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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

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. 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. Surgery can be safely performed in stage 3. Stage 4, medullary paralysis, is very deep CNS

reaction and help to dry up secretions. • Opioids to aid analgesia and sedation.

Stage 1

Loss of pain sensation

Stage 2

Combative excitement

Danger period— sympathetic stimulation

Induction

Recovery

Stage 3

Relaxation of skeletal muscles Loss of reflexes Pupil dilation

Anaesthesia

Maintenance

Danger severe CNS depression

Severe CNS depression

Stage 4

Death

FIGURE 27.1  Stages of general anaesthesia.