(Mode of Action of Drugs)

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Chapter

1.4

Chapter

2.1

General

Anaesthetics

General

Anaesthetics

General anaesthetics are the group of
drugs which bring about a reversible loss of
pain sensation and consciousness. The depth
of anaesthesia appropriate for the conduct
of surgical procedures can be achieved by a
wide variety of drugs, either alone or by a
combination of drugs, each drug for a spe-
cific purpose. General anaesthetics can be
administered by a variety of routes, but in-
tra-venous or inhalation administration is
preferred, because the effective dose and the
time course of action are more predictable
when these techniques are used.

The general anaesthetics are divided into
two main groups (table 2.1.1).

INHALATIONAL ANAESTHETICS

NITROUS OXIDE (N 2 O)

It is a colourless, odourless, noninflammable
gas which is approx. 1½ times heavier than
air. It is non-irritating with a sweet taste.

Nitrous oxide is used for induction and
maintenance of anaesthesia. It is widely
used as carrier gas for other volatile agents
in general anaesthesia. The usual concentra-

tion used is 70 percent N 2 O + 30 percent O 2

along with some muscle relaxants or other

potent anaesthetics. Nitrous oxide, if admin-

istered along with air, it produces a stage of

excitement and delirium and also produce

amnesia. Hence, the name ‘laughing gas.’

It is eliminated unchanged from the

body, via the lungs. Despite its high fat solu-

bility, it is rapidly eliminated through lungs

within 2 to 5 minutes after its withdrawal.

Nitrous oxide, due to its analgesic action

in subanaesthetic concentration, is employed

for minor operation like tooth extraction, for

obstetrical analysis, painful procedures such

as changing dressing of burns. It is cheap and

very commonly used.

Prolonged administration of nitrous ox-

ide as in cases of tetanus, may cause bone

marrow depression and agranulocytosis.

CYCLOPROPANE

It is a colourless gas with sweet odour and

taste, available as liquid under pressure. It

produces analgesia without loss of conscious-

ness in 1 to 2 percent concentration, in 6 to 8

percent it produces loss of consciousness while