Introduction
Local anesthetics (LAs) are medicines that block nerve conduction when applied to a specific area. They cause a reversible loss of sensation without affecting consciousness. The order of sensory loss is pain → temperature → touch → pressure → motor power. These drugs are extremely important for minor surgeries, dental procedures and regional anesthesia.
Chemistry
Local anesthetics are weak bases and have three major components:
- A hydrophilic amino group
- A lipophilic aromatic group
- An intermediate linkage (either ester or amide)
This structural difference helps in classifying LAs into ester-linked and amide-linked drugs.
Classification of Local Anesthetics
1. Based on Clinical Use
a) Surface (Topical) Anesthetics
Cocaine, lignocaine, tetracaine, benzocaine, oxethazaine, proparacaine.
b) Injectable Anesthetics
- Short acting: Procaine, chloroprocaine
- Intermediate acting: Lignocaine, mepivacaine, prilocaine, articaine
- Long acting: Bupivacaine, tetracaine, ropivacaine, dibucaine
2. Based on Structure
- Ester-type: Procaine, chloroprocaine, benzocaine, tetracaine, cocaine
- Amide-type: Lignocaine, mepivacaine, prilocaine, bupivacaine, articaine, ropivacaine
Mechanism of Action
Local anesthetics block voltage-gated sodium channels in nerve membranes. This prevents the initiation and spread of nerve impulses.
Key Features
- Block is frequency-dependent: rapidly firing nerves are blocked earlier.
- Action depends on tissue pH. Alkaline pH increases penetration because more drug is in unionized form.
- Inflamed or infected tissue has acidic pH → less unionized drug → reduced effectiveness.
- Small fibres are blocked before large fibres.
- Myelinated nerves are blocked faster than unmyelinated ones.
- Sensory fibres are blocked earlier than motor fibres.
Factors Affecting Action
- Diameter and myelination of nerve
- Tissue pH and infection
- Drug concentration and dose
- Use of vasoconstrictors
Use of Vasoconstrictor with Local Anesthetics
Adrenaline is commonly added to prolong action.
Advantages
- Slows absorption → prolonged duration
- Reduces bleeding in the surgical field
- Decreases systemic toxicity
Disadvantages / Contraindications
- Risk of ischemia and gangrene in end-artery areas: fingers, toes, penis, ear lobule, nose tip
- Systemic effects: tachycardia, arrhythmia, rise in BP
- Avoid in hypertension, CCF, arrhythmias, ischemic heart disease, hyperthyroidism
- May delay wound healing
Pharmacological Actions
1. Nervous System
- Peripheral nerves: Early block of autonomic fibres → pain → temperature → touch → pressure → motor.
- CNS: High doses cause stimulation (tremors, convulsions) followed by depression and respiratory failure.
2. Cardiovascular System
- Decrease in abnormal pacemaker activity
- Reduced contractility and heart rate
- Bupivacaine is highly cardiotoxic and may cause cardiovascular collapse
- Lignocaine is useful in treating ventricular arrhythmias
Pharmacokinetics
- Ester LAs are rapidly metabolized by plasma cholinesterase
- Amide LAs are metabolized in the liver
- Not effective orally due to first-pass metabolism
- Reduce lignocaine dose in liver disease
Adverse Effects
- CNS: tremor, headache, convulsions, CNS depression, coma
- CVS: bradycardia, hypotension, arrhythmias
- Allergic reactions (more common with ester LAs)
- Methaemoglobinaemia with prilocaine
- Mucosal irritation (notably with cocaine)
Techniques of Local Anesthesia
1. Surface Anesthesia
Application on skin/mucosa using lignocaine, benzocaine, tetracaine. Useful in eye procedures, endoscopy, urethral procedures, ENT diagnostics.
2. Infiltration Anesthesia
Drug injected into tissue to block nerve endings. Used for suturing, abscess drainage, minor surgical removal. Avoid in infected areas.
3. Conduction Block
a) Field Block
Injection around operative area to block sensory nerves.
b) Nerve Block
Injection close to major nerves: brachial plexus, cervical plexus, sciatic, femoral, intercostal blocks.
4. Spinal Anesthesia
LA injected into subarachnoid space (L2–L3 or L3–L4). Useful for lower abdominal, pelvic and lower limb surgeries.
Complications
- Headache due to CSF leakage
- Hypotension (due to sympathetic block)
- Respiratory paralysis
- Rare: meningitis, nerve injury
5. Epidural Anesthesia
LA injected into epidural space. Used in labour analgesia. Ropivacaine preferred due to less motor block and lower cardiotoxicity.
6. Intravenous Regional Anesthesia (Bier’s Block)
Lignocaine or prilocaine injected into a limb vein after applying tourniquet.
Drug Interactions
- Propranolol increases lignocaine toxicity by reducing hepatic clearance
- Procaine metabolite (PABA) reduces effectiveness of sulphonamides
Detailed Notes:
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