Which of the following local anesthetics is the most common cause of methemoglobinemia?

EMLA Cream (Lignocaine + Prilocaine)

Mechanism of action of d-tubocurarine is:

Competitive, nondepolarizing block at the Nm cholinergic receptor

Noncompetitive, depolarizing block at the Nm cholinergic receptor

Non-competitive, nondepolarizing block at the Nm cholinergic receptor

Competitive, depolarizing block at the Nm cholinergic receptor

An 18 years old woman in the active stage of labour requests an epidural anaesthesia for delivery. Immediately following the epidural injection of 12ml of 2% lidocaine, the patient complains of lip numbness and becomes apprehensive. What is the presumptive diagnosis?

Systemic toxicity to drug administered

A patient undergoing a minor surgical procedure is given lignocaine injection. Assertion: Local anaesthetics act by blocking nerve conduction. Reason: Small fibers and non-myelinated fibers are blocked more easily than large myelinated fibers.

Both Assertion and Reason are true, and Reason is not the correct explanation for Assertion

Assertion is false, but Reason is true

Both Assertion and Reason are true, and Reason is the correct explanation for Assertion

Assertion is true, but Reason is false

Which is correct about the anesthetic drugs X and Y in the image shown? (Recent NEET Pattern 2016-17)

Drug X is more fast acting than Y

Drug X and Y have equally fast onset of action

Drug X and Y have equally fast onset of action but potency of X is more than Y

Drug Y is more fast acting than X

Ester Local Anesthetics for NEET-PG: High-Yield Anesthesiology Lessons

Ester LAs - The Quick Spritzers

Contain an ester linkage (-COO-) in their intermediate chain.
Metabolism: Rapidly hydrolyzed in plasma by pseudocholinesterase.
- Cocaine is an exception (also liver esterases).
Allergy: Higher potential due to metabolite PABA (para-aminobenzoic acid).
- Cross-allergenicity within the ester group is common.
Systemic toxicity: Less likely than amides due to rapid breakdown.
Duration of action: Generally shorter.
Examples: Procaine, Cocaine, Tetracaine, Benzocaine, Chloroprocaine. 📌 Mnemonic: Papa Can Take Baby Care.

⭐ Chloroprocaine: shortest duration, rapid metabolism; preferred in obstetrics.

Prototypic amide and ester local anesthetic structures

MOA & PK - Block & Vanish

Mechanism of Action (MOA): Block
- Reversibly block voltage-gated Na+ channels intracellularly.
- Preferentially bind to open/inactivated channels (use-dependency).
- Prevent Na+ influx, blocking nerve impulse conduction.
- Non-ionized form crosses nerve membrane; ionized form is active.
Pharmacokinetics (PK): Vanish
- Rapidly hydrolyzed by plasma pseudocholinesterases (butyrylcholinesterase).
  - Results in characteristically short duration of action.
  - Metabolite: Para-aminobenzoic acid (PABA) → potential for allergic reactions. 📌 "Esters are Pests (PABA)"
- Minimal hepatic metabolism (cocaine is an exception, also metabolized by liver esterases).
- Onset: Governed by pKa (↓pKa = ↑non-ionized form = faster onset).
- Duration: ↑ with protein binding & lipid solubility; vasoconstrictors (e.g., adrenaline) prolong duration & ↓ systemic toxicity.

Ester vs Amide Local Anesthetic Structure

⭐ Allergic reactions to local anesthetics are significantly more common with esters due to their metabolism to PABA (para-aminobenzoic acid) derivatives.

The Ester Crew - Meet the Agents

All esters are hydrolyzed by plasma pseudocholinesterases to para-aminobenzoic acid (PABA), which can cause allergic reactions. They generally have a shorter duration of action than amides.

📌 Esters have one 'i' in their name (e.g., Procaine, Cocaine, Tetracaine, Benzocaine, Chloroprocaine).

Agent	Potency	Onset	Duration	Key Uses / Notes
Cocaine	Medium	Medium	Medium	Topical (ENT), potent vasoconstrictor (blocks NE reuptake), Schedule II drug.
Procaine	Low	Slow	Short	Infiltration, nerve blocks (largely historical). High PABA metabolite (allergy).
Chloroprocaine	High	V. Fast	V. Short	Epidural (obstetrics), spinal. Rapid hydrolysis, low systemic toxicity.
Tetracaine	High	Slow	Long	Spinal, topical (corneal). Potent, longer acting ester.
Benzocaine	Low	Fast	Short	Topical ONLY (sprays, lozenges). ⚠️ Risk of methemoglobinemia.

Side Effects & Allergies - The Ester Sting

Systemic Toxicity (dose-dependent):
- CNS: Early (circumoral numbness, tinnitus, metallic taste); Late (seizures, coma).
- CVS: Hypotension, bradycardia, arrhythmias, cardiac arrest. (Cocaine: unique vasoconstriction, hypertension).
Management of LA Systemic Toxicity (LAST):
- Stop LA injection, ABCs (Airway, Breathing, Circulation), Oxygen.
- Seizures: Benzodiazepines.
- Lipid Emulsion (20%): Bolus 1.5 mL/kg IV, then 0.25 mL/kg/min infusion.
Allergic Reactions (📌 PABA = Problem):
- More common with esters due to Para-aminobenzoic acid (PABA) metabolite.
- Range: Skin rash, urticaria, angioedema to anaphylaxis.
- Generally no cross-allergy with amide LAs.

⭐ Allergic reactions to ester local anesthetics are primarily due to their metabolite, para-aminobenzoic acid (PABA), not the parent drug itself.

Ester vs Amide Local Anesthetics Chemical Structures

High‑Yield Points - ⚡ Biggest Takeaways

Metabolized by plasma pseudocholinesterase; PABA metabolite is responsible for allergic reactions.

Generally have a shorter duration of action compared to amide local anesthetics.

Cocaine is unique: causes vasoconstriction (blocks NE reuptake) and CNS stimulation.

Procaine: Low potency, short duration, high incidence of allergy; prototype.

Tetracaine: Potent, long-acting ester, commonly used for spinal anesthesia.

Chloroprocaine: Rapid onset, very short duration, low toxicity; ideal for obstetrics.

Benzocaine: Used topically only; associated with methemoglobinemia.

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