certain ethnic groups may
have a genetic predisposition for a prolongation of paral-
ysis (Box 28.2).
Pharmacokinetics
Suxamethonium, like the non-depolarising NMJ
blockers, is metabolised in the serum, although metab-
olism is dependent on the liver to produce the needed
plasma cholinesterases. Individuals with hepatic impair-
ment may experience prolonged effects of this drug.
Onset of action is usually within 1 minute, and duration
of effect is 10 to 12 minutes. Most of the metabolites are
excreted in the urine. Individuals with renal impairment
may be at risk for increased toxicity from the drugs.
Suxamethonium crosses the placenta. Effects on breast-
feeding are not known.
Contraindications and cautions
The contraindications and cautions for suxamethonium
are the same as for non-depolarising NMJ blockers.
Suxamethonium is often used during caesarean sections,
but accurate timing is necessary to prevent serious effects
on the fetus. In addition, suxamethonium should be used
with caution in individuals with fractures
because the
muscle contractions it causes might lead to additional
trauma
; in individuals with narrow-angle glaucoma or
penetrating eye injuries
because intraocular pressure
increases
; and in individuals with paraplegia or spinal
cord injuries,
which could cause loss of potassium from
the overstimulated cells and hyperkalaemia.
Extreme caution is necessary in the presence of
genetic or disease-related conditions causing low plasma
cholinesterase levels, such as cirrhosis, metabolic dis-
orders, carcinoma, burns, dehydration, malnutrition,
hyperpyrexia, thyroid toxicosis, collagen diseases
and exposure to neurotoxic insecticides.
Low plasma
cholinesterase levels may result in a very prolonged
paralysis because suxamethonium is not broken down
in the plasma and continues to stimulate the receptor
site, leading to a need for prolonged support after use of
the drug is discontinued.
Adverse effects
The adverse effects of suxamethonium are the same as
those for non-depolarising NMJ blockers. In addition,
suxamethonium is associated with muscle pain related
to the initial muscle contraction reaction. A non-
depolarising NMJ blocker may be given first to prevent
some of these contractions and the associated discom-
fort. Aspirin also alleviates much of this pain after
the procedure. Malignant hyperthermia, which may
occur in susceptible individuals, is a very serious con-
dition characterised by massive muscle contraction,
sharply elevated body temperature, severe acidosis and,
if uncontrolled, death. This reaction is most likely with
suxamethonium, and treatment involves dantrolene (see
Chapter 25) to inhibit the muscle effects of the NMJ
blocker.
Clinically important drug–drug interactions
Potential drug–drug interactions for suxamethonium
are the same as for the non-depolarising NMJ blockers.
Suxamethonium and paralysis
Suxamethonium is broken down in the body by
cholinesterase, an enzyme found in the plasma.
Several conditions may cause the body to produce
less of this enzyme, including cirrhosis, metabolic
disorders, carcinoma, burns, dehydration, malnutrition,
hyperpyrexia, thyrotoxicosis, collagen diseases
and exposure to neurotoxic insecticides. If plasma
cholinesterase levels are low, the serum levels of
suxamethonium remain elevated and the paralysis can
last much longer than anticipated.These people need
support and ventilation for long periods after surgery.
There is also a genetic predisposition to low plasma
cholinesterase levels. People should be asked whether
they or any family member has a history of either low
plasma cholinesterase levels or prolonged recovery from
anaesthetics. Alaskan Eskimos belong to such a genetic
group, and they are especially likely to suffer prolonged
paralysis and inability to breathe for several hours after
suxamethonium has been used for surgery. If there is
no other drug of choice for these people, special care
must be taken to monitor their response and ensure their
breathing for an extended postoperative period.
Cultural considerations
BOX 28.2
Prototype summary: Suxamethonium
Indications:
As an adjunct to general anaesthesia; to
facilitate endotracheal intubation; to induce skeletal
muscle relaxation during surgery or mechanical
ventilation.
Actions:
Combines with ACh receptors at the motor
endplate to produce depolarisation; this inhibits
neuromuscular transmission, causing a flaccid
paralysis.
Pharmacokinetics:
Route
Onset
Duration
IV
30–60 secs
4–6 mins
T
1/2
:
2 to 3 minutes; metabolised in the tissues,
excreted unchanged in the urine.
Adverse effects:
Muscle pain, related to the
contraction of the muscles as a first reaction;
respiratory depression, apnoea.
