An increasing number of patients are implanted with continuous-flow left ventricular assist devices (LVAD) for the treatment of severe congestive heart failure. In parallel with this growing experience has been an increase in knowledge of how these devices alter cardiac physiology and the important implications this has for cardiac function. Echocardiography offers the ability to provide serial noninvasive evaluation before and after LVAD implantation to document these changes, guide management decisions, and identify LVAD dysfunction. The authors detail a comprehensive assessment of LVAD function by transthoracic echocardiography.
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Continuous-Flow Left Ventricular Assist Devices
LVADs are designed to augment intrinsic LV function by the use of a mechanical pump operating in parallel with the patient’s native circulation. The pump magnifies the effect of LV contraction, effectively reducing left-heart filling pressures and increasing systemic circulation. The four main LVAD components are (1) the inlet cannula, (2) the pump, (3) the outflow graft, and (4) the external controller and battery pack. The inlet cannula inserted into the LV apex draws blood into the pump, which is located either within the pericardial space or distant from the cardiac chambers in a surgically created preperitoneal pocket. The outflow graft deposits the blood from the pump into the proximal ascending or descending thoracic aorta depending on pump type and surgeon’s preference. The external controller and battery pack are attached to the pump by wires (the drive line) through a surgically created track.
Pump technology has moved from bulkier devices requiring a reservoir chamber and a pusher-plate mechanism providing pulsatile flow to smaller devices that use a central spinning rotor to provide continuous flow. This rotor may be shaped to accelerate the blood circumferentially as in centrifugal pumps, or in an axial direction along the direction of the rotor’s axis using the Archimedes screw principle ( Figure 1 ). The effect of the pump is to draw blood through the LVAD circuit, decompressing the left ventricle. Durability has been increased with a second-generation single-moving-component design, which has been furthered by third-generation improvements using electromagnetic or hydrostatic forces to suspend the rotor, providing a “noncontact” pump. Mean survival is increased, while morbidity is decreased, for patients with severe heart failure implanted with continuous-flow LVADs compared with those implanted with older pulsatile devices.
Currently, three continuous-flow LVADs are US Food and Drug Administration approved. Of these, the HeartMate II (Thoratec Corporation, Pleasanton, CA) is the only surgically implanted continuous-flow LVAD with indications for bridge to cardiac transplantation and more recently destination therapy for patients who are not candidates for transplantation.
In contrast, the Impella (Abiomed, Danvers, MA) and TandemHeart (Cardiac Assist, Inc., Pittsburgh, PA) are percutaneous continuous-flow LVADs approved for short-term use only (TandemHeart, 6 hours; Impella, 5 days). They are generally considered temporary bridges either to myocardial recovery or to longer term support devices. The Impella continuous–axial flow LVAD is placed percutaneously through a femoral artery retroaortic approach in patients with severe heart failure requiring temporary stabilization and provides only limited support. The pump is located within a catheter in the LV cavity and ejects blood into the aortic root. The device is initially positioned with the aid of transesophageal echocardiography such that the inflow is below the aortic annulus but not obstructed by the septum or anterior mitral leaflet, and the outflow is above the sinuses of Valsalva in the aorta. Blood flow through the device can be detected with color Doppler. Follow-up transthoracic echocardiography can be used to confirm these findings ( Figure 2 ). The main contraindications for this device include the presence of aortic valve replacement, severe aortic valve calcification, and peripheral arterial disease.
Alternatively, the TandemHeart is a continuous extracorporeal centrifugal flow pump with a transseptal catheter drawing blood from the left atrium to the pump and then an outflow catheter returning blood to the femoral artery ( Figure 3 ). The necessity of large arterial and venous catheters as well as transseptal puncture has limited the use of this device. The main contraindications for this device include right ventricular (RV) dysfunction, severe peripheral vascular disease, and the presence of a ventricular septal defect causing hypoxemia from a right-to-left shunt.
Continuous-flow LVADs that are currently available in the United States are listed in Table 1 . For the purposes of this review, a general approach to transthoracic echocardiographic assessment of surgically implanted continuous-flow LVADs is used instead of focusing on a particular device.
