Understanding VV and VA ECMO

All right, hello everybody. My name is Eddie Watson and this is ICU Advantage. If this is your first time to the channel, welcome. For the rest of you guys, welcome back. Now in this lesson here I'm going to be talking about the differences between VV and VA ECMO. Now, I did cover this briefly in the last lesson I did talking about what is ECMO, but I feel understanding the differences between these two deserved more time and dedication to it, so here's this video here. So to start off, again, ECMO stands for extracorporeal membrane oxygenation, and it's used for both severe respiratory as well as cardiac failure for those patients that have a high expected mortality. Now, I did discuss the indications as well as the contraindications in that previous lesson, And while very similar in many ways, VV and VA ECMO are also quite different from one another, from how they provide support to the patient, as well as some of the considerations and physiological effects of that support. And hence the point of this lesson is to be able to go over this a little bit more in depth so that you guys have a better understanding of this. So we know that there's two types of ECMO. The two main types of ECMO that we use are venovenous or our VV ECMO, and this is our respiratory support. And then we have veno-arterial or VA ECMO, and this is our cardiac support. Now, both of these types of ECMO use almost all the same equipment, with the major exception being the type of cannula that we use. But the main thing that makes them so different from one another is where the blood is going to be returned. And both VV and VA ECMO are considered dual cannula configurations, which I'm going to explain more here in just a minute. Alright, so with that said, let's begin to talk about our VV ECMO. So in venovenous or VV ECMO, we're going to be draining blood from a vein, oxygenating it, and removing carbon dioxide, and then returning the blood back via a vein to the right atrium. Now, since we're returning this blood as it enters the right atrium, and thus really ultimately before it goes on through the pulmonary circulation, we're essentially supporting the patient's gas exchange in the case where their lungs are just not able to do this themselves. Now, by doing this, this also allows us to provide protective lung ventilation, which is really going to give the lung time to rest and recover. And so we're going to be providing the oxygenated blood for the heart to pump past the failed lungs and onto the rest of the body. And so as a result, the patient has to have preserved cardiac function in order for this to work. So one of the things that's going to be really important for us to do is to monitor these patients on VV ECMO for right heart failure, especially in the cases of ARDS. And this is something that we call core pulmonol. Now if this happens, this may require us having to transition this patient from VV to VA ECMO, which I'm going to talk about here in a minute. So now for VV ECMO, let's talk about our different cannula configurations that we see. So there's really two main ways in which we can cannulate patients for VV ECMO. The first is going to be using two separate cannulas. These cannulas are going to be our drainage and our return. So now typically the drainage cannula is going to be placed in our patient's femoral vein. It's a very long cannula and we're going to position it with the tip of the cannula at the border of the inferior vena cava and the right atrium. Now our return cannula is usually going to be placed in the right internal jugular vein and this one we're going to have with the tip at the border of the superior vena cava and the right atrium. So we're going to be draining blood from the patient's femoral vein and then returning that blood up through the internal jugular. Now to ensure we have these cannulas in the right place we want to be doing this under fluoroscopy. Now the other main option for our cannulation is actually going to be using a dual lumen cannula. Now this consists of a single insertion in the right IJ vein and this is going to have benefit for our patients for their bleeding risk. And this is a large cannula and it has both the drainage and the return lumens in one cannula. Now this one's inserted down the superior vena cava and then sits across the right atrium. in both the inferior vena cava and the superior vena cava. Now this has multiple ports for drainage both in the superior and the inferior vena cava. And then it has the return port directed downward towards the tricuspid valve, essentially returning blood into the right ventricle. And so because of the design of this cannula, we're going to have both the drainage line as well as the return line going to the same spot. So because this cannula is very dependent on its position, we're really going to need to use fluoroscopy to help place it, as well as getting an echo to ensure that we have the proper direction of flow from that return line. So now let's talk about a problem that we find that's exclusive to VVEcmo, and it's something that we call recirculation. So to give you an idea of what that is, what this is. This is when we have oxygenated blood from the return cannula that's going to be pulled into the ECMO circuit by the drainage cannula. And effectively what this does is it decreases the level of support that's going to be provided by the ECMO and it really can be extensive in some cases. If you think about it, we now have oxygenated blood that really doesn't need the help of the ECMO circuit that's taking up the space of deoxygenated blood that could be running through there. Hence, we're going to be able