Short-Term Ventricular Assist Devices

For acute or short-term mechanical circulatory support, several devices are currently available. The intraaortic balloon pump (IABP) is the “original” short-term VAD and is frequently used for very short-term support in shock, often during revascularization procedures. It augments left ventricle (LV) output via balloon deflation in systole (decreasing afterload), and improved coronary perfusion by inflation during diastole. On transthoracic echocardiography, it can be viewed on parasternal long-axis and subcostal windows within the thoracic and abdominal aorta ( Video 26.1 ). Percutaneously placed VADs (PVADs) that are Food and Drug Administration (FDA) approved include the TandemHeart (CardiacAssist, Inc., Pittsburgh, Pennsylvania) and Impella system (Abiomed Inc., Danvers, Massachusetts). The TandemHeart is an extracorporeal centrifugal pump that draws blood out of the body through an inflow cannula positioned in the left atrium ( Video 26.2 ) (access via femoral vein and transseptal puncture) and delivers blood through an outflow cannula positioned in a femoral artery. The Impella is a catheter-based system that contains a microaxial continuous flow pump at its distal end and outflow cannula more proximally. The Impella catheter is placed via a femoral or axillary artery retrograde across the aortic valve such that the distal cannula lies in the LV and proximal outflow port lies in the ascending aorta ( Video 26.3 ). Echocardiographic imaging is useful prior to PVAD placement to identify contraindications to their use; for example, left atrial or left ventricular thrombus, severe aortic or mitral stenosis (Impella), or severe aortic regurgitation. Echocardiography may help guide placement of these devices, and assess proper catheter position and stability: the TandemHeart catheter should cross the interatrial septum, with the perforated end residing in the left atrium only. Prolapse of the perforated segment into the right atrium would result in desaturated venous blood being drawn in to the LVAD. The Impella catheter should be seen traversing the left ventricular outflow tract (LVOT) into the aortic root and ascending aorta. Serial echocardiography may also be used to assess the ventricular response to mechanical unloading.

Surgically implanted short-term extracorporeal VADs include the Thoratec Paracorporeal Ventricular Assist Device and CentriMag (Thoratec Corp., Pleasanton, California), which are pneumatically driven pulsatile and centrifugal continuous flow pumps, respectively. Similar to the TandemHeart, these devices have inflow cannulas placed in the chamber proximal to the failing ventricle (i.e., the left atrium), which draw blood out of the body via an extracorporeal pump and then into an outflow cannula that is surgically implanted into the vessel distal to the failing ventricle (i.e., the aorta). Echocardiography is used for preimplant evaluation and postimplant surveys for complications and/or myocardial recovery.

Long-Term Surgically Implanted Ventricular Assist Devices

The two currently FDA-approved continuous-flow left VADs are the HeartMate II (Thoratec Corp., Pleasanton, California) and heartware ventricular assist device Ventricular Assist System (Heartware International Inc., Framingham, Massachusetts). The HeartMate II is approved for both bridge to transplantation and destination therapy, while the Heartware device is approved for bridge to transplantation. Both devices have an inflow cannula implanted near the LV apex, a mechanical impeller, and outflow graft to the ascending aorta. The axillary flow impeller for the HeartMate II is implanted subdiaphragmatically, whereas the centrifugal flow Heartware impeller is intrapericardial ( Fig. 26.1A, B ). The impeller location influences echocardiographic imaging because of the shadowing and artifact produced, as described later. The remainder of this chapter will focus on long-term surgically implanted LVADs, with regard to echocardiographic imaging needed when planning for LVAD, during LVAD implantation, and post-LVAD placement.

Chest x-rays of continuous flow left ventricular assist devices.

(A) HeartMate II. Note the subdiaphragmatic position of the axillary flow pump, which limits subcostal echocardiographic views. (B) Heartware. Note the apical (intrapericardial) position of the centrifugal flow pump, which limits apical echocardiographic views.

