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Research Papers

Thromboresistance Comparison of the HeartMate II Ventricular Assist Device With the Device Thrombogenicity Emulation-Optimized HeartAssist 5 VAD

[+] Author and Article Information
Wei-Che Chiu, Gaurav Girdhar, Michalis Xenos, Yared Alemu, Jõao S. Soares, Shmuel Einav

Department of Biomedical Engineering,
Stony Brook University,
Stony Brook, NY 11794-8151

Marvin Slepian

Department of Biomedical Engineering,
Stony Brook University,
Stony Brook, NY 11794-8151;
Sarver Heart Center,
University of Arizona,
Tucson, AZ 85724

Danny Bluestein

Department of Biomedical Engineering,
Stony Brook University,
Stony Brook, NY 11794-8151
e-mail: danny.bluestein@stonybrook.edu

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received September 8, 2013; final manuscript received December 10, 2013; accepted manuscript posted December 16, 2013; published online February 5, 2014. Editor: Victor H. Barocas.

J Biomech Eng 136(2), 021014 (Feb 05, 2014) (9 pages) Paper No: BIO-13-1418; doi: 10.1115/1.4026254 History: Received September 08, 2013; Revised December 10, 2013; Accepted December 16, 2013

Approximately 7.5 × 106 patients in the US currently suffer from end-stage heart failure. The FDA has recently approved the designations of the Thoratec HeartMate II ventricular assist device (VAD) for both bridge-to-transplant and destination therapy (DT) due to its mechanical durability and improved hemodynamics. However, incidence of pump thrombosis and thromboembolic events remains high, and the life-long complex pharmacological regimens are mandatory in its VAD recipients. We have previously successfully applied our device thrombogenicity emulation (DTE) methodology for optimizing device thromboresistance to the Micromed Debakey VAD, and demonstrated that optimizing device features implicated in exposing blood to elevated shear stresses and exposure times significantly reduces shear-induced platelet activation and significantly improves the device thromboresistance. In the present study, we compared the thrombogenicity of the FDA-approved HeartMate II VAD with the DTE-optimized Debakey VAD (now labeled HeartAssist 5). With quantitative probability density functions of the stress accumulation along large number of platelet trajectories within each device which were extracted from numerical flow simulations in each device, and through measurements of platelet activation rates in recirculation flow loops, we specifically show that: (a) Platelets flowing through the HeartAssist 5 are exposed to significantly lower stress accumulation that lead to platelet activation than the HeartMate II, especially at the impeller-shroud gap regions (b) Thrombus formation patterns observed in the HeartMate II are absent in the HeartAssist 5 (c) Platelet activation rates (PAR) measured in vitro with the VADs mounted in recirculation flow-loops show a 2.5-fold significantly higher PAR value for the HeartMate II. This head to head thrombogenic performance comparative study of the two VADs, one optimized with the DTE methodology and one FDA-approved, demonstrates the efficacy of the DTE methodology for drastically reducing the device thrombogenic potential, validating the need for a robust in silico/in vitro optimization methodology for improving cardiovascular devices thromboresistance.

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Figures

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Fig. 1

Illustrations of the exterior and interior features of (a) HMII and (b) HA5 and (c) Flow-loop for the in vitro experiments. The exterior features of HMII (a)i and HA5 (b)i; the length of the VADs are 71 mm and 81 mm, and the maximum diameter are 30 mm and 43 mm for HA5 and HMII, respectively. The interior features and the detailed components of HMII (a)ii and HA5 (b)ii; both VADs share similar overall design, which encloses the stationary flow straightener and diffuser, a single rotational impeller—both impellers actuated via electromagnetic fields. The flow straighteners for both VADs are composed of three blades, 120 deg apart. The spinning direction of the impeller of HMII and HA5 are opposite. 90 ml of flow-loops (c) were assembled for the in vitro experiments, consisting of two tube segments with inner diameter of 1/2" connected to the VADs, and a segment of flow-resistor-tube, with the inner diameter of 1/4", was connected to the flow-tubes with two tapered flow reducers, combining in series two reducing connectors (1/4"–3/8" and 3/8"–1/2").