to... oxygenate less new blood and really provide less support for these patients. Now there's a few different reasons on why this would be happening. Some of the causes can be things like our cannula configuration and positioning, and essentially the closer in proximity that we have the two cannulas, the more recirculation that we're going to see. Also, higher flow rates and cannula size can impact this. Now we do see that higher flow rates do correlate with higher recirculation, really depending on the cannula type, size, and its position. Also, changes in intrathoracic, intracardiac, and intraabdominal pressure can impact this. So having increased pressure, whether it be from a pneumo, tamponade, increased abdominal pressure, that this can really impede the venous return back to the heart and thus can lead to recirculation. And then finally, the direction of ECMO flow can impact this. And so... in the two cannula configuration, if we're draining from the inferior vena cava and returning to the superior vena cava right atrium, that this results in less recirculation and therefore it's our preferred cannulation strategy. Now that said, we do see a pretty markedly decrease in recirculation in our dual lumen cannulas, but it can still occur with these cannulas and usually it's related to malposition. Now we can identify recirculation in many different ways, but really trending our preoxygenator saturation as well as our patient's arterial oxygen saturation is going to be our biggest indicator. If we see an increasing pre and a decreasing arterial saturation, then this could indicate recirculation. All right, so let's go ahead and move in and talk about the pathophysiology of VV ECMO. And the first thing that I want to talk about here is going to be our oxygenation. And so here, our patient's oxygenation is really going to consist of a mix of blood, both from the ECMO circuit. and their own native blood flow. So both the ECMO and our patient's lung function are going to contribute to this oxygenation, and the overall mix really varies from patient to patient. Now, by the time this blood enters the aorta, it's essentially mixed together, so we can actually assess the oxygenation that's going on in our patients, either through radial or femoral, on really either side of the body. Now, it's important to know, though, that VV ECMO is not going to achieve normal oxygenation in our patients, but we can achieve... adequate oxygenation, even if they have no native lung function. And really the reason for this is that the flow of the ECMO pump is not going to be at the same level of what our patient's normal cardiac output is. And so as fast as we run the ECMO pump, there's still going to be additional native blood flow going past these non-functioning lungs. This is going to lead to mixing with the oxygenated blood that we provide via the ECMO and is going to decrease that effectiveness. And so as you can imagine, there's really many factors that are going to go into determining oxygenation in these patients. But one of the biggest contributors is our hemoglobin concentration. Now, I do actually dive into this more in depth than a previous lesson that I did in the lesson on hemodynamics. So I'm actually going to link to that here if you guys want more understanding of why this is. But it's really going to be important that we're maintaining an adequate, if not higher than normal, hemoglobin. So now let's transition and actually talk about hemodynamics in these patients. So. Now the most important thing to know here is that VV ECMO does not influence hemodynamics. So essentially the volume of blood that we're going to be draining from the inferior vena cava is then going to be replaced in equal volume to the superior vena cava in the right atrium. And so as a result, our right atrial pressure, our left ventricular end diastolic pressure, our patient's blood pressure, as well as their afterload are all going to remain unaffected. Now, that said, once we actually have these patients running and we have them in this protective lung ventilation, that this is likely going to be reducing intrathoracic pressure from where they were before, which, as we talked about, is going to increase that venous return to the heart and could improve our patient's cardiac output. Now, along with that, if the patients were in a profound respiratory acidosis, once we begin clearing that CO2 with our sweep gas and we correct that acidosis, we could actually see more effective working of our vasopressors. And could lead to a decrease in the amount of pressors that we're using. But the actual ECMO flow itself is not going to be contributing to our patient's hemodynamics. Now that said, on the subject of hemodynamics, the drainage of this system is going to be dependent on our patient's preload. All right, and the last thing that I want to talk about here is actually our strategies with anticoagulation. Now with VV ECMO, we're typically able to run these patients with decreased or sometimes even eliminated anticoagulation. And this really helps reduce the risk of bleeding complications in these patients. Now it is important to know though that coagulopathy still may exist in these patients. Now this is due to the interaction of the blood and really the clotting factors and platelets interacting with the circuit and the oxygenator. So this coagulopathy still can exist even if we're running them with no anticoagulation. And so with these decreased anticoagulation strategies, we're going to be less concerned with the dislodgement of fibrin and clots that we see on the return post-oxygenator side of our circuit. This is because if they do break off, they're