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Wednesday, August 6, 2025

Local injectable anesthetics


1. Definition and Overview

Local injectable anesthetics are pharmaceutical agents administered by injection to temporarily block the transmission of nerve impulses in a localized area. These drugs produce reversible loss of sensation, often without affecting consciousness, making them essential in various medical procedures such as minor surgeries, dental interventions, diagnostic manipulations, and childbirth.

Unlike general anesthetics, local injectable anesthetics act peripherally, by targeting nerve fibers at the site of administration, blocking sodium ion influx during depolarization, which prevents the propagation of nerve impulses. Their onset, potency, and duration vary according to their chemical structure, lipid solubility, protein binding, and pKa values.

These agents are divided into two main categories based on their intermediate chain: amide-type and ester-type anesthetics. Amides are more commonly used in modern clinical practice due to their longer duration of action and reduced allergenic potential.

2. Mechanism of Action

Local anesthetics inhibit the function of voltage-gated sodium channels (Nav1.7, Nav1.8, etc.) on the neuronal membrane. When administered locally via infiltration, field block, nerve block, or spinal injection, they bind to the intracellular portion of the sodium channel in its active or inactivated state.

This blockage prevents the initiation and propagation of action potentials along sensory (A-delta and C fibers) and, at higher doses, motor nerves. By increasing the threshold for electrical excitability and delaying repolarization, the affected region becomes numb, unresponsive to painful stimuli.

Local anesthetics are more effective in small, unmyelinated fibers (C fibers) than in large, myelinated ones, explaining their selective ability to block pain over other sensations and motor function. The onset and duration of action depend on:

  • Lipid solubility: enhances potency

  • pKa: affects onset speed

  • Protein binding: correlates with duration

  • Vasodilatory properties: influence systemic absorption and duration

3. Types and Classification

3.1 Amide-Type Local Anesthetics (Metabolized in Liver)

  1. Lidocaine (Xylocaine)

  2. Bupivacaine (Marcaine)

  3. Ropivacaine (Naropin)

  4. Mepivacaine (Carbocaine)

  5. Prilocaine (Citanest)

  6. Articaine (primarily used in dental anesthesia)

These drugs are more stable in solution, less likely to cause allergic reactions, and exhibit longer duration of action due to hepatic metabolism.

3.2 Ester-Type Local Anesthetics (Metabolized by Plasma Esterases)

  1. Procaine (Novocain)

  2. Tetracaine (Pontocaine)

  3. Chloroprocaine (Nesacaine)

  4. Benzocaine (used mostly topically but injectable forms exist in specialty uses)

Ester anesthetics have shorter half-lives and are more likely to cause allergic reactions due to their metabolism to para-aminobenzoic acid (PABA).

4. Clinical Indications

  1. Minor surgical procedures (suturing, skin lesion removal)

  2. Dental procedures (tooth extraction, cavity preparation)

  3. Regional anesthesia (epidural, spinal, nerve blocks)

  4. Obstetric procedures (labor analgesia)

  5. Orthopedic interventions (joint injections)

  6. Emergency medicine (wound care, laceration repair)

  7. Diagnostic procedures (lumbar puncture)

  8. Chronic pain management (nerve block therapy)

  9. Cardiac arrhythmias (lidocaine IV in emergency, though not localized)

The route and dose depend on the site, depth, expected duration of anesthesia, and patient comorbidities.

5. Routes of Administration

  • Infiltration anesthesia: directly into tissues (e.g., for skin lacerations)

  • Field block: around the operative area to block nerves before they reach the surgical site

  • Nerve block: near major nerves or plexuses (e.g., brachial plexus block)

  • Epidural anesthesia: into epidural space to anesthetize spinal roots

  • Spinal (intrathecal) anesthesia: directly into cerebrospinal fluid for lower body surgeries

  • Intravenous regional anesthesia (Bier block): anesthetic into isolated limb via IV

  • Intra-articular: within joints, post-operatively

6. Pharmacokinetics

  • Absorption: Determined by site vascularity and presence of vasoconstrictors (epinephrine prolongs duration and reduces systemic toxicity)

  • Distribution: Initially high to vessel-rich organs (brain, heart); then redistributed to fat and muscle

  • Metabolism:

    • Amides: hepatic microsomal enzymes (CYP1A2, CYP3A4)

    • Esters: plasma pseudocholinesterases

  • Excretion: Metabolites excreted via kidneys

Time to onset: influenced by pKa and pH of tissue
Duration: varies from 30 minutes (lidocaine) to 4–8 hours (bupivacaine)

7. Adverse Effects

7.1 Local Effects

  • Pain at injection site

  • Hematoma

  • Infection

  • Nerve damage (rare)

