Membrane pressure drop, pressure gradient or simply pressure drop (dP or ΔP) is defined as the difference between pressure of the blood at the inlet of the membrane lung (ML), pre-membrane pressure, P_PRE or P_IN, and pressure of the blood at the outlet of the membrane lung or post-membrane pressure, P_POST or P_OUT (picture 1). Displayed in bold characters the definitions and acronyms according to the ELSO Nomenclature in Extracorporeal Life Support (Conrad et al. 2018).

ΔP = P_PRE – P_POST

Sometimes pressure drop has been improperly referred to as “transmembrane pressure”; this definition, to be specific, relates with the pressure gradient across the wall of the hollow fibers between the blood phase and the gas phase.