going to be trapped by our patient's pulmonary circulation. All right, so that was a good overview of VV ECMO. Let's go ahead and move on now and actually talk about venoarterial or VA ECMO. Now in the case of VA ECMO, we're going to be draining blood from a vein. oxygenating it and removing carbon dioxide just like VV, this time we're going to be returning it to an artery towards our patient's aorta. And so here what we're effectively creating is a right to left shunt and it's going to be providing two main benefits. First it's going to give a stable flow of oxygenated blood to perfuse our patient's organ and body and it's also going to be unloading the failing heart by reducing preload. So essentially we're providing perfusion of oxygen for the body when the heart's really not able to do it itself while also unloading and resting that heart so it can recover. So once again, let's talk about the cannula configurations. Now again, there are two main ways in which we would cannulate patients for VA ECMO. The first is again with two separate cannulas. Now first, our drainage or venous cannula is usually going to be placed in the femoral vein, although we can also place it in the right IJ. And here we're going to have the tip of the cannula in the right atrium. And this is where we'll be draining the deoxygenated blood. And then our return or arterial cannula is going to be placed in the opposite femoral artery with the tip of the cannula in the common iliac artery. And so our oxygenated blood is going to be returning back through an artery. And so this configuration here is going to provide flow down the opposite limb, but also provide retrograde flow up the aorta. Now, because of the size of the large cannula in the leg, the perfusion distal to that. return arterial cannula is actually going to be reduced. And so we're actually going to have a risk of limb ischemia. And so because of this, we're actually going to insert a reperfusion cannula distally to that return arterial cannula. And this is what's going to provide perfusion to that leg. Now, it is a smaller size cannula, so it is at increased risk for being dislodged and rapid exsanguination and or injury of that limb and tissue for that patient. So it's really important that we, especially this reperfusion line, make sure that we keep an eye on it and prevent it from becoming dislodged. Now, the other type of cannulation is something that's less common, but it's something that we call central cannulation. And so this is actually going to require an open chest. The patient's going to be left with an open sternotomy, and we're going to have... cannulas or grafts that are inserted directly into the RA as well as the proximal portion of the ascending aorta. And so here again, we'll be draining blood from that right atrium, but this time returning the blood directly to the ascending aorta. Now this one, we're going to see more common post-cardiotomy, so our open heart patients that are unable to come off bypass. So those are the different ways that we'd cannulate patients for VA ECMO. So now let's go ahead and move on and talk about the pathophysiology of VA ECMO. First thing we're going to talk about is oxygenation. And the first thing to really mention is unlike VV ECMO, there is no risk of recirculation as we have drainage and return lines in separate systems. Now, the big thing that I do want to talk about with oxygenation is something that we call the mixing cloud phenomenon. Now, to understand this, we have to know that in many patients on VA ECMO, that they still remain with some left ventricular output. And this is going to be causing antigrade native blood flow down the aorta. Now this native blood flow is going to meet and mix with the ECMO blood flow coming retrograde somewhere between the ascending aorta and the renal arteries. Now we have the coronaries and potentially the first few branches off the aorta that are going to be perfused with native blood flow potentially. So unlike VVECMO, we're actually going to have to use normal ventilation really to ensure that we have adequate oxygenation of this native blood flow. Now this native blood flow is still going to be dependent on a functioning respiratory system. So if this is failing, this really could result in hypoxemia of the heart and the brain, despite having good perfusion pressure. And so knowing this, it's really going to be imperative that we're monitoring for changes that really might indicate that this mixing cloud could be risking perfusion for our patients. Now, one of the primary ways that we do this is we actually have, if you look at the first branch coming off the aortic arch, this vessel actually goes up and splits. Part of it becomes our carotid and perfuses part of our brain. And the other part becomes the subclavian and goes on to perfuse the right arm of this patient. And this is that very first branch. off the aortic arch. So it's really going to be important that we have a right radial A-line, as well as a pulse ox on the right hand, in order to monitor the oxygenation. And we want to use this to trend the pulse ox on that side, as well as potentially getting blood gases, if need be, to let us know that we're having an issue with declining oxygen perfusion on that very first branch off the aortic arch. Now, in addition to that, we also want to monitor the pulse pressure that we see, so this is the difference between our systolic and diastolic, as this could be a sign of increasing left ventricular output, which again, if we're having problems with our oxygenation in our lungs, this is going to be creating more forward flow and causing a further shifting back of that mixing