Types of VADs
Ventricular assist devices are mechanical pumps that take over the function of the damaged ventricle in order to re-establish normal hemodynamics and end-organ blood flow. In addition, VADs unload the native heart allowing it to rest and, in some cases, the heart can recover function. They can be used as short-term support (days) or as long-term support (weeks or months). They have several components (Figure 1) and can support the right, left or both ventricles. Devices are implanted through a median sternotomy; in a LVAD, the inflow cannula is connected to the apex of the left ventricle (LV) and the outflow cannula is connected to the ascending aorta, whereas in a right VAD (RVAD), the inflow cannula is connected to either right atrium or ventricle and the outflow cannula is connected to the pulmonary artery. The pumping chamber can be placed outside the patient's body (extra- or para-corporeal devices) or within the abdomen in a preperitoneal position immediately under the diaphragm or above the diaphragm in the pericardial space (intracorporeal devices). Specific characteristics of each device will be explained in the following sections.
Table 1 summarizes and characterises short-term devices. These types of VADs are usually used in patients who present in cardiogenic shock after an acute myocardial infarction or acute myocarditis or post-cardiotomy shock. The implantation of a VAD unloads the ventricle in order to diminish myocardial work, allowing a potential myocardial recovery. Patients who fail to demonstrate myocardial recovery within 7 days should be considered for conversion to a long-term device. In transplant-ineligible patients, device withdrawal should be considered if destination therapy (DT) with a long-term implantable device is not an option.
Table 2 shows the general characteristics of intermediate- and long-term VADs. This type of device can be classified in many different ways, for example, from the engineering point of view, which includes many device features. This classifies VADs in levels or generations ( Table 2 ).
The first generation of devices includes pulsatile volume displacement pumps (Figure 2). These devices are engineered with a pumping chamber and two valves (outflow and inflow valves) that fill and empty cyclically and are capable of generating a stroke volume of around 65–83 ml. The pumps are driven by either pneumatic or electrical drive systems. Examples of this type of devices are the Thoratec pneumatic VAD (pVAD) and implantable VAD (IVAD; Thoratec Laboratories Corp., CA, USA), the HeartMate IP1000, VE and XVE (Thoratec Corp.), the Novacor (World Heart Corp., CA, USA) and the LionHeart LVD2000 (Arrow International, PA, USA).
LVAD: Left ventricular assist device; pVAD: Pneumatic assist device.
Figures courtesy of Thoratec Laboratories Corp., CA, USA; World Heart Corp., CA, USA; and Arrow International, PA, USA.
The Thoratec pVAD is a pneumatically driven, polyurethane sac enclosed in a plastic housing designed for long-term use. This VAD is indicated as both a bridge-to-transplantation (BTT) and a bridge-to-recovery (BTR) device. It can provide uni- or bi-ventricular support. The external position of the pump allows device exchange in cases of malfunction, thrombus or infection. Furthermore, this also allows use in smaller patients who are poor candidates for implantable devices. Patients require systemic anticoagulation for the duration of the Thoratec VAD support.
The Thoratec IVAD is a small, simple and versatile IVAD with the same features as the Thoratec pVAD, except that longer support is anticipated owing to the intracorporeal position. The US FDA approved it in 2004 for circulatory assistance as BTT or BTR.
The HeartMate is an implantable device designed for long-term circulatory support and is approved by the FDA as a BTT and DT.[6–8] It offers only support for the LV (i.e., LVAD). There are three types of HeartMate devices. The first one is the implantable, pneumatic (IP)-LVAD, which is powered and controlled by an external pneumatic drive console that rests on a wheeled cart. The second is the vented, electric (VE)-LVAD, which contains an electric motor within the blood pump housing. It receives external power and control signals from an external microprocessor via a vented driveline. The third is the XVE HeartMate, which is an improved version of the VE HeartMate due to reported device durability issues of the original model and is currently the only type of HeartMate pump commercially available. The three systems have porcine valves and textured blood-contacting surfaces that become covered by a 'pseudoneointimal' layer. This results in a very low incidence of thromboembolic events and, therefore, patients do not require systemic anticoagulation. In addition, patients supported with the HeartMate XVE have shown a lower incidence of gastrointestinal bleeding than patients supported with pulsatile devices (e.g., HeartMate II, VentrAssist and DeBakey). This can be explained, in part, because HeartMate XVE does not require anticoagulation. However, device durability remains an important limitation, with a probability of device exchange or fatal device failure of 17.9 and 72.9% at 1 and 2 years, respectively.
