Three-dimensional extent of flow stagnation in transcatheter heart valves

Abstract

<jats:p> The recent unexpected discovery of thrombosis in transcatheter heart valves (THVs) has led to increased concerns of long-term valve durability. Based on the clinical evidence combined with Virchow's triad, the primary hypothesis is that low-velocity blood flow around the valve could be a primary cause for thrombosis. However, due to limited optical access in such unsteady three-dimensional biomedical flows, measurements are challenging. In this study, for the first time, we employ a novel single camera volumetric velocimetry technique to investigate unsteady three-dimensional cardiovascular flows. Validation of the novel volumetric velocimetry technique with standard planar particle image velocimetry (PIV) technique demonstrated the feasibility of adopting this new technique to investigate biomedical flows. This technique was used to quantify the three-dimensional velocity field in the vicinity of a validated, custom developed, transparent THV in a bench-top pulsatile flow loop. Large volumetric regions of flow stagnation were observed in the neo-sinus throughout the cardiac cycle, with stagnation defined as a velocity magnitude lower than 0.05 m s <jats:sup>−1</jats:sup> . The volumetric scalar viscous shear stress quantified via the three-dimensional shear stress tensor was within the range of low shear-inducing thrombosis observed in the literature. Such high-fidelity volumetric quantitative data and novel imaging techniques used to obtain it will enable fundamental investigation of heart valve thrombosis in addition to providing a reliable and robust database for validation of computational tools. </jats:p>

In Vitro Thrombogenicity Testing of Biomaterials

“…Short‐term and continuous supraphysiologic shear stresses—, e.g., in artificial heart valves, stents, blood pumps and dialysis tubings—can induce conformational changes in blood plasma proteins (e.g., von Willebrand factor), platelet and leukocyte activation (e.g., platelet‐leukocyte aggregation) as well as lysis of platelets (release of soluble activators and microparticle formation) and erythrocytes (hemolysis) . On the other hand, low shear forces, recirculation and areas of stagnation—occurring, e.g., in the neo‐sinus of transcatheter heart valves and in secondary flow areas of centrifugal blood pumps—can also induce thrombotic processes . Evaluating these processes under appropriate (and varying) shear conditions is particularly relevant when the device reached a certain level of development since hemodynamics and device geometry can influence the blood–material interactions in the near vicinity but also for downstream activation processes …”

On the Impact of Subaperture Sampling for Multispectral Scalar Field Measurements

“…The result is a compact camera capable of sampling volumetric information in instantaneous snapshots, thereby enabling many possibilities in the realm of flow diagnostics. The Advanced Flow Diagnostics Laboratory (AFDL) of Auburn University has constructed many plenoptic cameras for use in PIV [16][17][18][19][20] and PTV [21][22][23], with a recent focus on scalar-field diagnostics, such as background-oriented schlieren [24], chemiluminescence [25], and pyrometry [25]. Although volumetric reconstructions can be performed using a thin-lens assumption, a volumetric calibration procedure provides many advantages.…”

“…Thus, every camera samples both angular and spatial content (i.e., the light field). A few methods exist to capture multiple views per camera: (1) fiber optic bundles that branch from the main sensor with independent optics and mounting, effectively creating miniature cameras, have been used for optical coherence tomography [5,6] and background oriented schlieren (BOS) [7]; (2) relay prisms and mirrors in front of the primary objective produce either a stereoscope [8] or a quadscope [9], which have been demonstrated for a variety of high-speed diagnostic techniques, including tomo-PIV [10,11], LIF [9,12], and multi-color LIF [13,14]; (3) insertion of a microlens array between the primary objective and imaging sensor creates a plenoptic camera [15], which has been used for a wide variety of applications, including PIV [16][17][18][19][20], PTV [21][22][23], background-oriented schlieren [24], chemiluminescence [25], and pyrometry [25].…”

Hemodynamics inside the neo‐ and native sinus after TAVR: Effects of implant depth and cardiac output on flow field and coronary flow

“…A lower THV implant depth favors flow stasis in the neo‐sinus (the space between the native and the THV leaflets). This increases thrombosis severity 14,15 and enhances leaflet thrombosis 3,16‐18 found in as many as 40% of patients 16,18,19 . Latter reduces leaflet motion and worsens valve hemodynamics 14 .…”

“…Latter reduces leaflet motion and worsens valve hemodynamics 14 . These findings have led to increased concerns of ischemic attacks, stroke, and long‐term valve durability 19,20 …”