Dental Corner: Using intraoral regional anesthetic nerve blocks

Sep 01, 2004

Local anesthesia and regional anesthetic nerve blocks have been used for decades in human dentistry, but incorporating intraoral regional anesthetic blocks into veterinary dental and oral surgical procedures did not gain acceptance until the mid-1990s.1-3 Regional anesthetic blocks, when combined with general anesthesia, provide preemptive analgesia to patients during and after painful procedures. Because regional anesthetic blocks produce complete analgesia before oral surgery, the amount of a general anesthetic needed to maintain a surgical plane of anesthesia is reduced. This reduction ameliorates the hypotensive and other adverse effects of inhalant anesthetics, and the savings from the reduced use of these inhalants (especially sevoflurane) over the course of a year can be great. Moreover, recovery from anesthesia is smoother when a patient has been provided complete analgesia, which reduces the chances of self or iatrogenic injury. And the need for immediate postoperative narcotics is also reduced or eliminated.

How regional anesthesia works

Regional anesthesia refers to blocking the nerve supply to a part of the body, as opposed to local anesthesia in which the anesthetic infiltrates the affected area. Intraoral regional anesthetic nerve blocks are used for a specific area of the mouth and can be used even if only one tooth in the area is to be operated on. Local anesthetic blocks, or splash or ligament blocks, may be used if only one tooth is to be operated on, but they are not as effective as regional blocks, in my experience.

The key to successful regional anesthesia lies in a thorough understanding of the appropriate neuroanatomy. Branches of the maxillary nerve supply sensory innervation to the maxillary teeth, and branches of the mandibular nerve innervate the mandibular teeth. The various regional blocks described below can be used alone or in combination to block sensory impulses from any or all areas of the dental arches.

Figure 1. The proper needle placement for the cranial infraorbital nerve block. The local anesthetic is injected into the opening of the infraorbital foramen. Note the location of the foramen, apical to the distal root of the third premolar.
Local anesthetics act on membrane channels to prevent nerve cell depolarization and to retard or prevent nerve impulse conduction. This mode of action is different from the analgesic properties of narcotics or nonsteroidal anti-inflammatory drugs, so the combination of local and systemic pain relief is not only complementary but also synergistic. The various local anesthetics differ in time of onset, duration of effect, and potential for toxicosis. An individual drug's effects can differ among species; for instance, cats generally have a lower threshold for toxicosis than dogs do. The local anesthetic of choice for intraoral regional anesthetic blocks is bupivacaine hydrochloride because of its prolonged duration of action. Although bupivacaine has a six- to 12-minute lag until the onset of analgesia, the analgesia will last four to six hours.

Performing the blocks

A small-gauge (25- or 27-ga), 1.5-in beveled hypodermic needle is required to minimize the trauma to nerves and blood vessels in the area of the block. Be sure to use a new needle, not the same one used to withdraw the anesthetic agent from the stock bottle. It is essential that aspiration for blood be done before any local anesthetic is injected. The potential for cardiac or acute central nervous system toxicosis greatly increases when these drugs are inadvertently administered intravenously. The total maximum dose of bupivacaine in a cat is 2 mg/kg, so do not inject more than 1 ml bupivacaine (0.5%) into an 11-lb (5-kg) cat during a single procedure. The maximum dose in dogs is 5 mg/kg. The toxic effects that can be observed in a patient that has received too much bupivacaine are cardiovascular arrhythmias and, less commonly, central nervous system signs.