The Novacor LVAD is a portable, implantable, electric, dual pusher plate device designed for long-term cardiac support. The pump housing is constructed of a smooth polyurethane pump sac with gelatin-sealed inflow and outflow polyester grafts containing porcine bioprosthetic valves. The Novacor shares many similarities with the HeartMate system, including an external drive system with a portable power pack option. The Novacor LVAD device requires systemic anticoagulation to prevent thromboembolism (risk: 5–7%). The incidence of primary device failure is very rare. The Novacor LVAD was approved as a BTT therapy by the FDA, the CE Mark and in Japan; however, this device has been discontinued.
The LionHeart LVAD is a totally implantable device that lacks a percutaneous lead and is powered by a transcutaneous energy transmission system and requires the patient to carry a rechargeable, extracorporeal battery pack. It was mainly designed to be used as DT. The Clinical Utility Baseline Study (CUBS) trial, a nonrandomized, observational European study, has shown a 1- and 2-year survival of 39 and 22%, respectively, in patients undergoing a LionHeart LVAD implantation. This fully implantable device caused fewer infectious complications than other devices as a DT. It was approved by CE Mark in 2003, and it is not currently commercially available.
The second-generation devices (Figure 3) include implantable, continuous flow, rotary pumps with axial flow that offer several advantages over pulsatile flow pumps. Some of the advantages are the smaller size that reduces the risk of infections and simpler implantation (Figure 4). There are fewer moving parts, absence of valves to direct blood flow, smaller blood-contacting surfaces and reduced energy requirements that enhance simplicity and durability. These pumps have an internal rotor within the blood flow path that is suspended by contact bearings, which imparts tangential velocity and kinetic energy to the blood. The net action results in generation of a net pressure rise across the pump. An external system driver connected by a percutaneous lead powers it. Some of the greatest limitations of this type of device are hemolysis, ventricular suction and thrombus formation and pump stoppage. Examples of these devices are the HeartMate™ II (Thoratec Inc.), the Jarvik 2000® (Jarvik Heart, Inc., NY, USA) and the DeBakey® LVAD (MicroMed Technology, Inc.). The HeartMate II is a rotary pump smaller than the first-generation devices, principally due to the elimination of the sac or reservoir necessary in pulsatile pumps. This device has two cannulas (inflow and outflow) without valves. It has smooth surfaces in the outlet and inlet stators but requires anticoagulation. Clinical experience shows that the HeartMate II LVAD provides excellent support in the outpatient setting, with significant improvement in functional capacity. In a multicenter, prospective study, 42% of 133 heart transplant candidates supported with HeartMate II underwent HT within the 6 months of support, with an overall 6-month survival of 75% and 1-year survival of 68%. More recently, John et al. reported an improved 6-month survival of 86.9% and 3% incidence of device malfunction in a mean duration support of 6 months. The FDA has already approved its use as a BTT therapy. In addition, the HeartMate II has demonstrated great device reliability and significantly reduced device noise. It also has low thrombogenicity and low thromboembolic risk that make it a good device option for DT. Currently, there is an ongoing trial in DT patients comparing the use of HeartMate XVE and HeartMate II that will elucidate this indication.
Figures courtesy of Thoratec Laboratories Corp., CA, USA; MicroMed Technology Inc., NY, USA; and Jarvik Health Inc, NY, USA.
The MicroMed DeBakey® (bottom) shows a smaller size than first-generation devices, such as Novacor® LVAD (left) and Thoratec® (right).
Image courtesy of MicroMed Technology Inc.
The DeBakey LVAD is a small rotary pump with similar characteristics to HeartMate II. Worldwide clinical experience has shown that this device is suitable for both BTT and DT therapy. The success as a BTT, reported in 150 patients, was between 50 and 66%, and device malfunction was observed in 3% of the population. Multicenter, nonrandomized BTT trials and a randomized DT trial (HeartMate XVE vs DeBakey) are ongoing.
