JSAL Anesthesia Practice Test

Onset of action for a local anesthetic depends on how quickly the drug diffuses to the nerve and blocks the sodium channels inside the nerve. Bupivacaine is highly lipid-soluble and strongly protein-bound, so it penetrates nerve membranes more slowly and takes longer to reach effective intraneural concentrations. That slower diffusion translates into a longer time before sensory blockade appears. For a standard nerve block with bupivacaine, the onset is typically in the range of 20–30 minutes. This slow onset is paired with a long duration of action, which is a common trade-off for this agent. Factors like concentration, exact site of injection, and whether a vasoconstrictor is used can tweak the timing, but the inherent property of greater lipid solubility underpins the longer onset.