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As the population ages, the prevalence of heart failure continues to escalate. Heart failure with reduced ejection fraction (HFrEF) comprises approximately 50% of the admission diagnoses for heart failure. The American College of Cardiology/American Heart Association categorizes heart failure into four stages labeled A to D. Patients with stage D heart failure have the worst outcomes. Clinically, they have persistent symptoms despite optimal doses of guideline-directed medical therapy and cardiac resynchronization devices when appropriate. These patients, classified as “advanced” or “end stage,” may be eligible for advanced therapies such as mechanical assist devices and cardiac transplantation. Unfortunately, the number of patients with stage D HFrEF far surpasses donor heart availability, such that cardiac transplantation is not an option for many patients, and alternative options must be considered. The ventricular assist device (VAD) is one such alternative.
The left ventricular (LV) assist device or LVAD is a battery-operated pump that conducts blood from the left ventricle via an inflow cannula implanted at the LV apex to the aorta by way of an outflow graft with direct anastomosis to the ascending aorta. In so doing, it augments cardiac output in the failing heart by reducing the afterload on the left ventricle and decreasing filling pressures, pulmonary artery pressures, and mitral regurgitation (MR; Fig. 175.1 ). Patients eligible for LVADs generally have stage D HFrEF (explained earlier) with New York Heart Association class III to IV symptoms and LV ejection fraction (EF) less than 25%, with or without cardiac resynchronization therapy with most often a dilated left ventricle.
According to the INTERMACS registry, more than 20,000 LVADs have been implanted in the United States with more than 2500 new implants occurring every year. VADs can be commissioned for short- (hours to days) or long- (months to years) term support. VADs are differentiated based on a number of factors: (1) location of implant (intracorporeal versus extracorporeal), (2) implantation approach (percutaneous versus surgical), (3) flow characteristics (pulsatile versus continuous), (4) pump mechanism (volume displacement, axial, centrifugal), and (5) ventricle supported (left, right, both).
Transthoracic echocardiography (TTE) is the noninvasive imaging modality of choice for the assessment of patients with continuous-flow (CF) LVADs. The purpose of this chapter is to focus on the echocardiographic imaging recommendations for CF LVADs. Most of the data supporting the use of echocardiography in these patients come from the experience with axial-flow pumps (HeartMate II), which are described in more detail later.
Three types of CF LVADs are currently approved for implantation in the United States: the Heartmate II (Abbott Laboratories), which houses an axial-based pump, and the HeartWare (Medtronic) and HeartMate III, both of which house a centrifugal-based pump. CF LVADs have been shown to have improved device performance and patient survival profiles compared with the earlier pulsatile-flow devices. The axial pump generates flow parallel to the axis of rotor or impeller rotation using a propeller in pipe mechanism while the centrifugal pump generates flow perpendicular to the axis of rotation with a spinning bladed disk. Studies based on two-dimensional (2D) and three-dimensional (3D) echocardiographic measurements have shown that the HeartWare Ventricular Assist System (HVAD) results in less of a reduction in LV chamber diameter and 3D volumes with increasing pump speed compared with the Heart Mate II pump. Both pumps, however, have been shown to provide similar overall flows in the normal working range of speed so that the differential shape changes seen in these studies during unloading may be attributable to the location of the pumps in the thorax. The HeartMate II pump is located subdiaphragmatically, likely resulting in inferior displacement of the LV apex because of the pull of the inflow cannula. In contrast, the HVAD and HeartMate III pumps are inserted intrathoracically at the LV apex, resulting in less distortion of the LV apex ( Fig. 175.2 ). LVADs can be used as (1) a bridge to cardiac transplantation, (2) a bridge to cardiac transplant candidacy, 3) a bridge to recovery, and (4) destination therapy.
Echocardiographic examination of LVAD candidates should include a comprehensive 2D, Doppler, and color Doppler assessment. Findings that may impact patient outcomes and device function include the presence of LV thrombus, any ascending aortic pathology, dilatation or dysfunction of the right ventricle, presence of significant tricuspid regurgitation (TR), presence of significant pulmonary hypertension, significant aortic or mitral stenosis, or regurgitation and presence of an interatrial communication. If significant MR is present, the mechanism needs to be elucidated. Primary MR may not improve with LV unloading, and the mitral valve may need to be repaired or replaced at the time of LVAD implantation. Similarly, significant TR may need to be intervened upon before LVAD placement. Significant aortic regurgitation (AR) is a very important comorbidity (see later) and usually requires either oversewing of the valve or valve replacement before LVAD placement. Interatrial septal defects should be ruled out with an agitated saline study before LVAD placement, and if clinical suspicion remains, the interatrial septum should be evaluated on transesophageal echocardiography (TEE) preoperatively. Unloading of the left ventricle after LVAD results in a decrease in both LV and left atrial (LA) pressures. Right atrial (RA) pressures, on the other hand, as described in Fig. 175.1 , may remain the same or increase in the presence of increased venous return after LVAD-assisted improvement in cardiac output. The pressure differential created across a defective interatrial septum forms the perfect milieu for paradoxical embolization or hypoxia (with right-to-left shunting), and both of these complications can occur immediately after implantation or months later. If found on preoperative imaging, these interatrial septal defects can be closed at the time of LVAD implantation.
Imaging patients after LVAD insertion can be challenging, especially during the immediate postoperative period. Generally, a standard echocardiographic protocol (2D and Doppler) is adopted with certain additions or modifications and off-axis views to adequately assess LVAD function. These modifications or additions are discussed sequentially later. The first screen of the stored echocardiogram on a patient with an LVAD should document the type of LVAD (e.g., HeartMate II, HeartWare, HeartMate III), the LVAD speed in revolutions per minute (rpm), and the power and pulsatility index of the device. Any changes made during the study should also be recorded ( Table 175.1 ).
Annotate LVAD Type and Speed Setting | |
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Parasternal long-axis view
|
Apical four-chamber view
Subcostal view
|
Right upper sternal view
Parasternal short-axis view
|
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