cloud. So now speaking of that pulse pressure, I do want to talk real quickly about our non-pulsatile flow. So really depending on the amount of unloading of the heart that we're able to do and the left ventricular output, most if not all of our flow is going to be supplied by our ECMO circuit. And so as a result, we're either going to have a limited pulse pressure or potentially even a flat A-line, which is just a single pressure reading, which is going to be our map. And so in these cases, we're really going to be shooting for a target map of like 60 or 65 based on that number that we see with the flat A-line. And so now let's actually talk about the hemodynamics of VA ECMO. So as I already kind of mentioned, our patient's blood pressure is going to be aided by the flow of ECMO, if not completely dependent on it, and it's really going to be important that we ensure good flow. Now if we see a decrease in preload, that this can impact drainage into the system and ultimately decrease flow rates. Now if our patients have decreased afterload or the resistance of the arterial vasculature, then we can see a reduction in our patient's MAP. And so really knowing this, the use of vasopressors and our fluid management is going to be essential in maintaining both adequate flow and adequate perfusion for these patients. Now, another thing that can come up with VA ECMO is left ventricular distension. So if we have a patient that has a very low left ventricular output or they have aortic regurgitation, they are at risk for distension of that left ventricle as well as pulmonary edema. And so if you can think about, we still have native blood flow going through the heart. So if that left ventricular output just is really low or even non-existent to where that aortic valve is an opening, we're going to see a backup of that blood in the system. As well as if we think about in the cases of the femoral cannulation where we have that retrograde blood flow coming back up the aorta, if we have regurgitation of that aortic valve, that's going to allow the blood flow to be coming back into the heart. again, if they don't have that left ventricular output in order to push it back out, that that can also rapidly increase distension of that ventricle. And so there's a couple things that we can do. If our main problem is not having enough output of that left ventricle, then sometimes our patients can benefit from having a second drainage cannula to add to the reduction of preload. This is actually something that I'm going to go in more in depth into in the next lesson. But another option, and actually a pretty easy fix nowadays with the cath lab, is we're seeing really good results. unloading that left ventricle with a device called an impella. Now this is a microaxial pump that we insert usually in the femoral artery and it goes and it sits across from the aorta across the aortic valve into the left ventricle and what it does is it creates flow from that left ventricle out into the aorta. And so like I said we're seeing really good results using the impella to unload in conjunction with VA ECMO. And then finally the last thing that I want to talk about is going to be our anticoagulation strategy. So on VA ECMO, maintaining therapeutic anticoagulation is really going to be essential. And so really unlike VV ECMO, we're really going to need to ensure that we remain therapeutic for these patients. As a result though, bleeding complications are going to be increased for them. And this goes for everything from GI bleeds, hemorrhagic strokes, as well as even just bleeding from the cannulocytes. Since we're providing flow into an artery and retrograde up the aorta, there's a very real risk of stroke if... a clot were to make it to the patient. And so if you remember back when we talked about VV ECMO, we talked about seeing fibrin or clot in the circuit. So if we were to see this pre-oxygenator, it's not a big deal because that's going to get trapped in that oxygenator. But what's going to be really important though is that thorough inspection post-oxygenator to identify any of those fibrin strands or clots that might be forming in the circuit and really ensuring that they haven't broken off. Because again, if they were to break off, then we really run that risk of our patient having a stroke from that embolotic event. So that kind of covers the overview of VA ECMO as well as kind of comparing it to VV ECMO. As you can see, even though they're quite similar in a lot of ways, they are very different from one another. And it's really important that you do understand these differences so that you can properly provide care for your patients. So I really hope that you guys liked this lesson. If you did, please leave me a like. It really goes a long way to support this channel and get it in front of as many people as possible. As well as if you'd be interested in more Critical Care videos such as this and you haven't already, make sure and subscribe down below and leave me a comment. Let me know what you thought of this. I really enjoy reading your comments and responding back to you guys. As well as a special shout out to the awesome Patreon members. In addition to the support that you provide for this channel, you guys receive additional content beyond what you just get here on the YouTube channel. So thank you guys so much for your support and if you'd be interested in showing additional support then head on over to the Patreon page and take a look. Otherwise, make sure to stay tuned for the next lesson. Although in the meantime, feel free to check out another couple awesome videos I'll link to right here. Thank you guys so much and you have a great day.

