Veterinary Medicine - January 2009 - (Page 25)

TABLE 1 Characteristics of Amide-linked and Ester-linked Local Anesthetics Amides Predominant metabolic pathway Tendency to cause allergic reactions Use in veterinary medicine Hepatic microsomal enzymes Low Common Esters Plasma cholinesterases Relatively high Rare sti e blocks, brachial plexus nerve blocks, intercostal nerve blocks, intrapleural nerve blocks, maxillary and mandibular nerve blocks, and epidural anesthesia and analgesia—in subsequent articles throughout this year. All of these techniques are easy to perform, do not require special equipment, and can greatly enhance the analgesic management of veterinary patients. DRUGS USED FOR LOCAL AND REGIONAL ANALGESIA AND ANESTHESIA Three general drug groups are used to produce regional anesthesia and analgesia in veterinary patients—local anesthetics, opioids, and alpha2 agonists. Local anesthetic drugs Local anesthetics are weak bases that are poorly soluble in water. Commercially available preparations are formulated as acidic hydrochloride salts to improve stability and water solubility. The pH of commercial preparations of local anesthetics ranges from 3.9 to 6.6. Most local anesthetics are marketed as racemic mixtures of left and right enantiomers. The enantiomers vary in pharmacokinetic, pharmacodynamic, and toxic properties.1,2 Structure and effects. Local anesthetic molecules consist of a lipophilic unsaturated aromatic ring and hydrophilic portion, usually a tertiary amine, separated by a connecting hydrocarbon chain. The lipophilic portion is essential for the anesthetic activity. Local anesthetics are categorized as amino esters or amino amides based on the chemical bond between the aromatic ring and the hydrocarbon chain of the molecule (Table 1). Although lidocaine (known as lignocaine in the United Kingdom) and bupivacaine are most commonly used in small-animal practice, a variety of local anesthetics are available, varying in their chemical structures, potency, onset of action, and duration of effect (Table 2).1,2 Local anesthetics block nerve conduction in all types of neurons, including all pain (A delta and C bers), sensory, motor, proprioceptive, and sympathetic nerve bers (Table 3). The minimum concentration of local anesthetic necessary to block conduction is higher for motor nerve bers than for sensory bers, so sensory anesthesia can occur without muscle blockade. Typically, autonomic preganglionic B bers are blocked rst. A delta and C sensory bers are blocked before and at lower concentrations than larger sensory A beta, motor A alpha, and proprioceptive A gamma bers. The order of blockade varies with anatomical location, the speci c nerve, and the local anesthetic used.1,2 The active form of local anesthetics, the nonionized base, diffuses across the axonal nerve membrane where it blocks the generation and conduction of nerve impulses by inhibiting voltage-gated sodium channels. The degree of drug ionization depends on the local anesthetic’s dissociation constant (pKa) and the surrounding tissue’s pH. When the pKa and pH are identical, 50% of the drug is ionized and 50% is nonionized. The dissociation constants of local anesthetics vary from 7.6 to 9.1, which means that less than 50% of local anesthetic exists in the active, nonionized form at the normal tissue pH of 7.4. The potency, speed of onset of nerve blockade, and duration of anesthesia are related to the degree of ionization of the local anesthetic molecule and, thus, lipid solubility. Alkalizing the local anesthetic by adding sodium bicarbonate increases the percentage that exists in the nonionized, lipid-soluble form and also reduces pain on injection. Thus, buffering the local anesthetic solution with sodium bicarbonate before administration may increase ef cacy as well as decrease pain on injection. Acidosis at the injection site, as occurs with tissue infection, increases the ionized portion of the drug, decreasing the local anesthetic’s ef cacy.1,2 Systemic uptake. Local anesthetics are rapidly absorbed across mucosal, pleural, and peritoneal surfaces, with high bioavailability. Absorption from epidural and subcutaneous sites is slower, with lower bioavailability. The ultimate plasma concentration is determined by the rate of systemic uptake and the rate of drug clearance. The systemic absorption of a local anesthetic is determined by the dose administered (volume and concentration), the drug’s protein binding and lipid solubility, the injection site’s vascularity, and the use of a vasoconstrictor. Higher doses and increased injection site vascularity will increase systemic absorption. All local anesthetics cause vasodilation, which makes systemic drug absorption more rapid. Epinephrine can be VETERINARY MEDICINE January 2009 25

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Veterinary Medicine - January 2009 Contents Leading Off Letters Research Updates Idea Exchange A Modified Subconjunctival Enucleation Technique in Dogs and Cats Local and Regional Anesthesia Techniques Author Guidelines Product Preview CE Form/Advertiser Index Marketplace/Classifieds Mind Over Miller

Veterinary Medicine - January 2009

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