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2# Local Anesthesia
1. Local anesthetics produce anesthesia primarily by blocking which ion channel?
Voltage-gated calcium channels
Voltage-gated sodium channels
Ligand-gated potassium channels
Chloride channels
Explanation:
All local anesthetics reversibly block voltage-dependent Na⁺ channels, preventing depolarization and action potential propagation.
2. Which sequence correctly describes the order of nerve fiber blockade produced by local anesthetics?
Motor → touch → pain → sympathetic
Pain → motor → touch → sympathetic
Sympathetic → pain & temperature → touch → motor
Touch → pain → sympathetic → motor
Explanation:
Small, unmyelinated fibers are blocked first: sympathetic, then pain/temperature, followed by touch and finally motor fibers.
3. Which technique provides anesthesia by applying the drug directly onto skin or mucosa?
Topical anesthesia
Infiltration anesthesia
Field block
Nerve block
Explanation:
Topical anesthesia acts on skin or mucosal surfaces and has a variable onset (20–60 minutes).
4. Sensory fibers are blocked before motor fibers because sensory fibers are:
Larger and heavily myelinated
Located deeper within nerves
More resistant to ischemia
Smaller and often unmyelinated
Explanation:
Pain fibers are small and unmyelinated, making them more sensitive to sodium channel blockade.
5. Which regional anesthesia technique involves injection into the epidural space?
Spinal anesthesia
Epidural anesthesia
Field block
Peripheral nerve block
Explanation:
Epidural anesthesia is administered outside the dura mater and provides segmental anesthesia with slower onset.
6. Which statement correctly compares epidural and spinal anesthesia?
Spinal has slower onset and longer duration
Epidural causes denser motor block than spinal
Spinal has faster onset but shorter duration
Epidural must always cause motor paralysis
Explanation:
Spinal anesthesia has rapid onset (2–5 min) with shorter duration, whereas epidural has slower onset and longer duration.
7. Which chemical component of local anesthetics allows penetration through the nerve membrane?
Lipophilic aromatic ring
Hydrophilic tertiary amine
Ionized base form
Ester linkage
Explanation:
The lipophilic aromatic ring allows the anesthetic to cross lipid nerve membranes.
8. Which property most strongly determines the potency of a local anesthetic?
pKa value
Protein binding
Degree of ionization
Lipid solubility
Explanation:
Higher lipid solubility enhances membrane penetration, increasing potency and intrinsic activity.
9. Which characteristic is typical of ester local anesthetics?
Metabolized slowly in the liver
Higher incidence of allergic reactions
Very stable in solution
Long duration of action
Explanation:
Ester anesthetics produce PABA metabolites, making allergic reactions more common.
10. Which amide local anesthetic can cause methemoglobinemia?
Lidocaine
Bupivacaine
Prilocaine
Ropivacaine
Explanation:
Prilocaine can induce methemoglobinemia due to its metabolites.
11. Absolute contraindication to adding epinephrine to a local anesthetic includes:
Fingers and toes
Epidural anesthesia
Dental infiltration
Scalp blocks
Explanation:
End-arterial areas (fingers, toes, nose, ear lobes) risk ischemia and gangrene with vasoconstrictors.
12. Why is local anesthesia less effective in infected tissues?
Increased blood flow washes the drug away
Sodium channels are upregulated
Protein binding is reduced
Low pH increases ionized fraction of the drug
Explanation:
Acidic environments shift LA toward the ionized form, impairing membrane penetration and efficacy.
13. Which local anesthetic has the highest cardiotoxicity?
Lidocaine
Bupivacaine
Prilocaine
Procaine
Explanation:
Bupivacaine binds strongly to cardiac sodium channels, causing severe arrhythmias in overdose.
14. Which factor most prolongs the duration of action of a local anesthetic?
High protein binding
Low lipid solubility
High pKa
Rapid metabolism
Explanation:
High protein binding prolongs duration by retaining the drug at the nerve site and lowering systemic toxicity.
15. Which local anesthetic is considered safest in the 2nd trimester of pregnancy?
Bupivacaine
Mepivacaine
Lidocaine
Articaine
Explanation:
Lidocaine (Category B) is considered safe during the second trimester.
16. What is the main purpose of adding sodium bicarbonate to a local anesthetic solution?
Increase protein binding
Reduce systemic toxicity
Prolong duration of action
Increase unionized fraction and speed onset
Explanation:
Buffering raises pH, increasing the unionized fraction of LA, improving nerve penetration and faster onset.