Cardiac Assist Devices

Durable, Implanted Ventricular Assist Devices

Most implanted VADs consist of a pump, an inflow cannula, an outflow cannula, a power supply, and a controller. The controller collects data from the pump and directs its action through a driveline. VADs are classified according to the location of these components. Extracorporeal VADs have pumps and controllers outside the body. Patients with paracorporeal VADs have pumps strapped close to the body but not inside it. Intracorporeal VADs have implanted pumps, with drivelines connecting to a paracorporeal or extracorporeal controller and batteries or another power supply.

Cardiac assist devices can support the LV as an LV assist device (LVAD), the right ventricle (RV) as an RV assist device (RVAD), or both ventricles as a biventricular assist device (BI-VAD). LVAD inflow cannulas are connected to left-sided chambers, most often the LV apex. The outflow graft cannula can be attached to the aorta in multiple places but is usually anastomosed, end to side, to the ascending aorta. RVADs involve right atrial (RA) or RV inflow and pulmonary artery outflow cannulation.

Devices are also characterized by mechanism. Early-generation cardiac VADs included a pneumatically compressed, distensible chamber or a pusher-plate mechanism to expel blood into the outflow cannula. These early-generation devices simulated cardiac pulsatile action through one-way inlet and outlet valves.

The second-generation VADs produce continuous flow by a rotating propeller and are called axial-flow pumps. The HeartMate II (Abbott, Abbott Park, IL) draws blood from the heart through an inflow cannula and propels it into the outflow graft cannula ( Fig. 16.1A ).

The third-generation continuous-flow VADs, HeartWare HVAD (Medtronic, Minneapolis, MN), and HeartMate 3, use centrifugal force to propel blood. Magnetic or hydrodynamic forces levitate and spin a disklike impeller rather than a propeller (see Fig. 16.1B ). The speed of the propeller or impeller (in revolutions per minute [rpm]) and the pressure difference between the inflow and outflow cannulas determine the rate of flow. There is greater flow during systole, when the aortic-LV pressure difference drops (i.e., LV pressure rises with ventricular contraction) than during diastole, when LV pressure falls with ventricular relaxation and the pressure difference increases.

The continuous-flow pumps provide greater flow at lower pressures than pulsatile pumps do. They also tend to be more durable because they lack valves (which wear out over time). ^, Centrifugal pumps lack bearings and may reduce the amount of hemolysis, and they appear to be associated with lower rates of reoperation for pump dysfunction. The HeartMate 3 device has an additional feature that rapidly and briefly alters pump speed, asynchronous to heart rate, with the intention of lowering rates of thrombosis.

Continuous-flow devices are called nonpulsatile VADs, but residual ventricular function and non-uniform pressure differences between the inflow and outflow cannulas produce some pulsatility. Centrifugal-flow devices, particularly the HeartMate 3, demonstrate greater pulsatility than the axial-flow devices. ^,

Percutaneous Mechanical Circulatory Support Devices

Percutaneous MCS devices are inserted through peripheral arteries and veins and can be placed relatively quickly to provide temporary support. The Impella Recover devices (Abiomed, Danvers, MA) consist of an axial-flow (propeller) pump within a catheter that is placed into the femoral artery, through the aorta, and across the aortic valve into the LV ( Fig. 16.2A ). The inflow port draws blood from the LV outflow tract (LVOT) and ejects it through the outflow port in the aorta. A right-sided Impella is placed into the RV and across the pulmonic valve; it draws blood from the RV outflow tract (RVOT) and pumps it into the proximal pulmonary artery. Several different sizes are available, corresponding to the amount of support in liters per minute. The larger devices have traditionally required surgical cutdown of a large artery for insertion.

The TandemHeart (LivaNova, London, UK) devices consist of a paracorporeal centrifugal-flow pump connected to an inflow catheter, which is placed through the venous system into the RA, and then into the left atrium (LA) across the interatrial septum, and an outflow cannula that is in the distal aorta through the femoral artery (see Fig. 16.2B ). The TandemHeart cannulas can also be positioned in the RA (inflow) and the pulmonary artery (outflow) to support the RV and can be connected to an oxygenator to provide lung support, similar to venovenous ECMO. The TandemHeart devices improve the cardiac index and pulmonary pressures in patients with decompensated heart failure after myocardial infarction and have been used to stabilize patients with incessant ventricular tachycardia.