All right, hello everybody. My name is Eddie Watson and this is ICU Advantage. If this is your first time to the channel, welcome. For the rest of you guys, welcome back.

Now in this lesson here I'm going to be talking about the differences between VV and VA ECMO. Now, I did cover this briefly in the last lesson I did talking about what is ECMO, but I feel understanding the differences between these two deserved more time and dedication to it, so here's this video here. So to start off, again, ECMO stands for extracorporeal membrane oxygenation, and it's used for both severe respiratory as well as cardiac failure for those patients that have a high expected mortality.

Now, I did discuss the indications as well as the contraindications in that previous lesson, And while very similar in many ways, VV and VA ECMO are also quite different from one another, from how they provide support to the patient, as well as some of the considerations and physiological effects of that support. And hence the point of this lesson is to be able to go over this a little bit more in depth so that you guys have a better understanding of this. So we know that there's two types of ECMO.

The two main types of ECMO that we use are venovenous or our VV ECMO, and this is our respiratory support. And then we have veno-arterial or VA ECMO, and this is our cardiac support. Now, both of these types of ECMO use almost all the same equipment, with the major exception being the type of cannula that we use. But the main thing that makes them so different from one another is where the blood is going to be returned. And both VV and VA ECMO are considered dual cannula configurations, which I'm going to explain more here in just a minute.

Alright, so with that said, let's begin to talk about our VV ECMO. So in venovenous or VV ECMO, we're going to be draining blood from a vein, oxygenating it, and removing carbon dioxide, and then returning the blood back via a vein to the right atrium. Now, since we're returning this blood as it enters the right atrium, and thus really ultimately before it goes on through the pulmonary circulation, we're essentially supporting the patient's gas exchange in the case where their lungs are just not able to do this themselves. Now, by doing this, this also allows us to provide protective lung ventilation, which is really going to give the lung time to rest and recover. And so we're going to be providing the oxygenated blood for the heart to pump past the failed lungs and onto the rest of the body.

And so as a result, the patient has to have preserved cardiac function in order for this to work. So one of the things that's going to be really important for us to do is to monitor these patients on VV ECMO for right heart failure, especially in the cases of ARDS. And this is something that we call core pulmonol.

Now if this happens, this may require us having to transition this patient from VV to VA ECMO, which I'm going to talk about here in a minute. So now for VV ECMO, let's talk about our different cannula configurations that we see. So there's really two main ways in which we can cannulate patients for VV ECMO. The first is going to be using two separate cannulas. These cannulas are going to be our drainage and our return.

So now typically the drainage cannula is going to be placed in our patient's femoral vein. It's a very long cannula and we're going to position it with the tip of the cannula at the border of the inferior vena cava and the right atrium. Now our return cannula is usually going to be placed in the right internal jugular vein and this one we're going to have with the tip at the border of the superior vena cava and the right atrium. So we're going to be draining blood from the patient's femoral vein and then returning that blood up through the internal jugular. Now to ensure we have these cannulas in the right place we want to be doing this under fluoroscopy.

Now the other main option for our cannulation is actually going to be using a dual lumen cannula. Now this consists of a single insertion in the right IJ vein and this is going to have benefit for our patients for their bleeding risk. And this is a large cannula and it has both the drainage and the return lumens in one cannula.

Now this one's inserted down the superior vena cava and then sits across the right atrium. in both the inferior vena cava and the superior vena cava. Now this has multiple ports for drainage both in the superior and the inferior vena cava. And then it has the return port directed downward towards the tricuspid valve, essentially returning blood into the right ventricle. And so because of the design of this cannula, we're going to have both the drainage line as well as the return line going to the same spot.

So because this cannula is very dependent on its position, we're really going to need to use fluoroscopy to help place it, as well as getting an echo to ensure that we have the proper direction of flow from that return line. So now let's talk about a problem that we find that's exclusive to VVEcmo, and it's something that we call recirculation. So to give you an idea of what that is, what this is.

