ECMO is all the rage, and ScanCrit is a fanatical believer. However, ECMO has its problems and challenges. The challenge we’ll look into here, is choosing between VV or VA ECMO – sometimes the choice is straightforward, other times not. And on VA ECMO – how do you circulate and oxygenate the whole body, not just from the waist down? Maybe you don’t have to choose? An article in the ASAIO Journal looks into hybrid configurations of ECMO.
Standard ECMO configurations and problems
ECMO is usually defined as either venovenous (VV) or venoarterial (VA) ECMO. Those are the two standard configurations. There are others, but we won’t go into that here. The configurations do what the names imply. Let’s look at them
Venovenous (VV) takes blood out from a central vein, oxygenates it, and puts it back into a central vein. It does not affect the arterial side directly, so there’s no circulatory support. VV is great for isolated pulmonary failure, but if the heart starts failing further down the line, you might run into trouble. You wish you were on VA.
Venoarterial (VA) takes blood out from a central vein, oxygenates it, and pumps it back into a central artery. So here you get oxygenated blood pumped back into the arterial system under pressure – so it also gives circulatory support. Sounds like you’re covered – as you have both oxygenation (lung support) and circulation (heart support). The problem is that the oxygenated blood gets pumped in somewhere in the descending aorta, running the normal direction down into the lower body (with the gut, kidneys and lower extremeties), but the upper body gets the oxygenated ECMO blood retrograde up the descending aorta – where it meets the native blood being pumped by the native heart somewhere along the line. Where they meet is called the mixing point or the watershed. If the native, failing heart starts recovering and pumping well again, but the lungs are still failing, you’ll end up with a native heart pumping badly oxygenated blood out into the aorta. The better it pumps, the worse the problem, as the watershed moves further down the aorta and the badly oxygenated blood from the native circulation will end up serving many vital organs, most notably the heart itself and the patient’s brain. You wish you were on VV.
This can be a dynamic situation, changing back and forth over days or hours, and changing from VV to VA or the other way around has traditionally meant setting up a whole new ECMO circuit and changing horses, so to speak. Costly and stressful. But maybe there’s a better way?
Hybrid ECMO configurations
In the ECMO circuit, you have one big line sucking blood from a central vein, blood goes into the rotary pump and gets pushed through the oxygenator before it goes through a big line back into the patient (either a central vein or a central artery).
What if you could change the set-up on the fly? This article looks into the hybrid configuration of splitting the line that goes back to the patient with a Y-piece, dividing it into one line to a central artery and one line into a central vein. Each line’s flow is controlled by a screw clamp on the line (in principle working like the roller clamp on a traditional i.v. drip). By adjusting the diameter of the line by adjusting the clamp, they are able to balance the flow pretty much any way they like.
The clamps are needed not only to adjust the ratio of flow to the vein vs the artery, but also to compensate for the pressure difference in the two outflow systems; the venous line being a low pressure system, while the arterial line being a high pressure system. But it seems to work out fine in the real world: this article is based on 21 cases of hybrid ECMO in 2012 and 2013.
Image with one drainage cannula (blue) here from the left femoral vein, going into the ECMO pump, continuing as a blue line on to the oxygenator. Out from the oxygenator as as the red line, immediately splitting into two red lines; one going into the jugular vein (down to the level of the right atrium), one going into the femoral artery (up into the descending aorta). These two red lines will have clamps to balance the venous/arterial return flow.
VVA or VAV?
They call these hybrid set-ups VVA (venovenous-arterial) or VAV (venoaretrial-venous) ECMO. They are essentially the same – the nomenclature just depends on the starting set-up: VV turns into VVA and VA turns into VAV.
These set-ups are sometimes a rescue set-up for having chosen the “wrong” ECMO set-up initially, but more often these hybrids can be the answer to the pure VV or VA ECMO’s shortcomings, and responding to the changing/dynamic situation a critical patient on ECMO sometimes presents.
Often, VV or VA will be the solution for patients needing ECMO, but in complex situations the VVA/VAV hybrids allow for flexibility not provided by either VV or VA.
Edit 21st dec 2016: Recent proposals have been to differentiate between VVA and VAV ECMO: VVA refers to two venous cannulas for suction, to improve drainage, and one output cannula: the arterial one. VAV refers to one venous cannula draining, and two output cannulas, one venous and one arterial. The nomenclature proposes starting with the drianage cannulas and using the ‘A’ as the first output cannula, so theoretically a VVAV set-up would be two venous drainage cannulas and two output cannulas, one arterial and one venous.
Possible case for VVA/VAV?
It’s an interesting article to look into, especially if you’re working in an ECMO centre, as this should probably be in your bag of tricks. It’s not that long ago that we put a young man with failing lungs on VV ECMO. He also had a beginning right heart failure, but it was thought to be initiated by his lung failure, with low pO2 giving vasoconstriction in his lung circulation.
And indeed, his pulmonary hypertension did initially improved on VV ECMO, but we had problems keeping up with his native cardiac output (and also problems attenuating his native cardiac output), so a lot of blood by-passed the ECMO circuit that was running at full speed, providing 4.5 L/min. So he was again hypoxemic, which again gave problems with pulmonary vasoconstricion and pulmonary hypertension. And so his heart started failing again. We were unsure of VA ECMO being the solution, because if his heart got better again and started pumping, his watershed would work against him, leaving his heart and brain with low DO2.
Although his hyperdynamic output was a challenge, adjusting VV and VA output through a VVA/VAV set-up might have helped, directing circulatory support or pre-pulmonary oxygenation as needed. So this was a dynamic situation where a hybrid set-up might have provided better control and support with the patient’s changing needs.
Edit November 29th, 2015:
Another very good review article on ECMO cannulation strategies:
Cannulation strategies for percutaneous extracorporeal membrane oxygenation in adults, Clin Res Cardiol, Nov 2015.