Local and regional anesthesia techniques, Part 1: Overview and five simple techniques
In this first article in a four-part series, these clinicians describe five easy-to-perform anesthetic techniques to improve patient care: infiltration anesthesia, splash blocks, digital nerve blocks, intravenous regional anesthesia, and soaker-type catheters.
Local and regional anesthesia techniques in veterinary practice have many advantages. These techniques
Provide effective preemptive and multimodal analgesia
Reduce the amount of inhalational agent needed to maintain anesthesia, resulting in improved cardiopulmonary stability during
anesthesia
Modulate the sympathetically driven stress response to surgical trauma
Reduce the development of central sensitization.
Multimodal analgesia can be provided with combinations of local anesthetics, opioids, and alpha2 agonists.
In this article, we give an overview of drugs used for local and regional anesthesia and then discuss five anesthetic techniques—infiltration
anesthesia, splash blocks, digital nerve blocks, intravenous regional anesthesia, and soaker-type catheters. We will discuss
additional techniques—intra-articular stifle 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
TABLE 1: Characteristics of Amide-linked and Ester-linked Local Anesthetics
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
TABLE 2: Local Anesthetics and Their Physical, Chemical, and Pharmacodynamic Properties
Local anesthetics block nerve conduction in all types of neurons, including all pain (A delta and C fibers), sensory, motor,
proprioceptive, and sympathetic nerve fibers (Table 3). The minimum concentration of local anesthetic necessary to block conduction is higher for motor nerve fibers than for sensory
fibers, so sensory anesthesia can occur without muscle blockade. Typically, autonomic preganglionic B fibers are blocked first.
A delta and C sensory fibers are blocked before and at lower concentrations than larger sensory A beta, motor A alpha, and
proprioceptive A gamma fibers. The order of blockade varies with anatomical location, the specific nerve, and the local anesthetic
used.1,2
TABLE 3: Types of Neurons Blocked with Local Anesthetics
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 efficacy 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 efficacy.1,2
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