This is when we have oxygenated blood from the return cannula that's going to be pulled into the ECMO circuit by the drainage cannula. And effectively what this does is it decreases the level of support that's going to be provided by the ECMO and it really can be extensive in some cases. If you think about it, we now have oxygenated blood that really doesn't need the help of the ECMO circuit that's taking up the space of deoxygenated blood that could be running through there. Hence, we're going to be able to... oxygenate less new blood and really provide less support for these patients.

Now there's a few different reasons on why this would be happening. Some of the causes can be things like our cannula configuration and positioning, and essentially the closer in proximity that we have the two cannulas, the more recirculation that we're going to see. Also, higher flow rates and cannula size can impact this. Now we do see that higher flow rates do correlate with higher recirculation, really depending on the cannula type, size, and its position.

Also, changes in intrathoracic, intracardiac, and intraabdominal pressure can impact this. So having increased pressure, whether it be from a pneumo, tamponade, increased abdominal pressure, that this can really impede the venous return back to the heart and thus can lead to recirculation. And then finally, the direction of ECMO flow can impact this.

And so... in the two cannula configuration, if we're draining from the inferior vena cava and returning to the superior vena cava right atrium, that this results in less recirculation and therefore it's our preferred cannulation strategy. Now that said, we do see a pretty markedly decrease in recirculation in our dual lumen cannulas, but it can still occur with these cannulas and usually it's related to malposition.

Now we can identify recirculation in many different ways, but really trending our preoxygenator saturation as well as our patient's arterial oxygen saturation is going to be our biggest indicator. If we see an increasing pre and a decreasing arterial saturation, then this could indicate recirculation. All right, so let's go ahead and move in and talk about the pathophysiology of VV ECMO. And the first thing that I want to talk about here is going to be our oxygenation.

And so here, our patient's oxygenation is really going to consist of a mix of blood, both from the ECMO circuit. and their own native blood flow. So both the ECMO and our patient's lung function are going to contribute to this oxygenation, and the overall mix really varies from patient to patient. Now, by the time this blood enters the aorta, it's essentially mixed together, so we can actually assess the oxygenation that's going on in our patients, either through radial or femoral, on really either side of the body.

Now, it's important to know, though, that VV ECMO is not going to achieve normal oxygenation in our patients, but we can achieve... adequate oxygenation, even if they have no native lung function. And really the reason for this is that the flow of the ECMO pump is not going to be at the same level of what our patient's normal cardiac output is.

And so as fast as we run the ECMO pump, there's still going to be additional native blood flow going past these non-functioning lungs. This is going to lead to mixing with the oxygenated blood that we provide via the ECMO and is going to decrease that effectiveness. And so as you can imagine, there's really many factors that are going to go into determining oxygenation in these patients. But one of the biggest contributors is our hemoglobin concentration.

Now, I do actually dive into this more in depth than a previous lesson that I did in the lesson on hemodynamics. So I'm actually going to link to that here if you guys want more understanding of why this is. But it's really going to be important that we're maintaining an adequate, if not higher than normal, hemoglobin.

So now let's transition and actually talk about hemodynamics in these patients. So. Now the most important thing to know here is that VV ECMO does not influence hemodynamics.

So essentially the volume of blood that we're going to be draining from the inferior vena cava is then going to be replaced in equal volume to the superior vena cava in the right atrium. And so as a result, our right atrial pressure, our left ventricular end diastolic pressure, our patient's blood pressure, as well as their afterload are all going to remain unaffected. Now, that said, once we actually have these patients running and we have them in this protective lung ventilation, that this is likely going to be reducing intrathoracic pressure from where they were before, which, as we talked about, is going to increase that venous return to the heart and could improve our patient's cardiac output. Now, along with that, if the patients were in a profound respiratory acidosis, once we begin clearing that CO2 with our sweep gas and we correct that acidosis, we could actually see more effective working of our vasopressors.

And could lead to a decrease in the amount of pressors that we're using. But the actual ECMO flow itself is not going to be contributing to our patient's hemodynamics. Now that said, on the subject of hemodynamics, the drainage of this system is going to be dependent on our patient's preload. All right, and the last thing that I want to talk about here is actually our strategies with anticoagulation. Now with VV ECMO, we're typically able to run these patients with decreased or sometimes even eliminated anticoagulation.

