Cardiogenic shock problems to be addressed:
A. Decreased oxygen delivery (Low cardiac output)
1. Increased preload
2. Decreased contractility
3. Increased afterload
B. Poor perfusion (Hypotension)
C. Increased oxygen consumption
Figure 1: Breaking Down Cardiogenic Shock Hemodynamic Problems
Classifying Mechanical Circulatory Support
Left ventricular support devices should be initiated with the goal to reverse the hemodynamic abnormalities above and be able to fully support the patients to allow for complete cardiac rest if needed.
Left Ventricular Support Devices
*VA ECMO: Veno-arterial extracorporeal membrane oxygenation
1. Impella CP
General Information
This device is a pump that takes blood out of the left ventricle and into the aorta with continuous flow. It has a motor that pumps blood through a cannula that sits across the aortic valve. The size of the cannula determines the maximum flow that can be achieved through the cannula. It is a 9Fr cannula and can achieve up to 4.3lpm of flow, but usually achieves around 3.5 - 4lpm depending on the conditions.
Placement:
The Impella CP is inserted through the femoral artery and advanced across the aortic valve and into the left ventricular. It has a pigtail at the end of the catheter that helps keep the device in place so that it does not advance inappropriately. The inlet should be 3.5cm from the aortic valve (AV) when visualized on echo.
Physiological Benefits of the Device: (Figure 2)
The Impella CP takes blood out of the LV independent of the heart and pumps it into the aorta which does 3 things.
1. It decreases the end diastolic volume in the LV (EDV) by pumping the blood out of the ventricle.
2. It increases cardiac output (Flow) to the body.
3. It increases the pressure in the aorta (AOP).
Reducing the LV EDV decreases the radius of the LV which decreases wall tension (see LV afterload post). It also reduces the LV end diastolic pressure (EDP) which reduces the transmural pressure across the ventricle which also decreases wall tension. The decrease in wall tension decreases afterload, decreases microvascular resistance, and improves coronary blood flow. This also decrease the amount of work the LV must do to eject blood. The removal of volume from the LV will usually drop the LV low on the Starling curve and will reduce the pulse pressure in the LV. This is intentional to allow for cardiac rest. The decrease in wall tension and mechanical work, along with the increase in coronary perfusion, will increase oxygen delivery and decrease oxygen demand.
Increasing the aortic pressure (AOP) will increase the MAP and improve CPO. Additionally, it will increase diastolic coronary filling and improve oxygen supply.
Increasing cardiac output (Flow) will increase oxygen delivery and also increase CPO.
Figure 2: Physiological Benefits of the Impella CP
Benefits of the Device:
The benefit of using this device is that it is less invasive than the other options listed. It can be placed in the Cath Lab quickly and does not require an arterial cutdown or graft.
Limitations of the device:
This device only offers up to 3.5-4.3 liters of flow and for some patients this may not be enough support. If the support is not adequate, then inotropes/vasopressors will have to be used which will not allow the benefit of true cardiac rest. This device also requires a 14Fr sheath and placement in the femoral artery. This puts them at increased risk for leg ischemia and limits their ability to mobilize. The recommended diameter of the femoral artery is ≥ 6mm. A new Impella CP is being made that is expandable in the body and can be placed with a smaller sheath and reduce leg ischemia.
2. Impella 5.5
General Information:
The Impella 5.5 offers all the benefits of the Impella CP but can offer up to 5.5L of flow. This ensures that it will provide the full support needed for most patients. It can offer more flow because the cannula is much larger at 21Fr.
Placement:
The larger size which means that it must be placed in a different location. The most common placement is in the axillary artery with a graft. A graft can also be sewn directly on the aorta, if needed, and just like the CP, it crosses the aortic valve. The graft size recommended is 10mm x 20cm which equates to 31Fr. The placement should be confirmed with echo and the inlet should be 5cm from the aortic valve. The Impella 5.5 does not have a pigtail.
Physiological Benefits of the Device:
This is the same as the Impella CP. The Impella 5.5 takes blood out of the LV independent of the heart and pumps it into the aorta. It also decreases the EDV and increases the flow and AOP.
As mentioned with the Impella CP, reducing the EDV decreases the radius of the LV which decreases wall tension (see LV afterload post). It also reduces EDP which reduces the transmural pressure across the ventricle and decreases wall tension. The decrease in wall tension decreases afterload, decreases microvascular resistance, and improves coronary blood flow. This also decreases the amount of work the LV must do to eject blood. The removal of volume from the LV will drop the LV lower on the Starling curve and will reduce the pulse pressure in the LV. This is intentional to allow for cardiac rest. The decrease in wall tension and mechanical work, along with the increase in coronary perfusion, will increase oxygen delivery and decrease oxygen demand.
Increasing the aortic pressure (AOP) with increase the MAP which will improve CPO, and it will increase diastolic coronary filling and improve oxygen supply.
