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General anaesthesia is required when there is some reason why the other forms of anaesthesia cannot be used or when the operation needs to be done inside the abdomen, chest or head.
General anaesthesia consists of three parts – narcosis, analgesia and muscle relaxation.
Narcosis means that the patient is rendered unconscious; analgesia refers to the need to dull the sensations of pain that are slightly but incompletely diminished by the drugs causing the patient to sleep; muscle relaxation refers to the need to paralyse the patient’s muscles so that the surgeon can gain access to the site of operation.
Inducing and maintaining narcosis
There are two different ways of inducing and keeping a patient asleep. Going off to sleep needs to be quick and stress-free. It has been likened to an aeroplane taking off. The usual method of giving an anaesthetic is to put the patient to sleep with an intravenous induction agent, and then to keep them asleep with an inhalational agent.
An intravenous induction agent is a drug that is injected into a vein and is rapidly transported to the brain by the circulation. Within a minute or two a patient goes from being wide awake to deeply unconscious. The most commonly used intravenous drugs are thiopentone (which is still used after nearly 60 years of being first introduced), propofol and etomidate.
A continuous infusion of propofol can also be used to keep the patient asleep (maintenance of anaesthesia), but this needs to be carefully calculated and is best given with an electronic syringe that can be programmed to administer the drug continuously into the circulation via a drip.
The other types of drug that will produce narcosis are called inhalational agents. These are introduced into the gas stream being delivered from the anaesthetic machine to the patient as a vapour. This is the usual method of keeping a patient asleep during the anaesthetic, but can also be used to induce anaesthesia. This requires that the patient breathe the gas in through a mask on the face, but this technique is not used much in adults because of the unpleasant feeling of having to breathe through a facemask while still awake.
It is often the preferred method of putting children to sleep however, as they do not like the needle prick required for an intravenous induction.
Analgesia
Even though a patient may be anaesthetised he/she can feel pain that may not have the usual emotional effects, but can cause the body to react badly. During anaesthesia very powerful drugs are used to prevent this pain, and one of the side effects of these drugs is that they depress the patient’s ability to breathe. The anaesthetist has to thus monitor the patient’ breathing very carefully, and if it is inadequate, has to support it.
Muscle relaxation
The third type of drug commonly used by anaesthetists is the group called muscle relaxants. These were originally derived from South American arrow poisons, but today they are manufactured as synthetic drugs. Although they paralyse the patient’s muscles they do not have any effect on the brain, and are thus never given unless the patient is anaesthetised.
A paralysed patient cannot of course breathe as the respiratory muscles are also paralysed, and in these circumstances the anaesthetist thus has to place the patient on a ventilator.
The effect of general anaesthetic on the body:
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Preparations in theatre