7.2 Central Nervous System Toxicity

Due to systemic absorption or overdose

  • Lightheadedness

  • Dizziness

  • Metallic taste

  • Tinnitus

  • Circumoral numbness

  • Seizures

  • Coma

7.3 Cardiovascular Toxicity

Especially with bupivacaine and large doses

  • Hypotension

  • Bradycardia

  • Ventricular arrhythmias

  • Cardiac arrest

7.4 Allergic Reactions

  • More common with esters due to PABA

  • Rashes, urticaria

  • Anaphylaxis (rare)

7.5 Methemoglobinemia

  • Seen with prilocaine, benzocaine, articaine

  • Oxygen-carrying capacity is impaired

  • Cyanosis unresponsive to oxygen therapy

  • Treatment: methylene blue

8. Contraindications

  • Known hypersensitivity to the agent or class

  • Severe hepatic impairment (for amides)

  • Pseudocholinesterase deficiency (for esters)

  • Cardiac conduction abnormalities (caution with bupivacaine)

  • Local infection at injection site

  • Coagulopathy (for spinal or epidural routes)

  • Certain neurological disorders (risk with neuraxial anesthesia)

9. Drug Interactions

  1. Antiarrhythmics (Class I-B): Additive cardiotoxicity with lidocaine

  2. Beta-blockers: Reduced hepatic clearance of amides

  3. Cimetidine: Inhibits metabolism of amide anesthetics

  4. Other CNS depressants: Additive sedative effects

  5. Sulfonamides: Compete with ester metabolism (PABA antagonism)

  6. Nitrates: Increased risk of methemoglobinemia with prilocaine

  7. Epinephrine combinations: Should not be used in end-arterial areas (fingers, nose, penis)

10. Special Precautions and Monitoring

  • Resuscitation readiness: especially when using high doses or performing nerve blocks

  • Cardiac monitoring: continuous ECG during large block procedures

  • Pulse oximetry and blood pressure: for all regional anesthetics

  • Avoid intravascular injection: aspiration and slow injection technique mandatory

  • Test dosing: recommended for epidural administration

  • Lipid emulsion availability: to manage systemic toxicity (especially bupivacaine)

11. Local Anesthetic Systemic Toxicity (LAST)

Life-threatening condition, typically from intravascular injection or overdose

  • CNS symptoms: agitation, tremors, seizures, coma

  • Cardiac symptoms: bradycardia, hypotension, QRS widening, asystole

  • Treatment:

    • Airway management

    • Benzodiazepines for seizures

    • 20% lipid emulsion IV (intralipid)

    • Advanced cardiac life support (ACLS) protocol if arrest occurs

    • Avoid vasopressin and calcium channel blockers during resuscitation

12. Clinical Pearls

  • Bupivacaine has longer duration but greater cardiotoxicity

  • Lidocaine has faster onset and less toxicity, often used with epinephrine

  • Ropivacaine is less cardiotoxic than bupivacaine, ideal for continuous epidural infusions

  • Prilocaine carries risk of methemoglobinemia

  • Articaine offers excellent bone penetration, used widely in dentistry

  • Tetracaine used for spinal anesthesia due to long duration

13. Emerging Research and Trends

  • Liposomal formulations (e.g. liposomal bupivacaine): extended-release local anesthetics providing pain relief for up to 72 hours after a single administration

  • Perineural adjuvants: clonidine, dexmedetomidine, dexamethasone to prolong block duration

  • Non-opioid multimodal analgesia: expanding the use of regional blocks to reduce opioid dependence

  • Nerve stimulator and ultrasound guidance: improving safety and efficacy of nerve blocks

  • Gene polymorphisms: affecting metabolism and responsiveness to local anesthetics

14. Use in Special Populations

Pediatrics

  • Weight-based dosing crucial

  • Lower maximum allowable doses

  • Increased sensitivity to toxic effects

Geriatrics

  • Decreased hepatic clearance

  • Start with lower doses

  • Risk of hypotension in spinal blocks

Pregnancy

  • Decreased protein binding increases free drug

  • Enhanced effect due to hormonal and anatomic changes

  • Epidural and spinal anesthetics are commonly used in labor

Liver Disease

  • Avoid amides or reduce dosage

  • Monitor closely for signs of CNS toxicity

Renal Impairment

  • May prolong the elimination of certain metabolites

  • Monitor for systemic accumulation

15. Agents Summary Snapshot (without table)

  • Lidocaine: moderate duration, fast onset, low toxicity, used for infiltration, nerve blocks, epidural, IV regional

  • Bupivacaine: long duration, high potency, slower onset, high cardiotoxicity, used in spinal, epidural, nerve blocks

  • Ropivacaine: similar to bupivacaine but with less cardiotoxicity

  • Mepivacaine: moderate onset, moderate duration, used for infiltration and nerve blocks

  • Prilocaine: moderate duration, risk of methemoglobinemia

  • Tetracaine: potent, long-acting, primarily for spinal anesthesia

  • Procaine: short duration, low potency, historical use

  • Articaine: used mainly in dental anesthesia due to good bone diffusion



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