Thromboembolism and the attendant risk of cardioembolic stroke remains an impediment to the development of prosthetic cardiovascular devices. In particular, altered haemodynamics are implicated in the acute blood cell damage that leads to thromboembolic complications, with platelet activation being the underlying mechanism for cardioemboli formation in blood flow past mechanical heart valves (MHVs) and other blood re-circulating devices. In this work, a new modeling paradigm for evaluating the cardioembolic risk of MHVs is described. In silico fluid-structure interaction (FSI) approach is used for providing a realistic representation of the flow through a bileaflet MHV model, and a Lagrangian analysis is adopted for characterizing the mechanism of mechanically induced activation of platelets by means of a mathematical model for platelet activation state prediction. Additionally, the relationship between the thromboembolic potency of the device and the local flow dynamics is quantified by giving a measure of the role played by the local streamwise and spanwise vorticity components. Our methodology indicates that (i) mechanically induced activation of platelets when passing through the valve is dependent on the phase of the cardiac cycle, where the platelet rate of activation is lower at early systole than late systole; (ii) local spanwise vorticity has greater influence on the activation of platelets (R>or=0.94) than streamwise vorticity (R>or=0.78). In conclusion, an integrated Lagrangian description of key flow characteristics could provide a more complete and quantitative picture of blood flow through MHVs and its potential to activate platelets: the proposed "comprehensive scale" approach could represent an efficient and novel assessment tool for MHV performance and may possibly lead to improved valve designs.