Use of Local Anesthetics

Local anesthetics are usually eliminated by hydrolyzing or breaking apart the drug molecule. This metabolic hydrolysis is catalyzed by hepatic enzymes or enzymes circulating in the plasma (e.g., the plasma cholinesterase). Once metabolized, the kidneys excrete the polar drug metabolites. 

The primary clinical uses of local anesthetics according to their method of administration and specific indications are presented here. 

Topical anesthesia has also been used to improve motor function in some patients with skeletal muscle hypertonicity resulting from a cerebrovascular accident (CVA) or head trauma.53 In this situation, a local anesthetic (e.g., 20% benzocaine) can be sprayed on the skin overlying hypertonic muscles, and then various exercises and facilitation techniques can be performed to increase and improve mobility in the affected limbs. The rationale of this treatment is that it temporarily decreases abnormal or excessive excitatory feedback of cutaneous receptors on efferent motor pathways so that normal integration and control of motor function can be reestablished. Preliminary evidence has suggested that repeated application of this 

Generic Name Trade Name(s) Onset of Action Duration of Action Principle Use(s) 

Articane Septocaine Rapid Intermediate Peripheral nerve block 

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Local anesthesia involves the blockade of nerve conduction in order to stop sensation. Because local anesthetics act on all nerve fibers they may also temporarily create motor paralysis. The usefulness of local anesthetics is their ability to completely block axonal transduction, which is reversible and without any apparent lasting effects. 

Sympathetic block is one use of local anesthetics. at is the best location to apply the local anesthetic ... 

Physical therapists may encounter the use of local anesthetics in several patient situations because of their various clinical applications. For example, therapists may be directly involved in the topical or transdermal administration of local anesthetics. As discussed earlier, repeated topical application of local anesthetics may help produce long-term improvements in motor function in patients with skeletal muscle hypertonicity, so therapists may want to consider incorporating topical anesthetics into the treatment of certain patients with CNS dysfunction. Therapists may also administer local anesthetics transdermally, using the techniques of iontophoresis and phonophoresis. Agents such as lido-caine can be administered through this method for the treatment of acute inflammation in bursitis, tendinitis, and so on. 

The use of in vitro fertilization has raised the question of whether the use of local anesthetics during oocjde removal is innocuous or not. Pharmacological concentrations of anesthetic agents are found in folhcular fluid (44). No clinical effects have been noted, but knowledge of the behavioral effects of lidocaine on offspring in rats must cause some concern (SEDA-15,117). 

A further recommendation in pediatric PM studies is the use of a local anesthetic, such as Emla cream, whenever possible. Not only will this reduce patient discomfort but it will also ameliorate anxiety from both potential subjects and parents, thus reducing reluctance to study participation. Use of local anesthetics requires planning as application is require well in advance of the venipuncture. 

The use of local anesthetic agents or hyaluronidase prior to administering the labeled preparation have been shown to negatively affect lymphatic uptake. 

Cardiovascular System Following systemic absorption, local anesthetics decrease electrical excitabdity, conduction rate, and force of contraction. Most local anesthetics also cause arteriolar dilation. Untoward cardiovascular effects usually are seen only after high systemic concentrations are attained and effects on the CNS are produced. However, on rare occasions, lower doses of some local anesthetics wUl cause cardiovascular collapse and death, probably due to either an action on the pacemaker or the sudden onset of ventricular fibrillation. Ventricular tachycardia and fibrillation are relatively uncommon consequences of local anesthetics other than bupivacaine. The use of local anesthetics as antiarrhythmic drugs is discussed in Chapter 34. Untoward cardiovascular effects of local anesthetic agents may result from their inadvertent intravascular administration, especially if epinephrine also is present. 

The following discussion summarizes the pharmacological and physiological consequences of the use of local anesthetics categorized by method of administration. 

When investigating diffusible ions it is necessary to consider that treatment of the specimen before cryo-fixation may affect the results. For example, dicing tissue into small pieces before cryofixation is not suitable for highly metabolically active tissues such as the heart. It has been shown that in this tissue redistribution of ions can occur in times as short as 10 s. Similarly, in other tissues any delay between harvesting and fixation gives the possibility of element redistribution. In addition it has been shown that treatments such as the use of local anesthetics when taking skin biopsy samples can also affect elemental content. 

One of the first uses of local anesthetics (LA) for anesthesia was in the late nineteenth century with William Halsted reporting a mandibular block and brachial plexus block using cocaine [37,38]. The chemical structure of local anesthetics in clinical use consists of an aromatic (lipophilic) benzene ring linked to an amino group (hydrophflic) via either an ester or an amide intermediate chain. The intermediate link classifies the local anesthetic as either an ester (procaine, chloroprocaine, tetracaine, and cocaine) or an amide (lidocaine, prilocaine, mepivacaine, bupi-vacaine, etidocaine, and ropivacaine). 

These cases stress the importance of keeping methemoglobinemia in mind when desaturation occurs after the use of local anesthetics. 

Local anesthetics, such as lidocaine, are metabolized in the liver and excreted by the kidneys. Acute or chronic deterioration of renal function can lead to their inadequate clearance. Use of local anesthetics is not contraindicated in CKD patients with eGFR <50 ml/min, but dosages should be kept to a minimum and the interval between injections adequately extended. 

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