As the number of plastic surgical procedures performed under local anesthesia continues to grow, a thorough understanding of local anesthetic strategies has turn out to be important. Furthermore, emergency care of lacerations, avulsions along with other acute injuries also necessitates an adequate grasp of local anesthesia. It's important to obtain informed consent prior to using local anesthesia. Discussion of the risks and advantages of the surgery alone is not sufficient. Anesthetic-related issues like adverse reactions, systemic toxicity, nerve damage, hematoma and pain both during and after the injection should be addressed.
Mechanism of Action
Local anesthetics exert their effect by temporarily blocking nerve conduction. This is achieved by interference with influx of sodium ions through the sodium channel. This leads to a slowing of the rate of membrane depolarization, a lowering of the threshold potential, and the inhibition of propagation of the action potential down the length of the axon. The smallest unmyelinated sensory nerves (C fibers) are affected first. The motor nerves are usually larger and myelinated, and are unaffected or only mildly affected by the actions of local anesthetics at the doses commonly utilized.
Pharmacodynamics
Local anesthetics can be classified based on their molecular structure as either amides or esters. The amides, such as lidocaine, are metabolized in the liver by microsomal enzymes and excreted in the urine. The esters, such as cocaine, are quickly metabolized by plasma pseudocholinesterase into PABA and excreted in the urine.
Local anesthetics are acidic, in the pH range of 5-7 . Their pH further decreases with the addition of epinephrine to the anesthetic answer. Once they enter the tissue, the body's bicarbonate buffer system converts the acidic answer to a more basic form. This is the active, uncharged form of the drug that can diffuse through the plasma membrane of the neurons. Bupivicaine, with its higher pKa, has a slower onset of action than lidocaine, which has a lower pKa. Acidic tissue, such as a hypoxic or infected wound, increases the fraction of ionized drug, thus delaying the onset and decreasing