Local anesthesia of the soft tissues by the infiltration technique occurs almost immediately with all of the local anesthetics. As more tissue penetration becomes necessary, the intrinsic latency of onset previously discussed plays a greater role. Generally, in dentistry, for any given drug, the onset time required is shortest with an infiltration block, longer with a peripheral (minor) nerve block, and longest for topical anesthesia. Onset of action is also affected by the dissociation constant (pKa) and the pH at which two forms of the molecule, charged and uncharged, exist in equivalent amounts. In other words when pKa equals pH, half of the anesthetic molecules present will be charged and half uncharged. A lower pKa, closer to a physiologic pH of 7.4, will result in faster diffusion through tissues and into nerve fibers. This is especially important when dealing with an infected tooth where the pH of the surrounding tissues may be lower than the physiologic pH. In these cases, mepivacaine may be the agent of choice given its pKa of 7.6 compared to that of lidocaine at 7.9.

The duration of anesthesia varies greatly with the regional technique performed. This profile may differ for different agents, depending on their intrinsic pharmacologic properties. For example, lidocaine (1 %) with epinephrine (1:200,000) has a duration of 416 min with infiltration, 178 min with ulnar nerve block, 156 min with epidural anesthesia, and 94 min with spinal block.

The quality, onset time, and duration of a local anesthetic block may be improved by increasing the dose of the agent by using a higher concentration or greater volume. Increases in dosage must be limited by anesthetic toxicity; however, for consistently effective local anesthetic block, an adequate concentration and volume must be administered as close to the target nerve(s) as possible.

In a survey of local anesthetic usage in pediatric patients by Florida dentists, Cheatham and Primosch found that younger children received much higher dosages of local anesthetics than older children. Frequently, the amount of local anesthetic administered to young children far exceeded the manufacturers’ maximum recommended dose [7, 8].

During any dental treatment for children but especially during pediatric sedation, the maximum dose of local anesthetic to be administered to the child must be carefully calculated and recorded prior to its administration. Table 5.1 shows the recommended dosages for each local anesthetic commonly used in pediatric dentistry. Note that the dosages shown are lower than those recommended by the manufacturers. The rationale for these differences will be discussed in a subsequent section of this chapter.

Onset time, duration, and quality of block are also affected by the addition of vasoconstrictor agents to the local anesthetic solution. Vasoconstrictor agents such as epinephrine decrease the rate of drug absorption by decreasing blood flow to the tissues, prolonging the duration of the anesthesia produced, and increasing the frequency with which adequate anesthesia is attained and maintained. Toxic effects of local anesthetics are reduced as a result of delay in absorption into the circulation. Onset time of anesthesia is sometimes shortened as well.