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Local Anesthetic
Drugs and their effects
| Question | Answer |
|---|---|
| Local anesthetics | Reversibly block nerve conduction, restricted region, for minor surgeries on skin and subcutaneous tissue, can be given during labor or diagnostic procedures, pathologic conditions pain relief, no loss of consciousness |
| What are the classification and the drugs associated? | Esters: Procaine , Chloroprocaine, Cocaine, Tetracaine Amides: Lidocaine (lignocaine),Dibucaine, Bupivacaine, Mepivacaine, Ropivacaine, Prilocaine |
| Lipophilic and hydrophilic properties | hydrophilic portion exists in ionized and nonionized forms. Only nonionized form penetrate neuronal membranes to reach binding sites of sodium channels.Inflammation and acidosis decrease the pH of tissues,increasing the ionization. larger doses needed |
| Esters | relatively unstable in solution, rapidly hydrolyzed in the body by plasma cholinesterase One of the main breakdown products is para-amino benzoic acid (PABA) which is associated with allergic phenomena and hypersensitivity reactions |
| Amides | relatively stable in solution, slowly metabolised by hepatic amidases. Hypersensitivity reactions are extremely rare Therefore, in current clinical practice esters have largely been superseded by the amides |
| What other effects can local anesthetics have? | • Anti-inflammatory and antibacterial • Antimetastatic properties • Neuropathic pain • Acute postsurgical pain and postoperative ileus after colorectal surgery |
| Mechanism of action | Block the voltage gated Na+ channels, blocking nerve conduction. Thus, preventing the initiation and propagation. Use-dependent or phasic block. Depress cardiac pacemaker activity, excitability, conduction, cause arteriolar dilatation(hypotension) |
| How does the presence of ester / amide bond in LA molecule governs its biotransformation? | Ester type (tetracaine): hydrolyzed by plasma and liver esterases, hydrolyzed rapidly, shorter duration of action. Amide (lidocaine/ lignocaine): degraded by hepatic enzymes, long duration of action |
| What can be given along with LA to prolong its action? | Adrenaline as it will constriction of blood vessels → decreases tissue perfusion. Slow absorption into CVS will prolong its action, reduce its systemic toxicity, reduces bleeding at injection site |
| Contraindications for Adrenaline | Absolute contraindication: Infiltration around end-arteries Relative contraindications: Patient with cardiac arrhythmias, uncontrolled hypertension |
| Advantages of giving a vasoconstrictor with LA | • Prolongs action of LA by ↓ rate of removal from local site into circulation. • Contact time of LA with nerve fibre is prolonged. • Enhanced intensity of nerve block. • ↓ systemic toxicity of LA (↓ absorption). • More bloodless field for surgery. |
| Disadvantages of giving a vasoconstrictor with LA | • ↑ chances of subsequent local edema & tissue necrosis. • Delayed wound healing (↓ O2 supply + ↑ O2 consumption) • ↑ BP possibility. • Arrhythmia possibility. |
| Adverse Effects and Interactions with CNS | produce CNS stimulation (restlessness, tremor and euphoria) followed by inhibition (drowsiness and sedation). Other effects include headache, paresthesias, and nausea. |
| Adverse Effects and Interactions with CVS | hypotension and cardiac depression. Most Las are vasodilators. |
| Adverse Effects and Interactions with Periphery | blockade of autonomic ganglia and neuromuscular transmission and potentiate the effect of neuromuscular blocking drugs (e.g., atracurium). Thus must be used with great caution in patients with myasthenia gravis |
| What are the routes of administration of LA | Topical Anaesthesia Infiltration Anaesthesia Conduction block: Field block Nerve block Epidural Anaesthesia Spinal Anaesthesia Intravenous regional anaesthesia (IVRA) |
| Topical rout of administration | applied on the mucous membrane of skin. Corneal surface, mucosa of mouth, nose, pharynx, trachea etc. are easily anesthetized. |
| Infiltration Anesthesia | Dilute solution of LA is injected under the skin to reach sensory nerve terminals around the site of surgery. The treatment is done in the same place where anesthetic is deposited. |
| Nerve block | Drug is injected close to the nerve or big nerve trunks Eg. brachial, sciatic, femoral nerve, radial, ulnar nerves Also used for tooth, eye, limbs, abdominal wall, fracture setting, ribs, neuralgias, persistent hiccup. |
| Field block | LA is injected s.c in the surrounding area of the nerve so that all other nerves coming to a particular field are blocked. Done for herniorrhaphy, appendectomy, dental procedures, scalp stitching, surgery on fore arm or legs. |
| Spinal anesthesia | LA is injected into the subarachnoid space between L2-L3 or L3-L4.Lower abdomen&lower limbs are anaesthetized & paralyzed. |
| Epidural anesthesia | LA is injected in the epidural space in the lumbar, thoracic / cervical region to provide anesthesia |
| Intravenous Regional Anesthesia | The LA is injected into a vein of a limb (occluded by a tourniquet) |
| Intra – articular | The LA is injected into a joint cavity by orthopedic surgeons. |
| CNS adverse effects | tongue numbness, sleepiness, mild headache, visual and auditory disturbances. At high dose, twitching may occur, CNS stimulation may lead to convulsions at higher dose |
| CVS adverse effects | block cardiac Na+ channels, depress abnormal cardiac pacemaker activity, excitability and conduction. Except cocaine, most produces hypotension. • Bupivacaine is cardio toxic, can cause tachycardia and fibrillation |
| Blood adverse effect | Methemoglobinemia, after large doses prilocaine |
| Allergic reactions | ester type are metabolized to PABA derivatives, which can be allergic. Dermatitis, rashes, asthma (rare) |
| Lidocaine | topical administration |
| Benzocaine | lozenges. Used for stomatitis, sore throat, painful ulcers. topical administration |
| Procaine | unsuitable as a surface anesthetic. Poor penetrating power, unwanted central effects; hence superseded by lidocaine and prilocaine |
| Mepivacaine | Infiltration anesthesia |
| Lignocaine | Infiltration Anesthesia, common Epidural anesthesia. Used for only for Upper Limb orthopedic surgeries in intravenous reginal anesthesia |
| Bupivacaine | Infiltration Anesthesia common Epidural anesthesia |
Created by:
David wiid
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