Duration of Action and Operative Pain Control

As discussed in Chapter 3, there are several physical properties that determine the local anesthetic’s onset and duration of action. Table 5-2 lists the properties for the local anesthetic agents that are currently available. The drug’s pK_a determines the anesthetic’s distribution of cations and anions. The lower the pK_a, the more anions are present in base form for better penetration through the lipid rich nerve, which provides a more rapid onset of action. The protein-binding capacity and the lipid solubility of the anesthetic are related to its duration of action. The lipid solubility also determines the anesthetic’s potency. Bupivacaine has the highest percentage of protein binding and is the most lipid-soluble of all the local anesthetics; it therefore has the longest duration and is the most potent. However, bupivacaine has the highest pK_a and therefore the slowest onset of action. The vasodilating properties of anesthetics also play a significant role in both the potency and duration of action. Mepivacaine and prilocaine have the least vasodilating effects, allowing the anesthetic to remain in the area of disposition longer, especially following a nerve block^3,4 (Table 5-3). For this reason, both anesthetics are quite effective without a vasoconstrictor, and both are good alternatives when a vasoconstrictor is contraindicated.

TABLE 5-2

Physical Properties of Local Anesthetics

LOCAL ANESTHETIC	PKA*	VASODILATING†	T ‡	LIPID SOLUBILITY§	PROTEIN BINDING‖ (5%)
Lidocaine	7.7	1	1.6	2.9	65
Mepivacaine	7.7	0.8	1.9¶	0.8	75
Prilocaine	7.7	0.5	1.6	0.9	55
Bupivacaine	8.1	2.5	3.5	27.5	95
Articaine	7.8	1¶	.75	1.5	54

^*pK_a, Dissociation constant; rate of onset.

^†Vasodilating lidocaine given value of 1.

^‡Half-life (hours).

^§Lipid solubility oil/water solubility; intrinsic potency, increased penetrability.

^‖Protein binding duration of action

^¶Estimated.

Modified from Haveles B: Applied pharmacology for the dental hygienist, ed 6, St Louis, 2011, Mosby.

The duration of local anesthetic agents is divided into three main categories, which are influenced by the presence or absence of a vasoconstrictor:

Table 5-4 lists the local anesthetics by their duration of action categorized by short, intermediate, and long acting, and both pulpal and soft tissue anesthesia. Figure 5-1 illustrates the duration of action for soft tissue anesthesia following the administration of a nerve block, and Figure 5-2 illustrates the duration of action for pulpal anesthesia following the administration of a nerve block.

Duration of anesthesia varies among patients depending on individual response to anesthetic, accuracy of anesthetic administration, vascularity of tissue, variation of anatomic structure, and injection technique:

Posttreatment Pain Control

For effective pain management, the selection of an appropriate anesthetic agent for a dental procedure should take into consideration posttreatment pain control. For half mouth (two quadrants) nonsurgical periodontal therapy, profound anesthesia and a longer appointment time may be necessary. Although nonsurgical periodontal therapy usually does not require posttreatment pain control, long duration agents may be needed on occasion for some individuals if posttreatment discomfort is anticipated.⁵ Most dental procedures performed by the dentist and dental hygienist fall within the intermediate range of pain control. Examples of procedures that may require postoperative pain control may include, but are not limited to, extractions (impacted third molars) and periodontal surgeries. If the dental hygienist is administering anesthesia for a non–dental hygiene procedure, consultation with the dentist before selecting the local anesthetic is appropriate. Anesthetics that provide longer posttreatment durations of pulpal and soft tissue anesthesia are 0.5% bupivacaine 1 : 200,000 epinephrine (long acting), and 4% prilocaine 1 : 200,000 epinephrine (which can provide intermediate to long) (see Table 5-4). Longer acting local anesthetic agents should be avoided for children and individuals with special needs to prevent the possibility of self-mutilation by accidentally biting or chewing on the lip or tongue.

Patient Health Assessment and Current Patient Medications

Local anesthetics used in dentistry today are reliable, and when administered correctly produce effective pain control with little toxicity.⁶ However, certain medical conditions, such as cardiovascular disease, liver disease, hyperthyroidism, brittle diabetes, allergies, etc. and drug interactions, such as tricyclic antidepressants, cimetidine, beta blockers, etc. may influence the type and volume of anesthetic or vasoconstrictor that the patient may safely receive. The patient’s medical history must be thoroughly evaluated and discussed at each dental hygiene appointment to determine if a relative or absolute contraindication exists for the local anesthetic agent or vasoconstrictor. A relative contraindication means that the administration of the offending drug may be used judiciously, and an absolute contraindication means the offending drug should not be administered to the individual under any circumstances. Once the dental hygienist determines if any contraindications exist utilizing consultations as needed, the maximum recommended dose (MRD) for the individual patient can be determined. The maximum reco/>