Which factor most contributed to a prolonged neuraxial block when using bupivacaine described as lasting 8 to 12 hours with a large volume and epinephrine added?

Which factor most contributed to a prolonged neuraxial block when using bupivacaine described as lasting 8 to 12 hours with a large volume and epinephrine added?

• A. Short duration of action
• B. Addition of epinephrine
• C. Small volume of local anesthetic
• D. Low concentration

Answer: (B)

The key idea is that how long a neuraxial local anesthetic lasts is largely determined by how quickly it is absorbed into the bloodstream. Epinephrine added to bupivacaine causes vasoconstriction at the injection site, which reduces local blood flow. This slows the drug’s systemic uptake, keeping more of it near the nerve fibers for a longer time and thereby extending the duration of both sensory and motor blockade. While a large volume can increase the spread of the block to more nerve roots, the vasoconstrictive effect of epinephrine has the greater impact on duration, making the prolonged 8–12 hour effect most attributable to its addition. If epinephrine were not used, the block would typically wear off sooner; using a smaller volume or lower concentration would also shorten duration, not prolong it.

The key idea is that how long a neuraxial local anesthetic lasts is largely determined by how quickly it is absorbed into the bloodstream. Epinephrine added to bupivacaine causes vasoconstriction at the injection site, which reduces local blood flow. This slows the drug’s systemic uptake, keeping more of it near the nerve fibers for a longer time and thereby extending the duration of both sensory and motor blockade. While a large volume can increase the spread of the block to more nerve roots, the vasoconstrictive effect of epinephrine has the greater impact on duration, making the prolonged 8–12 hour effect most attributable to its addition. If epinephrine were not used, the block would typically wear off sooner; using a smaller volume or lower concentration would also shorten duration, not prolong it.