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.
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.
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 the efficacy of local anesthetics.
A vasoconstricting agent like epinephrine, is often added to local anesthetic solutions. This provides the following advantages:
Premixed solutions containing epinephrine are acidified even further than plain local anesthetics. This increased acidity delays the onset of action and is more painful on injection. There's no utility in using higher than 1:100,000 epinephrine solutions. No additional vasoconstrictive benefit is offered, whereas the risk of toxicity increases in a dose-dependent manner. Adequate hemostasis relies significantly on allowing adequate time for the vasoconstrictive effects to occur. This usually takes 7-10 minutes.
Contraindications to the use of epinephrine-containing solutions include patients with unstable angina, cardiac dysrhythmias, severe uncontrolled hypertension, or pregnant patients with placental insufficiency. Relative contraindications include hyperthyroidism and concurrent use of MAOI or tricyclic antidepressants. When contraindicated, phenylephrine (1:20,000) may be substituted, however it's not as efficient as epinephrine.
Sodium bicarbonate may be added to local anesthetics in order to alkalinize the answer. This neutralization of the low pH creates a solution that is less irritating to the tissues and less painful on administration. The limiting factor in the addition of bicarbonate is the tendency for the lipid soluble agents, such as bupivicaine, to precipitate at the more neutral pH values. As a result, bicarbonate may be added to lidocaine but ought to usually not be used with bupivicaine.
Lidocaine is the most widely utilized local anesthetic. It's ready as a 1% (10 mg/ml) or 2% (20 mg/ml) solution with or with out epinephrine. Its duration of action is about 1.5 hours without epinephrine, and this is doubled to 3 hours with the addition of epinephrine to the solution (1:100,000). Lidocaine may also be utilized as a dilute answer (0.2%-0.5%) for certain procedures like a rhytidectomy. This answer is adequately anesthetizing and vasoconstrictive.
A commonly utilized dilute answer, the modified Klein solution, may be ready as follows: 20 ml of 2% lidocaine, 5 ml of sodium bicarbonate, and 1 ml of 1:1,000 epinephrine all mixed in 500 ml of lactated Ringer's answer. The maximum safe dose for plain lidocaine is reported as 3-4 mg/kg. With the addition of epinephrine, this increases to 7 mg/kg. Recent research, nevertheless, refutes this number, providing evidence for a much higher maximal safe dose-up to 35 mg/kg when combined with epinephrine.
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