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How do factors such as dosage of local anesthetic, addi-tion of vasoconstrictors, carbonation and pH adjust-ment, mixtures of local anesthetics, and pregnancy influence local anesthetic blockade?
Dose of Local Anesthetics As the dosage of local anesthetics is increased, by administration of either a larger volume or a more concentrated solution, the probability and duration of satisfactory anesthesia increases and the time to onset of blockade is shortened.
Addition of Vasoconstrictors Epinephrine, norepinephrine, and phenylephrine are the vasoconstrictors frequently added to local anesthetic solutions. Of these, epinephrine is the most frequently used. Vasoconstrictors decrease local blood flow and thereby impair uptake of the drug, which allows more molecules to remain at the nerve to act on it. In this way, the degree and duration of blockade are enhanced. Lidocaine with 1:200,000 epinephrine is optimal for epidural and intercostal blockade. Although epinephrine extends the duration of action of local anesthetics when employed for peripheral nerve blocks and infiltration anesthesia, it does not significantly prolong the duration of epidural anesthesia produced by bupivacaine or etido-caine. The high lipid solubility of bupivacaine and etidocaine allow for storage by, and extended release of drug from, adipose tissue, thereby prolonging the block beyond the influence of the vasoconstrictor mechanism.
Carbonation and pH Adjustment Sodium bicarbonate may be added to local anesthetic solutions in an attempt to decrease the onset time. Alkalinizing an anesthetic solution increases the amount of drug in the free base form, which increases diffusion of the drug through nerve sheaths and membranes. The latency of both lidocaine and bupivacaine can be decreased in this way. This method has proven to be successful in the epidural space, but has met with varying results when applied to brachial plexus blockade.
Local Anesthetic Mixtures The use of mixtures of local anesthetic solutions for regional anesthesia has become popular in recent years, especially for ambulatory proce-dures. Mixtures of local anesthetic solutions such as chloroprocaine and bupivacaine offer theoretical clinical advantages, owing to the rapid onset and low systemic toxicity of chloroprocaine and the long latency of action of bupivacaine. Anesthesiologists should be aware, however, that there are no existing data that show that local anes-thetic toxicities are independent of each other. Mixtures of local anesthetics should be considered to have roughly additive toxic effects.
Pregnancy The spread of epidural and spinal anesthesia has been reported to be greater in pregnant women than nonpregnant women. This was originally attributed to mechanical factors, such as engorged epidural veins. A study by Moller et al. (1992) has demonstrated a more rapid onset of action and an increased sensitivity to local anesthetics during pregnancy. Such studies suggest that hormonal changes may result in increased sensitivity of nerve to local anesthetics. Thus the dosage of local anesthetics should be reduced in patients in all stages of pregnancy.
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