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Question: Identify the correct statements regarding pharmacokinetics of locally injected LA.
- The local anestetic effect is a result of systemic absorption the drug from the local site of injection
- Higher the pka, quicker the onset of action
- Higher the protein and tissue binding, longer the duration of action
- Decrease in local tissue pH due to infection hastens the speed of action and increases the potency
- Potency is related to lipid solubility of the drug
Answers:
- false
- false
- true
- false
- true
Explanation:
Important pharmacokinetic properties of local anaesthetics:
- Only the unionised form of LA can penetrate the lipid cell membrane, and produce its effect. Therefore the drug which is more unionised at physiological pH will reach its target site more quickly than the drug which is less so. All LAs have a pka of >7.4 and are ionised at physiological pH to varying degrees. The higher the pka, the more ionised the drug and slower the onset or vice versa. For example, lidocaine with pka of 7.9 has quicker onset than bupivacaine (pka 8.1)
- This explains reduced effectiveness and delayed onset of LA action in infected tissue (low pH= more ionised drug = less unionised drug available to act on the membrane)
- Higher protein binding/ tissue bindings make the drug unavailable for diffusion away from the site of action and metabolism, prolonging the duration of action.
- Local anaesthetics vary in their potency, as the result of differences in lipid solubility, which enhances diffusion through nerve sheaths and neural membranes. For example, bupivacaine is more lipid-soluble and potent than articaine, allowing it to be formulated as a 0.5% concentration (5 mg/mL) rather than a 4% concentration (40 mg/mL).
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The ‘EDAIC MCQs and answers with simplified explanations’ section covers a different topic each day with the intent that candidates can prepare one small topic each day, making steady progress in their exam preparation.
