Which of the following local anesthetics is an aminoester: (a) tetracaine, (b) lidocaine, (c) bupivacaine, (d) mepivacaine, or (e) etidocaine?

Tetracaine is the Aminoester Among the Listed Local Anesthetics

The correct answer is (a) tetracaine, which is the only aminoester local anesthetic among the options listed. 1

Classification of Local Anesthetics

Local anesthetics can be chemically divided into two main categories based on their chemical structure:

1. Aminoesters:

   • Contain an ester linkage between the aromatic portion and the intermediate chain
   • Examples include procaine, chloroprocaine, and tetracaine 1
   • Metabolized by plasma cholinesterase (hydrolyzed in blood) 1, 2
   • More likely to cause allergic reactions 2

2. Aminoamides:

   • Contain an amide linkage between the aromatic portion and the intermediate chain
   • Examples include lidocaine, mepivacaine, bupivacaine, prilocaine, and etidocaine 1
   • Metabolized primarily in the liver 1, 2
   • Cause fewer allergic reactions than aminoesters 2

Analysis of the Options

Let's examine each option:

• (a) Tetracaine: An aminoester local anesthetic with high potency and long duration of action (180-600 minutes) 3, 1
• (b) Lidocaine: An aminoamide local anesthetic with intermediate potency and duration of action 1, 4
• (c) Bupivacaine: An aminoamide local anesthetic with high potency and long duration of action 1, 4
• (d) Mepivacaine: An aminoamide local anesthetic with intermediate potency and duration of action 1, 4
• (e) Etidocaine: An aminoamide local anesthetic with high potency and long duration of action 1, 4

Clinical Relevance of the Classification

The distinction between aminoesters and aminoamides is important clinically for several reasons:

• Metabolism: Aminoesters are hydrolyzed by plasma cholinesterase, while aminoamides undergo hepatic metabolism 1, 2
• Allergic potential: Aminoesters have a higher potential for allergic reactions compared to aminoamides 2
• Toxicity profiles: Though both classes can cause CNS and cardiac toxicity, their specific profiles differ 4, 5

Pharmacological Properties of Tetracaine

Tetracaine (the correct answer) has several distinctive properties:

• Maximum recommended dose: 1.5 mg/kg without epinephrine 3
• Duration of action: 180-600 minutes (one of the longest-acting local anesthetics) 3
• High lipid solubility, which contributes to its high potency 5
• Higher cytotoxicity compared to many other local anesthetics due to its high lipophilicity 5

In summary, tetracaine is the only aminoester among the options listed. The other options (lidocaine, bupivacaine, mepivacaine, and etidocaine) are all aminoamide local anesthetics.