Local anesthesia

Forms of local anesthesia. Local anesthetics are applied via different routes, including infiltration of the tissue (infiltration anesthesia] or injection next to the nerve branch carrying fibers from the region to be anesthetized (conduction anesthesia of the nerve, spinal anesthesia of segmental dorsal roots), or by application to the surface of the skin or mucosa (surface anesthesia]. In each case, the local anesthetic drug is required to diffuse to the nerves concerned from a depot placed in the tissue or on the skin. 

Topical anesthesia Local use of drugs of this kind on the mucous membranes of the nose, mouth, larynx, tracheobrachial tree, eyes, urinary tract, and gastrointestinal tract causes superficial anesthesia. 

Local anesthetics are used for topical anesthesia, local infiltration, peripheral nerve block, paravertebral anesthesia, intravenous block also known as regional anesthesia, epidural block, and spinal i.e. subarachnoid blockade. The local anesthetics may be divided into two main groups, the esters and the amide-type agents. 

Local anesthetics have poorly understood effects on inflammation at sites of injury, and these anti-inflammatory effects may contribute to improved pain control in some chronic pain syndromes. At the concentrations used in spinal anesthesia, local anesthetics can inhibit transmission via substance P (neurokinin-1), NMDA, and AMPA receptors in the secondary afferent neurons (Figure 26-1). These effects may contribute to the analgesia achieved by subarachnoid administration. Local anesthetics can also be shown to block a variety of other ion channels, including nicotinic acetylcholine channels in the spinal cord. However, there is no convincing evidence that this mechanism is important in the acute clinical effects of these drugs. High concentrations of local anesthetics in the subarachnoid space can interfere with intra-axonal transport and calcium homeostasis, contributing to potential spinal toxicity. 

BLIND AND PAINFUL EYE Retrobulbar injection of either absolute or 95% ethanol may provide relief from chronic pain associated with a blind and painful eye. Retrobulbar chlorpro-mazine also has been used. This treatment is preceded by administration of local anesthesia. Local infiltration of the ciliary nerves provides symptomatic relief from pain, but other nerve fibers may be damaged, causing paralysis of the extraocular muscles, including those in the eyelids, or neuroparalytic keratitis. The sensory fibers of the ciliary nerves may regenerate, and repeated injections sometimes are needed. 

Local anesthetics are used to block pain conduction by nerves. They are used for infiltration anesthesia, local nerve blocks, spinal nerve blocks and epidural nerve blocks. The first local anesthetic was cocaine, which has largely been replaced by synthetic agents. 

The onset of action is fast (within 60 seconds) for the intravenous anesthetic agents and somewhat slower for inhalation and local anesthetics. The induction time for inhalation agents is a function of the equiUbrium estabUshed between the alveolar concentration relative to the inspired concentration of the gas. Onset of anesthesia can be enhanced by increasing the inspired concentration to approximately twice the desired alveolar concentration, then reducing the concentration once induction is achieved (3). The onset of local anesthetic action is influenced by the site, route, dosage (volume and concentration), and pH at the injection site. 

Local anesthetics produce anesthesia by blocking nerve impulse conduction in sensory, as well as motor nerve, fibers. Nerve impulses are initiated by membrane depolarization, effected by the opening of a sodium ion channel and an influx of sodium ions. Local anesthetics act by inhibiting the channel s opening they bind to a receptor located in the channel s interior. The degree of blockage on an isolated nerve depends not only on the amount of dmg, but also on the rate of nerve stimulation (153—156). 

Specific Local Anesthetic Agents. Clinically used local anesthetics and the methods of appHcation are summarized in Table 5. Procaine hydrochloride [51-05-8] (Novocain), introduced in 1905, is a relatively weak anesthetic having along onset and short duration of action. Its primary use is in infiltration anesthesia and differential spinal blocks. The low potency and low systemic toxicity result from rapid hydrolysis. The 4-arninobenzoic acid... 

Lidocaine hydrochloride [73-78-9] (Xylocaine), is the most versatile local anesthetic agent because of its moderate potency and duration of action, rapid onset, topical activity, and low toxicity. Its main indications are for infiltration, peripheral nerve blocks, extradural anesthesia, and in spinal anesthesia where a duration of 30 to 60 min is desirable. Because of its vasodilator activity, addition of the vasoconstrictor, epinephrine, increases the duration of action of Hdocaine markedly. It is also available in ointment or aerosol preparations for a variety of topical appHcations. 