| Name | Manufacturer | Pump type/location | Anastomosis sites: inflow/outflow | Pump speed range (rpm) | Maximum pump flow (L/min) |
|---|---|---|---|---|---|
| Second generation | |||||
| HeartMate II | Thoratec Corporation (Pleasanton, CA) | Axial/preperitoneal | LV apex/aortic root | 6,000−15,000 | 10 |
| Jarvik 2000 | Jarvik Heart, Inc. (New York, NY) | Axial/intrapericardial | LV apex/aortic root or descending thoracic aorta | 8,000−12,000 | 8 |
| MicroMed DeBakey | MicroMed Technology, Inc. (Houston, TX) | Axial/preperitoneal | LV apex/aortic root | 7,500−12,500 | 10 |
| Third generation | |||||
| HVAD | HeartWare International, Inc. (Framingham, MA) | Centrifugal/intrapericardial | LV apex/aortic root | 2,000−3,000 | 10 |
| DuraHeart | Terumo Medical Corporation (Somerset, NJ) | Centrifugal/intrapericardial | LV apex/aortic root | 1,200−2,400 | 8 |
| Incor | Berlin Heart AG (Berlin, Germany) | Axial/preperitoneal | LV apex/aortic root | 5,000−10,000 | 5 |
| Levacor | WorldHeart, Inc. (Salt Lake City, UT) | Centrifugal/preperitoneal | LV apex/aortic root | 1,000−3,000 | 10 |
| Percutaneous | |||||
| Impella | Abiomed (Danvers, MA) | Axial catheter based/LV | Retroaortic | 25,000−51,000 10,000−31,000 | 2.5 5 |
| TandemHeart | Cardiac Assist, Inc. (Pittsburgh, PA) | Axial/extracorporeal | Left atrium/femoral artery | 3,000−7,500 | 4 |
Continuous-Flow Left Ventricular Assist Devices
LVADs are designed to augment intrinsic LV function by the use of a mechanical pump operating in parallel with the patient’s native circulation. The pump magnifies the effect of LV contraction, effectively reducing left-heart filling pressures and increasing systemic circulation. The four main LVAD components are (1) the inlet cannula, (2) the pump, (3) the outflow graft, and (4) the external controller and battery pack. The inlet cannula inserted into the LV apex draws blood into the pump, which is located either within the pericardial space or distant from the cardiac chambers in a surgically created preperitoneal pocket. The outflow graft deposits the blood from the pump into the proximal ascending or descending thoracic aorta depending on pump type and surgeon’s preference. The external controller and battery pack are attached to the pump by wires (the drive line) through a surgically created track.
Pump technology has moved from bulkier devices requiring a reservoir chamber and a pusher-plate mechanism providing pulsatile flow to smaller devices that use a central spinning rotor to provide continuous flow. This rotor may be shaped to accelerate the blood circumferentially as in centrifugal pumps, or in an axial direction along the direction of the rotor’s axis using the Archimedes screw principle ( Figure 1 ). The effect of the pump is to draw blood through the LVAD circuit, decompressing the left ventricle. Durability has been increased with a second-generation single-moving-component design, which has been furthered by third-generation improvements using electromagnetic or hydrostatic forces to suspend the rotor, providing a “noncontact” pump. Mean survival is increased, while morbidity is decreased, for patients with severe heart failure implanted with continuous-flow LVADs compared with those implanted with older pulsatile devices.
Currently, three continuous-flow LVADs are US Food and Drug Administration approved. Of these, the HeartMate II (Thoratec Corporation, Pleasanton, CA) is the only surgically implanted continuous-flow LVAD with indications for bridge to cardiac transplantation and more recently destination therapy for patients who are not candidates for transplantation.
In contrast, the Impella (Abiomed, Danvers, MA) and TandemHeart (Cardiac Assist, Inc., Pittsburgh, PA) are percutaneous continuous-flow LVADs approved for short-term use only (TandemHeart, 6 hours; Impella, 5 days). They are generally considered temporary bridges either to myocardial recovery or to longer term support devices. The Impella continuous–axial flow LVAD is placed percutaneously through a femoral artery retroaortic approach in patients with severe heart failure requiring temporary stabilization and provides only limited support. The pump is located within a catheter in the LV cavity and ejects blood into the aortic root. The device is initially positioned with the aid of transesophageal echocardiography such that the inflow is below the aortic annulus but not obstructed by the septum or anterior mitral leaflet, and the outflow is above the sinuses of Valsalva in the aorta. Blood flow through the device can be detected with color Doppler. Follow-up transthoracic echocardiography can be used to confirm these findings ( Figure 2 ). The main contraindications for this device include the presence of aortic valve replacement, severe aortic valve calcification, and peripheral arterial disease.
Alternatively, the TandemHeart is a continuous extracorporeal centrifugal flow pump with a transseptal catheter drawing blood from the left atrium to the pump and then an outflow catheter returning blood to the femoral artery ( Figure 3 ). The necessity of large arterial and venous catheters as well as transseptal puncture has limited the use of this device. The main contraindications for this device include right ventricular (RV) dysfunction, severe peripheral vascular disease, and the presence of a ventricular septal defect causing hypoxemia from a right-to-left shunt.
