Tài liệu CLINICAL PHARMACOLOGY 2003 (PART 20) ppt

18

Anaesthesia and neuromuscular

block

SYNOPSIS

The administration of general anaesthetics and

neuromuscular blocking drugs is generally

confined to trained specialists. Nevertheless,

nonspecialists are involved in perioperative

care and will benefit from an understanding of

how these drugs act. Doctors from a variety of

specialties use local anaesthetics and the

pharmacology of these drugs is discussed in

detail.

General anaesthesia

Pharmacology of anaesthetics

Inhalation anaesthetics

Intravenous anaesthetics

Muscle relaxants: neuromuscular blocking

drugs

Local anaesthetics

Obstetric analgesia and anaesthesia

Anaesthesia in patients already taking drugs

Anaesthesia in the diseased, the elderly and

children; sedation in intensive therapy units

General anaesthesia

Until the mid-19th century such surgery as was

possible had to be undertaken at tremendous speed.

Surgeons did their best for terrified patients by

using alcohol, opium, hyoscine,1

or cannabis. With

the introduction of general anaesthesia, surgeons

could operate for the first time with careful delib￾eration. The problem of inducing quick, safe and

easily reversible unconsciousness for any desired

length of time in man only began to be solved in

the 1840s when the long-known substances nitrous

oxide, ether, and chloroform were introduced in

rapid succession.

The details surrounding the first use of surgical

anaesthesia were submerged in bitter disputes on

priority following an attempt to take out a patent

for ether. The key events around this time were:

• 1842 — W. E. Clarke of Rochester, New York,

administered for a dental extraction. However,

this event was not made widely known at the

time.

• 1844 — Horace Wells, a dentist in Hartford,

Connecticut, introduced nitrous oxide to

produce anaesthesia during dental extraction.

• 1846 — On October 16 William Morton, a Boston

dentist, successfully demonstrated the

anaesthetic properties of ether.

• 1846 — On December 21 Robert Liston

performed the first surgical operation in England

under ether anaesthesia.2

1

A Japanese pioneer of about 1800 wished to test the

anaesthetic efficacy of a herbal mixture including

solanaceous plants (hyoscine-type alkaloids). His elderly

mother volunteered as subject since she was anyway

expected to die soon. But the pioneer administered it to his

wife for, 'as all three agreed, he could find another wife, but

could never get another mother' (Journal of the American

Medical Association 1966 197:10).

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18 ANAESTHESI A AND N E U R O M U S C U L A R BLOC K

• 1847 — James Y. Simpson, professor of

midwifery at the University of Edinburgh,

introduced chloroform for the relief of labour

pain.

The next important developments in anaesthesia

were in the 20th century when the appearance of

new drugs both as primary general anaesthetics

and as adjuvants (muscle relaxants), new apparatus,

and clinical expertise in rendering prolonged anaes￾thesia safe, enabled surgeons to increase their range.

No longer was the duration and type of surgery

determined by patients' capacity to endure pain.

STAGES OF GENERAL ANAESTHESIA

Surgical anaesthesia is classically divided into four

stages: analgesia, delirium, surgical anaesthesia

(subdivided into four planes), and medullary

paralysis (overdose). This gradual procession of

stages was described when ether was given to un￾premedicated patients, a slow unpleasant process.

Ether is obsolete and the speed of induction with

modern inhalational agents or intravenous anaes￾thesia drugs makes a detailed description of these

separate stages superfluous.

Balanced surgical anaesthesia (hypnosis with

analgesia and muscular relaxation) with a single

drug requires high doses that will cause adverse

effects such as slow and unpleasant recovery, and

depression of cardiovascular and respiratory func￾tion. In modern practice, different drugs are used to

attain each objective so that adverse effects are

minimised.

DRUGS USED

The perioperative period may be divided into three

phases and in each of these a variety of factors will

determine the choice of drugs given:

2

Frederick Churchill, a butler from Harley Street, had his leg

amputated at University College Hospital, London. After

removing the leg in 28 seconds, a skill necessary to

compensate for the previous lack of anaesthetics, Robert

Listen turned to the watching students, and said "this

Yankee dodge, gentlemen, beats mesmerism hollow". That

night he anaesthetised his house surgeon in the presence of

two ladies. Merrington W R1976 University College

Hospital and its Medical School: A History. Heinemann,

London.

Before surgery, an assessment is made of:

• the patient's physical and psychological

condition

• any intercurrent illness

• the relevance of any existing drug therapy.

All of these may influence the choice of anaesthetic

drugs.

During surgery, drugs will be required to provide:

• unconsciousness

• analgesia

• muscular relaxation when necessary

• control of blood pressure, heart rate, and

respiration.

After surgery, drugs will play a part in:

• reversal of neuromuscular block

• relief of pain, and nausea and vomiting

• other aspects of postoperative care, including

intensive care.

Patients are often already taking drugs affecting

the central nervous and cardiovascular systems and

there is considerable potential for interaction with

anaesthetic drugs.

The techniques for giving anaesthetic drugs and

the control of ventilation and oxygenation are of

great importance, but are outside the scope of this

book.

Before surgery (premedication)

The principal aims are to provide:

Anxiolysis and amnesia. A patient who is going to

have a surgical operation is naturally apprehensive

and this anxiety is reduced by reassurance and a

clear explanation of what to expect. Very anxious

patients will secrete a lot of adrenaline (epineph￾rine) from the suprarenal medulla and this may

make them more liable to cardiac arrhythmias

with some anaesthetics. In the past, sedative pre￾medication was given to virtually all patients under￾going surgery. This practice has changed dramatically

because of the increasing proportion of operations

undertaken as 'day cases' and the recognition that

sedative premedication prolongs recovery. Sedative

premedication is now reserved for those who are

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