Local anesthetics and counter irritants

local anaesthetics and counter irritants

1. Local anaesthetics and counter irritants Dr. SHINDE VIRAJ JR-II, Dept. of pharmacology, GMC ,Nagpur.

2. Overview

3. Comparative features of general and local Anaesthesia General anaesthesia Local anaesthesia Site of action CNS Peripheral nerve Area of body involved Whole body Restricted area Consciousness Lost Unaltered Care of vital functions Essential Usually not needed Physiological trespass High Low Poor health patient Risky Safer Use in non cooperative patient Possible Not possible Major surgery preferred Cannot be used Minor surgery Not preferred Preferred

4. Introduction Reversibly block the impulse conduction Provide transient loss of sensation Performing minor surgery Neither cause loss of consciousness nor need a proper maintenance of vital functions during surgery

5. History First local anaesthetic –COCAINE, was serendipitously discovered Albert Neimann first isolated in 1860 Sigmund Freud studied cocaine’s physiological actions Carl Koller introduced it in ophthalmological surgery

6. More intense & longer lasting anaesthesia Bind α 1 acid glycoprotein in plasma Not hydrolysed by plasma esterases Rarely cause hypersensitivity Less intense and short duration anaesthesia Hydrolysed by plasma esterases Higher risk of hypersensitivity Amide LAs Ester LAs

7. Classification Amide type Longer acting bupivacaine , levo-bupivacaine , etidocaine , ropivacaine , dibucaine Intermediate acting lidocaine , mepivacaine , prilocaine

8. Ester type Longer acting tetracaine Intermediate acting cocaine Short acting procaine, chloroprocaine , benzocaine * , butamben *,

9. Miscellaneous pramoxine *, dyclonine *, oxethazaine *

10. Mechanism of action LAs block nerve conduction by decreasing the entry of Na + ions during upstroke of action potential. As the concentration of the LA is increased, the rate of AP and maximum depolarisation decreases causing slowing of conduction. Finally local depolarisation fails to reach the threshold potential and conduction block ensues.

11. Effects of pH on LA LA action is strongly pH dependent, being more at alkaline pH and less at acidic pH. LA remain partly ionised and partly unionised at physiological pH. The unionised form is needed for its diffusion through axon membrane.

12. Inside the axon it is reionised and it is the cationic form of LA which binds to the LA receptor inside the channel. LA are less effective in infected tissue.

13. Prolongation of action by vasocontrictors Anything that delays the absorption of LA into the circulation will prolong its action and reduce its systemic toxicity. Adrenaline is most commonly used and it prolongs duration of LA action.

14. Pharmacokinetics The presence of the ester or the amide bond in the LA molecule governs its biotransformation and its liability to cause hypersensitivity reactions. The ester type are hydrolysed by plasma esterases and also by liver esterases . The amide type LA are degraded by hepatic microsomes

15. Uses and types of LA Topical anaesthesia Mucous membrane, damaged skin, wounds, or burns. Facilitate endoscopic procedures and to reduce pain of haemorrhoids and anal fissures. Tetracaine ( 2 %), lidocaine ( 2-5 %), benzocaine ( 5 %), dyclonine ( 0.5-1 %), pramoxine (1 %).

16. Infiltration anesthesia Dilute solution of LA is injected under the skin to reach sensory nerve terminals. Infiltration is used for minor surgical procedures like incision or excisions . Lidocaine ( 1 %), bupivacaine ( 0.25 %), procaine( 0.5-1 %).

17. Conduction block anaesthesia Field block LA is injected subcutaneously in the surrounding area of the nerve so that all other nerves coming to a particular field are blocked.

18. Nerve block LA is injected around anatomically localised nerve trunk Nerve block lasts longer than field block or infiltration anaesthesia .

19. Central nerve block anaesthesia Epidural block anaesthesia and Spinal block anaesthesia

20. Epidural block anaesthesia Also referred as peridural block Injecting LA in the lumbar, thoracic or cervical region. Caudal block is also a epidural block but in the caudal region.

21. Spinal block anaesthesia Is also referred as subarachnoid or intrathecal block It is produced by injecting a suitable LA in the spinal subarachnoid space between L2 and L3 or L3 and L4. Anaesthetise lower abdomen and hind limbs.

22. Intravenous regional anaesthesia Also referred as Bier’s blocks Used mainly for upper limb and for orthopaedic procedures.

23. Adverse effects Central nervous system Low doses:- tongue numbness, sleepiness, mild headache, visual and auditory disturbances Higher concentration:- nystagmus and muscular twitching Profound CNS stimulation may lead to convulsions.

24. Cardiovascular system Primary site of action is myocardium where decrease in electrical excitability, conduction rate and force of contraction occurs. Excepting cocaine, most LAs produce hypotension. Bupivacaine can cause ventricular tachycardia and fibrillation.

25. Blood Administration of large dose of prilocaine may lead to accumulation of the metabolite orthotoluidine . Orthitoluidine oxidises haemoglobin to methaemoglobin Lead to cyanosis

26. Allergic reactions Ester type LAs are metabolised to PABA derivatives And are responsible for allergic reaction like contact dermatitis, rashes and asthma

27. Neuromuscular junction In large doses LAs block neuromuscular transmission and enhance the effects of d- tubucurarine Is the result of blockade of the nicotinic receptor ion channel.

28. Counterirritants Definition – Irritants which produce a remote effect which tends to relieve pain and inflammation in deeper organs

29. Mechanism of counterirritation Area of skin supplied by nerves from the same segment as deeper organ from which pain impulses are coming the cutaneous impulses obscure deeper sensation Generally massaged to releive headache , muscular pain { torticollis, backache,sprain } , joint pain , colics .

30. Examples Volatile oils { essential oils} Turpentine oil – Used as counterirritants in the form of liniments or stupes Clove oil – Cotton swab for toothache Eucalyptus oil – Pain balms Camphor – Pain balms Thymol -Pain balms Menthol - Mustard seeds – Rubefacient and counterirritant Capsicum – Powerful irritant { hot in taste } Included in some counterirritant preparations

31. VICKS VAPORUB: Menthol 2.8%, camphor 5.25%, thymol 0.1 % turpentine oil 5.5% ointment . IODEX: Methylsalicylate 5%, iodine 4% nonstaining ointment . AMRUTANJAN: Eucalyptus oil 17%, camphor 10%, thymol 1 %, menthol 4.5%, methylsalicylate 7% ointment . CAPSIGYL-D: Capsaicin 0.075%, methyl salicylate 20 %,menthol 10%, camphor 5%, eucalyptus oil 5%, diclofenac 1%gel. Volini gel – diclofenac diethylamine 1.16% w/w , menthol 5% w/w, methyl salicylate 10% w/w lineseed oil 3% w/w