Genesis of "negative pressure" during hanging drop; the answer is positive pressure.

Abstract

A drop of saline placed within the hub of an epidural needle is sucked in when the needle is advanced and penetrates through a polyurethane foam cube. This phenomenon might be explained by the release of compressed air inside of the needle into the air. The positive pressure inside of the needle may be generated by advancing of the needle. To prove this hypothesis, the pressure inside of the needle was measured during needle advancement, and it increased gradually and then dropped to zero suddenly when the needle fully penetrated the polyurethane foam block. We can speculate that the same phenomenon occurs during hanging-drop method of epidural anesthesia, and this may occur regardless of whether epidural subatmospheric pressure exists or not.