The CentriMag (Abbott, Abbott Park, IL) consists of a paracorporeal, centrifugal pump with a magnetically levitated impeller. It can be connected with catheters placed in any of the cardiac chambers to provide left (LA or LV inflow, aortic outflow), right (RA or RV inflow, pulmonary artery outflow), biventricular, or cardiopulmonary support. The CentriMag has been used to provide temporary support of the RV in patients with a durable LVAD.

Total Artificial Heart

The current version of the total artificial heart, the temporary Total Artificial Heart (TAH-t; SynCardia Systems, Tucson, AZ) replaces the RV and LV with pulsatile pumps and metallic atrioventricular valves anastomosed to the native atria. The pumping component is attached by drivelines to an extracorporeal controller and battery ( Fig. 16.3 ). The SynCardia device is approved by the U.S. Food and Drug Administration (FDA) as a bridge to transplantation.

Preimplantation Assessment

Specific criteria for placement of an LVAD as destination therapy include an LV ejection fraction (LVEF) of less than 25% to 30%, most often measured by echocardiography. Although the hemodynamic criteria refer to invasively determined measurements, echocardiography can provide noninvasive assessment of the cardiac index and intracardiac pressures. A small ventricular size (<6.3 cm) portends a worse outcome after LVAD implantation, possibly because patients with smaller cavity sizes and severe LV dysfunction suffer from infiltrative cardiomyopathies.

Echocardiography identifies factors that warrant interventions, complicate cardiac assist device placement, or support the use of one type of device over another ( Table 16.2 ). It is particularly important to detect thrombi that can embolize after an MCS device is placed. Use of an ultrasound enhancing agent (i.e., microbubble contrast) may be necessary to exclude ventricular thrombi in patients with inadequate images, and TEE should be used to examine LA and RA appendages in patients with atrial fibrillation or atrial flutter. Intracardiac shunting produces systemic hypoxemia and/or paradoxical emboli after LVAD placement with left-sided decompression and a potential right-to-left pressure gradient. Intracardiac shunts should be repaired at the time of implantation. High LA pressure may prevent passage of agitated saline contrast across a patent foramen ovale (PFO), even after a Valsalva maneuver, leading to false-negative examination results for shunting at the atrial level.

Moderate or greater aortic regurgitation (AR) complicates the use of MCS devices. Placement of an outflow cannula in the ascending aorta or an Impella Recover device across the aortic valve may worsen baseline AR, leading to circulation of blood from the LV to the pump, to the aorta, and back into the LV. High LV pressure and low diastolic pressure in the aortic root from severe AR can reduce coronary perfusion pressure, which is exacerbated by the fact that the TandemHeart draws blood from the LA and ejects it into the distal aorta (bypassing the coronary ostia).

Preimplantation grading of the degree of AR is complicated in patients with end-stage heart failure, who commonly have low systemic pressure and high LV diastolic pressure. The low aortic-to-LV pressure gradient may artificially decrease the amount of regurgitation through an incompetent aortic valve. In the case of greater than moderate AR, the aortic valve should be replaced, repaired, or (rarely) oversewn. Although significant mitral stenosis must be corrected at the time of implantation of an LVAD or Impella device because it impedes flow into the device, aortic stenosis does not necessarily require intervention because the aortic valve is bypassed. However, severe aortic stenosis, aortic valve oversewing to prevent AR, and aortic valve cusp fusion prevent cardiac output in the setting of LVAD pump failure. Significant mitral regurgitation should be ameliorated by a functioning LVAD and does not represent a pitfall for implantation.

