Aspirin has limited ability to modulate shear-mediated platelet activation associated with elevated shear stress of ventricular assist devices

Abstract

Continuous flow ventricular assist devices (cfVADs) while effective in advanced heart failure, remain plagued by thrombosis related to abnormal flows and elevated shear stress. To limit cfVAD thrombosis, patients utilize complex anti-thrombotic regimens built upon a foundation of aspirin (ASA). While much data exists on ASA as a modulator of biochemically-mediated platelet activation, limited data exists as to the efficacy of ASA as a means of limiting shear-mediated platelet activation, particularly under elevated shear stress common within cfVADs. We investigated the ability of ASA (20, 25 and 125 μM) to limit shear-mediated platelet activation under conditions of: 1) constant shear stress (30 dynes/cm(2) and 70 dynes/cm(2)); 2) dynamic shear stress, and 3) initial high shear exposure (70 dynes/cm(2)) followed by low shear exposure - i.e. a platelet sensitization protocol, utilizing a hemodynamic shearing device providing uniform shear stress in vitro. The efficacy of ASA to limit platelet activation mediated via passage through a clinical cfVAD system (DeBakey Micromed) in vitro was also studied. ASA reduced platelet activation only under conditions of low shear stress (38% reduction compared to control, n=10, p<0.004), with minimal protection at higher shear stress and under dynamic conditions (n=10, p>0.5) with no limitation of platelet sensitization. ASA had limited ability (25.6% reduction in platelet activation rate) to modulate shear-mediated platelet activation induced via cfVAD passage. These findings, while performed under "deconstructed" non-clinical conditions by utilizing purified platelets alone in vitro, provide a potential contributory mechanistic explanation for the persistent thrombosis rates experienced clinically in cfVAD patients despite ASA therapy. An opportunity exists to develop enhanced pharmacologic strategies to limit shear-mediated platelet activation at elevated shear levels associated with mechanical circulatory support devices.

Keywords: Aspirin; Mechanical circulatory support; Platelets; Shear; Thrombosis; Ventricular assist devices.

Copyright © 2016 Elsevier Ltd. All rights reserved.

Figures

Figure 1. Constant and dynamic shear stress waveforms

Gel-filtered platelets (GFP) were exposed to a) variable magnitudes of constant shear stress for variable durations and b) dynamic shear stress waveforms representing a single exposure in the DeBakey VAD.

Figure 2. The effect of ASA on platelets exposed to arachidonic acid (AA)

a) Platelet activation in time after treatment with AA alone (black), ASA and then AA (gray) or ASA alone (white). b) Platelet activation rate (PAR) after 10 min in the same conditions as a) (n = 4, *p < 0.01).

Figure 3. The effect of ASA on constant shear-mediated platelet activation

The effect of ASA (25 or 125 μM) was investigated after exposure to a) 30 dyne/cm2 and b) 70 dyne/cm2 (n = 10).

Figure 4. The effect of ASA on dynamic shear-mediated platelet activation

The effect of ASA (25 or 125 μM) was investigated after exposure to the dynamic waveforms extracted from simulation in the DeBakey VAD corresponding to the a) 30th (Dynamic_30%) and b) 50th percentile (Dynamic_50%) of the PDF function (n=10).

Figure 5. Platelet sensitization after in vitro ASA treatment

a) For both 20 μM ASA-treated and control platelets, PAS was measured after initial 40 s exposure to 70 dyne/cm2, and during subsequent exposure 1 dyne/cm2. b) Sensitization PAR rates for ASA-treated and control platelets were calculated from PAS values subsequent to 70 dyne/cm2 and 1 dyne/cm2 pre-exposure (n = 10, *p < 0.01).

Figure 6. Platelet activation in DeBakey VAD 2h post-in vivo ASA treatment

a) PAS was measured for 1000 mg ASA-treated platelets and untreated platelets recirculated for 30 min through the VAD, with b) PAR determined from the slope of lines fit to PAS values (n = 7, p < 0.001).