Experimental Flow Field Assessment – Digital Particle Image Velocimetry (DPIV)

The thrombogenic and haemolytic potential of the heart valve substitute can be assessed using a combination characterization techniques: digital particle image velocimetry (DPIV), computational fluid dynamics (CFD), and ex-vivo methods (e.g. blood loops).

DPIV (using digital particle image velocimetry) is an experimental flow field assessment technique that it is used to determine areas of high shear stress and stagnation in the vicinity of prosthetic valve when subjected to hypo, normal an hypertensive flow physiological flow conditions.  Under any flow conditions, when a device is positioned in a vessel, it will modify the natural flow field which can generate high shear stress (inducing haemolysis) and/or stagnation areas (inducing thrombosis).

The ViVitro Labs DPIV procedure and equipment can be used to test prosthetic valve and many other endovascular devices. DPIV test method has been validated with the ViVitro Labs Pulse duplicator using a standard configuration.  Test method would need to be validated for other configuration.

Test Procedure

Velocity maps of the flow in 3 parallel planes in the immediate vicinity of the prosthetic cardiac valve with pressure and flow waveforms conditions consistent with a low and elevated cardiac output (e.g. 2 l/min and 7 l/min) at 70 beats per min as listed in table 3 of ISO 5840-1. A PIV Laser (65mJ, 15 Hz) is mounted on a traverse micrometric displacement system, and a camera is set perpendicular to the laser sheet. For each plane, image pairs (phase-locked, double-pulse images) are acquired every 0.0536 s (16 phases per cardiac cycle) and repeated 40 times (or more), which has been found sufficient to reach a statistical convergence for the velocities. The test fluid is seeded with particles. Particle area density is controlled, and pulse separation time (∆t) is adjusted to ensure quality cross-correlation results. The PIV system is synchronized with the pressure-flow measurements using an external trigger delivered by ViVitro SuperPump controller.

Test Method Validation

• An interoperator study was performed with 2 operators and 2 PIV calibrations. Each operator captured 10 groups of 40 cycles at nominal conditions.
• Velocities measured by the PIV system were converted to flow and compared to the results of the flowmeter. This analysis was performed on the interoperator data that was used for the measurement uncertainty.
• Factors influencing the results were identified and controlled: Temperature of test fluid, Particle density, Particle displacement and S/N ratio, Fluid viscosity, Time delay between images, Laser and camera alignment, Flow rate, Ambient light.
• Using the results of the interoperator study, an uncertainty budget was defined for the measurement of velocity by the PIV system. The expanded velocity uncertainty with coverage factor k=95% is ±0.07m/s

Equipment

• ViVitro Pulse Duplicator + specific valve fixtures (specific site section)
• Aortic PIV test chamber or other defined by study sponsor
• Aortic PIV test chamber (surgical valve only) – customization required for any other device
• Laser, 65 mJ, 15 Hz, Litron, Nano S 65-15 PIV
• Camera, Andor Zyla 5.5
• Light sheet optics 1.5” BSL/NWL/Evergreen
• PIV software DynamicStudio (current version)
• Synchronizer
• Computer

Measured Parameters

• Velocity
• Shear stress (viscous shear stress, Reynolds shear stress)
• Regions of stagnation
• Qualitative analysis: Turbulent flow region, flow separation, vortex formation, jets
• Calculated particle residence time (if required)

Scope

Applications

• Transcatheter valves; TAVI, TMVR
• Biological, Polymeric, Mechanical valves: Rigid or soft
• Venous valves and conduits
• Valvular conduits
• Vena Cava Filters
• Assist ventricular devices
• Any device implant in flow models

Service Levels

• Standard Service
• Full Service with ISO 17025

Applicable Standards

• ISO 5840-2:2021
  Cardiovascular implants — Cardiac valve prostheses — Part 2: Surgically implanted heart valve substitutes

• ISO 5840-3:2021
  Cardiovascular implants — Cardiac valve prostheses — Part 3: Heart valve substitutes implanted by transcatheter techniques

• ISO 5840-1:2021
  Cardiovascular implants — Cardiac valve prostheses — Part 1: General requirements

Related Tests

• Pulsatile Flow Testing – Hydrodynamic Performance
• Steady Flow Testing – Hydrodynamic Performance
• Accelerated Wear Testing (AWT)
• Experimental Flow Field Assessment – Echo Doppler – Bernoulli’s Equation Verification

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