Local anaesthesia is generally used when loss of consciousness is neither necessary nor desirable and also as an adjunct to major surgery to avoid high-dose general anaesthesia. It can be used for major surgery, with sedation, though many patients prefer to be unconsciousness. It is invaluable when the operator must also be the anaesthetist, which is often the case in some parts of the developing world.
Local anaesthetics may be used in several ways to provide:
• Surface anaesthesia, as solution, jelly, cream or lozenge.
• Infiltration anaesthesia, to paralyse the sensory nerve endings and small cutaneous nerves
• Regional anaesthesia.
Regional anaesthesia requires considerable knowledge of anatomy and attention to detail for both success and safety.
Nerve block means to anaesthetise a region, which may be small or large, by injecting the drug around, not into, the appropriate nerves, usually either a peripheral nerve or a plexus. Nerve block provides its own muscular relaxation as motor fibres are blocked as well as sensory fibres, although with care differential block, affecting sensory more than motor fibres, can be achieved. There are various specialised forms: brachial plexus, paravertebral, paracervical, pudendal block. Sympathetic nerve blocks may be used in vascular disease to induce vasodilatation.
Intravenous. A double cuff is applied to the arm, inflated above arterial pressure after elevating the limb to drain the venous system, and the veins filled with local anaesthetic, e.g. 0.5-1% lidocaine without adrenaline (epinephrine). The arm is anaesthetised in 6-8 min, and the effect lasts for up to 40min if the cuff remains inflated. The cuff must not be deflated for at least 20 minutes. The technique is useful in providing anaesthesia for the treatment of injuries speedily and conveniently, and many patients can leave hospital soon after the procedure. The technique must be meticulously conducted, for if the full dose of local anaesthetic is accidentally suddenly released into the general circulation severe toxicity and even cardiac arrest may result. Bupivacaine is no longer used for intravenous regional anaesthesia as cardiac arrest caused by it is particularly resistant to treatment. Patients should be fasted and someone skilled in resuscitation must be present.
Extradural (epidural) anaesthesia is used in the thoracic, lumbar and sacral (caudal) regions. Lumbar epidurals are used widely in obstetrics and low thoracic epidurals provide excellent analgesia after laparotomy. The drug is injected into the extradural space where it acts on the nerve roots. This technique is less likely to cause hypotension than spinal anaesthesia. Continuous analgesia is achieved if a local anaesthetic, often mixed with an opioid, is infused through an epidural catheter.
Subarachnoid (intrathecal) block (spinal anaesthesia). By using a solution of appropriate specific gravity and tilting the patient the drug can be kept at an appropriate level. Sympathetic nerve blockade causes hypotension. Headache due to CSF leakage is virtually eliminated by using very narrow atraumatic 'pencil point' needles.
Serious local neurological complications, for example infection and nerve injury, are extremely rare.
Opioid analgesics are used intrathecally and extradurally. They diffuse into the spinal cord and act on its opioid receptors (see p. 333); they are highly effective in skilled hands for postsurgical and intractable pain. Respiratory depression may occur. The effect begins in 20 min and lasts about 5 h. Diamorphine or other more lipid-soluble opioids, such as fentanyl, may be used.
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