Anaesthetics, local Lignocaine

Infiltration anaesthesia is applied fan-shaped, with as few needle punctures as possible, in close proximity of the wound or the skin area to be treated. An aspiration should always take place to avoid intravascular injection. Suitable alternatives are lidocaine (lignocaine) or prilocaine for injection 5-10 mg/ml, with or without adrenaline. When making an incision of an abscess it is sometimes difficult to use a local anaesthetic if there is a pronounced inflammatory reaction, since the effect of the anaesthetic is reduced due to an increased acidity level. While adrenaline reduces bleeding and delays dispersion of the anaesthetic, local anaesthetic/adrenaline combinations are contraindicated for local anaesthesia of digits, on the face or where the skin survival is at risk. 

The site of drug action means where a drug acts and mechanism means how the drug acts. Drug which act only at the site of application (i.e. localized region) are termed as local or topical action for example, ointments, paste, creams and certain other local preparations used externally produce only local effect. The local anaesthetics like lignocaine, procaine produce anaesthesia (local) in a localized region only. 

Lignocaine (lidocaine) A local anaesthetic and voltage-operated Na+ channel blocker. 

Drugs with a chromophore such as that of procaine include procainamide and proxymetacaine. It should be noted that local anaesthetics such as bupivacaine and lignocaine do not fall into this category since they are aromatic amides and the lone pair on the nitrogen atom is not fully available due to electron withdrawal by the adjacent carbonyl group. 

Jellies are transparent or translucent, non-greasy medicated semi-solid preparation used externally, sometime containing local anaesthetic agent also e.g. Lignocaine jelly. 

Local anaesthetics are readily absorbed through mucous membranes and damaged skin. These are weak bases and at tissue pH diffuse through the connective tissue and cellular membranes to reach the nerve fibres where ionization can occur. Amide type local anaesthetics (lignocaine, bupivacaine) are metabolised in the liver and in some cases the kidneys. These are considerably protein bound. For certain procedures the duration of action is prolonged by adding... 

Dental anaesthesia—The total amount of local anaesthetics injected is much smaller (20-80 mg of lignocaine) than that used for othr purpose. Lignocaine (2%) with adrenaline (1 80,000) is the standard local anaesthetic preparation used in dentistry which produces good soft tissue and pulpal anaesthasia and also reduce postextraction bleeding. 

Nerves differ in their sensitivity to local anaesthetics. When lidocaine (lignocaine) is applied to a mixed peripheral nerve the onset of the block is in the order, vasodilatation (B fibres), loss of pain and temperature (C and A6 fibres), muscle spindle reflex (Ay fibres), motor and pressure (A(3 fibres) and large motor and proprioception (Aa fibres). This phenomenon is called differential block. There are other minor variations in this ranking order among the local anaesthetics. The basis of differential block is thought to be the result of variability in the sensitivity of different nerves to the same agent. 

Ambient pH in the extracellular fluid (ECF) is approximately 7.4 but the value varies and this determines the proportions of ionised and unionised local anaesthetic drug. A decrease in ambient pH will increase the amount of ionised drug and reduce the unionised fraction available for transfer across the cell membrane. A common example of this is when infection or inflammation reduces the ambient pH. In the case of lidocaine (lignocaine), a fall in tissue pH from 7.4 to 7.0 will halve the amount of unionised drug. This has obvious implications for efficacy. Similar effects occur following repeated administrations of acidified local anaesthetic solutions. 

Local anaesthetics are rarely associated with localised nerve damage. There have been a number of reports of prolonged motor and sensory deficits after large doses of chloroprocaine. This is believed to be related to the antioxidant sodium bisulphite. Radiculopathy has been reported following the subarachnoid administration of lidocaine (lignocaine) 5%. 

Figure 2.5 An outline of the known metabolic pathways of the local anaesthetic lignocaine...

Unconscious patients are usually catheterised and, in this case, the sample may be contaminated with the catheter lubricant which frequently contains lignocaine as a local anaesthetic. Urine is ideal for qualitative screening as it is available in large volumes, and usually contains higher concentrations of drugs or poisons than blood. The presence of drug metabolites can be used to assist identification if chromatographic techniques which can separate them are used. A 50-ml sample is sufficient for a comprehensive series of tests, and no preservative should be added. 

Local anaesthetics benzocaine, butacaine, chloroprocaine, cinchocaine, cocaine, cyclomethycaine, dimethisoquin, diperodon, dyclonine, lignocaine... 

Calamine and astringents (aluminium acetate, tannic acid) may help. Local anaesthetics do not offer any long-term solution and since they are liable to sensitise the skin they are best avoided lignocaine is least troublesome in this respect. Topical doxepin... 

Impaired liver function, whether due to primary cellular insufficiency or to low liver blood flow as in cardiac failure, may both delay elimination and allow higher peak plasma concentrations of both types of local anaesthetic. This is likely to be important only with large or repeated doses or infusions. These considerations are important in the management of cardiac arrhythmias by i.v. infusion of lignocaine (lidocaine) (see p. 502). 

Cardiovascular drugs. Those that possess local anaesthetic properties [quinidine, procainamide, lignocaine (lidocaine)] and certain 3-blockers (propranolol, oxprenolol) interfere with acetylcholine release and may aggravate or reveal myasthenia gravis. 

Coleman M, Kelly DJ. Local anaesthetic toxicity in a pregnant patient undergoing lignocaine-induced intravenous regional anaesthesia. Acta Anaesthesiol Scand 1998 42(2) 267-9. 

Local anaesthetics Lidocaine hydrochloride Minims Lignocaine and Fluorescein Local anaesthesia POM medicine for administration (not for sale or supply)... 

It is regarded as an all-purpose soluble local anaesthetic agent. The onset and duration of action is almost similar to that of lignocaine. 

Lignocaine has already been diseussed under local anaesthetics and it is also found to possess anti-arrhythmic actions. There are a few other synthetic compormds which exhibit lignocaine-like properties, e.g., mexiletine hydrochloride tocainide which shall be discussed as rmder ... 

Giaufre E, Bruguerolle B, Morisson-Lacombe G, Rousset-Rouviere B. The influence of di-azepam on the plasma concentrations of bupivacaine and lignocaine after caudal injecticn of a mixture of the local anaesthetics in children. BrJ Clin Pharmacol (1988) 26, 116-18. 

Fig. 5. a Chemical structure of procaine hydrochloride, 2-diethylaminoethyl-4-aminoben-zoate hydrochloride, an ester local anaesthetic, b Structure of lignocaine hydrochloride, iV-diethylaminoacetyl-2,6-xylidine hydrochloride monohydrate, an amide local anaesthetic... 

Amethocaine (tetracaine), bupivacaine, Ugnocaine (lindocaine), prilocaine, and procaine FIA Lignocaine in local anaesthetic preparation Bupivacaine and procaine 5X10" M, lignocaine 7X10- M (205)... 

Two methods which are quite widely applicable to many local anaesthetics are non-aqueous titration (perchloric acid to crystal violet, p. 792) and titration with sodium nitrite (0 5 g in 75 ml water and 10 ml hydrochloric acid, titrating with O IM nitrite and determining the end-point using the dead-stop technique, p. 867). A few materials such as benzocaine may be determined by both methods but in most cases either one or the other is applicable. Lignocaine hydrochloride, for example, gives a satisfactory end-point by the non-aqueous method but cannot be determined by titration with nitrite procaine hydrochloride on the other hand is satisfactorily titrated wdth nitrite but gives rise to a precipitate during titration in non-aqueous medium which obscures the end-point. Certain compounds such as amethocaine hydrochloride cannot be determined by either method. 

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