ECMO or Impella, or ECMO and Impella? With increased focus on mechanical support for acute severe heart failure and cardiac arrest, there’s more research looking into which type of mechanical assistance that’s most appropriate. Each type of assist device has its own pros and cons.
The most used assist device is currently the intra-aortic balloon pump (IABP). The pros are that it’s relatively easy to place and it doesn’t do much harm. No heparin needed. The con is that it doesn’t do that much good either as it’s not very good at supporting the circulation, as we wrote on here. So the last couple of years real mechanical support devices have gotten more in vouge. Mostly ECMO, but also devices like the Impella. Here’s a short look at the devices and a few interesting articles.
The Impella is a miniature axial flow assist device, meaning it’s a cork screw in a pipe attached to the end of a wire inserted via the femoral artery, feeding the cork screw device up the aorta and into the left ventricle so the Impella sits with one end in the left ventricle outflow tract and the other end in the aorta, just distal to the aortic valve. Rotating quickly, the original generates 2,5 litres of flow. A slightly newer – but also fatter, requiring a bigger arterial entry hole – version generates 5 litres of flow, and the latest generation, the Impella CP has pretty much the size of the 2.5 version, but generates around 4 litres of flow.
Pros: relatively easy to place. Less bleeding and anticoagulant use than ECMO. It offloads the left ventricle. Cons: Needs x-ray for placement. Sensitive to displacement (it is just a few centimeters long and has pressure sensors in the ventricle and aortic end. These pressures should be different. If they’re equal, it means the whole device is sitting inside the ventricle or in the aorta, and it stops. That’s not good if you have total heart failure, aka cardiac arrest. This also makes the system difficult to use for transport. Finally, it doesn’t offload the right ventricle.
ECMO devices are quite flexible. They can be used as venovenous devices for lung “dialysis”, or in different forms of venoarterial set-ups that also supports the circulation. Usually you can get around 4.5-5 litres of flow per minute, enough to fully support circulation, but the filling through the venous catheter can be a challenge. The patient needs to be volume loaded to keep feeding the venous catheter.
The standard venoarterial set-up is a ‘peripheral’ venoarterial set-up. Meaning the venous catheter is placed in the vena cava, close to the right atrium, to suck blood out from the venous side, and pump it back into the aortic catheter placed through the femoral artery into the descending aorta. This bypasses the heart and the lungs. But it also places strain on the left ventricle with the retrograde flow through the aorta against the aortic valve. If there is very poor (or no) left ventricular activity, the left ventricle will slowly get filled with blood, even if the aortic valve is ‘patent’, and get volume overloaded.
This LV overloading will make it much harder for the left ventricle to get started again. Also, the overloading will work its way retrograde and fill the left atrium and then build pressure throughout the lung circulation, getting your lungs wet. One way to overcome this, is to puncture the atrial septum, to allow the overload to flow straight into the right atrium, and from there get sucked back into the ECMO circuit. This doesn’t help the overloaded left ventricle, but it keeps the lungs from getting wet.
ECMO and IMPELLA
The other way to help alleviate this problem, is to place an impella device at the same time as having the patient on ECMO. This supports the circulation, offloads the LV and oxygenates the blood. We do love our gadgets, but this combination brings even more complexity to the situation.
The link below is to a case report on a young woman with acute, severe heart failure as a complication to myocarditis. After being put on ECMO, her lungs got wet and she needed her pulmonary circulation unloaded using an Impella device. This ‘ventricular decompression’ immediately improved the situation. Unfortunately, the demanding set-up ended with the Impella malfunctioning and the patient dying. Still, the case report is an interesting look into the possibilities of circulatory support and combining ECMO and Impella.
Comparing circulatory support
If the patient doesn’t need respiratory support, an Impella can support the circulation on its own. In this study, comparing different circulatory support systems on man’s best friend, they found the Impella was better for unloading the heart and getting it back into rhythm after VF.
Below, in this next comparison on our frinds the pigs, they found no significant difference between AV ECMO, TandemHeart and Impella. Naturally, the Impella provided the least amount of circulatory support as they tested the 2.5 model. Still, even this 2.5 model with added NA infusion, provided enough circulation keep blood pressures adequate – if that is a meaningful target. Still, an interesting comparison to read as an introduction to circulatory support devices.
Both ECMO and Impella are used at our institution, and with increased focus on circulatory support, I found the above articles interesting with regards to understanding and choosing devices, and how they might work together.