OK. Time for the First T...

Metadata

• Author: @UCSFFresnoECMO on Twitter
• Full Title: OK. Time for the First T...
• Category: #tweets
• URL: https://twitter.com/UCSFFresnoECMO/status/1625583768722366471

Highlights

• OK. Time for the first tweetorial where we’ll start with the absolute basics:
  Title: The circuit and its pressures
  Level: Basic
  Author: @thind888
  Before diving right into it, an excellent primer on ECMO from @MLSCourse is well worth a read
  https://t.co/STXstEwRj6 (View Tweet)
• The figure attached shows the basic ECMO circuit. The blood flows serially through the following components:
  1)Access/drainage/venous cannula
  2)Pump
  3)Oxygenator
  4)Return/arterial cannula
  🚨Important: Circuit pressures are negative pre-pump and positive post-pump.
  (View Tweet)
• The cannulae: Components (1) and (4)
  The drainage and return cannulae allow a portal for blood to flow from and to the patient.
  The cannulae are wire-reinforced to prevent collapse in the presence of extreme negative pressures. This also allows curvature without kinking.
  
  (View Tweet)
• All cannulas impose a resistance (R) to blood flow causing a pressure change (∆P) across its length. The degree of ∆P is governed by Poiseuille’s law
  Key things to note:
  For a given flow
  • ∆P & R are inversely proportional to 4th power of radius
  • ∆P & R are ∝ to the length
    
    (View Tweet)
• Since blood is being "sucked through" a drainage cannula, ∆P across the cannula results in negative pressures at its proximal end.
  Similarly, ∆P across the return cannula creates positive pressures at its proximal end.
  In general, we like these ∆Ps to be as low as possible.
  
  (View Tweet)
• The pump: Component 2
  Older ECMO systems used 'roller pumps'. These are still utilized in CPB & RRT systems. In these, the rollers squeeze/milk the blood through the tubing in a particular direction.
  Since these are rarely used for ECMO, we won't be discussing them further https://t.co/zPIguGxa67 (View Tweet)
• Most modern ECMO systems utilize 'centrifugal pumps'. These contain a rotating impeller that accelerates the blood & ejects it radially
  With these, the user sets the pump speed (RPMs). The actual blood flow generated by that pump speed is dependent on several factors. https://t.co/N3LqNyTO0y (View Tweet)
• Important features of centrifugal pumps:
  (i) They are preload dependent:
  For a given pump speed,
  An increase in pre-pump resistance (e.g. drainage cannula occlusion) results in a drop in blood flow.
  (ii) They are afterload sensitive:
  As post-pump resistance ⬆️, blood flow drops. (View Tweet)
• This can be seen with return cannula occlusion or clots in the oxygenator
  (iii) They are non-occlusive:
  When the pump is stopped, retrograde flow is possible from post-pump to pre-pump side if the post-pump pressure is higher than pre-pump pressure (e.g. stopped pump in VA-ECMO) (View Tweet)
• This is why it is taught that the circuit should never left unclamped with the pump stopped!

  Time for a breather. We will return tomorrow with part B where we’ll discuss the oxygenator and various circuit pressures.
  Feel free to chime in with comments/criticisms. ([View Tweet](https://twitter.com/UCSFFresnoECMO/status/1625583786183254022))
  

• @JonahRubinMD @vaszochios @iceman_ex @EMinMiami @civestallman @MauraWalshEMCCM @ECMOdaily @EcmoNinja @FOAMecmo @ECMOprincess @ELSOOrg @mtECMO @rbarbosa91 @ArgaizR @PBSherren @EduardArgudo @JenelleBadulak @imcrit @ThinkingCC @JoeTonnaMD @MazenOdish @intheICU @kshekar01 @msiuba (View Tweet)