Anaesthesia Health Dictionary

Anaesthesia: From 3 Different Sources


Absence of all sensation; insensibility. The term most commonly refers to anaesthesia that is induced artificially for medical purposes. Two types of anaesthesia are used: local (see anaesthesia, local) and general (see anaesthesia, general).

Damage to nerve tissues by injury or disease can produce anaesthesia in a localized area.

Health Source: BMA Medical Dictionary
Author: The British Medical Association
The loss or absence of sensation or feeling. Commonly used to describe a reversible process which allows operations and painful or unpleasant procedures to be performed without distress to the patient.

The speciality of anaesthesia broadly covers its provision for SURGERY, intensive therapy (intensive care), chronic pain management, acute pain management and obstetric analgesia. Anaesthetists in Britain are trained specialists with a medical degree, but in many countries some anaesthetists may be nurse practitioners working under the supervision of a medical anaesthetist.

The anaesthetist will assess the patient’s ?tness for anaesthesia, choose and perform the appropriate type of anaesthetic while monitoring and caring for the patient’s well-being, and, after the anaesthetic, supervise recovery and the provision of post-operative pain relief.

Anaesthesia may be broadly divided into general and local anaesthesia. Quite commonly the two are combined to allow continued relief of pain at the operation site after the patient awakens.

General anaesthesia is most often produced by using a combination of drugs to induce a state of reversible UNCONSCIOUSNESS. ‘Balanced’ anaesthesia uses a combination of drugs to provide unconsciousness, analgesia, and a greater or lesser degree of muscle relaxation.

A general anaesthetic comprises induction, maintenance and recovery. Historically, anaesthesia has been divided into four stages (see below), but these are only clearly seen during induction and maintenance of anaesthesia using inhalational agents alone.

(1) Onset of induction to unconsciousness

(2) Stage of excitement

(3) Surgical anaesthesia

(4) Overdosage

Induction involves the initial production of unconsciousness. Most often this is by INTRAVENOUS injection of a short-acting anaesthetic agent such as PROPOFOL, THIOPENTONE or ETOMIDATE, often accompanied by additional drugs such as ANALGESICS to smooth the process. Alternatively an inhalational technique may be used.

Maintenance of anaesthesia may be provided by continuous or intermittent use of intravenous drugs, but is commonly provided by administration of OXYGEN and NITROUS OXIDE or air containing a volatile anaesthetic agent. Anaesthetic machines are capable of providing a constant concentration of these, and have fail-safe mechanisms and monitors which guard against the patient’s receiving a gas mixture with inadequate oxygen (see HYPOXIC). The gases are adminstered to the patient via a breathing circuit either through a mask, a laryngeal mask or via ENDOTRACHEAL INTUBATION. In recent years, concerns about side-effects and pollution caused by volatile agents have led to increased popularity of total intravenous anaesthesia (TIVA).

For some types of surgery the patient is paralysed using muscle relaxants and then arti?cially ventilated by machine (see VENTILATOR). Patients are closely monitored during anaesthesia by the anaesthetist using a variety of devices. Minimal monitoring includes ELECTROCARDIOGRAM (ECG), blood pressure, PULSE OXIMETRY, inspired oxygen and end-tidal carbon-dioxide concentration – the amount of carbon dioxide breathed out when the lungs are at the ‘empty’ stage of the breathing cycle. Analgesic drugs (pain relievers) and local or regional anaesthetic blocks are often given to supplement general anaesthesia.

Volatile anaesthetics are either halogenated hydrocarbons (see HALOTHANE) or halogenated ethers (iso?urane, en?urane, des?urane and sevo?urane). The latter two are the most recently introduced agents, and produce the most rapid induction and recovery – though on a worldwide basis halothane, ether and chloroform are still widely used.

Despite several theories, the mode of action of these agents is not fully understood. Their e?cacy is related to how well they dissolve into the LIPID substances in nerve cells, and it is thought that they act at more than one site within brain cells – probably at the cell membrane. By whatever method, they reversibly depress the conduction of impulses within the CENTRAL NERVOUS SYSTEM and thereby produce unconsciousness.

At the end of surgery any muscle relaxant still in the patient’s body is reversed, the volatile agent is turned o? and the patient breathes oxygen or oxygen-enriched air. This is the reversal or recovery phase of anaesthesia. Once the anaesthetist is satis?ed with the degree of recovery, patients are transferred to a recovery area within the operating-theatre complex where they are cared for by specialist sta?, under the supervision of an anaesthetist, until they are ready to return to the ward. (See also ARTIFICIAL VENTILATION OF THE LUNGS.) Local anaesthetics are drugs which reversibly block the conduction of impulses in nerves. They therefore produce anaesthesia (and muscle relaxation) only in those areas of the body served by the nerve(s) affected by these drugs. Many drugs have some local anaesthetic action but the drugs used speci?cally for this purpose are all amide or ester derivatives of aromatic acids. Variations in the basic structure produce drugs with di?erent speeds of onset, duration of action and preferential SENSORY rather than MOTOR blockade (stopping the activity in the sensory or motor nerves respectively).

The use of local rather than general anaesthesia will depend on the type of surgery and in some cases the unsuitability of the patient for general anaesthesia. It is also used to supplement general anaesthesia, relieve pain in labour (see under PREGNANCY AND LABOUR) and in the treatment of pain in persons not undergoing surgery. Several commonly used techniques are listed below:

LOCAL INFILTRATION An area of anaesthetised skin or tissue is produced by injecting local anaesthetic around it. This technique is used for removing small super?cial lesions or anaesthetising surgical incisions.

