4

Local Anesthetics Commonly Used in Dentistry: Assessment, Analysis, and Associated Dental Management Guidelines

LOCAL ANESTHETIC CLASSIFICATION AND PHARMACOTHERAPEUTICS

There are two distinct types of local anesthetics: amides and esters. Amides are subdivided into amides with epinephrine and amides without epinephrine (Table 4.1).

Table 4.1 Summary: Local Anesthetics (LAs)

Classification of Amides with Epinephrine

• 2% lidocaine with 1:100,000 epinephrine (Xylocaine)
• 4% prilocaine HCL with 1:200,000 epinephrine (Citanest Forte)
• 0.5% bupivacaine with 1:200,000 epinephrine (Marcaine)
• 4% septocaine with 1:100,000 and 1:200,000 epinephrine (Articaine)
• 0.5% Ropivacaine with 1:200,000 epinephrine (Naropin)

Amide with Levonordefrin (NeoCobefrin)

• 2% mepivacaine HCL (Carbocaine) with 1:20,000 levonordefrin (NeoCobefrin), a sympathomimetic amine

Classification of Amides Without Epinephrine

• 3% mepivacaine HCL (Carbocaine)
• 4% prilocaine HCL (Citanest Plain)

Classification of Ester Local Anesthetics

• Injectable propoxycaine and procaine (Ravocaine)
• Topical benzocaine

Metabolism of Local Anesthetics

Metabolism of all amide local anesthetics other than septocaine does not begin until those local anesthetics reach the liver where they are metabolized and then, for the most part, excreted through the renal system.

Septocaine (Articaine) is unique among amide-type local anesthetics in the way that it is metabolized. Septocaine is actually a hybrid of both an amide and an ester class anesthetic because of the presence of both an amide and an ester intermediate chain in its chemical composition. Biotransformation of 90–95% of septocaine begins immediately upon the drug entering the blood stream where the plasma carboxyesterase enzymes initiate the metabolic breakdown process via hydrolysis of the ester chain to its primary metabolite, articainic acid, which is inactive. The remainder of septocaine (5–10%) is metabolized in the liver by the hepatic microsomal enzymes. Amides have a high rate of first pass metabolism as the local anesthetic (LA) passes through the liver. Slow absorption from tissue is less likely to result in toxicity. If toxicity occurs, it often results from accidental parenteral injection or due to LA overdose.

Ester local anesthetics are metabolized in the plasma, by plasma cholinesterase, and plasma cholinesterase is synthesized in the liver. There is no real advantage to using ester local anesthetics over the amides in a cirrhotic patient. Benzocaine as a topical anesthetic is the only ester used today in dentistry and, as further discussed in this chapter, benzocaine products should not be used in children less than 2 years of age.

Age is one factor that alters the pharmacokinetics of local anesthetics (LAs). The half-life of LAs can be altered in the elderly because of decreased hepatic blood flow from disease processes such as liver disease or congestive heart failure (CHF). These disease processes can also impair the ability of the liver to produce enzymes. All these factors contribute to elevated levels of amide local anesthetics in these patients compared to patients with normal liver function.

Hormonal changes during pregnancy are primarily responsible for the enhanced potency of local anesthetics. Thus, pregnancy can also alter the metabolism of LAs, and the dose of LAs should be reduced by 30%, regardless of the trimester of pregnancy.

Factors Affecting Onset and Duration of Local Anesthetics

The onset and duration of action depends on multiple factors:

Local Anesthetics and Cross-Reactivity