Jan 08 2024 11 mins 20
Welcome back to the tasty morsels of critical care podcast.
Today we’ll talk about one of the niche and shall I say “advanced” in inverted commas therapies in intensive care practice. ECMO. And to be precise we’ll be talking about VV ECMO. Indeed saying that you are “putting someone on ECMO” is a woefully incomplete sentence as the support and physiological difference between venovenous ECMO and venoarterial ECMO is really rather profound.
The post will be an intentionally broad description of the therapy and perhaps less on the nuances of managing a patient on VV ECMO, as at fellowship exam level I suspect you’d only be expected to have an overview of what it it is, what it can (and can’t do) and when to ask for it. I acknowledge the glaring gaps in the post and the likely criminal omission of the oxygen carrying capacity calculation. It would be fair to call this an idiot’s guide. And given that these posts are generated from my own notes then we all know who the idiot in that title it refers to is.
We’ll start at its simplest level, which is how i try to describe to friends and non medical people about how ECMO works. Blood is removed from the veins in one pipe and put through an artificial lung type device where CO2 is removed and Oxygen added, then blood is returned to the veins via a second pipe. If you’re lungs don’t work so well then the device can replace a lot of their function in the short term. Lay person explanation ends.
The degree to which we can replace lung function, primarily the degree to which we can oxygenate, is determined by the amount of the venous return coming back to the heart we can divert through the machine. Let’s say the cardiac output is a healthy 5L/min. That means that 5L/min is being ejected from the left ventricle and 5L/min is returning to the right ventricle. If the lungs aren’t working well then we need to capture at least 60% or so of this venous return and stick it through the oxygenator in order to maintain tolerable saturation of haemoglobin with oxygen. So in our example we’ll have to be siphoning off at least 3L/min from the venous return, putting it through the oxygenator and returning it back to the right side of the heart. With me so far?
It is at this stage that we immediately run into one of the physics challenges of VV ECMO. Pulling off 3L/min of blood requires pipes of substantial diameter. Typically these are in the 23 to 27Fr range. (ie 8-9mm internal diameter). You want to place this drainage pipe somewhere where there is a high flow of blood in a large vessel capable of accommodating it. Typically this will be in the SVC or the IVC, typically reached by an insertion point in the IJ or femoral vein respectively. It becomes really quite tricky to drain more than 3L/min of blood (or 60% of the venous return) with a single pipe as you can really only drain either the SVC (venous return from the upper body) or the IVC (venous return from the lower body) and as should be obvious the venous return from the body is split between these. In addition to the limitations of the physical size of the pipes you have to remember that the vessels within which these pipes are placed are not rigid fixed stented things, they dilate and contract in response to intravascular volume and intravascular tone. If you try to suck blood out of them with too much negative pressure the vessels will collapse around the pipe blocking all the holes and stopping all drainage.
All this to say that oxygenation is determined by the proportion of venous return we can divert through the ECMO machine. And capturing that venous return should be the priority when it comes to deciding on drainage pipe size and placement. once the blood is out of the body and through the oxygenator it turns out that it’s quite east to get it back into the budy. Pushing blood back into the body is much easier and can be done with a much smaller pipe. Pulling is harder than pushing in this context.
The key factor in returning blood back to the body in VV ECMO is 1) it has to return to a vein, hence the second V in VV ECMO and 2) it needs to return to the venous circulation at a healthy distance from the drainage pipe. It would be bad form to return 3L/min of beautifully oxygenated blood directly into the inlet holes of the drainage pipe and through the circuit for a second entirely pointless run. We call this re circulation and we get around it by placing the tip of our return pipe somewhat remote to the access pipe. For example we could drain blood from the SVC and return it to the IVC or if both of our pipes were in the IVC we could ensure that the return of blood happens much closer to the heart in the IVC or even in the RA but importantly a healthy distance away from where the drainage pipe is in the IVC.
Why would one want to initiate such a therapy? There are a number of indications, or should i say circumstances where VV ECMO might have a role. A reasonable list for examination purposes might run as follows.
- Refractory hypoxaemia is a good coverall term but better teased out into some specifics below;
- ARDS where you can’t maintain safe settings on a ventilator or safe numbers in terms of oxygenation and CO2 clearance
- of note you can squeeze in almost every pathologic lung condition under the ARDS umbrella so think pneumonia, pulm vasculitis etc…
- this is the vast majority of VV ECMO runs.
- primary graft dysfunction post lung transplant
- refractory asthma
- a bronchopleural fistula where continuing to ventilate with pressure is probably not the best thing for healing the hole in the lung.
- ARDS where you can’t maintain safe settings on a ventilator or safe numbers in terms of oxygenation and CO2 clearance
- refractory hypercapnoea where the hypercapnoea is causing life threatening problems despite a thorough effort to fix it
- the occasional slightly bonkers airway surgery where you may have no means to oxygenate or ventilate for a substantial period of time
There is lots of debate and indeed variations in practice on when you might initiate VV ECMO. There are a number of published criteria for when ECMO is indicated but you have to remember that a single PaO2 of 6.5kPa on 100% does not actually tell you how sick a patient is or if you’ve truly exhausted your conventional management. Either way at the end of a viva question on management strategies for severe ARDS, once you’ve been through high PEEPs, permissive hypercapnoea, diuresis, proning, and a nuanced discussion on steroids and nitric you should probably mention VV ECMO.
It’s worth noting some reasons when doing VV ECMO is not a great idea. As with almost all intensive care organ supports there’s not much point in adding it if you don’t have a way to fix the underlying organ. For example, if you have ARDS from pneumonia we can probably fix that, however if you have end stage COPD we can’t fix that. Adding the device will not change things. There are some programs who will use VV ECMO as a bridge to transplant but this is beyond the scope of this post. But it is commonly used in the immediate post lung tx phase when the new lungs are a bit heavy, wet and not working too well.
If your circulation is falling apart and you’re on 2mcg/kg/min of noradrenaline and the LV is clapped out from septic cardiomyopathy then it’s hard to see how fixing the hypoxic part of the multiorgan failure is going to turn things around. That being said if you’re hypoxic with a struggling RV then adding VV ECMO might be enough to correct the circulatory issues simply by fixing the hypoxia and hypercarbia.
If you wanted to end your SAQ with a flourish and add some complications then a convenient top 5 might run as follows
- bleeding
- bleeding
- bleeding
- clotting
- bleeding
I am of course being a little bit facetious here but i’m not that far off the mark. Anticoagulation is typically used to keep the VV ECMO circuit running but even on those without anticoagulation they still bleed. And the bleeding is often spontaneously into non compressible sites like the pleura, GI tract or retroperitoneum.
References:
Oh chapter 41
