Table of Contents
Local anesthetics-Toxicity and Dosage
Toxicity; How much is too much?
The maximum dose for the various local anesthetic solutions is between 70 mg to 500 mg for an average 70 kg (154 lb) patient. Of course, the maximum dose is dependent upon the age, weight and health of the patient, the type of solution used, and whether vasoconstrictor is present or not. These anesthetic agents are distributed in concentrations that are appropriate to their toxicity and their anesthesia producing qualities. In the US and Canada, the carpules (cartridges) that these drugs are distributed in contain 1.8 ml of solution. Articaine carpules contain 1.7 ml. In some other countries, particularly the UK, carpules may contain 2.2 ml.
Since people vary in age, weight and health, the maximum dose of any given drug that an individual can tolerate varies widely, however, an average maximum dose can be computed arithmetically. The maximum dose (for a normal adult weighing 154 pounds) for Articaine is 500 mg. The maximum dose for lidocaine and mepivicaine is 300 mg. The maximum dose of prilocaine is 400 mg, and the maximum dose of bupivicaine is 90 mgm.
These maximum doses are for the anesthetic salts alone and do not take into account the toxic effects of vasoconstrictors which may or may not accompany the anesthetic in dental anesthesia carpules.
Calculating the number of mg from the percentage of the solution
Calculating the number of milligrams of anesthetic agent from the percentage of the solution is confusing to most people. What follows is a simple explanation of how the calculation is made. Keep in mind that all calculations are based on weight rather than volume:
There are 1000 grams of water in a liter. If we remove 1 ml (1 gm) of water and add 1 gm of anesthetic salt, the solution contains 1 gm solid/1000 gm solution. This works out to a .1% solution of anesthetic.
1/1000 = .001
.001 X 100% = 0.1%
Thus, if one adds 10 grams of the anesthetic to 990 ml of water, this equals 10 gm anesthetic/1000gm of solution = 1/100 = 1% of dissolved anesthetic. We now have a 1% solution.
Therefore, each percent of anesthetic concentration translates to 10 mg of anesthetic per milliliter. For example, a 4% solution of prilocain contains 40 mg of anesthetic salt per ml while a .5% solution of bupivicaine contains 5 mg per ml.
(Note that the volume of the original solution is no longer exactly equal to one liter. This is not a particularly important point since it is the ratio by weight of the anesthetic salt to water in the solution which is relevant.)
A 2% solution contains 20 mg of anesthetic agent per milliliter which means that each 1.8 ml cartridge contains 36 mg of agent. In the case of a standard carpule of 2% lidocaine, the maximum dose for a 70 kg (154 lb) adult works out to 500/36 = 13.8 carpules delivered at one time. For children, it works out to about 1/3 to 1/2 that number depending on their weight. These doses are not considered lethal. They are simply the doses at which an average person begins to feel toxic systemic effects from the anesthetics which may include CNS (Central Nervous System) effects of sedation, light headedness, slurred speech, shivering or twitching or, in rare cases, seizures; or cardiovascular effects such as hypotension (low blood pressure).
The maximum dose calculation above does NOT take into account any vasoconstrictor in the carpule. The cardiovascular effects of the vasoconstrictor are frankly more pronounced than the toxic effects of the anesthetic itself. Whenever an anesthetic carpule contains a vasoconstrictor, it is the vasoconstrictor in combination with the toxicity of the anesthetic which limits the recommended doses.
The incidence of toxicity to local anesthetics in the dental setting is extremely rare and generally revolve around very unusual patient centered physiologic abnormalities rather than poor anesthetic technique on the part of the dentist.
The most frequent dose related toxic effect in the dental setting is nervousness and high heart rate, due not to the effect of the anesthetic itself, but rather to the systemic effect of the vasoconstrictor.
The following table is based on The Handbook of Local Anesthesia (Malamed, S.F.). All carpules of anesthesia except articaine contain 1.8 ml of anesthesia. A carpule of articaine contains 1.7 ml.
The third column indicates maximum doses of that particular anesthetic for the average 154 lb (70 kg) patient. The fourth column indicates the maximum number of carpules taking into consideration the presence (or absence) of vasoconstrictor.