Fig. 52.1
The HeartMate III LVAS implanted and external components
The Full MagLev rotor eliminates the need for fluid or mechanical bearings, thereby avoiding wear of the single moving component. A single stator with back-iron poles, copper coils, and position sensors controls the rotation and levitation of the rotor. The radial position and rotational speed of the rotor are actively and independently controlled by measuring the position of a permanent magnet in the rotor and controlling the current in the drive and levitation coils. The attraction of the rotor’s permanent magnets to the iron poles passively resists movement of the rotor in the axial direction. There are relatively large blood flow gaps between the rotor and the housing (◘ Fig. 52.2). The gaps on the side of the rotor (radial) are approximately 0.5 mm; on the top and bottom (axial), the gap is 1.0 mm, which is 10–20 times greater than with a hydrodynamic bearing. Computational fluid dynamic analysis demonstrates well-organized flow fields across a wide range of flow (2–10 L/min), and surface shear forces are low compared to other types of pumps. An additional benefit of Full MagLev technology is that the large blood flow gaps are maintained regardless of rotor speed, even when not rotating. Therefore, it is possible to operate the pump at low speeds, which may be important for partial left ventricular assistance, right ventricular assistance, or weaning from support.
Fig. 52.2
Internal diagram of the HeartMate III showing the blood flow gaps and the magnetic fields
The pump is designed with large blood flow gaps to avoid stasis of blood and to reduce damage to blood components. The low hydraulic resistance in the gaps avoids stasis of blood in those regions. Low shear stresses reduce damage to blood components, which can minimize adverse events such as thromboembolism, hemolysis, and bleeding. Furthermore, since minor radial and axial divergence of the rotor is acceptable, levitation is maintained during patient physical activities, and an artificial pulse is facilitated. The artificial pulse is created as the rotor speed periodically decreases and increases from the set speed, permitting pressure and flow changes resulting from the LVAD (◘ Fig. 52.3). The preset artificial pulse rate is 30 times per minute and is asynchronous with respect to the heart.
Fig. 52.3
A diagram showing the artificial pulse generated by the HeartMate III control
It has been established that a pulse during support with a continuous-flow LVAD is not necessary for adequate organ function and survival, but there are possible advantages for having some arterial pulsatility [6]. Changing flow within a rotary pump allows for a more complete washing of the blood-contacting surfaces for prevention of stasis. An artificial pulse may also help to avert adverse events, such as aortic insufficiency and bleeding from arteriovenous malformations.
In addition to reduction of shear and increased washing of the pump surfaces, enhanced hemocompatibility is also achieved by texturing internal pump surfaces, except for the rotor and rotor well. Surfaces textured with sintered titanium microspheres promote the adhesion of circulating cells and the development of a stable biological lining that reduces thromboembolic risk and the level of required anticoagulation therapy.
52.3 Clinical Experience
A multicenter clinical study evaluating the HeartMate III LVAS to meet the CE Mark requirements for clinical approval has been completed. CE Mark approval was granted in October of 2015. The trial involved 50 patients at ten centers in six countries and involved patients needing bridge-to-transplant support or support as destination therapy [7, 8]. The trial was designed to include “all-comers” with severe heart failure defined as New York Heart Association (NYHA) class IIIB or IV; ACC/AHA Stage D heart failure; ejection fraction <25%, cardiac index <2.2 l/min/m2 without inotropes, or inotrope dependent on optimal medical management; or listed for heart transplant.
At the 6-month study end point, the performance goal of 88% survival was surpassed, with 92% of patients alive. Compared to the Seattle Heart Failure model, support with the HeartMate III reduced mortality risk by 66%. Two patients underwent successful heart transplant. There was a statistically significant improvement in NYHA class (p < 0.0001), quality of life (p < 0.001), and 6-minute walk distance (p < 0.0001). Adverse events included stroke and infection; however, there were no instances of device thrombosis or failure, and there was no hemolysis. In the USA, the 6-month results of a randomized multicenter study have recently been released. The Multicenter Study of MagLev Technology in Patients Undergoing Mechanical Circulatory Support Therapy with HeartMate III (MOMENTUM 3) has been conducted to compare clinical outcomes with the Full MagLev™ centrifugal flow HeartMate III LVAS versus the axial-flow HeartMate II LVAS in patients with advanced heart failure refractory to standard medical therapy. Patients were randomly assigned, in a 1:1 ratio, to receive the centrifugal-flow pump or the axial-flow pump. From September 2014 through October 2015, a total of 294 patients underwent randomization; 152 patients were assigned to the HeartMate III group and 142 to the HeartMate II group. One patient in the centrifugal-flow pump group and four in the axial-flow pump group did not undergo implantation in accordance with the protocol inclusion criteria. The remaining patients – 151 who underwent implantation of the centrifugal-flow pump and 138 who underwent implantation of the axial-flow pump – were included in the per-protocol population. The primary end point of event-free survival at 6 months was achieved in a higher percentage of patients in the centrifugal-flow pump group than in the axial-flow pump group (86.2% versus 76.8%). Study results demonstrated that non-inferiority was established of the HeartMate III LVAS as compared to the HeartMate II LVAS (absolute difference, 9.4 percentage points; 95% lower confidence boundary, −2.1; P < 0.001 for non-inferiority). The trial also specified a test for superiority, and it was demonstrated that at 6 months, the HeartMate III LVAS was superior to the HeartMate II LVAS (hazard ratio, 0.55; 95% confidence interval [CI], 0.32–0.95; two-tailed P = 0.04 for superiority). The rate of reoperation for pump malfunction was significantly lower in the HeartMate III group than in the HeartMate II. Only one patient (0.7%; 95% CI, 0 to 3.6) in the HeartMate III group underwent pump replacement (electrical malfunction), whereas 11 patients (7.7%, 95% CI, 3.9–13.4) in the HeartMate II group underwent either a device exchange (nine patients) or device removal with urgent transplantation (two patients) (P = 0.002) as a result of pump thrombosis. No significant differences between the two groups in the rates of death or disabling stroke resulted. The Kaplan-Meier estimate of the rate of actuarial event-free survival was significantly higher in the HeartMate III group (86%; 95% CI, 80–92) than in the HeartMate II (77%; 95% CI, 70–84; two-tailed P = 0.03 by the log-rank test) (◘ Fig. 52.4). No patients in the centrifugal-flow pump group had suspected or confirmed pump thrombosis, whereas 14 patients (10.1%) in the axial-flow pump group suffered from 18 thrombotic events (P < 0.001). The data from the MOMENTUM 3 show that implantation of the fully magnetically levitated centrifugal continuous-flow pump HeartMate III was associated with a higher rate of survival-free of disabling stroke or survival-free of reoperation at 6 months after implantation compared to implantation of the continuous-flow pump HeartMate II among patients with advanced heart failure, irrespective of their eligibility for transplantation. The incremental benefits associated with the centrifugal-flow pump observed in this 6-month analysis were due to the absence of suspected or confirmed pump thrombosis leading to surgical pump exchange or urgent transplantation [9].
