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Anaesthesia is the loss of the sensations of touch, pressure, pain and temperature in any part of the body, or in the whole of it. This may be due to disease or injury to the nerves carrying impulses for these sensations to the brain, or to damage from disease or injury to the brain itself. However, most people think of anaesthesia mainly in the context of surgery.
Drugs are used to cause general anaesthesia (a 'GA') or local anaesthesia (a 'local'). General anaesthesia is associated with a loss of consciousness, which is quite different from simply being asleep.
Anaesthetics are now safely given to patients who would formerly have been thought too frail or seriously ill. Monitoring equipment can give a continuous indication of the vital processes, including heartbeat and breathing. Any change in these immediately prompts remedial action.
In this way the members of an operating team, as well as the anaesthetist, are continually aware of the patient's condition and can respond to any change. Modern anaesthetic methods aim to keep the patient quietly 'asleep', while other drugs are used to remove the sensation of pain, to relax muscles and to avoid too much loss of blood.
Premedication with drugs (the 'pre-med') produces a calm, relaxed state of mind, before and after the anaesthetic. It is common for patients to wake up after their operation with no recollection of being taken to the operating theatre.
Spinal anaesthesia is a major form of local anaesthesia, performed by injecting an anaesthetic drug between two of the vertebrae of the lower back into the cerebrospinal fluid. This blocks transmission in the nearby spinal nerves, causing a complete loss of feeling from that point down the body. It is used when it is desirable to avoid the risk of a general anaesthetic, such as in older people or those with heart or lung problems. Hip replacement operations and Prostate gland removal (prostatectomy) are commonly performed under spinal anaesthesia.
Epidural anaesthesia involves injecting the drug into the epidural space surrounding the spinal cord but not into the fluid, which is inside its protective covering called the dura. The needle is passed into the space between two of the spinal bones in the small of the back (lumbar vertebrae). A fine plastic tube is then passed through the needle. Its end is left in the epidural space so that anaesthetic can be injected from time to time as needed.
Epidural anaesthesia is popular for childbirth because, although highly effective in the relief of pain, it has no effect on the contractions of the womb or on the respiratory centre of the baby. It is a skilled procedure requiring the services of an experienced anaesthetist. It is valuable for long-term anaesthesia, but diminishes the voluntary assistance the mother-to-be can give, and forceps have to be used more often than in deliveries without anaesthetic.
It is also safer than general anaesthesia, especially if this has to be given urgently to an unprepared patient who may have eaten recently and who will be liable to vomit - a dangerous complication during full general anaesthesia.