The epidural sieve and MBC (minimal blocking concentration): an hypothesis.

Certain painful stimuli-of which those associated with placental abruptio and impending rupture of a uterine scar are examples-can penetrate an apparently well-established lumbar epidural block. As a possible explantation of this anomaly, it is suggested that, when used in the concentrations currently employed, the local anaesthetic reaches nerve axons only slightly above the minimal blocking concentration (MBC) of the drug, and that a more powerful stimulus can provoke an impulse which may be conducted through the "blocked" segment of axon. The beneficial aspects of the "epidural sieve" should be welcomed and no attempt made to eliminate the phenomenon.