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2 SBAs on antimicrobials and respiratory drugs
Viva
3 viva questions
Why is once-daily gentamicin better than three times a day?
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Because efficacy and toxicity depend on different parts of the curve. Killing is concentration-dependent, so a single large dose gives the high peak:MIC ratio that predicts it. Toxicity depends on sustained exposure — uptake into renal tubular and cochlear cells is saturable — so it tracks the trough, which once-daily dosing keeps low. The prolonged post-antibiotic effect means the sub-MIC period costs nothing.
Would you give a beta-lactam by continuous infusion in severe sepsis?
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There is a coherent pharmacodynamic argument for it. Beta-lactam killing depends on the percentage of the interval spent above MIC, and continuous infusion maximises that. It is most attractive where the organism has a high MIC and where the volume of distribution is expanded or renal clearance augmented, both of which shorten time above MIC. A loading dose is still needed, because an infusion alone reaches target slowly.
Why does lidocaine fail when injected into an abscess?
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Local anaesthetics are weak bases and only the unionised fraction crosses the nerve membrane. Infected tissue is acidic, so a much larger proportion of the drug is ionised and cannot penetrate. Increased blood flow in inflamed tissue also carries drug away faster. The solution is to block proximally, in tissue of normal pH.