Continuous-flow LVADs that are currently available in the United States are listed in Table 1 . For the purposes of this review, a general approach to transthoracic echocardiographic assessment of surgically implanted continuous-flow LVADs is used instead of focusing on a particular device.
| Name | Manufacturer | Pump type/location | Anastomosis sites: inflow/outflow | Pump speed range (rpm) | Maximum pump flow (L/min) |
|---|---|---|---|---|---|
| Second generation | |||||
| HeartMate II | Thoratec Corporation (Pleasanton, CA) | Axial/preperitoneal | LV apex/aortic root | 6,000−15,000 | 10 |
| Jarvik 2000 | Jarvik Heart, Inc. (New York, NY) | Axial/intrapericardial | LV apex/aortic root or descending thoracic aorta | 8,000−12,000 | 8 |
| MicroMed DeBakey | MicroMed Technology, Inc. (Houston, TX) | Axial/preperitoneal | LV apex/aortic root | 7,500−12,500 | 10 |
| Third generation | |||||
| HVAD | HeartWare International, Inc. (Framingham, MA) | Centrifugal/intrapericardial | LV apex/aortic root | 2,000−3,000 | 10 |
| DuraHeart | Terumo Medical Corporation (Somerset, NJ) | Centrifugal/intrapericardial | LV apex/aortic root | 1,200−2,400 | 8 |
| Incor | Berlin Heart AG (Berlin, Germany) | Axial/preperitoneal | LV apex/aortic root | 5,000−10,000 | 5 |
| Levacor | WorldHeart, Inc. (Salt Lake City, UT) | Centrifugal/preperitoneal | LV apex/aortic root | 1,000−3,000 | 10 |
| Percutaneous | |||||
| Impella | Abiomed (Danvers, MA) | Axial catheter based/LV | Retroaortic | 25,000−51,000 10,000−31,000 | 2.5 5 |
| TandemHeart | Cardiac Assist, Inc. (Pittsburgh, PA) | Axial/extracorporeal | Left atrium/femoral artery | 3,000−7,500 | 4 |
Preimplantation Echocardiographic Assessment
Preoperative echocardiography is important in assessing eligibility for LVAD therapy in a patient with severe heart failure, as well as an indicator for potential postoperative complications ( Table 2 ).
| Echocardiographic feature | Postoperative complication |
|---|---|
| Severe RV systolic dysfunction | Low pump flows/failure |
| Grade III–IV tricuspid regurgitation and RV spherical remodeling (diameter-to-length ratio > 0.6) | Severe RV dysfunction after LVAD implantation |
| LV apical thrombus | Inflow cannula placement complications, thrombosis of pump/cannulae and thromboembolism |
| Aortic root aneurysm or significant atherosclerosis | Outflow graft (connecting the outlet cannula of the pump and the aorta) placement complications and thromboembolism |
| Interatrial septal defect | Right-to-left shunting and oxygen desaturation |
| Moderate or severe aortic valve regurgitation | Reduced systemic circulation |
| Mitral stenosis | Low pump flows/failure |
The elements of this examination are similar to a standard transthoracic echocardiographic study, with the addition of “off-axis” imaging and views focusing on sites for graft anastomosis. In this regard, the aortic root or descending thoracic aorta and the LV apex are imaged for the presence of atheroma and thrombus, respectively. Before LVAD insertion, aortic root aneurysms must be repaired, and LV apical aneurysms, if present may require resection.
Furthermore, particular attention is paid to structural changes and hemodynamic abnormalities before LVAD placement. Marked RV enlargement and elevated afterload (pulmonary vascular resistance > 5.9 Wood units) predict RV failure after LVAD implantation. Resultant RV spherical remodeling and severe tricuspid valve regurgitation have been associated with a fourfold to fivefold increased risk for RV failure after LVAD implantation. These preoperative features provide guidance on which patients may benefit from tricuspid annuloplasty versus biventricular support. Preplanned placement of biventricular support improves survival compared with delayed placement after LVAD failure.
Mitral stenosis is important to quantify and correct before LVAD implantation, because of reductions in preload and effective cardiac output. Those found to have moderate to severe aortic regurgitation should have this corrected either by oversewing the valve or valve replacement before LVAD placement. This is secondary to worsening regurgitation after implantation, creating a “short circuit” that bypasses systemic circulation and reduces cardiac output. Color Doppler and agitated intravenous saline injection should be used to detect the presence of an interatrial communication as significant hypoxemia from right-to-left shunt may occur after left atrial pressures are reduced with an LVAD.
Postimplantation Echocardiography
Postoperative echocardiography focuses on demonstrating significant deviations from normal cardiac physiology, because the left ventricle now is continuously unloaded by flow into the LVAD pump rather than by means of pulsatile ejection across the aortic valve. However, many of the postprocedural echocardiography windows and views will be the same as those for any standard examination but with modifications that will now be discussed.
Protocol
Imaging windows are selected on the basis of LVAD type, as pump position and its connection to the left ventricle and circulation may differ. Pump speed should be annotated on acquired images to aid in comparison of serial studies. Standard machine settings are used, as recommended by the American Society of Echocardiography for the performance of transthoracic echocardiography. Blood pressure and body surface area should also be noted for hemodynamic and chamber measurements. Doppler evaluation with both continuous-wave Doppler to detect high-velocity LVAD flows and pulsed-wave Doppler to localize the origin of flow should be performed from multiple acoustic windows to ensure that maximum velocity is recorded ( Figure 4 , Videos 1–3 ). Pulsed-wave and continuous-wave Doppler also help differentiate LVAD flow from reverberation artifact that may be “colorized” by color Doppler. This artifact may appear as a discrete line of turbulent color superimposed across the cardiac image or may fill the entire view ( Figure 5 , Video 4 ).
