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P A R T 4
Drugs acting on the central and peripheral nervous systems
in the movement of gas in the opposite direction—out
of the tissues and back to expired air.
Anaesthetic gases include ethylene (red cylinder) and
nitrous oxide (blue cylinder). (See Table 27.1.)
Therapeutic actions and indications
Nitrous oxide is a very potent analgesic. However, it is
the weakest of the gas anaesthetics and the least toxic.
It moves so quickly in and out of the body that it can
actually accumulate and cause pressure in closed body
compartments such as the sinuses. Because nitrous oxide
is such a potent analgesic, it is used frequently for dental
surgery and during labour. It does not cause muscle
relaxation. Nitrous oxide is usually combined with other
agents for anaesthetic use.
Ethylene is less toxic than most of the other gas
anaesthetics. It is not associated with bronchospasm,
and there is usually less postanaesthetic vomiting with
this drug.
Pharmacokinetics
All of these agents have a rapid onset of action, usually
within 1 to 2 minutes, and a rapid recovery period.
Timing of recovery depends on the other drugs being
used.
Contraindications and cautions
Nitrous oxide and ethylene can block the reuptake of
oxygen after surgery and cause hypoxia. Because of this
reaction, it is always given in combination with oxygen.
Susceptible individuals should be monitored for signs of
hypoxia, chest pain and stroke.
Adverse effects
As with other general anaesthetics, people need to be
monitored for skin integrity when they are not able to
move for periods of time. Ethylene can leave the person
with a headache and a very unpleasant taste in the
mouth. Nitrous oxide can cause acute sinus and middle
ear pain, bowel obstruction and pneumothorax because
it so rapidly moves into and accumulates in closed
spaces. Because nitrous oxide inactivates vitamin B
12
,
people should also be monitored for low vitamin B
12
levels, including neurological, immune and haemato
logical complications.
Clinically important drug–drug interactions
Caution should be used if these drugs are combined with
any other drug that causes CNS depression. If halothane
and ketamine are used in combination, severe cardiac
depression with hypotension and bradycardia may
occur. If these agents must be used together, the person
should be monitored closely.
V
olatile
liquids
Inhaled anaesthetics also can be
volatile liquids
—liquids
that are unstable at room temperature and release gases.
These gases are then inhaled by the person. Therefore,
volatile liquids act like gas anaesthetics.
Most of the volatile liquids in use are halogenated
hydrocarbons such as desflurane (
Suprane
), isoflurane
(
Aerrane, Forane
), methoxyflurane (
Penthrox
) and sevo-
flurane (
Sevorane
). (See Table 27.1.)
Therapeutic actions and indications
Desflurane is widely used in outpatient surgery because
of its rapid onset and quick recovery time.
Isoflurane is widely used to maintain anaesthesia
after inductions. It can cause muscle relaxation.
Sevoflurane is used in outpatient surgery as an
induction agent and is rapidly cleared for quick recovery.
Pharmacokinetics
Desflurane and isoflurane have a rapid onset—also within
1 to 2 minutes—and rapid recovery—usually within 15 to
20 minutes. Sevoflurane, the newest of the volatile liquids,
has a very rapid onset of action—within 30 seconds—and
a very rapid clearance—lasting only about 10 minutes.
These drugs are all cleared through the lungs.
Contraindications and cautions
Desflurane use should be avoided in people with respira-
tory problems and in those with increased sensitivity
because of its irritation to the airways and tendency to
cause respiratory depression.
In addition, it is not recom-
mended for induction in children
because of its irritation
of the airways.
Isoflurane and sevoflurane should be used
with caution in individuals with respiratory depression
to avoid severe respiratory depression.
All of these drugs
have the potential to trigger malignant hyperthermia and
Prototype summary: Nitrous oxide
Indications:
Induction and maintenance of
anaesthesia.
Actions:
Depresses the CNS to produce anaesthesia
and analgesia.
Pharmacokinetics:
Route
Onset
Duration
IV
1–2 mins
20 mins
T
1/2
:
minutes; not metabolised, excreted in the lungs.
Adverse effects:
Cardiovascular depression,
respiratory depression, apnoea, earache, sinus pain,
vomiting, malignant hyperthermia.
