Brett Beckman
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The use of regional nerve blocks for oral surgery in dogs and cats is synonymous with quality patient care.
This article details techniques that can be incorporated into companion-animal-hospital dentistry service to maximize safety,
care and comfort for patients undergoing oral surgical procedures.
This discussion suggests adopting less misleading names for the regional nerve blocks, relying on simple anatomical regions
blocked rather than traditional confusing nomenclature.
Detailed descriptions of regional and local nerve blocks commonly used to provide analgesia for oral surgery in veterinary
patients have appeared in numerous publications.
Complete analgesia to the targeted tissue and a reduction in the concentration of the inhalant anesthetic are two significant
benefits of proper administration of local agents. Hypotension, bradycardia and hypoventilation are complications that can
be minimized by the use of local and regional blocks for oral surgery. Local blocks eliminate the perception of pain, making
surgical anesthetic depths unnecessary.
Agents
This author commonly uses lidocaine and bupivicaine combined in the same syringe for regional oral-nerve blocks. Lidocaine
is not desirable as a sole agent due to its limited effect post administration (one to two hours). Bupivicaine requires administration
much earlier in the anesthetic event than lidocaine in order to be effective prior to surgical tissue manipulation. These
limitations can be minimized or eliminated by using them in combination. The quick onset of lidocaine, coupled with bupivicaine's
extended duration of effect (up to eight hours), make them a good choice for combined use in regional nerve blocks. The agents
may be used with or without epinephrine.
Although rare, complications are possible with inadvertent systemic administration or systemic uptake of locally delivered
local-anesthetic agents. Central nervous system excitement, including seizures, followed by depression, apnea, severe cardiovascular
compromise, dysrhythmias and hypotension are all possible. Direct toxicity to skeletal muscle and anaphylactic reactions are
possible with local administration. The author has never experienced any of these complications at the doses recommended in
this article.
Dosing considerations
Table 1: Recommended infusion volumes
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A maximum dose of 1.0 mg/kg of each agent in combination in the same syringe is recommended. Care should be exercised, especially
in smaller patients and cats, so that maximum doses are not exceeded. The ratio of lidocaine: bupivicaine is 1:4 when using
concentrations of 2 percent and 0.5 percent, respectively. The total volume used depends on the size of the patient and the
number of sites that require analgesia. The maximum number of sites would be four if the patient requires surgical manipulation
in all four quadrants.
Table 2: Sample calculation for regional nerve block for oral surgery in a small patient
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Table 1 shows recommended infusion volumes per site based on patient size. Please note that these volumes represent both agents
mixed together. Table 2 shows a sample calculation for a 4-kg dog.
Table 3: Sample calculation for regional nerve block for oral surgery in a large patient
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Physiologic parameters will determine if the block is effective. If blood pressure, heart rate and respiration rate increase
with surgical manipulation, the block either was not correctly placed or hasn't had the proper amount of time for onset (five
minutes). If enough time has elapsed and the attending veterinarian believes the time for onset is not an issue, then the
block may be repeated as long as the maximum total dose is not exceeded. It should be noted that the low ph of inflamed tissue
will render local anesthetics less effective.
Opiates
Products
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Based on human research, the use of local analgesics and opiates in combination provides extended duration of action. The
addition of morphine or buprenorphine to a regional brachial plexus block for limb amputation demonstrated close to a twofold
increase in duration of effect over patients receiving only bupivicaine in one study. A similar model was used in another
study where researchers demonstrated that the addition of buprenorphine to the regional block provided significant increases
in duration compared to giving the buprenorphine intramuscularly.
The author currently uses opiates in the lidocaine/bupivicaine mixture for procedures where increased duration of effect are
desirable. Patients that are particularly difficult to medicate, either in the hospital or at home, may benefit from this
combination.
Chronic pain states, as seen in canine or feline stomatitis, may make post-operative pain management challenging. Morphine
may be used as a portion of the premedication protocol at 0.5 mg/kg IM 20 minutes prior to induction. One-fourth of that same
volume is added to the local mixture per site to be blocked. Hydromorphone is administered at 0.1 mg/kg as a portion of the
premedication protocol. One-fourth of that same volume is added to the local mixture per site to be blocked. Buprenorphine
may be used as a premedication at 0.015 mg/kg following the same volume recommendations for local infiltration as with the
examples above.
Morphine and hydromorphone are pure mu agonists and are preferred for management of severe pain. Buprenorphine, a partial
mu agonist, is a good choice for mild to moderate pain. All three agents may be used in either species, but pure mu agonists,
in particular hydromorphone, are known to cause hyperthermia in cats. If pure mu agonists are used either locally or systemically
in cats, temperature should be monitored during and up to five hours post procedure.
Equipment
None of the regional nerve blocks for oral surgery in dogs and cats need any special equipment to employ. A tuberculin syringe
with a 5/8-in., 25-gauge needle is used for patients 4 kg or less. Three-or 6-ml syringes with 22-25 gauge, 3/4-in. needles
are used for patients more than 4 kg, depending upon the volume to be infused. The correct dose of the desired agents is drawn.
The needle is advanced to the desired location. Aspiration is performed to ensure the needle is not in a vessel and the agent
is dispersed. If one encounters a vessel, the needle is redirected and aspiration repeated. Once extravascular placement is
confirmed, the agent is administered. Be careful not to advance the needle during injection to avoid inadvertent vessel penetration.
Directional terms
Photo 1: Mesial and distal are locational terms used to describe proximity to a certain tooth. In this example, the arrows
are pointing to regions both mesial and distal to the mandibular left third premolar.
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An introduction of two important directional terms is indicated prior to our discussion of the individual nerve blocks. These
two terms are used when describing direction adjacent to an individual tooth (Photo 1). The first term is mesial. It describes
the area adjacent to a particular tooth that is toward the maxillary or mandibular midline (where the incisors of the right
and left arcades meet). A similar, more familiar, term is rostral. It refers to direction in a larger context, considering
the skull in general rather than an individual tooth. Distal is the term used to describe the area adjacent to a tooth away
from the midline. Caudal is a similar less-descriptive term.
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