The Jarvik 2000 is a miniature axial flow pump (diameter: 2.4 cm; length: 5.5 cm). The pump is placed within the LV, different to the HeartMate II (placed in the abdomen in a preperitoneal position) or the DeBakey LVAD (placed within the thorax in the pericardial space). The avoidance of a pump pocket decreases the incidence of serious device-related infections. It also requires anticoagulation; however, the risk of hemolysis is minimal. Clinical experience has demonstrated that this device provides safe and effective therapy for HF patients, both as a BTT and DT.[18,19] This pump optimally performs when it acts as a true assist device by partially unloading the LV, thereby allowing reverse remodeling and native ventricular ejection to contribute to the remainder of the cardiac output. The first patient to receive the Jarvik 2000 as DT was supported continuously for more than 6 years. No internal device components failed during an accumulative support time of 59 years. Furthermore, the freedom from LVAD system failure due to external components was 95% at 1–4 years. Clearly, such proven reliability and durability make the Jarvik 2000 promising for long-term use in selected patients.
The third generation of devices includes centrifugal continuous-flow pumps with an impeller or rotor suspended in the blood flow path using a noncontact bearing design, which uses either magnetic or hydrodynamic levitation (Figure 5). The levitation systems suspend the moving impeller within the blood field without any mechanical contact, thus eliminating frictional wear and reducing heat generation. This feature promises longer durability and higher reliability with low incidence of device failure and need for replacement. Usually, magnetic levitation devices are larger owing to the need for complex position sensing and control system that increases requirements for a large pump size. Examples of third-generation devices are the VentrAssist™ (Ventracor Ltd., Sydney, Australia), DuraHeart™ (Terumo, Inc., MI, USA), the HVAD (HeartWare Corp., FL, USA) and the EVAHEART™ LVAS (Sun Medical Technology Research Corporation, Nagano, Japan).
Figures courtesy of Ventracor Ltd, Sydney, Australia; Terumo Inc., MI, USA; Heartware Corp., FL, USA; and Sun Medical Technology Research Corp., Nagano, Japan.
The VentrAssist LVAD is a centrifugal pump with hydrodynamic levitation of the impeller. The absence of magnetic levitation and monitoring systems results in smaller pump size. Initial clinical experience showed that the VentrAssist LVAD is an efficacy and reliable device in BTT or DT patients. In a prospective, multicenter study, overall survival in 33 patients undergoing a device implantation as a BTT therapy was 82% at 5 months (39.4% transplanted and 42.4% transplant eligible) and the incidence of device failure was 15%. On a cohort of 16 patients receiving the device as a DT therapy, the median support time was 330 days and the 1-year survival was 80%, with 2-year survival approaching 60%.
The DuraHeart is a centrifugal pump with magnetically levitated impeller along with hydrodynamic bearings in case of failure of the magnetic levitation system. The European clinical experience in 55 BTT patients, presented at the International Society of Heart and Lung Transplantation meeting in 2008, showed that survival rate was 86% at 6 months and 77% at 1 year, with a maximum support time of 2.75 years. Of the ten patients who died, the majority of deaths were due to fatal intracerebral bleeding or nontraumatic subdural hematoma, most probably associated with excessive anticoagulation. After implementing less intensive anticoagulation, Kaplan–Meier survival rate at 1 year for the latest 44 patients was 85%, showing significantly improved survival compared with that of the initial 11 patients (45%; p < 0.0001). There was no incidence of pump mechanical failure, pump thrombosis or hemolysis throughout the support duration.
The HVAD is a centrifugal continuous flow pump with magnetic and hydrodynamic bearings characterized by the small size (diameter: 4 cm; height: 2 cm) but with up to 10 l/min flow capacity. This feature allows the implantation of the device in the pericardial cavity without need for creation of a preperitoneal pocket. In a European and Australian multicenter trial, 20 BTT patients underwent implantation of the HVAD. Actuarial survival at 1 year was 80%. Additionally, some cases have experienced myocardial recovery with successful device explantation and maintain of left ventricular function 6 months postexplant.[26,27] Other clinical trials are currently ongoing.
The EVAHEART LVAD is a centrifugal pump characterized by a unique thromboresistant coating over its blood-coating surfaces that promises potential reduction of thromboembolic and bleeding events, which still constitute some of the most important adverse events and cause of death in patients undergoing VAD implantation. Japanese clinical experience in 14 BTT patients has demonstrated excellent efficacy and reliability with a 6-month survival of 91% and 1 and 2-year survival of 78%. The maximum support time reported was 866 days without device failure events and thrombus formation. The mortality rate was 14% (two patients) due to cerebral bleeding.
Expert Rev Cardiovasc Ther. 2009;7(9):1067-1077. © 2009 Expert Reviews Ltd.
Cite this: The Future is Here: Ventricular Assist Devices for the Failing Heart - Medscape - Sep 01, 2009.