This article throws light upon the seventeen major classical local anaesthetics. They are: 1. Cocaine 2. Procaine 3. Chloroprocaine 4. Tetracaine (Amethocaine) 5. Lidocaine (Lignocaine) 6. Mepivacaine 7. Bupivacaine 8. Hexylcaine 9. Proparacaine (Proxymetacaine) 10. Oxybuprocaine (Benoxinate) 11. Benzocaine (Ethyl Amino Benzoate) 12. Butamben (Butylaminobenzoate) 13. Cinchocaine (Dibucaine) and few others.
1. Cocaine:
Local anaesthetic potency is 3 times (range 2-6) that of procaine. Onset of action is about 5 minutes (range 2-10 min) and duration 20 to 45 minutes (topical) and up to 2 hours (spinal). It is vasoconstrictor and mydriatic agent. The drug was once choice anaesthetic for all uses. It is now exclusively recommended for topical anaesthesia; 2-4% solution for ophthalmic anaesthesia, and 5-10 % for other mucosal uses.
Vasoconstrictor potential restricts its prolonged ophthalmic use as it favours ischemia and corneal ulceration. It is advantageous on other mucosal uses as it reduces operative bleeding.
High tissue irritancy disfavours its use as infiltration agent. Systemic toxicity potential is at-least 4 times that of procaine (range 4 – 1 6). It is a scheduled drug so not freely available for use; there is high abuse (addiction) potential to its psychomitic actions (i.e. euphoria).
2. Procaine:
Local anaesthetic potency (LAP) is low compared to other agents. Onset of action is slow about 5 to 10 minutes (range 5- 20 mins). Duration of action is 30 to 90 minutes (range 15 – 120 mins). Adrenaline raises duration of action almost by 50- 100%. It is practically non-irritant to tissues; effective concentrations being 1/150th of tissue irritant concentration.
Systemic toxicity is low as compared to other agents; parakeets appear to be highly sensitive to its lethal effects, and horses appear to be highly sensitive to its convulsive effects. The drug is very popular in India for all types of anaesthesia in animal except topical one.
It is topically ineffective due to its poor lipid solubility that reduces its penetration through mucus membranes. Procaine, or preferably procainamide, is an effective anti-arrhythmic agent.
The drug is not recommended for use while the patient is on sulfonamide therapy. It is desirable to avoid its use during infections, unless adequate antimicrobial therapy has been instituted, as it would favour growth of pathogens by providing their growth factor PABA.
3. Chloroprocaine:
It is similar to procaine except:
(i) LAP 1 to 2.4 times more,
(ii) Onset of action faster,
(iii) Duration up to 1 hour,
(iv) Toxicity potential is equal to or lesser than that of procaine, and
(v) Used in humans as 3% solution for regional intravenous anaesthesia. The drug is topically ineffective like procaine.
4. Tetracaine (Amethocaine):
Tetracaine is longacting and one of most potent ester type local anaesthetics. LAP is at-least ten times (range 10-25) that of procaine. Onset of action about 5 mins (range 3-10 mins). It is used as longacting ophthalmic agent (duration 30-60 mins) or longacting injectable anaesthetic (duration 400-450 minutes; range 200-600 mins).
Local toxicity potential is 8-10 times while systemic toxicity is 10-30 times that of procaine. Tetracaine has antimicrobial activity against Candida sp., pseudomonas Sp. and staphylococcus Sp. The drug is one of choice agents for ophthalmic and spinal anaesthesia in humans where long duration effect is required.
5. Lidocaine (Lignocaine):
LAP of lidocaine is about 2 times (range 1.5-4) that of procaine. Onset of action is about 5 mins (range 2-5 mins) and duration about 2 hours (range 0.75-3 hrs.). Adrenaline raises the duration up to about 5 hours (e.g. epidural use in cattle). It is up to twice as toxic (usual stated value 1-1.5) as procaine. It is a multipurpose local anaesthetic with worldwide popularity.
Lidocaine is reported to be an immunosuppressant agent for both humoral as well as cell-mediated responses. The drug is also used as an anti-arrhythmic agent. It is available in Indian market as topical and injectable preparations.
6. Mepivacaine:
LAP is 2-4 times that of procaine. Onset of action is more rapid or equal to that with lidocaine. Duration of action ranges from 30 to 120 minutes. It is 1.5 to 2 times more toxic than procaine. Adrenaline raises its duration of action 2-8 fold.
Mepivacaine is not as effective topical anaesthetic as lidocaine. Its use is not considered safer in obstetrical practice as it tends to be more toxic to neonate due to its high ionization at acidic pH of neonatal blood that favours its accumulation in the neonate. The drug in veterinary practice has been approved for use in equines.
7. Bupivacaine:
LAP is 7 times (range 6-8) that of procaine. Onset of action is slow about 10-15 minutes (range 4-30 mins). Duration of action is longer about 400-450 mins (range 200- 600 mins). It is 6 times (range 5-7) more toxic than procaine. It has been recommended as post-operative topical analgesic, superior to phenylbutazone, in de-beaked chicken; sponging of cut portion of the beak with bupivacaine (1: 1 with DMSO) increases feed intake .