The total U.S. market value for the anesthetic agents Hsted was 299.9 million ia 1990 (162). General inhalation agents, valued at 154.5 million, comprised over half (51.5%) of the 1990 market. General iv anesthetics were valued at 111.5 million (37.2%). Local iajectable agents, at 33.9 million, represented the smallest portion of the market (11.3%). U.S. sales for selected anesthesia pharmaceuticals are given ia Table 6. 

E. Ericksson, Illustrated Handbook of Local Anesthesia, W. B. Saunders Philadelphia, Pa., 1979. 

Clinical evaluation is underway to test transvenous electrodes. Transvenous leads permit pacemakers to be implanted under local anesthesia while the patient is awake, greatly reducing recovery time and risk. As of 1996, the generation of implantable defibrillators requires a thoracotomy, a surgical opening of the chest, in order to attach electrodes to the outside of the heart. Transvenous electrodes would allow cardiologists to perform pacemaker procedures without a hospital or the use of general anesthesia. 

Commercially available electrotransport systems are bulky and limited to acute appHcations (96). One example, the Drionic system used for the treatment of hyperhidrosis (excessive perspiration), is presoaked in water for 30 min before each 20- to 30-min treatment. Another system, the Phoresor, approved for the deHvery of Hdocaine [137-58-6] for local anesthesia, and of dexamethasone [50-02-2] for treatment of local inflammation such as bursitis or tendinitis, is powered by a 9 V replaceable battery and features a disposable, fiHable dmg electrode. 

Toxic Reactions of the Skin Irritation is the most common reaction of the skin. Skin irritation is usually a local inflammatory reaction. The most common skin irritants are solvents dehydrating, oxidizing, or reducing compounds and cosmetic compounds. Acids and alkalies are common irritants. Irritation reactions can be divided into acute irritation and corrosion. Necrosis of the surface of the skin is typical for corrosion. Acids and alkalies also cause chemical burns. Phenols, organotin compounds, hydrogen fluoride, and yellow phosphorus may cause serious burns. Phenol also causes local anesthesia, in fact it has been used as a local anesthetic in minor ear operations such as puncture of the tympanous membrane in cases of otitis. ... 

Butterworth JF, Strichartz GR (1990) Molecular mechanisms of local anesthesia a review. Anesthesiology 72 722-734... 

Although the use of barbiturates and miscellaneous sedatives and hypnotics for sedation has largely been replaced by the antianxiety drugs (see Chap. 30), they occasionally may be used to provide sedation before certain types of procedures such as cardiac catheterization or the administration of a local or general anesthesia Sedative doses usually given during daytime hours, may be used to treat anxiety and apprehension. Fhtients with chronic disease may require sedation, not only to reduce anxiety, but also as an adjunct in the treatment of their disease... 

Sate the uses of local anesthesia, methods of administration, and nursing responsibilities when administering a local anesthetic. 

List several drugs used in local and general anesthesia. 

Topical anesfliesia involves die application of die anes-flietic to die surface of the skin, open area, or mucous membrane The anesthetic may be applied wifli a cotton swab or sprayed on the area This type of anesthesia may be used to desensitize the skin or mucous membrane to the injection of a deeper local anesthetic. In some instances, topical anesthetics may be applied by the nurse ... 

Local infiltration anesthesia is die injection of a local anesthetic druginto tissues. This type of anesfliesia may be used for dental procedures, die suturing of small wounds, or making an incision into a small area, such as that required for removing a superficial piece of tissue for biopsy. 

Regional anesthesia is the injection of a local anesthetic around nerves so that the area supplied by these nerves will not send pain signals to the brain. The anesthetized area is usually larger than the area affected by... 

Spinal anesthesia is a type of regional anesthesia that involves the injection of a local anesthetic drug into the subarachnoid space of die spinal cord, usually at the level of the second lumbar vertebra There is a loss of feeling (anesdiesia) and movement in the lower extremities, lower abdomen, and perineum. 

A conduction block is a type of regional anesthesia produced by injection of a local anesthetic drug into or near a nerve trunk. Examples of a conduction block include an epidural block (injection of a local anesthetic into the space surrounding the dura of the spinal cord) a trails sacral (caudal) block (injection of a local anesthetic into the epidural space at the level of the sacrococcygeal notch) and brachial plexus block (injection of a local anesdietic into the brachial plexus). Epidural, especially, and trailssacral blocks are often used in obstetrics. A brachial plexus block may be used for surgery of the arm or hand. 

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