NERVE BLOCKS Local anaesthetic is injected close to a nerve or nerve plexus, often using a peripheral nerve stimulator to identify the correct point. The anaesthetic di?uses into the nerve, blocking it and producing anaesthesia in the area supplied by it.

SPINAL ANAESTHESIA Small volumes of local anaesthetic are injected into the cerebrospinal ?uid through a small-bore needle which has been inserted through the tissues of the back and the dura mater (the outer membrane surrounding the spinal cord). A dense motor and sensory blockade is produced in the lower half of the body. How high up in the body it reaches is dependent on the volume and dose of anaesthetic, the patient’s position and individual variation. If the block is too high, then respiratory-muscle paralysis and therefore respiratory arrest may occur. HYPOTENSION (low blood pressure) may occur because of peripheral vasodilation caused by sympathetic-nerve blockade. Occasionally spinal anaesthesia is complicated by a headache, perhaps caused by continuing leakage of cerebrospinal ?uid from the dural puncture point.

EPIDURAL ANAESTHESIA Spinal nerves are blocked in the epidural space with local anaesthetic injected through a ?ne plastic tube (catheter) which is introduced into the space using a special needle (Tuohy needle). It can be used as a continuous technique either by intermittent injections, an infusion or by patient-controlled pump. This makes it ideal for surgery in the lower part of the body, the relief of pain in labour and for post-operative analgesia. Complications include hypotension, spinal headache (less than 1:100), poor e?cacy, nerve damage (1:12,000) and spinal-cord compression from CLOT or ABSCESS (extremely rare).

Health Source: Medical Dictionary
Author: Health Dictionary
n. loss of feeling or sensation in a part or all of the body. Anaesthesia of a part of the body may occur as a result of injury to or disease of a nerve; for example in trauma. The term is usually applied, however, to the technique of reducing or abolishing an individual’s sensation of pain to enable surgery to be performed. This is effected by administering drugs (see anaesthetic).

General anaesthesia is total unconsciousness, usually achieved by administering a combination of injections and gases (the latter are inhaled through a mask). Local anaesthesia abolishes pain in a limited area of the body and is used for minor operations, particularly many dental procedures. It may be achieved by injections of substances such as lidocaine close to a local nerve, which deadens the tissues supplied by that nerve. Local anaesthesia may be combined with intravenous sedation. An appropriate injection into the spinal column produces *spinal anaesthesia or *epidural anaesthesia in the lower limbs or abdomen. Regional anaesthesia, usually of a limb, is achieved by encircling local anaesthetic solutions or direct application of anaesthetic to one or more peripheral nerves.

Health Source: Oxford | Concise Colour Medical Dictionary
Author: Jonathan Law, Elizabeth Martin

Epidural Anaesthesia

See ANAESTHESIA.... epidural anaesthesia

Spinal Anaesthesia

See under ANAESTHESIA.... spinal anaesthesia

Local Anaesthesia

Loss of sensation produced in a part of the body to stop pain while a person is examined, investigated or treated (see also ANAESTHESIA). The anaesthesia is e?ected by giving drugs in a local area temporarily to stop the action of pain-carrying nerve ?bres. To anaesthetise a large area, a nerve block is done. Various drugs are used, depending on the depth and length of local anaesthesia required.... local anaesthesia

Regional Anaesthesia

See ANAESTHESIA – Local anaesthetics.... regional anaesthesia

Anaesthesia, Dental

Loss of sensation induced in a patient to prevent pain during dental treatment.

For minor procedures, a local anaesthetic (see anaesthesia, local) is injected either into the gum at the site being treated or into the nerve a short distance away (called a peripheral nerve block).

In addition, topical anaesthetics are often used on the gums.

For more complicated procedures, such as periodontal (gum) surgery and multiple tooth extractions, general anaesthesia is carried out (see anaesthesia, general).... anaesthesia, dental

Anaesthesia, General

Loss of sensation and consciousness induced to prevent the perception of pain throughout the body during surgery. General anaesthesia is usually induced by intravenous injection of a barbiturate drug and maintained by inhalation of anaesthetic gases such as halothane, which may be introduced into the lungs via an endotracheal tube. During the anaesthetic, the pulse, blood pressure, and other vital signs are continuously monitored.

General anaesthetics have become much safer, and serious complications are rare.

However, severe pre-existing diseases such as lung or heart disorders increase the risks.

Minor after effects such as nausea and vomiting are usually controlled effectively with antiemetic drugs.... anaesthesia, general

Anaesthesia, Local

Loss of sensation induced in a limited region of the body to prevent pain during examinations, diagnostic or treatment procedures, and surgical operations. Local anaesthesia is produced by the administration of drugs that temporarily interrupt the action of pain-carrying nerve fibres.

Local anaesthetics applied topically before injections or blood tests include sprays and skin creams and ointments.

These are often used for children.

For minor surgical procedures, such as stitching of small wounds, local anaesthesia is usually produced by direct injection into the area to be treated.

To anaesthetize a large area, or when local injection would not penetrate deeply enough into body tissues, a nerve block may be used.

Nerves can also be blocked where they branch off from the spinal cord, as in epidural anaesthesia, which is widely used in childbirth, and spinal anaesthesia, which is used for surgery on the lower limbs and abdomen.

Serious reactions to local anaesthetics are uncommon.

Repeated use of topical preparations may cause allergic rashes.... anaesthesia, local




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