What effect does increased blood flow have on local anesthetic duration?

What effect does increased blood flow have on local anesthetic duration?

• A. Shortens
• B. Lengthens
• C. No effect
• D. Increases onset time

Answer: (A)

The main idea is that duration of a local anesthetic depends on how quickly the drug is cleared from the injection site. Increased blood flow speeds the removal of the anesthetic from around the nerve into the bloodstream, so the local concentration at the nerve drops sooner and the block wears off earlier. That’s why duration is shortened when blood flow is higher. Onset time is determined mainly by diffusion of the drug to the nerve and is not driven by clearance, so increased blood flow does not meaningfully delay onset; the key effect is the faster clearance leading to a shorter duration.

The main idea is that duration of a local anesthetic depends on how quickly the drug is cleared from the injection site. Increased blood flow speeds the removal of the anesthetic from around the nerve into the bloodstream, so the local concentration at the nerve drops sooner and the block wears off earlier. That’s why duration is shortened when blood flow is higher. Onset time is determined mainly by diffusion of the drug to the nerve and is not driven by clearance, so increased blood flow does not meaningfully delay onset; the key effect is the faster clearance leading to a shorter duration.