Lab 4

Local anesthetics

Local anesthetics: drugs used to produce transient and reversible loss of sensation in a circumscribed area of the body, interfering with nerve conduction without loss of consciousness.

Mechanism of action: Blocking of Na –channels which are responsible for depolarization so no depolarization and no action potential ( i.e rising threshold of action potential and slowing impulse conduction

The effect of local anesthetic on nerve depend on: 1- diameter of the nerve: the smaller, the rapid onset of action. 2-state of myelination: mylinated n.f are more rapidly anesthetized. 3-position of fibers ( cord, root, plexus) 4-Frequency of conduction: the faster the frequency, the more rapid onset of anesthesia.

Classification of local anesthetics: 1-chemical: a-Ester: hydrolyzed by liver and plasma cholinesterase. ( procaine, cocaine) b-Amid : dealkylated in the liver.( lignocain) Benzene ring ester or amide amine group

2- clinical classification: A-Surface anesthesia ( topical) B-infiltration anesthesia ( intradermal) C-regional : 1- I.V 2-nerve block ex: pudendal nerve block D- epidural or extsadural. E- subarachnoid ( intrathecal or spinal anesthesia)

Notes: 1-L.A are not effective in case of abscess because inflammation lead to acidosis that prevent ionization of the cationic amine group so prevent binding to receptor. 2-to prolong the duration of action of L.A vasoconstrictor agents are added like adrenaline but not for areas supplied by end arteries like fingers or male genetalia. To terminate the action of NA α-blockers are used as phentolamine.

3-to avoid systemic side effects of NA in patients with CVS diseases, felyspressin another vasoconstrictor agent is used. 4-in case of impaired liver function, large repeated doses of amide compounds may predispose to toxicity. 5- Lignocain is one of antiarrhythmic drugs ( class1b)

Adverse effects of L.A: in case of systemic absorption: 1-CNS: parasthesia, visual disturbances, convulsions. 2- CVS: hypotension, brady cardia. 3- Respiratory: hyperventilation, alkalosis and resp. failure.

Procedure: 1- 2 volunteers 2- 1 st : received 3 I.D injections of normal saline, lignocaine 2%, lignocaine2%+ adrenaline1: in the flexor aspect of the forearm. Then blebs and surrounding areas are pricked with sterile needle and the volunteer indicate if he feel the pricks, and no. of pricks felt is recorded. the procedure repeated after 5, 10,15 minutes. Comparision between the onset and duration of anesthesia is done. 3-2 nd : apply topical lignocaine on lips and lignocain spray on tongue. After 5,10,15 minutes the volunteer describe sensation for light touch, deep touch,pain and taste ( sweet, salt,sour,bitter)

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