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Development of pediatric ventricular assist devices (VADs) has significantly lagged behind that of adult devices. This frustrating reality is reflected by the fact that the Berlin Heart EXCOR VAD is currently the only approved pediatric-specific device in the USA. An alternative option is an off-label use of adult continuous-flow VADs, such as HeartMate II (HMII), which inevitably causes patient-device size mismatch in small children. We sought to conduct in vitro hemocompatibility testing in a pediatric flow condition, with a specific aim to provide benchmark values for future pediatric device development. Given the aforementioned fact that both pulsatile and continuous-flow devices are being used in the pediatric population, we opted to test both types of devices in the present study. The EXCOR and HMII blood pumps were tested using bovine blood under constant hemodynamic conditions (flow rate, Q = 2.5 ± 0.25L/min; differential pressure across the pump, ΔP = 68 ± 5mm Hg). Hemolysis was measured by Harboe assay. There was a steady increase in plasma free hemoglobin during in vitro testing, with a statistically significant difference between 5 and 360 min for both EXCOR (P 0.0001) and HMII (P 0.001). However, the degree of an increase in plasma free hemoglobin was more significant with HMII (P 0.001). Normalized index of hemolysis for EXCOR and HMII were 0.003 ± 0.0026g/100 L and 0.085 ± 0.0119g/100 L, respectively. There was also a steady increase in platelet activation detected by CAPP2A antibody using flow cytometry, with a statistically significant difference between 5 and 360 min for both devices (P 0.05). The degree of an increase in platelet activation was similar between the two devices (P = 0.218). High molecular weight von Willebrand factor (HMW vWF) multimer degradation measured by immunoblotting was evident for both devices, however, it was more pronounced with the EXCOR. EXCOR blood samples from all three time points (120, 240, and 360 min) were significantly different from the baseline (5 min), whereas only 360 min samples had a significant difference from the baseline with the HMII. In conclusion, we have observed similarities and differences in hemocompatibility profiles between the EXCOR and HMII, both of which are commonly used in the pediatric population. We anticipate the benchmark values in the present study will facilitate future pediatric VAD development.

Clinical outcomes from ventricular assist devices (VADs) have improved significantly during recent decades, but bleeding episodes remain a common complication of long-term VAD usage. Greater understanding of the effect of the shear stress in the VAD on platelet aggregation, which is influenced by the functional activity of high molecular weight (HMW) von Willebrand factor (vWF), could provide insight into these bleeding complications. However, because VAD shear rates are difficult to assess, there is a need for a model that enables controlled shear rates to first establish the relationship between shear rates and vWF damage. Secondly, if such a dependency exists, then it is relevant to establish a rapid and quantitative assay that can be used routinely for the safety assessment of new VADs in development. Therefore, the purpose of this study was to exert vWF to controlled levels of shear using a rheometer, and flow cytometry was used to investigate the shear-dependent effect on the functional activity of vWF. Human platelet-poor plasma (PPP) was subjected to different shear rate levels ranging from 0 to 8000/s for a period of 6 h using a rheometer. A simple and rapid flow cytometric assay was used to determine platelet aggregation in the presence of ristocetin cofactor as a readout for vWF activity. Platelet aggregates were visualized by confocal microscopy. Multimers of vWF were detected using gel electrophoresis and immunoblotting. The longer PPP was exposed to high shear, the greater the loss of HMW vWF multimers, and the lower the functional activity of vWF for platelet aggregation. Confocal microscopy revealed for the first time that platelet aggregates were smaller and more dispersed in postsheared PPP compared with nonsheared PPP. The loss of HMW vWF in postsheared PPP was demonstrated by immunoblotting. Smaller vWF platelet aggregates formed in response to shear stress might be a cause of bleeding in patients implanted with VADs. The methodological approaches used herein could be useful in the design of safer VADs and other blood handling devices. In particular, we have demonstrated a correlation between the loss of HMW vWF, analyzed by immunoblotting, with platelet aggregation, assessed by flow cytometry. This suggests that flow cytometry could replace conventional immunoblotting as a simple and rapid routine test for HMW vWF loss during in vitro testing of devices.