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P A R T 4
Drugs acting on the central and peripheral nervous systems
a risk of abnormal reaction to the CNS-depressing
and muscle-relaxing effects of these drugs.
•
Cardiovascular factors
: Underlying vascular disease,
coronary artery disease or hypotension, which put
people at risk for severe reactions to anaesthesia,
such as hypotension and shock, dysrhythmias and
ischaemia.
•
Respiratory factors
: Obstructive pulmonary disease
(e.g. asthma, chronic obstructive pulmonary disease,
bronchitis), which can complicate the delivery of gas
anaesthetics, as well as the intubation and mechanical
ventilation that must be used in most cases of general
anaesthesia.
•
Renal and hepatic function
: Conditions that interfere
with the metabolism and excretion of anaesthetics
(e.g. acute renal failure, hepatitis) and could result
in prolonged anaesthesia and the need for continued
support during recovery. Toxic reactions to the
accumulation of abnormally high levels of anaesthetic
agents may even occur.
Balanced anaesthesia
With the wide variety of drugs available, the therapeutic
effects required need to be balanced with the potential
for adverse effects. This is accomplished by
balanced
anaesthesia
—the combining of several drugs, each with
a specific effect, to achieve analgesia, muscle relaxa-
tion, unconsciousness and amnesia rather than using
one drug. Balanced anaesthesia commonly involves the
following agents:
•
Preoperative medications
, which may include the use
of anticholinergics that decrease secretions to facilitate
intubation and prevent bradycardia associated with
neural depression.
•
Sedative–hypnotics
to relax the person, facilitate
amnesia and decrease sympathetic stimulation.
•
Antiemetics
to decrease the nausea and vomiting
associated with the slowing of GI activity.
•
Antihistamines
to decrease the chance of allergic
reaction and help to dry up secretions.
•
Opioids
to aid analgesia and sedation.
Many of these drugs are given before the general anaes-
thetic is administered as
pre-medication
to facilitate
the process. Some are continued during surgery to aid
the general anaesthetic, allowing therapeutic effects at
lower doses. For example, people may receive a neuro-
muscular junction (NMJ) blocker (Chapter 28) to stop
muscle activity and a rapid-acting intravenous general
anaesthetic to induce anaesthesia, and then a gas general
anaesthetic to balance the anaesthetic effect during the
procedure and allow for easier recovery. Careful selec-
tion of appropriate general anaesthetic agents, along
with monitoring and support of the person, helps to
alleviate many problems.
Administration of general anaesthesia
General anaesthesia is delivered by a specialist doctor
(anaesthetist) trained in the delivery of these potent
drugs along with intubation, mechanical ventilation and
full life support. During the delivery of anaesthesia, the
person can go through predictable stages (Figure 27.1),
referred to as the depth of anaesthesia:
Stage 1, the analgesia stage, refers to the loss of pain
sensation, with the person still conscious and able to
communicate.
Stage 2, the excitement stage, is a period of excitement
and often combative behaviour, with many signs of
sympathetic stimulation (e.g. tachycardia, increased
respirations and blood pressure changes).
Stage 3, surgical anaesthesia, involves relaxation of
skeletal muscles, return of regular respirations, and
progressive loss of eye reflexes and pupil dilation.
Surgery can be safely performed in stage 3.
Stage 4, medullary paralysis, is very deep CNS
depression with loss of respiratory and vasomotor
centre stimuli, in which death can occur rapidly. If a
person reaches this level, the anaesthesia has become
too intense and the situation is critical.
General anaesthesia administration is also divided
into three phases: induction, maintenance and recovery.
Stage 1
Stage 2
Stage 3
Loss of
pain sensation
Combative
excitement
Danger period—
sympathetic stimulation
Relaxation of skeletal muscles
Loss of reflexes
Pupil dilation
Stage 4
Death
Maintenance
Induction
Severe CNS
depression
Danger
severe CNS
depression
Anaesthesia
Recovery
FIGURE 27.1
Stages of general
anaesthesia.