It is widely used in human practice mainly for injection based local anaesthesia. It has been recommended for use during labour or during post-operative period as it tends to produce more sensory than motor nerve blockade. It is more cardio-toxic than lidocaine;
(i) High binding affinity with myocardial sodium channel sites, and
(ii) Induces medullary medicated cardiac arrhythmias. The drug is marketed in India as 0.5% solution with or without adrenaline.
8. Hexylcaine:
LAP is about 4 times (range 2-8) that of procaine with onset relatively faster. It is a longacting ester with duration 3-6 hours (range 3-12 hours). Its relative toxicity is 4 times that of procaine, and intravenous toxicity comparable to that of cocaine.
Over dosage on epidural use may led to permanent posterior paralysis. It has been recommended for use in cattle, horses and dogs. Topical effectiveness is comparable to that of cocaine. It is preferred epidural or paravertebral block anaesthetic for long- duration operations e.g. rumenotomy and cesarean section.
9. Proparacaine (Proxymetacaine):
LAP is similar or slightly greater than tetracaine. Onset very fast within a minute (1-3 mins range) with duration short about five minutes (range 3-15 mins). Immediate tissue irritation is little or negligible but may lead to delayed local irritation several hours after topical use. It is considered as drug of choice for topical anaesthesia including corneal use.
10. Oxybuprocaine (Benoxinate):
Action spectrum of benoxinate (0.4%) is comparable to that of proparacaine (0.5%) suggesting its potency is nearly 20% more than that of proparacaine. It is non-irritant to eye as its pH is neutral. It is available in Indian market, and is a choice agent (like proparacaine) for ophthalmic anaesthesia topically.
11. Benzocaine (Ethyl Amino Benzoate):
LAP is weaker than procaine so is its toxicity i.e.about 1/10 of cocaine. It is comparatively non-irritant and recommended for use on skin and non-eye mucus membranes including oral use in gastritis (ulcers and gastric carcinoma.). 2% solution (in equal parts of ethanol and water) is used in painful conditions of mouth and throat. 5-10% as ointment or dusting powder is used on ulcerated surfaces, burns and wounds or for intractable pruritis.
It is used as fast-acting (within 30 sees) fish immobilizing agent (50 mg/L water) for safe handling of fish for weighing and/or for injections. Being poorly soluble in water, it provides sustained local anaesthetic action when used on wounds. Benzocaine absorbs UV light and has been used in sunscreen creams and lotions.
12. Butamben (Butylaminobenzoate):
It is similar to benzocaine except its water solubility is lesser (1: 7000) than that of bezocaine (1: 2500). It is used topically as 1-2% as ointment, cream, sprays or suppository. Butamben picrate (1% ointment) is used for pruritis and to relieve pain due to minor burns. Picric and moiety confers some antiseptic potential to the preparation, and tends to stain animal fibres.
13. Cinchocaine (Dibucaine):
It is a rarely used drug – (i) highly unstable decomposes on boiling and in presence of alkalies, and (ii) its toxic potential is highest 30-40 times (range 16-100) that of procaine. LAP is 15-20 times that of procaine. It is one of the longest acting local anaesthetics with duration of action lasting several hours. It can be used as cream or ointment on skin for pruritis, rashes or dermatoses.
14. Prilocaine:
LAP is 1.5 to 2 times that of procaine. Onset is fast and duration 0.75 to 3 hours. Its toxicity is 50% lesser than that due to lidocaine on subcutaneous use. It is useful in intravenous regional blocks owing to low pKa favouring intracellular tissue accumulation (see Table 4) with higher Vd so decreasing CNS toxicity.
It is not safer for obstetrical practice:
(i) Acidic blood pH of neonates would favour its accumulation in their blood, and
(ii) Its 0-toludine metabolite causes methemoglobinemia, and fetal erythrocytes are more susceptible to hemolysis due to their decreased resistance to oxidant stress and due to their deficient enzyme activity to convert methemoglobin into ferrous state.
15. Etidocaine:
It is a fast (3-5 mins) and longacting (3-10 hours) lidocaine- like local anaesthetic. It is about 4.5 (range 3-6) times as toxic as procaine; intravenous toxicity is equal to that of bupivacaine while subcutaneous toxicity potential is nearly 50% that of bupivacaine. As compared to other amides, it produces stronger motor nerve blockade.
16. Ropivacaine:
Ropivacaine is a newer local anaesthetic that is similar to bupivacaine except it is:
(i) Less cardio-toxic
(ii) Slightly less potent, and
(iii) More motor sparing.
It is a longacting local anaesthetic like bupivacaine, and has been recommended for humans as epidural and regional anaesthetic.
17. Dyclonine:
It is a non-ester and non-amide linkage containing local anesthetic with duration of action comparable to procaine but onset is faster. Compared to procaine, it is topically effective due to better penetration through skin and mucus membranes. It can be used for all types of local anaesthesia as 0.5 to 1% aqueous solution.