We Are Back With Another...
Highlights
- We are back with another tweetorial. In the last one, we talked about the anatomy of the ECLS circuit and the importance of various pressures. This one is all about the flow.
Title: Determinants of ECLS flow: deeper dive
Level: Medium
Author: @thind888 https://t.co/5J6jEtlx86 (View Tweet)
- Quick recap: since modern circuits use centrifugal pumps, flow is a dependent variable and not something that is set by the user. Instead, the user sets pump speed (RPMs) & circuit resistance affects flow
Compare this to dialysis circuits (roller pumps) where flow is set by user https://t.co/Acsz7W5xVV (View Tweet)
- Why do we care about flow?
- In VA ECMO: flow ∝ hemodynamic support and a certain minimum amount of flow is needed to satisfy perfusion needs
- In VV ECMO: increasing ECMO flow to cardiac output ratio improves oxygenation (but may also augment recirculation) (View Tweet)
- Factor 1: The cannulae
They are, perhaps, the most important factor.
Smaller cannulae offer higher resistance and thus result in lower flow - and vice-versa
It is often thought that the drainage cannula is much more important for determining flow than the return cannula. (View Tweet)
- Although there is some merit to this thinking, it is not entirely accurate.
Strictly speaking, for a given pump speed (RPM), flow generated depends on the pressure difference across the pump (see image).
Hence, the resistance offered by either cannula is equally important.
(View Tweet)
- So why do we care more about the size of the drainage cannula?
It's because negative pressures are more damaging to the blood than positive pressures. Exact thresholds are unknown but it's advisable to keep venous pressure (P1) > —100mmHg, & post-pump pressures (P2/3) < 300mmHg.
(View Tweet)
- This is why drainage cannula is typically chosen to be larger than return cannula.
The expected pressure drop across a particular cannula can be looked up from the flow charts provided by the manufacturer (images).
(View Tweet)
- Factor 2: Gravity
This is a minor factor and as such does not affect flow directly. However, it does affect relative venous (P1) and post-pump (P2/3) pressures
Elevating the patient with respect to the pump reduces venous pressure but increases post-pump pressures & vice-versa (View Tweet)
- Factor 3: Tubing length
The standard adult tubing has an inner diameter of 3/8" - roughly equal to a 30 Fr cannula.
According to Poiseuille’s law, resistance ∝ L, but inversely proportional to 4th power of R. Nonetheless, drastic changes in length can be clinically relevant
(View Tweet)
- Factor 4: Oxygenator thrombosis
The oxygenator is the most common site of circuit thrombosis. Apart from deterioration in gas exchange, this can present as increased resistance on the post-pump side (increased P2), and hence lower flow for a given pump speed. (View Tweet)
- Factor 5: Vascular pressures
- Higher CVP will increase P1 (make it less negative) and increase flow for a given pump speed. However, this effect is small since the range of CVP is much smaller than the range of P1
- In VA ECMO, arterial hypertension ⬆️P2/P3 and ⬇️flows (View Tweet)
- Factor 6: Miscellaneous
A mechanical obstruction in any other part of the circuit will ⬇️flows
E.g. clot in cannulae, external compression/clamping of ECMO tubing etc.
If an unexpected drop in flows is encountered, analysis of pressure trends may help identify the cause
/FIN (View Tweet)
- @JonahRubinMD @vaszochios @iceman_ex @EMinMiami @civestallman @MauraWalshEMCCM @ECMOdaily @EcmoNinja @FOAMecmo @ECMOprincess @ELSOOrg @mtECMO @rbarbosa91 @PBSherren @EduardArgudo @JenelleBadulak @imcrit @ThinkingCC @JoeTonnaMD @MazenOdish @intheICU @kshekar01 @msiuba @MLSCourse (View Tweet)