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Fig. 2

Illustrations of mesh elements and the seeded platelet particles for the simulations of HA5 (a) and HMII (b) and the ROIs' definitions of HMII (c) and HA5 (d)- (a)i and (b)i indicates the side views of the volume meshes of the HeartAssist 5 (HA5) and HeartMate II (HMII) simulations, and the red dashed line indicates the mesh interfaces between the stationary and moving volume meshes. The HA5 mesh had approximately 9.3 × 106 elements, and the HMII mesh had approximately 17 × 106 elements. The yellow arrows indicate the spinning direction of the moving volume meshes, and the red arrows indicate the flow direction for both VADs- the HA5 spins clockwise along the flow, while the HMII spins counterclockwise. (a)ii and (b)ii approximately 31,000 and 32,000 platelets (red particles) were seeded and released from the upstream of the HA5 and HMII, accordingly. (c) and (d) six different ROIs were defined for both VADs (i.e., flow straightener ROI (red), rear hub ROI (yellow), impeller-shroud gap ROI (purple), impeller blade ROI (green), front hub ROI (orange), and diffuser ROI (navy blue)). The flow straightener ROI was defined from the entry tip to the tail of flow straightener blades as cylinder. The rear hub ROI was defined as the cylindrical area from the tail of the flow straightener blades to the head of impeller blades. The impeller-shroud gap ROI was defined as the annular area between the tip of the impeller blades and the shroud. The impeller blade ROI was considered as the area between each impeller blades. The front hub ROI covered the cylindrical area from the tail of impeller blades to the head of the diffuser blades. The cylindrical area from the head of the diffuser blades to the exit tip of the diffuser was defined as the diffuser ROI.

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Fig. 3

PDF Results from the Overall Devices of HMII and HA5- The PDF results of the overall regions of HMII and HA5 populated a similar slow SA range of both VADs; however, the distribution of the tail region (riskier high SA) showed higher thrombogenic potential in the HMII

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Fig. 4

PDF Results at ROI regions—PDF results obtained from the flow straightener ROI populated a similar low SA range in both VADs, and similar tail region distributions were found. The PDF results from the rear hub ROI populated a similar low SA range between two VADs; however, higher probability density (thrombogenic potential) was found at the HMII tail region comparing to the distribution of the HA5 tail region. Very different PDF distributions between the HMII and HA5 were found at the impeller-shroud gap ROI; the main mode of the HA5 is left-shifted (toward lower SA range) compared to the HMII, and higher distribution at the HMII tail region (riskier high SA range) comparing to the HA5. PDF results from the impeller blade ROI indicated that the main modes of both VADs populated a similar low SA range; however, the tail region of HMII's PDF showed higher density at higher SA range than the HA5. The PDF results from the front hub ROI showed that the main mode of HMII was right-shifted (toward higher SA range) comparing to HA5's main mode, and an additional mode was observed at the tail region of HMII comparing to HA5. The PDF result from the diffuser ROI indicated the main mode of HA5 was right-shifted compared to HMII, but very similar distribution was found at the tail region of both VADs.

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Fig. 5

Stagnant platelet trajectories and recirculation zones in HMII and HA5- (a) The recirculation zones and stagnant platelet trajectories were observed at the downstream of the flow straightener blades of HMII, with approximately 2 mm of the average eddy diameter. (b) Several entrapped platelet trajectories were observed at the entry of the impeller blades of the HMII, and closely following the rotational motion of impeller. (c) Fewer stagnant trajectories were observed at the rear hub region of the HA5 comparing to the HMII, and no recirculation zone was found. (d) No entrapped platelet trajectory was observed at the entry of the HA5 impeller. These platelet trajectories were represented as a time average of their residence time (184 ms).

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Fig. 6

2.5-fold differences of the thrombogenicity between HMII and HA5- The platelet activation rates (PARs) of HMII (red) and HA5 (blue) after 30 min flow-loop experiments indicated significant 2.fivefold differences. The PARs of HMII and HA5 were 1 × 10−4/min and 4 × 10−5/min, accordingly (p < 0.05; n = 10).

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Fig. 7

Markedly lower stress accumulation of platelet trajectories flow through impeller-shroud gap of HA5 representative shear stress and exposure time of five platelet trajectories flow through the impeller-shroud gap of (a) HMII and (b) HA5 VADs are shown. The platelet trajectories flowing through the HA5 has lower shear stress magnitude and shorter exposure time comparing to the trajectories flowing through the HMII.

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