And this really helps reduce the risk of bleeding complications in these patients. Now it is important to know though that coagulopathy still may exist in these patients. Now this is due to the interaction of the blood and really the clotting factors and platelets interacting with the circuit and the oxygenator.

So this coagulopathy still can exist even if we're running them with no anticoagulation. And so with these decreased anticoagulation strategies, we're going to be less concerned with the dislodgement of fibrin and clots that we see on the return post-oxygenator side of our circuit. This is because if they do break off, they're going to be trapped by our patient's pulmonary circulation. All right, so that was a good overview of VV ECMO. Let's go ahead and move on now and actually talk about venoarterial or VA ECMO.

Now in the case of VA ECMO, we're going to be draining blood from a vein. oxygenating it and removing carbon dioxide just like VV, this time we're going to be returning it to an artery towards our patient's aorta. And so here what we're effectively creating is a right to left shunt and it's going to be providing two main benefits.

First it's going to give a stable flow of oxygenated blood to perfuse our patient's organ and body and it's also going to be unloading the failing heart by reducing preload. So essentially we're providing perfusion of oxygen for the body when the heart's really not able to do it itself while also unloading and resting that heart so it can recover. So once again, let's talk about the cannula configurations.

Now again, there are two main ways in which we would cannulate patients for VA ECMO. The first is again with two separate cannulas. Now first, our drainage or venous cannula is usually going to be placed in the femoral vein, although we can also place it in the right IJ. And here we're going to have the tip of the cannula in the right atrium. And this is where we'll be draining the deoxygenated blood.

And then our return or arterial cannula is going to be placed in the opposite femoral artery with the tip of the cannula in the common iliac artery. And so our oxygenated blood is going to be returning back through an artery. And so this configuration here is going to provide flow down the opposite limb, but also provide retrograde flow up the aorta. Now, because of the size of the large cannula in the leg, the perfusion distal to that. return arterial cannula is actually going to be reduced.

And so we're actually going to have a risk of limb ischemia. And so because of this, we're actually going to insert a reperfusion cannula distally to that return arterial cannula. And this is what's going to provide perfusion to that leg.

Now, it is a smaller size cannula, so it is at increased risk for being dislodged and rapid exsanguination and or injury of that limb and tissue for that patient. So it's really important that we, especially this reperfusion line, make sure that we keep an eye on it and prevent it from becoming dislodged. Now, the other type of cannulation is something that's less common, but it's something that we call central cannulation.

And so this is actually going to require an open chest. The patient's going to be left with an open sternotomy, and we're going to have... cannulas or grafts that are inserted directly into the RA as well as the proximal portion of the ascending aorta.

And so here again, we'll be draining blood from that right atrium, but this time returning the blood directly to the ascending aorta. Now this one, we're going to see more common post-cardiotomy, so our open heart patients that are unable to come off bypass. So those are the different ways that we'd cannulate patients for VA ECMO.

So now let's go ahead and move on and talk about the pathophysiology of VA ECMO. First thing we're going to talk about is oxygenation. And the first thing to really mention is unlike VV ECMO, there is no risk of recirculation as we have drainage and return lines in separate systems. Now, the big thing that I do want to talk about with oxygenation is something that we call the mixing cloud phenomenon.

Now, to understand this, we have to know that in many patients on VA ECMO, that they still remain with some left ventricular output. And this is going to be causing antigrade native blood flow down the aorta. Now this native blood flow is going to meet and mix with the ECMO blood flow coming retrograde somewhere between the ascending aorta and the renal arteries. Now we have the coronaries and potentially the first few branches off the aorta that are going to be perfused with native blood flow potentially. So unlike VVECMO, we're actually going to have to use normal ventilation really to ensure that we have adequate oxygenation of this native blood flow.

Now this native blood flow is still going to be dependent on a functioning respiratory system. So if this is failing, this really could result in hypoxemia of the heart and the brain, despite having good perfusion pressure. And so knowing this, it's really going to be imperative that we're monitoring for changes that really might indicate that this mixing cloud could be risking perfusion for our patients. Now, one of the primary ways that we do this is we actually have, if you look at the first branch coming off the aortic arch, this vessel actually goes up and splits. Part of it becomes our carotid and perfuses part of our brain.