Increasing cardiac output (Flow) will increase oxygen delivery and increase CPO.
Figure 3: Physiological Benefits of the Impella 5.5 (Same as Figure 2)
Benefits of the Device:
The benefit of this device is that it will be able to fully support most patients. 5.5L of flow will support a BSA > 2.5m2. This will ensure that the patient can achieve cardiac rest. Another benefit is that it is placed in the ascending aorta or axillary artery and therefore allows the patient to sit up, get out of bed, and even ambulate.
Limitations of the device:
This device is more difficult to place compared to the Impella CP since it cannot be placed percutaneously and must be done surgically. It placed in the OR with a cutdown or during open sternotomy.
3. TandemHeart
General Information:
The TandemHeart is closer to VA-ECMO than an Impella. The motor/pump is outside the body, it involves tubing that is outside the body and the blood is pulled out of the body and then returned. The only difference from VA-ECMO is that it only supports the left side and therefore the patient must have good RV function. The TandemHeart offers up to 5lpm of flow. It is also like peripheral VA-ECMO because the blood return is opposite in direction to natural flow. The drainage cannula is inserted in the femoral vein, and it is placed using a wire across the atrial septum and into the left atrium. The oxygenated blood from the left atrium is pulled into the circuit and pumped back out into the femoral artery. The output is going up the femoral artery and will oppose the flow of the native heart. The drainage cannula in the left atrium provides venting of the left ventricle that will be discussed in more detail in the ECMO section.
Placement:
Placement can be done in either the Cath Lab or the Operating Room. The transeptal drainage cannula is 21Fr and goes in the femoral vein and a 15Fr or 17Fr arterial return cannula is placed in the femoral artery and they are attached to the circuit/pump.
Physiological Benefits of the Device:
The TandemHeart pumps blood from the left atrium to the left femoral artery and up the aorta. The removal of blood from the left atrium will reduce the volume in the left ventricle and therefore decrease LVEDV and LVEDP. This will reduce wall tension which is the true afterload of the LV. The decrease in wall tension will decrease mechanical work, decrease microvascular resistance, and increase coronary blood flow. The decrease in afterload, increase in coronary blood flow, and decrease in microvasculature resistance will increase oxygen delivery. The decrease in mechanical work will decrease oxygen demand.
The return of blood to the femoral artery will increase both the overall flow (cardiac output) and the overall perfusion (MAP). The increase in cardiac output will improve oxygen delivery and increase cardiac power (CPO). The increase in MAP will increase CPO and increase coronary perfusion.
Figure 4: Physiological Benefits of the TandemHeart
Benefits of the Device:
The benefit of this device is that it is easier to place than an Impella 5.5 and it has the benefit of also being an LV vent. In peripheral VA ECMO, a patient has retrograde return flow to the LV and the LV can get overdistended if the function is poor enough. This will lead to ischemia and increased work on the heart. A vent or unloading device is placed to ensure the LV does not get overdistended. For peripheral VA ECMO an Impella is placed to unload the LV, or a second cannula is placed in the LA or LV to drain the LV. The TandemHeart does not need a second cannula or device. Since the drainage cannula is placed across the atrial septum it is both the drainage and the vent.
Limitations of the device:
One of the concerns with this device is that it has reversal of arterial blood flow. Depending on the native LV function and respiratory status, there can be differential hypoxia (North-South Syndrome). Additionally the return cannula is in the femoral artery and leg ischemia can occur.
4. VA ECMO:
VA ECMO will support the left ventricle but since it also supports the RV it is discussed under biventricular support.
Summary of Left Ventricle MCS Devices and Hemodynamic Benefits
| Impella CP | Impella 5.5 | TandemHeart |
Flow | 3.5 – 4.3 lpm | 5.5 lpm | 3.5 – 5 lpm |
Sheath Size | 14Fr | 23Fr/10mm graft | Venous 21Fr Arterial 15-17Fr |
Inflow/Drainage | Left Ventricle | Left Ventricle | Left Atrium |
Outflow/Return | Proximal Aorta | Proximal Aorta | Distal Aorta |
Cannula/Circuit | Internal Cannula | Internal Cannula | External Circuit |
Directional Flow | Concordant | Concordant | Discordant |
Perfusion | ↑ | ↑↑ | ↑↑ |
Preload (LVEDV) | ↓ | ↓↓ | ↓↓ |
LVEDP | ↓ | ↓↓ | ↓↓ |
Afterload (Wall Tension) | ↓ | ↓↓ | ↓↓* |
Coronary Perfusion | ↑ | ↑ | ↑ |
Oxygen Delivery | ↑ | ↑↑ | ↑↑ |
Oxygen Demand | ↓ | ↓↓ | ↓ |
Cardiac Power | ↑ | ↑↑ | ↑↑ |
*Afterload may not decrease if poor native function due to reverse flow.
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