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Classification, Mechanism, Therapeutic Uses, and Effects of Local Anesthetics
Local anesthetics (LAs) are agents used to block sensation in a specific area of the body without affecting consciousness. They work by inhibiting sodium channels in nerve membranes, thereby preventing nerve impulse conduction.
Classification of Local Anesthetics
- Ester-linked Local Anesthetics
- Examples: Procaine, Chloroprocaine, Tetracaine, Benzocaine
- Shorter duration of action due to rapid hydrolysis by plasma esterases.
- Amide-linked Local Anesthetics
- Examples: Lidocaine (Lignocaine), Bupivacaine, Mepivacaine, Prilocaine, Ropivacaine
- Longer duration of action as they are metabolized in the liver.
Mechanism of Action
Local anesthetics block nerve impulse conduction by inhibiting sodium ion channels within neuronal membranes. By binding to these channels, they prevent the influx of sodium ions, which blocks depolarization and halts the propagation of action potentials. This leads to loss of sensation in the area without affecting consciousness.
Therapeutic Uses of Local Anesthetics
- Surface Anesthesia: Applied to skin or mucous membranes for minor procedures.
- Infiltration Anesthesia: Injected into tissues for local area anesthesia, commonly in dental procedures.
- Nerve Block Anesthesia: Injected near nerves to block sensation in larger areas.
- Spinal and Epidural Anesthesia: Administered near the spinal cord for anesthesia in larger body regions, often used in surgeries and labor.
Adverse Effects of Local Anesthetics
- Central Nervous System (CNS): Dizziness, lightheadedness, confusion; in severe cases, seizures and respiratory depression.
- Cardiovascular System: Hypotension, bradycardia, arrhythmias; Bupivacaine is known for cardiotoxicity at higher doses.
- Allergic Reactions: Ester-linked anesthetics may cause allergic reactions due to PABA metabolites.
Combination of Lignocaine with Adrenaline
Rationale
Adrenaline (epinephrine) acts as a vasoconstrictor, reducing blood flow at the site of injection. This slows the absorption of lignocaine, allowing it to remain effective at the site longer.
Advantages
- Prolonged Duration: Extended anesthesia duration due to slower absorption.
- Reduced Systemic Toxicity: Lower risk of systemic toxicity due to delayed absorption.
- Decreased Bleeding: Minimizes bleeding at the site, ideal for surgeries in vascular areas.
Contraindications
- End-Artery Areas: Avoid use in areas with limited blood supply (e.g., fingers, toes, nose, ears, or penis) to prevent tissue necrosis.
- Cardiovascular Conditions: Caution in patients with hypertension, ischemic heart disease, or arrhythmias, as adrenaline may exacerbate these conditions.
