High-speed particle image velocimetry to assess cardiac fluid dynamics in vitro: From performance to validation
Introduction
Abnormality in cardiac fluid dynamics is highly correlated with several heart conditions. This is particularly true in valvular heart diseases and congenital heart defects where changes in flow-field accompany significant variations in chambers’ pressure gradients. Quantifying the flow inside the heart conveys useful information regarding cardiac pumping function and its efficiency. Nevertheless, technological limitations related to imaging modalities sometimes preclude obtaining comprehensive information on intracardiac flow-fields attributable to diseases or implanted devices. The opacity of the cardiac chambers restrains technologies from using light sources to directly image the flow non-invasively. Furthermore, current imaging modalities such as echocardiography and magnetic resonance imaging (MRI) cannot achieve high temporal/spatial resolution required for in vivo imaging of the fast-paced intracardiac flow.
Particle Image Velocimetry (PIV) is a convenient technique for assessing cardiac fluid dynamics in vitro. With Digital PIV (DPIV), it is possible to quantitatively differentiate between normal and abnormal intracardiac flow fields in mock-up cardiac chambers. Additionally, this technique can be used to validate other imaging modalities with lower temporal/spatial resolution such as echocardiography and MRI. Since the natural cycle of the heart happens in less than one second, high-speed DPIV is a proper tool in providing accurate details for cardiovascular flow. To obtain reliable results, it is essential to precisely replicate the cardiac flow-field in vitro, which has been a major concern since early seventies [1]. Obtaining dependable results using high-speed DPIV is only possible along with an experimental setup that reproducibly generates a realistic environment similar to the heart chambers.
Different configurations have been described that imitate partial functionality of the cardiovascular system along with an imaging system to capture the flow-field [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Understanding the flow-field inside the heart chambers is challenging due to the fast pace of the flow, three dimensionality of the events, and complex deformability of the heart chambers that highly depends on compliance. As a result, lower speed imaging systems utilized in previous studies [6], [7], [8], [9], [10] are limited in extracting only particular details of the flow. Some investigators tried to overcome the frame-rate limitations of their imaging systems by averaging the velocity frames over multiple cardiac cycles [11], [12]. This approach suffers from the fact that no heart beat is similar to the other one, particularly with respect to the randomness of particle field and their 3D paths.
Here, we describe our novel experimental system that is particularly designed and developed for in vitro investigation of intracardiac flow fields. Pulsatile flow is developed through the systemic circulation using a transparent, compliant ventricular model with either prosthetic or bioprosthetic heart valves at mitral and aortic positions. Along with the heart-flow simulator, we use a high-speed, high-resolution DPIV system that is custom-designed at our laboratory. In this work, the fluid dynamics downstream of a novel mitral bioprosthesis is discussed as an example of the experiments being performed with this system.
Section snippets
Methods
We have previously developed a mitral bioprosthesis with dynamic saddle annulus that imitates natural mitral valve motion [14]. Two different prototypes of this heart valve were extensively evaluated in our heart-flow simulator with the aid of high-speed DPIV. The steps taken for assessment of fluid dynamics downstream these valves are described here. We have particularly focused on evolution of transvalvular vortex formation, Reynolds stress distribution, particle residence time, and kinetic
Results and discussions
In this section, we report the results of post-processing for both mitral prototypes. Although we emphasize on the difference in flow features between the two prototypes, the main focus of this article is on the methods developed for analyzing the flow based on high-speed imaging in a heart-flow simulator.
LV Velocity fields: The 70 beats-per-minute waveform resulted in 857 velocity frames with 1 ms time difference for a cardiac cycle. One velocity frame obtained during end-diastole is shown in
Conclusion
Artificial heart valves have been in use in United States since 1960, where many different models were granted market approval by the Food and Drug Administration (FDA). Engineering principles applied to the design and development of heart valves along with laboratory testing to assess the flow, leak, strength, and durability, resulted in different models of heart valves successfully implanted for the past 40 years. Performance of these valves is crucial to improve the quality of life and
Limitation
The silicone LV model cannot generate the twisting motion similar to the left ventricle, which may affect the flow-field. DPIV technique is performed in 2D, and cannot estimate the out-of-plane velocity. Hence, due to the complexity and three-dimensionality of the flow inside the LV model, variations in kinetic energy of the flow may be significantly affected if the third dimension of flow is also considered. The transmitral vortex ring is a three-dimensional flow structure whose 2D
Acknowledgments
This work was partially supported by the American Heart Association award No. 10BGIA4170011 and a Translational Research Award from Wallace H. Coulter Foundation to AK.
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