Techniques of local anaesthesia Surface anaesthesia (topical anaesthesia) Local anaesthetic is applied on the mucous membrane of the nose, mouth, eyes, throat, upper respiratory tract, esophagus, urethra, ulcers, burns, fissures etc. Tetracaine 2%, lignocaine 2-10%, cocaine 1-4%, benzocaine 1-2% etc are used for topical application. Surface anaesthetics are available as solution, ointment, jel, cream, spray, lozenges etc. Addition of adrenaline does not prolong the duration of surface anaesthesia because of poor penetration. Topical anaesthesia is also used in many diagnostic procedures like tonometry in eye and during endoscopies. Infiltration anaesthesia Local anaesthetic is injected directly into tissues to be operated blocks sensory nerve endings. Local anaesthetic is infiltrated into the skin, subcutaneous tissue or deeper structures, including intra-abdominal organs. The most frequently used local anaesthetics for infiltration are lignocaine (0.5 to 1%), procaine (0.5-1%) and bupivacaine (0.125-0.25%). Addition of adrenaline (1:200,000 – 250,000) to local anaesthetic prolongs the duration of anaesthesia. Infiltration anaesthesia is suitable only for small areas. The main disadvantage of infiltration is requirement of large amounts of the drug to anaesthetize relatively small area. It can be used for drainage of an abscess, excision of small swelling, suturing of cut wounds episiotomy etc. Infiltration anaesthesia is contraindicated, if there is local infection and clotting disorders. Field block anaesthesia is achieved by injecting the local anaesthetic subcutaneously which anaesthetizes the area distal to the injection. This principle is used in case of scalp, anterior abdominal wall, upper and lower extremities, in which a smaller dose produces larger area of anaesthesia. Nerve block: Local anaesthetic is injected very close to or around the peripheral nerve or nerve plexuses. It produces larger areas of anaesthesia than field block. Brachial plexus block for procedures on upper limb Cervical plexus block for surgery of the neck Intercostal nerve block for anterior abdominal wall surgery Sciatic and femoral nerve block for surgery distal to the knee. In this procedure the requirement of local anaesthetic is less than that of field block and infiltration anaesthesia. Spinal anaesthesia It is one of the most popular forms of anaesthesia. Local anaesthetic is injected into the subarachnoid space to anaesthetize spinal roots. Site of injection: Into the space between L2-3 or L3-4 below the lower end of the spinal cord. The level of anaesthesia is influenced by: (i) site of injection, (ii) amount of fluid injected, (iii) force of injection, (iv) specific gravity of the drug solution [hyperbaric (in 10% glucose), hypobaric (in distilled water), or isobaric] and (v) position of the patient – lying prone / lateral or tilted with head down position. Commonly used local anaesthetics: Lignocaine, tetracaine, bupivacaine etc. Addition of adrenaline to spinal anaesthetic increases the duration or intensity of block. Uses: Spinal anaesthesia can be used for surgical procedures below the level of umbilicus, i.e. lower limb surgery, caesarean section, obstetric procedures, prostatectomy, surgery on perineum, appendicectomy etc. Advantages of spinal anaesthesia are: No loss of consciousness, complete relaxation and good analgesia. Cardiac, pulmonary and renal disease patients tolerate spinal anaesthesia better than general anaesthesia. Complications of spinal anaesthesia 1. Headache is due to see page of CSF and can be reduced by using very fine needles. 2. Hypotension is due to blockade of the sympathetic vasoconstrictor fibres to blood vessels. Venous return to the heart is reduced due to paralysis of skeletal muscles in the legs. Hypotension is treated by raising the foot end and with sympathomimetics such as ephedrine, mephenteramine, metaraminal etc. 3. Respiratory paralysis It is due to paralysis of intercostal muscles. Respiratory failure may occur due to respiratory centre ischemia as a result of hypotension. 4. Septic meningitis and nerve injury are extremely rare at present, because of good anaesthetic practice. 5. Post operative urinary retention may occur. Contraindications are hypotension and shock, young children, vertebral abnormalities and sepsis in the region of lumbar puncture. Epidural anaesthesia: LA is injected into epidural space where it acts on spinal nerve roots. Lignocaine and bupivacaine are commonly used. It is safer but the technique is little difficult than spinal anaesthesia. Epidural anaesthesia is slower in onset than spinal. It requires a much larger amount of the drug. It is mainly useful in obstetric analgesia. Intravenous regional anaesthesia (Bier’s block) It is mainly used in anaesthetizing the upper limb. Lignocaine and prilocaine are commonly used. Local anaesthesia is injected into the vein of the limb whose blood flow is occluded by a tourniquet.
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