Left Ventricular Structure and Function

Severe left ventricular dysfunction, typically an ejection fraction less than 25%, is required to be a candidate for an LVAD. Therefore, the accurate quantification of left ventricular volumes at end diastole and systole is necessary using the biplane method of disks to allow calculation of left ventricular ejection fraction. Left ventricular size, measured on the parasternal long-axis view as the end-diastolic diameter, may also factor into the assessment of a patient’s candidacy for LVAD, as pre-LVAD end-diastolic diameters less than 6.3 cm may be associated with an increased risk of postoperative morbidity and mortality. The presence of left ventricular, particularly apical, thrombus, will also impact surgical planning, approach, and procedure. Evaluation of left ventricular function, size, and thrombus may be facilitated by the use of echocardiographic contrast agents.

Right Ventricular Structure and Function

Right ventricular size and systolic function, as well as tricuspid regurgitation, should be assessed on pre-LVAD echocardiography. Right ventricular dilation and dysfunction may influence medical and surgical management decisions regarding the need for biventricular support rather than LVAD alone, perioperatively, and more long term. A preoperative RV fractional area change (RVFAC) of less than 20% is associated with RV failure upon LVAD device activation. Additionally, right ventricular dysfunction and other clinical factors (such as dependence on inotropes, or elevated liver function tests) are markers of worse prognosis post-LVAD implantation. Currently, however, there is no single right ventricular parameter or clinical factor that accurately differentiates patients who will have a better or worse prognosis.

Valves

Valvular lesions that may potentially impair LVAD function are critical to identify and treat prior to or at the time of LVAD implantation. Moderate or severe mitral stenosis impairs left ventricular filling and, therefore, flow into the LVAD inflow cannula. Similarly, right-sided valvular stenosis will also impair filling of the left heart and LVAD inflow. In contrast, aortic stenosis, regardless of severity, typically does not impair LVAD function, as the outflow cannula bypasses the LVOT and aortic valve.

Careful attention must be given to the presence, mechanism, and severity of aortic regurgitation prior to LVAD implantation. Aortic regurgitation attenuates left ventricular unloading and systemic delivery of blood in the setting of an LVAD due to the creation of a loop of blood that travels through the LVAD inflow cannula, pump, then outflow graft into the ascending aorta, where it falls back into the LV through the regurgitant aortic valve. Significant regurgitation of right-sided valves is also a concern of pre-LVAD, as this may be a marker of right ventricular dysfunction, which is associated with a worse prognosis post-LVAD. Following LVAD implantation, tricuspid regurgitation could worsen due to changes in right ventricular geometry and tricuspid valve anatomy that result from over-decompression of the LV and shifting of the interventricular septum. Mitral regurgitation, however, typically improves as a result of an LVAD placement because of decompression of the LV both with regard to size, resulting in better mitral valve leaflet coaptation, and decline in pressures.

A mechanical aortic valve also needs to be identified pre-LVAD implantation and converted to a bioprosthetic valve at the time of LVAD placement to limit the risk of aortic valve thrombosis. Since LVAD outflow bypasses the native LVOT, a mechanical aortic valve would not open sufficiently in the setting of an LVAD and therefore be likely to thrombose. This is less of an issue for mechanical mitral valves, as the forward flow from left atrium to LV is maintained by the LVAD.

Endocarditis

Active infection is a contraindication to LVAD implantation; therefore, lesions suspicious for endocarditis, whether on valves or indwelling devices such as pacemaker/defibrillator leads or catheters, must be carefully evaluated.

Aorta

Since the LVAD outflow graft is typically implanted into the ascending aorta, attention should be given to the presence of aortic pathology, such as dilation, plaque, and dissection.

Congenital Heart Disease

Right-to-left shunts, such as a patent foramen ovale, atrial and ventricular septal defects, need to be identified prior to LVAD implantation because decompression of the left side of the heart by the LVAD may increase right-to-left shunting and sequelae, such as hypoxemia and paradoxic emboli. The evaluation for shunts is typically performed on the intraoperative transesophageal echocardiogram at the time of LVAD implantation. Detection of shunts is enhanced with agitated saline (“bubble”) contrast.