| Term | Definition |
|---|
| Definition | Drugs that reversibly block nerve conduction of sensory impulses from the periphery to the CNS Used to block pain sensation from specific areas of the body or sympathetic vasoconstrictor impulse to specific areas of the body |
| History | Cocaine 1860 (addiction and CNS stimulation) Procaine (Novocaine)®(1905) Lidocaine = Lignocaine (Xylocaine)®(1943) |
| Mechanisms of action | Block the initiation and propagation of action potential by preventing the voltage-dependent increase in Na+conductance |
| Chemistry | Local anaesthetic = Lipophilic gp + Intermediate chain (Ester/amide) + amine Ester versus Amide LA and duration of action Local Anaesthetics are weak bases Their action is pH dependent(increased at alkaline pH and vice versa) Reduced activity in inflamed tissues ? |
| LA action | Use-dependent more the channels are opened, the greater the block it becomes 'Size-dependent' block small-diameter fibre more readily |
| Procaine | Potency 1 Duration of action Short (1<h) |
| Lidocaine | Potency 4 Duration of action Medium (2 h) |
| Bupivacaine | Potency 16 Duration of action Long (> 2 h) |
| Tetracaine | Potency 16 Duration of action Long (> 2 h) |
| Local Anaesthetics | esters or amides |
| ESTERS Short Acting | Procaine |
| ESTERS Long Acting | Tetracaine |
| ESTERS Surface Acting | Benzocaine |
| AMIDES Long Acting | Bubivacaine |
| AMIDES Medium Acting | Lidocaine |
| Pharmacokinetics | Usually injected into the required area For topical applications (diffusion controls onset/offset) Systemic Absorptiondepends on many factors: Dose, Site of injection, Drug-tissue binding, blood flow, use of vasoconstrictor Vasoconstrictor (e.g. NA) can be used to decrease the systemic side effectsVasoconstrictors can prolong the action of short acting LA up to 50% more 1)Reduce systemic absorption 2)Enhance local neuronal uptake of LA 3)Inhibit the release of substance P by working on α2 receptors EXCEPTION: cocaine Long acting LA |
| Metabolism | Ester LA metabolized by plasma cholinesterases (pseudocholinesterases) procaine t1/2=1-2 min Amide LA metabolized in the liverby CYP450 |
| Clinical Uses | ●Surface anaesthesiaSkin or mucous membranes (Lidocaine; Tetracaine; Benzocaine)●Infiltration anaesthesiaInjected into tissues (minor surgery)Most LA can be used (with adrenaline or felypressin)●Nerve BlockInjected into the vicinity of the nerve trunkMost LA, ●Spinal anaesthesiaLA injected into CSF to act on spinal cord; mainly Lidocaine●Epidural anaesthesiaLA injected into epidural space to block spinal roots; mainly Lidocaine and Bubivacaine |
| Side Effects | A-CNS effect: light-headedness, sedation, confusion, agitation, tremors, convulsions, and respiratory depression B-CVS Effects: All LA except cocaine are vasodilators Arrhythmia; heart block; hypotension C-Hypersensitivity reactions: allergic dermatitis (rare) Hyperkalemia facilitates the cardiac toxicity of local anesthetics |
| Tetracaine (amethocaine) | Mainly for spinal and corneal anaesthesia Very slow onset and moderate tissue penetration |
| Mepivacaine | Similar to Lidocaine (rapid onset and good tissue penetration) |
| Benzocaine | As powder to dress painful skin ulcers |
Combine with other sets
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