In addition to
@ShockIndex's excellent explanation, I just read through this (
https://www.ahajournals.org/doi/full/10.1161/CIRCHEARTFAILURE.118.004905) which helped me visualize what he was talking about vis-a-vis north-south syndrome.
The article also talks about central cannulation VA ecmo which obviates the north-south issue and how you can achieve the same effect outside of the OR using different cannulation sites (e.g. the subclav artery) albeit with different potential issues there.
I also wound up going down a rabbit hole and started reading about how instead of doing IJ - Fem VV ECMO they will sometimes do a single cannulation, dual lumen V-V ECMO solely in the IJ/SVC for certain patients like severe CF patients who are awaiting lung transplant but are otherwise ambulatory and could get out of bed while on ecmo. Crazy stuff.
There are 3 unique complications that can occur with peripheral VA-ECMO, and understanding them means that you understand the physiology of the support.
1) Differential oxygenation / north-south: previously covered.
2) LV distension: It turn out that VA-ECMO is great at perfusing your body but terrible for a weak heart -ironic since it’s weak hearts that go on ECMO. The retrograde blood being blown up the aorta is a big afterload on the LV’s native CO. If you look at the pressure-volume loop of a heart on peripheral VA-ECMO, it’s almost identical to what is seen in aortic stenosis with a tall pressure spike needed to overcome isovolumetric contraction. This is no bueno for a stunned, hypocontractile myocardium as it increase stroke work (ie the area under the pressure-volume loop curve). You know there is a problem when the pulse pressure is <10 , PA pressures are rising from backward congestion, the AV valve is opening less than every 3rd beat, and the LV looks distended on echo. Thus, you often need an LV venting strategy; something that off loads the LV. That can be an IABP or percutaneous LVAD (ie Impella) that pulls blood out of the LV and injects it into the aorta. The Impella makes tons of physiologic sense but there is no evidence that it’s better than an IABP…yet. You can even do a surgical carheter vent to the LV if the patient came out of the OR peripherally cannulated (rare).
3) Lower extremity ischemia: That arterial cannula in the femoral artery threatens perfusion to the leg. A reperfusion cannula to the distal leg lessens the risk significantly but I’ve seen plenty of legs lost.
Central cannulation circumvents these issues because the cannula is placed in the proximal aorta and blood flows anterograde down the vessel. There is no mixing cloud, LV afterload, or big catheter in a little leg. However, central cannulation is more invasive and must be done in the OR by a CT surgeon. We mostly see central cannulation in patients coming out of cardiac surgery who couldn’t be separated from bypass. The ECMO is an extension of the bypass circuit minus a blood reservoir.
VV-ECMO is a different beast and none of these complications occur. While most of us trained on the 2-cannula technique (venous drainage from a femoral cannula with return to the RA via the IJ), we are increasingly using the dual lumen cannulas (Avalon and Crecent) for all of our VV cases. Here, a single long cannula is placed under fluoro into the IJ and extends across the RA with the tip near the hepatic IVC. Blood is drawn from holes in the distal tip and proximally through a hole in the SVC. The blood is pumped through one lumen to the oxygenator that delivers it via the other lumen to the RA where it is ejected out of a hole aimed at the TV.
These single cannulas has theoretic advantages such as less recirculation. This occurs when the oxygenated blood delivered by the return cannula is taken up by the venous drainage cannula instead of going to the patient. When using a 2-cannula method we see this when the cannulas from above and below migrate within 6-10 cm of each other. The single cannulas do not eliminate the problem as recirc can still happen- especially if the cannula rotates and ejects the oxygenated blood away from the TV.
FWIW, I was once an EP but now this is most of my day. During covid I had 14 patients on VV at one time. Now, I typically have 4-5 on VA and a VV here and there with tons of IABPs and Impellas. 3 years ago, I crashed a COVID peripartum cardiomyopathy onto VA ecmo while OB yucked out her baby via perimortum c-section. I had her on off and on my service for 9 months before she was finally discharged with a new heart. Watched her meet her 9-month old neurologically intact baby for the first time at discharge. Since then, no desire to work another ED shift. All the 60-hour work weeks, time away from family, etc - WORTH IT.
