A number of clinical, in vitro and computational studies have shown the potential for thromboembolic complications in bileaflet mechanical heart valves (BMHV), primarily due to the complex and unsteady flows in the valve hinges. These studies have focused on quantitative and qualitative parameters such as velocity magnitude, turbulent shear stresses, vortex formation and platelet activation to identify potential for blood damage. However, experimental characterization of the whole flow fields within the valve hinges has not yet been conducted. This information can be utilized to investigate instantaneous damage to blood elements and also to validate numerical studies focusing on the hinge’s complex fluid dynamics. The objective of this study was therefore to develop a high-resolution imaging system to characterize the flow fields and global velocity maps in a BMHV hinge. Subsequently, the present study investigated the effect of hinge gap width on flow fields in a St. Jude Medical BMHV. The results from this study suggest that the BMHV hinge design is a delicate balance between reduction of fluid shear stresses and areas of flow stasis during leakage flow, and needs to be optimized to ensure minimal thromboembolic complications. Overall, the current study demonstrates the ability of high-resolution Micro Particle Image Velocimetry to assess the fluid flow fields within the hinges of bileaflet mechanical heart valves, which can be extended to investigate micro-scale flow domains in critical regions of other cardiovascular devices to assess their blood damage potential.