Classification of the Frequency, Severity, and Propagation of Thrombi in the HeartMate II Left Ventricular Assist Device.

Computational fluid dynamics has become a dynamic tool in the development of ventricular assist devices (VADs) and as a predictor of thrombosis within these pumps. The genesis of thrombi could be in loco, due to deposition within the VAD, or upstream such as the left atrial appendage or inside the left ventricle. To calibrate our group's computational model, a retrospective analysis of 29 explanted HeartMate II (HMII) VADs due to suspected pump thrombosis (PT) from the University of Michigan was conducted. Thrombi in these pumps were characterized by their frequency, composition, severity, and physical distribution in five regions of the blood flow pathway. The outlet bearing/stator region had the highest frequency of deposition (≈72%), and the preponderance of thrombi appeared white and unlaminated in their microstructure. Conversely, 41.3% of VADs showed thrombus on the fore bearing of the HMII, and these formations tended to be red and laminated, indicating they formed in layers over time. Furthermore, the majority of clots observed in the fore bearing and outlet bearing/stator regions were partially occlusive in nature. Fourteen VADs presented PT in multiple regions and analysis of the data showed a statistically significant correlation (p < 0.01) between deposition in the fore bearing and subsequent thrombosis in the outlet bearing/stator; however, no other regions exhibited statistically significant correlations. This gives credence to the hypothesis that thrombi do not occur independently in multiple regions of the blood flow pathway in the HMII but may propagate downstream.