About Pressure Drop

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, PPRE or PIN, and pressure of the blood at the outlet of the membrane lung or post-membrane pressure, PPOST or POUT (picture 1). Displayed in bold characters the definitions and acronyms according to the ELSO Nomenclature in Extracorporeal Life Support (Conrad et al. 2018).

ΔP = PPRE – PPOST

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.

picture 1. Membrane lung (ML) or Membrane Oxygenator (MO), schematics: blood flow (BF) enters the venous side with a positive pressure (pre-oxygenator pressure – Ppre) and exits the arterial side, with a lower positive pressure (post-oxygenator pressure – Ppost), this pressure loss, named pressure drop, due to internal resistance related to hollow fibers configuration.

Pre-oxygenator and post-oxygenator pressures are useful to assess the resistance through the membrane lung. In relating Ohm’s Law to fluids, the pressure gradient (ΔP) across the ML is determined by the blood flow (BF) through the oxygenator and the resistance (R):

ΔP = BF x R

So, monitoring blood flow and pressures on the circuit, the ML internal resistance can then be derived as the function of ΔP and BF ratio (picture 2):

R= ΔP/BF

picture 2. Pressure drop ΔP and extracorporeal blood flow BF ratio to derive ML internal resistance.

ΔP INCREASE

An abnormally high pressure drop usually refers to increased pre-oxygenator pressure associated to decreased post-oxygenator pressure. Contemporary polymethylpentene PMP hollow fibers ML are characterised by very low pressure gradient. Under the assumption of a steady blood flow, a gradual (modest) ΔP (and R) increase is physiologic with the duration of ML use, due to progressive accumulation of fibrin strands with imbedded red blood cells and platelets on the surface of the fibers (picture 3).

picture 3. On the left, overviews of a gas exchange membrane of long-term used polymethylpentene  PMP oxygenators, close ups on the right; red star: a fiber (modified from Lehle K et  al. ASAIO Journal 2008).

A markedly increasing ΔP and R trend (with unchanged revolutions per minute – RMP – setting at the pump), or an abrupt rise in pressure gradient at any time, need to be promptly detected and investigated (picture 4). Look at the membrane! Assess oxygenator for clots (presence/extent, location, evolution) and their eventual negative impact on ML function, in terms of gas exchange (due to the reduction of the surface capable of gas transfer) and on blood components and coagulation (due to flow impairment), in order to establish the need for an oxygenator exchange, and its timing, elective vs emergent.

picture 4. What if membrane pressure drop rises or falls? Expected first line assessment and monitoring in the setting of abrupt  ΔP increase (left) and decline (right).

ΔP reduction

Pressure drop lowers, eventually slim to none, if pre and post-oxygenator pressures become more similar; distinction should be made between two situations (picture 4), both associated with lowering of extracorporeal blood flow in circuits equipped with centrifugal pumps:

  • ΔP reduction associated with pre-oxygenator + post-oxygenator pressures fall → check post-pump/pre-oxygenator section of the circuit for kinking/obstruction and assess proper pump operation;
  • ΔP reduction associated with pre-membrane + post-membrane pressures rise → check post-ML side of the circuit (return line and/or cannula) for kinking/obstructions, and rule out cannula displacement and complications involving the cannulated vessel.

References

Conrad SA, Broman LM, Taccone FS, Lorusso R, Malfertheiner MV, Pappalardo F, Di Nardo M, Belliato M, Grazioli L, Barbaro RP, McMullan DM, Pellegrino V, Brodie D, Bembea MM, Fan E, Mendonca M, Diaz R, Bartlett RH. The Extracorporeal Life Support Organization Maastricht Treaty for Nomenclature in Extracorporeal Life Support. A Position Paper of the Extracorporeal Life Support Organization. Am J Respir Crit Care Med. 2018 Apr 3. link http://bit.ly/2Ek64LX

Toomasian JM, Schreiner RJ, Meyer DE, Schmidt ME, Hagan SE, Griffith GW, Bartlett RH, Cook KE. A polymethylpentene fiber gas exchanger for long-term extracorporeal life support. ASAIO J. 2005 Jul-Aug;51(4):390-7. open access link http://bit.ly/2KzK6MQ

Lubnow M, Philipp A, Foltan M, Bull Enger T, Lunz D, Bein T, Haneya A, Schmid C, Riegger G, Müller T, Lehle K. Technical complications during veno-venous extracorporeal membrane oxygenation and their relevance predicting a system-exchange–retrospective analysis of 265 cases. PLoS One. 2014 Dec 2;9(12):e112316. open access link http://bit.ly/2KFkASe

Lehle K, Philipp A, Gleich O, Holzamer A, Müller T, Bein T, Schmid C. Efficiency in extracorporeal membrane oxygenation-cellular deposits on polymethylpentene membranes increase resistance to blood flow and reduce gas exchange capacity. ASAIO J. 2008 Nov-Dec;54(6):612-7. open access link http://bit.ly/2MQEA5c

Extracorporeal Life Support: The ELSO Red Book 5th Edition link http://bit.ly/2Nsuzw9

ECMO Specialist Training Manual 3rd Edition link http://bit.ly/2NvQ4wk

Manual de Emergencias en ECMO. Clínica Las Condes link http://bit.ly/2u0ioi9

M. Velia Antonini @foamecmo on social media; Trained as ECMO specialist, Cardiovascular Perfusionist & Sonographer, works as ICU Nurse in Parma & Lecturer in Perfusion techniques and Extracorporeal Support at UNIMORE, Italy. She is Social Media Editor for ELSO, is part of the Social Media council of ASAIO Journal & member of the SoMe team of ESICM, IFAD & SMARTmi. 

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