Around the block: local anesthesia for the dental patient (Proceedings)

It is fair to say that, anesthetically, dentistry can be very time intensive. Once the dental prophylaxis is completed, the patient's mouth has been charted, and a full series of intraoral radiographs is ready for evaluation, some patients may have already been under anesthesia for over an hour. A patient that will undergo multiple extractions, oral surgery, endodontics, etc. can be anesthetized for 2-3 hours.

As an adjunct to general anesthesia, regional nerve blocks can be added to increase patient safety by dramatically decreasing the amount of inhalant anesthetics needed for such long procedures. Nerve blocks will also significantly improve post-operative comfort.

During the pre-operative exam, it is often obvious when a procedure will include some level of discomfort. At this point, it is wise to plan on adding local anesthesia. The technician can calculate the dose of the chosen local anesthetic, prepare the syringe and label it accordingly.

Perform the regional block after the patient is under an appropriate level of general anesthesia, the monitoring devices are in place, the IV fluids are running and the condition of the patient is stable. This gives the block time to work before any pain is caused. It is infinitely better to prevent, than to treat pain.

There are a number of drugs used for local anesthesia, each varying in their dose, onset of action, duration, and toxicity. For simplicity, let's concentrate on two selections:

Lidocaine (2%) and Bupivicaine (0.5%).

Lidocaine

Depending on the reference, it is reported that the onset of action is anywhere from 5-15 minutes and most sources agree that its duration is approximately 1- 2 hours. It is also reported that the toxic dose is lower for cats than dogs.

Bupivicaine

Again, depending on the reference, onset of action has been reported as long as 20-30 minutes with a duration of 2.5-6 hours.

The above chart has been prepared to show the total calculated dose for each drug for dogs and cats when administering one or the other. If combining drugs use half of the volume of each.

Five common dental blocks

1.) Local infiltration

This method can be employed when only a small area requires anesthesia. The anesthetic agent can be injected into the gingiva, mucosa or periodontal ligament. When radiographs show bone loss around a specific tooth, a local anesthetic agent can be infiltrated apical to that tooth. Also, as an adjunct to an infraorbital block, the maxillary teeth can be blocked by infiltration palatal to that specific tooth.

2.) Infraorbital nerve block

The infraorbital foramen is easily palpated in the dog and cat. It can be found apical to the maxillary third premolar. Through this foramen passes several nerves that supply innervation to the maxillary arcade. Insert the needle through the buccal mucosa where it forms a crease and direct it towards the foramen in a rostrocaudal direction. Pass the needle until it is at the opening of the foramen. Aspirate in several planes. Slowly inject the agent in an attempt to bathe the exiting nerves supplying the rostral structures. When the more caudal teeth are to be blocked, finger pressure is held over the wheal of local to "force" the agent deeper into the foramen, thus blocking those structures.

Other sources recommend actually passing the needle just inside the foramen and instilling agent to block rostral structures and then advancing the needle gently into the foramen to instill more through the foramen (aspirating frequently). Digital pressure is used to prevent the local agent from then exiting from the foramen.

3.) Maxillary block

If you walk your needle around the most caudal aspect of the upper maxillary second molar, you will be able to pass the needle into the space under the eye. This is where the nerves pass to enter the infraorbital foramen. Make sure you use sharp, short needle with a finger stop employed so as to not advance the needle into the globe of the eye.

4. ) Mandibular block

When attempting to block the mandibular structures, the mandibular nerve needs to be blocked. There are two ways to accomplish this. To block this nerve intraorally, slide your index finger along the mandible caudal to the last molar until you feel an indentation. With the other hand, introduce the syringe until the needle is at the opening of the foramen. Aspirate then inject.

Another method is called the Transcutaneous Approach or the Extraoral Approach. A small area of hair is clipped and prepped at the angle of the mandible. The foramen is palpated as in the previous method. The canine mandible has a notch anterior to the angular process (the cat does not). Insert the needle through the skin until the needle hits the mandibular notch. Then gently walk the needle medially then gently guide the needle to your other index finger. Your needle should be trapped between the Inferior Alveolar Foramen and your finger. When you are sure of your placement, aspirate, then inject.

5.) Mental block

There are three mental foramina. The middle mental foramen is the largest. In large dogs this can be palpated apically between the first and second premolars. The needle is inserted under the submucosa just to the entrance of the foramen. Aspirate and inject. If the interest is in blocking the rostral mandibular structures put a minute's worth of finger pressure to ensure the agent anesthetizes the incisors and canine.

In the cat, the needle is introduced through the submucosa into the labial frenulum and directed caudally. Aspirate and inject. Some important rules for administering local anesthetic agents:

• Do not attempt to instill the local anesthetic agent until the patient is completely under general anesthesia.

• You can decrease the discomfort (sting) experienced by adding 1/10th the volume of Sodium Bicarbonate. For example, if you want to administer 0.9cc of Lidocaine, add 0.1cc Sodium Bicarbonate.

• When dosing small animals, you can increase the volume of local by diluting with Sterile Saline (not Sterile Water). For example, the patient only weighs 6 pounds and the drug chosen is Lidocaine, draw up 0.27cc Lidocaine, 0.02cc Sodium BiCarbonate and fill the remainder of the 1cc syringe with Sterile Saline. Volume is more important than concentration.

• If multiple sites are to be anesthetized, the total calculated dose must be divided by the number of sites.

• Change needles between sites. A sharp needle gives the anesthetist the best advantage for accurate placement. Dull needles require too much effort.

• Always, always aspirate prior to injecting.

• It is possible to provide both short acting and long duration by drawing up 50% of the calculated dose of Lidocaine and 50% of the dose of Bupivicaine. For example; for a 10 pound dog, draw up 0.9cc's of Marcaine and 0.2cc's of Lidocaine. This will insure that the toxic effects of each drug will not be additive.

• It is recommended to decrease the calculated amounts by 30%-40% for old or cachetic pets.

• Lidocaine can be purchased with or without Epinephrine. The epinephrine is added to provide vasoconstriction, this increasing affect and duration.

• Injection into infected tissue is contraindicated.

• Be aware that you may have the level of inhalant anesthesia low while working on one side of the pet. It is often necessary to increase the patient's plane of anesthesia before turning them. It is possible to keep the anesthetic level so light that moving the patient may begin to awaken them.

Finally, the technician experienced in the use of dental blocks will notice the positive impact for his/her patients. The anesthetic procedure will be more even, the recoveries will be smoother, and the patients will be ambulatory and will eat sooner post-operatively. Just as importantly, client satisfaction will increase because they will perceive the experience as less stressful for their pet.