Local Anaesthetic AgentsManjeet
April 2, 2013 — 1,272 views
Local Anaesthetic are agents which block conduction of impulses in nerves. When applied locally, they produce loss of sensation in the desired area.
Classification of drugs
Mechanism of action: Local Anaesthetic prevent the generation and conduction of impulses. This is produced by blocking voltage dependent sodium channels. So they decrease the permeability of cell membrane to sodium (membrane stabilizing effect). This prevents depolarization. As a result
1) Rise of action potential declines.
2) Impulse conduction slows.
3) Nerve conduction fails.
1. Effect on sensation: Initially, the local Anaesthetic block the sensation of pain and temperature. Later, they produce loss of sensation for touch and pressure. They produce blockade of smaller nerve fibers initially followed by large nerve fibers. Recovery occurs in the reverse order.
2. Central nervous system: Local Anaesthetic produce stimulation of CNS. This manifests as euphoria, restlessness and tremors. Addition to cocaine occurs mainly due to its euphoric effects.
3. Cardiovascular system: All local Anaesthetic except cocaine produce vasodilatation and so hypotension. But cocaine Anaesthetic produce a depressant effect on the myocardium.
4. Other actions: They produce relaxant effect on smooth muscles and neuromuscular blockade.
Absorption, fate and excretion: Local Anaesthetic are not absorbed from unbroken skin. But absorption can occur through mucous membranes. They are usually administered by subcutaneous infiltration. Vasoconstrictors like adrenaline prolong their duration of action. The local Anaesthetic are metabolized in the liver and plasma by hydrolysis.
1. Intolerance like dermatitis, asthmatic attack and anaphylactic reactions.
2. Cardiovascular symptoms like hypotension and cardiac arrest.
3. Central effects like euphoria, excitation, restlessness, tremors and convulsions.
1. Surface anesthesia for pain due to burns, fissures and ulcers.
2. Infiltration anesthesia to anaesthetize nerve ending by subcutaneous infiltration.
3. Nerve block anesthesia where it is injected close to a specific nerve.
4. Spinal anesthesia where it is injected into the subarachnoid space.
5. Systemic use for anti-arrhythmic effect.
COCAINE: It is natural alkaloid. It obtained from the leaves of coca tree, Erythroxylon coca. It produces a local anesthetic effect it produces a sympathomimetic effect (due to inhibition of noradrenalin reuptake) leading to:
1. Dilatation of pupil (mydriasis).
2. Rise in blood temperature.
3. Rise of blood pressure.
4. Rise in blood sugar level.
PROCAINE: Procaine is synthetic compound. It is derivative of Para amino benzoic acid (P.A.B.A.). It is not absorbed from mucous membrane. So it is not useful for local use. It has a vasodilator effect. Since it is a derivate of P.A.B.A., it decreases the antimicrobial activity of sulfonamides. So procaine should not be used with sulfonamides.
LIGNOCAINE: It is an acetanilide derivative. It is the most commonly used drug. It has a quick onset of action. Also it has a high degree of penetration. It may produce drowsiness but has no action of blood vessels. It can be used for all purposes including topical use. It is used in patients allergic to procaine and other local Anaesthetic.
TETRACAINE (Amethocaine): It is an ester of PABA. It is more potent. Also it is more toxic due to slow metabolism. On spinal injection, it produces prolonged effect. It is more suitable for topical use on eye, nose and throat. But it should not be used on injured, inflamed or vascular surfaces. Rapid absorption from these sites may produce serious complications and fatal effects.
BUPIVACAINE: It is four times more potent than lignocaine. Also it is long acting. It is used for infiltration, nerve block, epidural and spinal anesthesia of long duration. It is cardio-toxic and can produce ventricular tachycardia and cardiac depression.
ROPIVACAINE: It is a newer congener of bupivacaine. It is long acting but less cardio-toxic. Continuous epidural administration is used to relieve post-operative and labor pain. It can also be used for nerve block.
BENZOCAINE: It has very low solubility. So it is not absorbed from mucous membrane or abraded skin. Also it produces long lasting anesthesia without systemic toxicity. It is used as lozenge, dusting powder or suppository.
OXETHAZINE: It is a potent local anesthetic. Its main advantage is low degree of ionization even at very low pH values. So it can anesthetize gastric mucosa at its own low pH. So it is administered with antacids for gastritis, drug induced gastric irritation and gastroeosophagial reflux.