Local anesthetics advantages

Local anesthetics are used extensively on the mucous membranes in the nose, mouth, tracheobronchial tree, and urethra. The vasoconstriction produced by some local anesthetics, cocaine especially, adds a very important advantage to their use in the nose by preventing bleeding and inducing tissue shrinkage. Topical anesthesia permits many diagnostic procedures in the awake patient, and when it is combined with infiltration techniques, excellent anesthesia may be obtained for many... 

The class lb antiarrhythmic drug mexilitine is structurally related to the local anesthetic agent lidocaine and also shows a voltage- and frequency-dependent block of sodium channels. Mexilitine is not selective for any painrelevant subtype of sodium channel. As an advantage over lidocaine, mexilitine can be given orally. 

When applied locally to nerve tissue in appropriate concentrations, local anesthetics (Figure 23.2) reversibly block the action potentials responsible for nerve conduction. They act on any part of the nervous system and on every type of nerve fiber. Thus, a local anesthetic in contact with a nerve trank can cause both sensory and motor paralysis in the area innervated. The necessary practical advantage of the local anesthetics is that their action is reversible at clinically relevant concentra-... 

The addition of sodium bicarbonate increases the potency of the local anesthetics two to four times for direct application or injection into nerve tranks and probably for subdural injection and on application to mucous surfaces. This is due to the easier penetration of the free anesthetic bases, as compared with their salts. For these purposes, the usual solutions of the anesthetic salts may be mixed with an equal volume of 0.5% sodium bicarbonate solution, without loss of efficiency, and with a saving of one half of the anesthetic, and correspondingly smaller danger of accidents. The mixtures, however, do not keep well, and must be made just before use. Alkalinization or buffering has no advantage for hypodermic or intradermic injections, because these do not require much penetration. Procaine base dissolved by the aid of carbon dioxide is also more potent than the hydrochloride when applied to the cornea, but not for intramuscular injection. 

This involves considerable art, which must be learned in the clinic. It falls into two divisions (1) surface application to the mucous membranes, especially of the eye, nose, throat, and urethra and (2) injections about nerves, in different parts of their course and distribution, from their spinal roots to their ultimate fibrils. The advantages and disadvantages in comparison with general anesthesia and the selection of the local anesthetic agent also depend on clinical discrimination. Nervous, fearful, and excitable patients often suffer severely from apprehension, which also disposes toward accidents. They may be at least somewhat quieted by sedatives, morphine (0.015 g hypodermically) half an hour before the operation, or by barbiturates. The latter also tend to prevent convulsions. 

Infiltration anesthesia is the injection of local anesthetic directly into tissue without taking into consideration the course of cutaneous nerves. Infiltration anesthesia can be so superficial as to include only the skin. It also can include deeper structures such as intra-abdominal organs when these too are infiltrated. The advantage of infiltration anesthesia and other regional anesthetic techniques is that it is possible to provide satisfactory anesthesia without disruption of normal bodily functions. 

Clinically used local anesthetics are either esters or amides. Even drugs containing a methylene bridge, such as chlorpromazine (p.233) or imipramine (p.229) would exert a local anesthetic effect with appropriate application. Ester-type local anesthetics are subject to inactivation by tissue esterases. This is advantageous because of the diminished danger of systemic intoxication. On the other hand, the high rate of bioinactivation and, therefore, shortened duration of action is a disadvantage. 

The duration of action of local anesthetics is proportional to the time they are in contact with the nerve tissue. Consequently, any agent or procednre that keeps anesthetics at their site of action prolongs the period of anesthesia. In clinical practice formnlation of injectable local anesthetics with vasoconstrictors helps to localize the anesthetic at the desired site. Local vasoconstriction may also offer the advantage of slowing absorption into the systemic circnlation, which rednces the potential fiar... 

The adverse effects of local anesthetics are well established (3,4). The safety advantages claimed for newer agents have to be treated with much reserve. With increasing experience, discovery of optimal doses, and understanding of potency differences, the tolerability of... 

Benzyl alcohol is commonly used as a prc.servative in vials of injectable drugs in concentrations of I to 4% in water or saline solution. Benzyl alcohol has the added advantage of having a local anesthetic action. It is commonly used in ointments and lotions as an antiseptic in the treatment of various pruritic skin conditions. 

Tropacocaine has been used also as a local anesthetic in ophthalmic and pharyngological practice. It possesses an advantage over cocaine in being more stable in solution and easily sterilizable. 

Field block anesthesia is produced by subcutaneous injection of a solution of local anesthetic in order to anesthetize the region distal to the injection. For example, subcutaneous infiltration of the proximal portion of the volar surface of the forearm results in an extensive area of cutaneous anesthesia that starts 2-3 cm distal to the site of injection. The same principle can be applied with particular benefit to the scalp, the anterior abdominal wall, and the lower extremity. The drugs, concentrations, and doses recommended are the same as for infiltration anesthesia. The advantage of field block anesthesia is that less drug can be used to provide a greater area of anesthesia than when infiltration anesthesia is used. 

Current theories suggest that carbon dioxide potentiates the action of local anesthetics by initial indirect depression of the axon, followed by diffusion trapping of the active form of the local anesthetic within the nerve. Use of the carbonate salt appears to be one pharmaceutical modification of the classic local anesthetic agents that may result in significant clinical advantages. 

The first totally synthetic substitute was eucaine. It was synthesized by Harries in 1918 and retains many of the essential skeletal features of the cocaine molecule. The development of this new anesthetic partly confirmed the portion of the cocaine structure essential for local anesthetic action. The advantage of eucaine over cocaine is that it does not produce mydriasis and is not habit forming. Unfortunately, it is highly toxic. 

The benefits of giving fentanyl in the epidural space for improving intra-operative and post-operative analgesia are very clear. The analysis of current literature shows that the addition of fentanyl to local anesthetics for intra-operative and post-operative epidural analgesia is safe and advantageous. The reduction in the... 

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