The Next Generation of Ventricular Assist Devices
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Over the last decade, left ventricular assist devices (LVADs) have become a vital treatment in the advanced heart failure team's armamentarium, whether employed as a bridging therapy to cardiac transplantation or as long-term "destination therapy." The dramatic survival benefit with LVADs compared to medical therapy has led to a shift in the goals of LVAD care from simply surviving LVAD implantation to decreasing long-term morbidity and improving and maintaining end-organ function. As revealed in the ENDURANCE Destination Therapy trial, a randomized controlled trial of the HVAD (Heartware Inc., Framingham, MA) versus the Heartmate II (HMII) (Thoratec Corp., Pleasanton, CA), patients were plagued by adverse events including gastrointestinal bleeding, stroke, device thrombosis, and infection of the percutaneous driveline despite a greater than 70% two-year survival for both devices.1 Device companies and clinicians alike recognize that the adverse event profile of current LVAD technology is the chief impediment to expanding this therapy to a "less-sick" population. Fortunately, the nearly exponential increase in successful LVAD implants has instigated equally rapid advancements in LVAD technology such that many newer devices are poised to be evaluated in clinical trials. This article profiles two examples of next-generation LVADs the HeartMate 3 Left Ventricular Assist System and the Heartware MVAD.
HeartMate 3 Left Ventricular Assist System (LVAS)
The HeartMate 3 (HM3) LVAS (Thoratec Corp., Pleasanton, CA) is a centrifugal continuous-flow LVAD designed to improve hemocompatibility by decreasing platelet activation and red blood cell destruction in the hopes of reducing adverse events, such as bleeding, stroke, and pump thrombosis. The HM3 is significantly smaller than the HMII and employs a fully magnetically levitated rotor, even with no blood within the pump. Unlike a hydrodynamic bearing, there is no critical speed required to levitate the rotor, and as a result, the device provides a wide range of operating speeds capable of generating flows between 2.5 10.0 L/min. The rotor itself is uniquely designed with larger gaps (~1000 um) compared to current generation devices, which facilitate smoother transitions of blood through the pump, decreasing sheer stress and red blood cell trauma. The blood contacting surfaces are also textured to minimize blood component activation.
In response to concerns that a pulsatile circulation provides certain physiologic benefits, HM3 incorporates "artificial pulse" technology. Approximately every 2.0 seconds, the rotor speed will briefly drop below the set speed and then increase above the set speed, generating a flow pulse. The primary purpose of this feature is to allow complete washing of the pump and reduce stasis inside the rotor. Whether the small pulse pressure generated by this feature translates into less gastrointestinal bleeding or aortic insufficiency remains to be seen.
Additional features of the HM3 include improved ease of surgical implantation with a specifically engineered apical attachment for the pump, and a modular driveline that can be swapped out in the event of driveline malfunction without necessitating complete pump exchange. The HM3 is currently being implanted as an investigational device as part of the MOMENTUM 3 U.S. IDE Clinical Trial.2
Miniaturization of continuous-flow devices is propelled forward with the latest offering from Heartware Inc. (Framingham, MA), the MVAD is a mixed design pump with an axial impeller, but whose blood path exits the device perpendicular to the rotor's orientation. The pump is one-half the size of the existing HVAD and weighs only 92 grams. The pump continues to employ magnetic and hydrodynamic force to levitate the rotor. The impeller is made from a platinum alloy that is believed to be more biologically inert than the previous titanium alloys, and the pump has redesigned surfaces to reduce blood trauma and improve surface washing.
The search for pulsatility in a continuous-flow era continues; MVAD incorporates a technology termed qPulse. Within qPulse, there are four settings, including off, low, medium, and high, that the clinician can choose; this allows a reduction in rotor speed by a certain percentage over a specific time frame (e.g., 15% reduction in speed for five seconds at 10 second intervals). MVAD will have an operating speed between 12,000 and 20,000 rpm as the impeller size is smaller than earlier devices, but clinical settings are expected to be between 13,000 and 16,000 rpm. Similar to the HM3 device, peripheral components have been redesigned to include a modular driveline allowing controller/external driveline exchange without invasive surgery. Interestingly, a design similar to a telephone cable has been incorporated into the driveline to prevent direct trauma to the driveline exit site when patients inadvertently drop their controller. The controller will have the ability to directly accommodate a small and large battery without additional cables/cords; the batteries are estimated at 5 10 hours based on size and speed. Finally the controller has a new touch-screen interface with more patient friendly alarm features including a vibratory alarm.
Miniaturization of the durable device also promises alternative applications that will be tested in the upcoming years. These applications include transapical implantation with outflow graft placement across the aortic valve, transseptal placement into the left atrium and pediatric applications for the smallest recipients. The Conformité Européene (CE) mark clinical trial for MVAD began in the summer of 2015, with the U.S. Investigational Device Exemption (IDE) trial coming late in the winter of 2015/2016. Details are still forthcoming regarding study design for the U.S. trial, but the expected control device will likely be the HMII.
The newest generation of continuous-flow LVADs promises to be as mechanically durable as the generation of devices they will replace while still capable of providing full cardiac support. What remains to be seen is whether the devices' more biologically inert surfaces will reduce thrombosis risk, whether the unique designs improve flow characteristics and whether programmed (intermittent) pulsatility can further reduce adverse event rates. If the devices have an improved safety profile, the heart team can then begin to explore their use in the "less-sick" patient population.
- Pagani FD, Milano CA, Tatooles AJ, et al. HeartWare HVAD for the treatment of patients with advanced heart failure ineligible for cardiac transplantation: results of the ENDURANCE destination therapy trial. J Heart Lung Transplant 2015;34:S9.
- U.S. National Institutes of Health. MOMENTUM 3 IDE Clinical Study Protocol (HM3(TM)) (ClinicalTrials.gov website). 2015. Available at: https://www.clinicaltrials.gov/ct2/show/NCT02224755. Accessed 8/15/2015.
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