Mechanical prosthetic aortic valves should be replaced with a bioprosthesis at the time of implantation because reduced transaortic flow increases the risk of thrombosis. Transmitral flow, however, is preserved, and a functional mechanical mitral valve need not be replaced. ^,

Before implantation, identification and characterization of RV dysfunction is vitally important. RV failure is a significant cause of morbidity and mortality. LVAD placement produces a higher cardiac output and increased venous return to the RV. An RV with preexisting dysfunction may fail in the setting of increased preload. The LVAD influence on septal motion reduces RV function and can worsen baseline tricuspid regurgitation. However, decompression of the LV by a VAD can reduce pulmonary pressures and RV afterload, improving RV function. Identification of RV dysfunction can lead to a decision to delay durable LVAD implantation to optimize RV function through medical therapy or a temporary RVAD; to implant a TAH, a BIVAD, or an RVAD with an LVAD; or to forego durable MCS device implantation entirely. For patients with known RV dysfunction, planned early placement of an RVAD improves survival compared with delayed placement. In light of the potential worsening of RV function with increased tricuspid regurgitation after LVAD implantation, expert consensus documents recommend tricuspid repair if tricuspid regurgitation is greater than moderate.

TEE may be necessary to assess valvular anatomy and function and to exclude vegetations in patients with suspected endocarditis. Imaging of the ascending aorta may reveal aneurysms, dissections, or severe or mobile atheromatous plaques, all of which complicate placement of an outflow graft.

Periprocedural Guidance

Intracardiac echocardiography and TEE provide real-time visualization to direct cannula placement away from potentially obstructing cardiac structures. The Impella device can be guided to position the inflow port in the LVOT and the outflow port above the aortic valve. Echocardiography can guide transseptal puncture in TandemHeart insertion, preventing inadvertent puncture of the LA or RA wall or the aortic root.

As with other cardiac surgical procedures, TEE is useful in deairing the heart before taking the patient off bypass. Air bubbles arise at cannula anastomosis sites and can embolize if not detected before the aortic cross-clamp is removed. Anterior structures, including the ascending aorta, the RV, and anterior portions of the LV, should be examined for air bubbles ( Fig. 16.4 ).

Periprocedural TEE is useful to reassess the degree of AR, identify a potentially missed PFO, and guide initial speed settings (see Optimization Studies). After chest closure, periprocedural TEE can assess for changes in cannula orientation.

Postimplantation Surveillance and Complications

When there is no suspicion of MCS device dysfunction, longitudinal surveillance echocardiographic studies are part of protocols for many institutions that follow patients with cardiac assist devices ( Table 16.3 ). The protocols vary in the frequency and timing of echocardiography. The goals of these echocardiograms are to monitor baseline changes in native heart and device performance and to detect dysfunction before clinical manifestations. Examples include worsening AR with LV enlargement and detection of intracardiac thrombi. Surveillance echocardiography may be performed with speed changes to optimize devices with echocardiographic parameters directing the optimal setting (discussed later). In certain cases, improvement in cardiac function detected on surveillance echocardiography may warrant consideration for explantation.

Echocardiography is also performed for symptoms such as dyspnea, edema, and lightheadedness and for MCS device alarms ( Table 16.4 ). Current MCS devices have alarms tied to changes in power consumption and/or flow, although the latter are limited in accuracy. Alarms signal that dysfunction exists, but defining the cause of the dysfunction often requires an imaging study ( Table 16.5 ).

Overfilling

Regurgitation, obstruction, and malfunction cause overfilling and pressure increase in the supported ventricle. In pulsatile VADs, failure of the one-way valves leads to regurgitation. Cannula regurgitation can occur in continuous-flow devices when the systemic vascular resistance is elevated. Cannula obstruction can result from kinking, malposition, or thrombosis and can be partial, complete, or intermittent. VAD pumping mechanisms may become partially or completely obstructed, leading to increased shear stress and hemolysis.

There can be primary failure of VAD components: battery, controller, or pump. Pump failure can result from failure of the pumping mechanism but is more likely to result from pump thrombosis. The ensuing ventricular dilation and reduced cardiac output can lead to intracavitary stasis and thrombus formation in the LV or aortic root ( Fig. 16.5 ). Failure of the power supply or of the controller can be catastrophic unless the patient has adequate residual cardiac function and an unobstructed aortic outflow.