Effect of Hinge Gap Width of a St. Jude Medical Bileaflet Mechanical Heart Valve on Blood Damage Potential—An In Vitro Micro Particle Image Velocimetry Study

Despite the clinical success and widespread use of BMHVs, the reported incidence of thromboembolic complications is up to 4.3% per patient-years [1,2]. In BMHVs, clots are found in both upstream and downstream sides of the hinge geometry [2–4]. The hinge and pivot regions are known to cause blood element damage due to nonphysiological levels of low and high shear stresses within these regions [5–8]. However, methodologies for characterization of flow fields in the hinge region of a SJM BMHV is limited due to the 3D geometry involving in-plane butterfly-shaped design with curved-semicircular flow domain that has estimated depth and volume of 600 μm and 1.5 μl, respectively [9]. In addition, the temporal scales involved in leaflet opening and closure are within 20–40 ms, which requires optical setup with high temporal and spatial resolution, further limiting the choice of experimental approaches. Recently, Jun et al. [9] established an imaging system using 2D μPIV to obtain global flow fields in a BMHV hinge in aortic position during diastole. This study enabled detailed characterization of the hinge flow field that was difficult to achieve from previous experimental methods [6,10,11]. Novel features of this system included whole flow field characterization with high spatial resolution (64 μm), VSS fields, and a representation of the mating between the leaflet and hinge housing. The study focused on evaluating the blood damage potential during the diastolic phase in the aortic position, modeled under steady flow. Subsequently, the aortic diastolic leakage flow phase has been shown to have the highest thromboembolic potential of any phase of the cardiac cycle in BMHVs. However, high-resolution global velocity maps and flow characterization of different BMHV models under pulsatile conditions have not been investigated yet. Such information can allow better understanding of the blood damage potential from a fluid mechanics point of view by incorporating time varying acceleration of leaflet dynamics across three different hinge geometries [6,9–11]. To evaluate the performance and blood damage potential in BMHVs, parameters such as velocity magnitude, RSS, and VSS can be quantified from measured flow fields. The velocity magnitude can be used to assess the blood element washout capability of BMHV hinges. In addition, RSS and VSS levels can be correlated to shear stress induced blood damage [12–15]. Leverett et al. [14] demonstrated that red blood cell damage can be affected predominantly by VSS alone at thresholds of 150 N/m2. Subsequently, shear stress damage under turbulent flow was investigated by Lu et al. [15] using 2D laser Doppler anemometry, where a RSS level of 800 N/m2 was defined as an estimated threshold for hemolysis. Similarly, platelet activation threshold was also investigated, and VSS levels ranging from 30 to 100 N/m2 with exposure times of 25–1650 ms were shown to lead to platelet activation. The RSS threshold for platelet activation on a physiological time scale is still not clearly known [16]. Previous ex-vivo studies by Travis et al. [8] and Fallon et al. [5] evaluated markers of platelet damage and platelet activation within different BMHV pivot designs, hinge gap widths, and valve sizes. The studies reported significant differences in markers of platelet damage and coagulation levels between BMHVs with different hinge gap widths. However, these studies lacked detailed flow characterization to explain the observed blood damage levels. Subsequently, Leo et al. [6] performed laser Doppler velocimetry (LDV) measurements on SJM BMHVs with varying hinge gap widths under mitral flow condition and reported that the regular gap width showed less thromboembolic potential than prototypes with either a smaller or larger gap, based on qualitative flow fields, peak velocities, and RSS values. A numerical study by Simon et al. [17] revealed 3D flow fields under aortic flow condition comparing SJM BMHVs with larger and standard hinge gap widths. Simon et al. concluded that the larger hinge gap width indicated a lower propensity for platelet aggregation, due to the presence of elevated shear stresses and relatively poor washout during the leakage flow phase in the SJM BMHV with standard hinge gap width. In terms of the implant location, Simon et al. reported that the forward flow and leakage flow patterns were more favorable in the aortic position than the mitral position [17,18]. However, the analysis conducted using LDV relied on peak values of measured quantities due to low spatial resolution (203 μm). A major limitation with computational fluid dynamics (CFD) is that only a rotational motion with one degree of freedom can be included in the fluid–structure interaction model, which may not capture the translational effects of leaflet under pulsatile flow [17]. Therefore, the focus of the present study is to investigate the influence of hinge gap width on flow fields within BMHV hinges using a high-resolution μPIV system under pulsatile flow conditions and to relate these results to the potential to cause blood element damage and thrombotic complications. These findings can provide a better understanding of the local regions of potential blood element damage within the BMHV hinge, in combination with previous results from in vitro LDV and CFD studies [6,11,17]. In addition, the new PIV flow field data may be used for the validation of the CFD approaches developed for characterizing the BMHV hinge flow fields [17,19].