Local anaesthetic agents are classified into two main groups based on their chemical structure. Which of the following is a characteristic feature that distinguishes ester local anaesthetics from amide local anaesthetics?

## Classification and Metabolism of Local Anaesthetics ### Structural Basis Local anaesthetics are divided into two classes based on the type of linkage between the aromatic ring and the intermediate chain: - **Ester local anaesthetics**: contain an ester linkage (–COO–) - **Amide local anaesthetics**: contain an amide linkage (–NH–CO–) ### Key Metabolic Differences **Key Point:** The chemical structure determines the metabolic pathway and, consequently, the duration of action and potential for systemic toxicity. | Feature | Ester Local Anaesthetics | Amide Local Anaesthetics | |---------|--------------------------|-------------------------| | **Metabolism** | Plasma cholinesterase (pseudocholinesterase) | Hepatic microsomal enzymes (CYP450) | | **Duration of Action** | Shorter (30–60 min) | Longer (2–6 hours) | | **Allergic Potential** | Higher (PABA metabolite) | Lower (no PABA metabolite) | | **Examples** | Procaine, cocaine, tetracaine | Lidocaine, bupivacaine, prilocaine | | **Systemic Toxicity Risk** | Lower (rapid metabolism) | Higher (slower metabolism) | ### Clinical Significance **High-Yield:** Patients with pseudocholinesterase deficiency (genetic or acquired) cannot rapidly metabolize esters and are at risk for prolonged toxicity. Amides are safer in such patients but require hepatic function to be intact. **Clinical Pearl:** Esters are metabolized to PABA (para-aminobenzoic acid), a known allergen. This is why amides are preferred in patients with a history of local anaesthetic allergy. [cite:KD Tripathi 8e Ch 12]