And the other part becomes the subclavian and goes on to perfuse the right arm of this patient. And this is that very first branch. off the aortic arch. So it's really going to be important that we have a right radial A-line, as well as a pulse ox on the right hand, in order to monitor the oxygenation.

And we want to use this to trend the pulse ox on that side, as well as potentially getting blood gases, if need be, to let us know that we're having an issue with declining oxygen perfusion on that very first branch off the aortic arch. Now, in addition to that, we also want to monitor the pulse pressure that we see, so this is the difference between our systolic and diastolic, as this could be a sign of increasing left ventricular output, which again, if we're having problems with our oxygenation in our lungs, this is going to be creating more forward flow and causing a further shifting back of that mixing cloud. So now speaking of that pulse pressure, I do want to talk real quickly about our non-pulsatile flow. So really depending on the amount of unloading of the heart that we're able to do and the left ventricular output, most if not all of our flow is going to be supplied by our ECMO circuit. And so as a result, we're either going to have a limited pulse pressure or potentially even a flat A-line, which is just a single pressure reading, which is going to be our map.

And so in these cases, we're really going to be shooting for a target map of like 60 or 65 based on that number that we see with the flat A-line. And so now let's actually talk about the hemodynamics of VA ECMO. So as I already kind of mentioned, our patient's blood pressure is going to be aided by the flow of ECMO, if not completely dependent on it, and it's really going to be important that we ensure good flow.

Now if we see a decrease in preload, that this can impact drainage into the system and ultimately decrease flow rates. Now if our patients have decreased afterload or the resistance of the arterial vasculature, then we can see a reduction in our patient's MAP. And so really knowing this, the use of vasopressors and our fluid management is going to be essential in maintaining both adequate flow and adequate perfusion for these patients. Now, another thing that can come up with VA ECMO is left ventricular distension.

So if we have a patient that has a very low left ventricular output or they have aortic regurgitation, they are at risk for distension of that left ventricle as well as pulmonary edema. And so if you can think about, we still have native blood flow going through the heart. So if that left ventricular output just is really low or even non-existent to where that aortic valve is an opening, we're going to see a backup of that blood in the system. As well as if we think about in the cases of the femoral cannulation where we have that retrograde blood flow coming back up the aorta, if we have regurgitation of that aortic valve, that's going to allow the blood flow to be coming back into the heart. again, if they don't have that left ventricular output in order to push it back out, that that can also rapidly increase distension of that ventricle.

And so there's a couple things that we can do. If our main problem is not having enough output of that left ventricle, then sometimes our patients can benefit from having a second drainage cannula to add to the reduction of preload. This is actually something that I'm going to go in more in depth into in the next lesson. But another option, and actually a pretty easy fix nowadays with the cath lab, is we're seeing really good results. unloading that left ventricle with a device called an impella.

Now this is a microaxial pump that we insert usually in the femoral artery and it goes and it sits across from the aorta across the aortic valve into the left ventricle and what it does is it creates flow from that left ventricle out into the aorta. And so like I said we're seeing really good results using the impella to unload in conjunction with VA ECMO. And then finally the last thing that I want to talk about is going to be our anticoagulation strategy.

So on VA ECMO, maintaining therapeutic anticoagulation is really going to be essential. And so really unlike VV ECMO, we're really going to need to ensure that we remain therapeutic for these patients. As a result though, bleeding complications are going to be increased for them. And this goes for everything from GI bleeds, hemorrhagic strokes, as well as even just bleeding from the cannulocytes. Since we're providing flow into an artery and retrograde up the aorta, there's a very real risk of stroke if...

a clot were to make it to the patient. And so if you remember back when we talked about VV ECMO, we talked about seeing fibrin or clot in the circuit. So if we were to see this pre-oxygenator, it's not a big deal because that's going to get trapped in that oxygenator.

But what's going to be really important though is that thorough inspection post-oxygenator to identify any of those fibrin strands or clots that might be forming in the circuit and really ensuring that they haven't broken off. Because again, if they were to break off, then we really run that risk of our patient having a stroke from that embolotic event. So that kind of covers the overview of VA ECMO as well as kind of comparing it to VV ECMO. As you can see, even though they're quite similar in a lot of ways, they are very different from one another. And it's really important that you do understand these differences so that you can properly provide care for your patients.

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Transcript for:Understanding VV and VA ECMO

Transcript for:
Understanding VV and VA ECMO