MJDF Pharmacology & Pain Control 1 — Questions and Answers
Question 1: What is the mechanism of action of lidocaine as a local anaesthetic?
- Blocks voltage-gated sodium channels, preventing action potential propagation in nerve fibres (Correct answer)
- Blocks potassium channels
- Inhibits calcium influx at the neuromuscular junction
- Activates GABA receptors
Correct answer: Blocks voltage-gated sodium channels, preventing action potential propagation in nerve fibres
Lidocaine blocks voltage-gated Na⁺ channels in the non-ionised form, preventing membrane depolarisation and propagation of action potentials along sensory and motor nerve fibres.
Question 2: Why does local anaesthesia work less effectively in infected tissue?
- Acidic pH in infected tissue increases ionisation of LA, reducing membrane penetration (Correct answer)
- Bacteria directly inactivate the LA
- Increased blood flow washes away the LA immediately
- Infected tissue has fewer nerve endings
Correct answer: Acidic pH in infected tissue increases ionisation of LA, reducing membrane penetration
Infected tissue has a low pH; LA molecules (weak bases) are mostly in the ionised form at low pH, unable to cross the lipid nerve membrane, reducing effectiveness.
Question 3: What is the maximum safe dose of articaine with adrenaline for a 70 kg adult?
- 7 mg/kg (approximately 490 mg — about 5–6 cartridges of 4% articaine) (Correct answer)
- 3.5 mg/kg
- 10 mg/kg
- 1.5 mg/kg
Correct answer: 7 mg/kg (approximately 490 mg — about 5–6 cartridges of 4% articaine)
Articaine with adrenaline has a maximum dose of 7 mg/kg; at 70 kg, this is approximately 490 mg — equivalent to approximately 5–6 cartridges of 4% articaine (40 mg/ml, 1.7 ml per cartridge).
Question 4: What is the pharmacological reason for adding adrenaline to a local anaesthetic?
- Causes local vasoconstriction, reducing systemic absorption, prolonging duration, and improving depth of anaesthesia (Correct answer)
- Acts as a second anaesthetic
- Increases pH of the solution
- Prevents bacterial contamination
Correct answer: Causes local vasoconstriction, reducing systemic absorption, prolonging duration, and improving depth of anaesthesia
Adrenaline causes local vasoconstriction at the injection site, slowing systemic absorption, increasing LA concentration at the nerve, prolonging duration by 50–100%, and reducing systemic toxicity risk.
Question 5: Which group of patients requires special caution when administering adrenaline-containing local anaesthetic?
- Patients taking non-selective beta-blockers (risk of hypertensive crisis) (Correct answer)
- Patients taking ACE inhibitors
- Patients with well-controlled Type 2 diabetes
- Patients taking statins
Correct answer: Patients taking non-selective beta-blockers (risk of hypertensive crisis)
Non-selective beta-blockers (e.g., propranolol) block β₂-mediated vasodilation, leaving α₁ vasoconstriction unopposed when adrenaline is given, potentially causing a hypertensive crisis with reflex bradycardia.
Question 6: What is the appropriate management of an intravascular injection of local anaesthetic?
- Stop injection immediately, monitor vital signs, position supine, call 999 if systemic signs develop (Correct answer)
- Continue injection at a slower rate
- Administer atropine
- Give more LA to reverse the effect
Correct answer: Stop injection immediately, monitor vital signs, position supine, call 999 if systemic signs develop
If intravascular injection is suspected (tachycardia, metallic taste, CNS signs), stop immediately, position the patient supine, monitor closely, and summon emergency help if systemic toxicity signs develop.
MJDF Pharmacology & Pain Control 1
What is the mechanism of action of lidocaine as a local anaesthetic?