Local Anesthetic Review

Local anesthetics are linked to 50% of the deaths in the dental office. Use the smallest dose that will produce adequate anesthesia. Toxicity can be reached for any anesthetic by administering too much of the drug (especially as related to the patient's body weight), administering the drug to a sensitive individual, administering the drug into a blood vessel, or by improper drug combinations. Local anesthetic drugs affect the cardiovascular system, the nervous system, and local tissues. If the level of the drug is too high, it can become toxic causing a dangerous reaction in the nervous system, cardiovascular system, or in the local tissues. The rate of absorption and elimination of the drug is directly related to its toxic effects. The faster it is absorbed by the bloodstream and the slower the metabolism of the drug, the more toxic it is to the body.

Toxicity Equations

mg per cc. X 1.8 = mg per cartridge
patient weight X toxic limit for the drug = toxic limit in mg
toxic limit in mg # mg in cartridge = maximum cartridges allowed

Administer Less Than Maximum Cartridges Allowed or Less Than Maximum Dosage Allowed, Whichever Is Less.

Always take the weight of the patient into account.

Injection of even a small amount of anesthetic solution directly into a blood vessel can result in an immediate toxic level. It is critical to aspirate each time an anesthetic is administered into an area that is very vascular, but negative aspiration does not guarantee that the bevel of the needle is not in the vessel. However, if the practitioner aspirates multiple times during the slow injection of anesthetic, chances of injection into a vessel are reduced.

Toxic limits are for normal, healthy patients. Some patients will be more sensitive to drugs so they may react to an even smaller dose than someone else regardless of their weight. If the patient is overly sleepy or lethargic after administration of the local anesthetic, it may be a symptom of toxicity.

Any time the patient is taking another CNS depressant, the mixture of the drugs will reduce the toxic level for the anesthetic. Patients should be questioned as tactfully as possible prior to anesthetic administration if there have been any drugs (prescription, over the counter, or street contraband) ingested recently. If the dentist prescribes preoperative anxiety relieving drugs such as Valium or Demerol, the dose of local anesthetic should be monitored even more carefully.

Signs and symptoms of local anesthetic toxicity include: slurred speech, excitement, shivering, muscular twitching, and tremor of facial muscles and extremities. The patient may also feel numbness of the tongue (on the opposite side of a mandibular block or in maxillary anesthesia), warm, flushed skin, lightheadedness, dizziness, diminished sight, tinnitus, and disorientation. These signs and symptoms may not be present when using lidocaine and prilocaine. Toxic levels of these anesthetics usually produce mild sedation or drowsiness. If the patient indicates an excitement reaction, observation is usually all that is necessary. Do not dismiss a patient or leave the patient alone if they are exhibiting any reaction symptom. As the concentration of anesthetic in the bloodstream increases, the patient may go into a seizure. As with all seizures, the most important first aid measure is to place the patient in a position where they will not be hurt and move all dental instruments away from the area. Do not put anything in the patient's mouth. If the seizure continues and the patient has lost the ability to breathe, artificial respiration must be administered. If the seizure does not become self-limiting, it may be necessary to administer 5 mg of Valium. Watch the patient's vital signs, they may go into respiratory arrest. Usually if the patient is properly ventilated, the effect of the anesthetic will wear off and the patient should be able to breathe on his or her own after about 15 minutes.

The table below lists maximum recommended doses and toxic limits for the most common anesthetics (from Malamed Handbook of Local Anesthetics)

Before administering any anesthetic, calibrate the dose of anesthetic in the cartridge. The percent of the solution is the indicator of concentration. For example, 2% lidocaine is 20 mg of xylocaine per cc of the drug. Multiply this number by 1.8 (because of the cartridge containing 1.8 cc. of solution). 2% xylocaine is 20 mg per cc x 1.8cc = 36 mg per cartridge. So for a 180 lb patient the maximum dose is 2 mg/lb x 180 divided by 36 mg in the cartridge = 10 cartridges. But the maximum dose for this drug is 300 mg which is 8 cartridges. In the same patient, the maximum dose for citanest would be 5.5 cartridges.

Children have a smaller body weight, so the toxic level will be reached faster. Remember to take the patient's weight into account when figuring the maximum dose of any local anesthetic. For a 50 lb. child, using 2% lidocaine: 2 mg/lb x 50 divided by 36 mg in the cartridge = 2.7 cartridges.