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.
TABLE 4:Indications and Drug Dosages for Local and Regional Anesthesia Techniques
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All local anesthetics cause vasodilation, which makes systemic drug absorption more rapid. Epinephrine can be added to cause
vasoconstriction and slow systemic absorption, increasing the intensity and the duration of blockade and reducing the potential
for toxicity. In addition, epinephrine may have some local anesthetic effects itself.1 The usual concentration of epinephrine is 5 µg/ml, or 1:200,000. This concentration can be obtained by adding 0.1 ml of
1:1,000 (0.1 mg) epinephrine to 20 ml of the local anesthetic. Some commercially available local anesthetics already contain
epinephrine. Epinephrine can cause tissue necrosis along wound edges and, rarely, ventricular dysrhythmias. Never use epinephrine
to block distal extremities, including digits, ears, or tails, because of the potential for local tissue ischemia.1-3
Toxicity. Local anesthetics can cause severe toxic reactions after unintentional intravenous administration, vascular absorption of
an excessive dose, or ingestion of topical local anesthetic preparations. Intravenous injection of bupivacaine is highly toxic
to the cardiovascular system and central nervous system, resulting in agitation, muscular twitching, seizures, unconsciousness,
coma, respiratory arrest, cardiac depression, dysrhythmias, hypotension, and death. Always aspirate the syringe before injecting
any local anesthetic to avoid intravenous or intra-arterial injection.
Base dosage calculations on lean body weight, and always calculate the maximum safe dose to avoid toxicosis (Table 2). Animals with hepatic impairment or reduced hepatic blood flow (hypotension, cardiac failure), geriatric patients, and neonates
are more prone to develop toxicosis with amide-linked local anesthetics, so reduce doses by 40% to 60%. In addition, cats
should receive lower doses than dogs do since they are more likely to develop signs of toxicosis because they metabolize local
anesthetics more slowly and, to a lesser extent, because of their decreased ability for glucuronidation.1-3
Relative or absolute overdosage of local anesthetics results in agitation, muscular twitching, seizures, unconsciousness,
coma, respiratory arrest, cardiac depression, dysrhythmias, hypotension, and death.1,2 Some effects may be masked by general anesthesia, and dysrhythmias, hypotension, and respiratory arrest may be the only
observable signs of toxicosis. Allergic reactions can occur but are uncommon. Benzocaine and prilocaine, and less commonly
procaine and lidocaine, have been associated with the development of methemoglobinemia in dogs and cats.2 Cats are at increased risk for developing methemoglobinemia and Heinz-body anemia with benzocaine-containing products, including
Cetacaine (a combination of benzocaine, butamben, and tetracaine), so these products should be avoided in cats.2
When given in high volumes epidurally or spinally, local anesthetics can cause excessive blockade of sympathetic neurons,
resulting in hypotension, and excessive blockade of motor neurons, resulting in paralysis of intercostal muscles and the diaphragm.1,2
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