Ned H. C. Hwang

Particle image velocimetry study of pulsatile flow in bi-leaflet mechanical heart valves with image compensation method

Particle Image Velocimetry (PIV) is an important technique in studying blood flow in heart valves. Previous PIV studies of flow around prosthetic heart valves had different research concentrations, and thus never provided the physical flow field pictures in a complete heart cycle, which compromised their pertinence for a better understanding of the valvular mechanism. In this study, a digital PIV (DPIV) investigation was carried out with improved accuracy, to analyse the pulsatile flow field around the bi-leaflet mechanical heart valve (MHV) in a complete heart cycle. For this purpose a pulsatile flow test rig was constructed to provide the necessary in vitro test environment, and the flow field around a St. Jude size 29 bi-leaflet MHV and a similar MHV model were studied under a simulated physiological pressure waveform with flow rate of 5.2xa0l/min and pulse rate at 72xa0beats/min. A phase-locking method was applied to gate the dynamic process of valve leaflet motions. A special image-processing program was applied to eliminate optical distortion caused by the difference in refractive indexes between the blood analogue fluid and the test section. Results clearly showed that, due to the presence of the two leaflets, the valvular flow conduit was partitioned into three flow channels. In the opening process, flow in the two side channels was first to develop under the presence of the forward pressure gradient. The flow in the central channel was developed much later at about the mid-stage of the opening process. Forward flows in all three channels were observed at the late stage of the opening process. At the early closing process, a backward flow developed first in the central channel. Under the influence of the reverse pressure gradient, the flow in the central channel first appeared to be disturbed, which was then transformed into backward flow. The backward flow in the central channel was found to be the main driving factor for the leaflet rotation in the valve closing process. After the valve was fully closed, local flow activities in the proximity of the valve region persisted for a certain time before slowly dying out. In both the valve opening and closing processes, maximum velocity always appeared near the leaflet trailing edges. The flow field features revealed in the present paper improved our understanding of valve motion mechanism under physiological conditions, and this knowledge is very helpful in designing the new generation of MHVs.

On the closing sounds of a mechanical heart valve.

In the 1994 Replacement Heart Valve Guidance of the U.S. Food and Drug Administration (FDA), in-vitro testing is required to evaluate the potential for cavitation damage of a mechanical heart valve (MHV). To fulfill this requirement, the stroboscopic high-speed imaging method is commonly used to visualize cavitation bubbles at the instant of valve closure. The procedure is expensive; it is also limited because not every cavitation event is detected, thus leaving the possibility of missing the whole cavitation process. As an alternative, some researchers have suggested an acoustic cavitation-detection method, based on the observation that cavitation noise has a broadband spectrum. In practice, however, it is difficult to differentiate between cavitation noise and the valve closing sound, which may also contain high-frequency components. In the present study, the frequency characteristics of the closing sound in air of a Björk-Shiley Convexo-Concave (BSCC) valve are investigated. The occluder closing speed is used as a control parameter, which is measured via a laser sweeping technique. It is found that for the BSCC valve tested, the distribution of the sound energy over its frequency domain changes at different valve closing speeds, but the cut-off frequency remains unchanged at 123.32± 6.12 kHz. The resonant frequencies of the occluder are also identified from the valve closing sound.

A Parametric Study of Hertzian Contact Stresses in Pyrolytic Carbon/Graphite Composite

A mechanical heart valve (MHV) prosthesis made of pyrolytic carbon (PyC) coated graphite is a long-term implant that operates continuously for lifetime in patients body. The ceramic-like PyC coating is brittle and is prone to contact damages. The high-level local contact stress may lead to the initiation of microcracks, as well as accelerated propagation of preexisting cracks. In this paper, the effect of changing coating parameters on Hertzian contact stresses is investigated for a trilayer PyC/graphite laminate undergoing a spherical indentation test, by means of a finite element analysis. It is shown that by suitably changing Youngs modulus, Poissons ratio, and the thickness of the coating material, the stress levels at critical locations can be greatly modified. This can be accomplished effectively within a small range of variations of the coating parameters. Low Youngs modulus, large Poissons ratio, and relatively thick coating are found to be beneficial in general. The results obtained may be useful in the design of MHV components for improving resistance against contact-induced damages by elaborately controlling the deposition process of PyC coating.

A Fracture Mechanics Model for Cavitation Damage in Mechanical Heart Valve Prostheses

In the 1994 Replacement Heart Valve Guidance of the Food and Drug Administration (FDA), both cavitation and damage tolerance analyses are required for mechanical heart valves (MHV). Cavitation results from a sequence of events. First, vaporous bubbles are generated in the blood flow. They then collapse to form high-speed micro liquid jets, striking against the solid valve boundary, and subsequently causing pits on the surface. Micro cracks may initiate around the pitted areas under repeated jet impacts superimposed with cyclic loading. These events are, of course, closely related to the shape and size of an MHV. The factors specified in the current FDA guidance and considered by many authors for calculating working stresses, including static stresses, dynamic stresses, residual stresses, and stress concentrations, appear to be inadequate. The local high pressure caused by cavitation jets is another important factor which may aggravate crack propagation. The present paper is aimed at quantitatively assessing the influence of cavitation jets on the safe service life of an MHV using a damage tolerance approach. A new fracture mechanics model for estimating service lives of MHV prostheses is proposed, in which bubble dynamics and cavitation phenomenon are incorported. Numerical results show that the local high pressure is dominant, and is large enough to cause a crack to propagate at a greater rate, and resulting in much shorter fatigue life for an MHV.

On Accelerated Fatigue Testing of Prosthetic Heart Valves

Accelerated testing (AT) of prosthetic heart valves allows simulation of wear and fatigue sustained by the replacement heart valves, and to estimate the valves’ life expectancy in human body. At accelerated test rates, sufficient amounts of data can be collected within a reasonably short time period, after repeated opening and closing cycles, to predict the valve durability. The U.S. Food and Drug Administration (FDA) Replacement Heart Valve Guidance (Version 4.1, 1994) requires that mechanical heart valves (MHV) must be tested at least 600 million cycles (equivalent to 15 years in vivo), while biological heart valve prostheses (BHV) must be tested at least 200 million cycles (equivalent to 5 years in vivo) in pulsatile flow simulators. The cyclic test must meet two basic FDA requirements: 1) the test valve open and close fully each cycle; and 2) the average transvalvular pressure is kept at least 100 mmHg at closure. At accelerated test rates, the valves were subjected to non-physiologic dynamic force loads and often damaged under excessive conditions, such as cavitation. AT may pinpoint early flaws in the design and in the manufacturing processes, and deflects regions of materials weakness. Hence the design of AT must follow the principles of engineering testing such as the law of dynamic similarities. One must first identify dimensionless parameters that are physiologically meaningful and those much be specific to heart valve testing. The main goal of this paper is to present an AT system and an experimental protocol so that in vitro accelerated testing may be carried out without creating these excess forces on the test valves and to predict the durability of prosthetic heart valves with physiological considerations.

Measurement of Dynamic Stresses in a Mechanical Heart Valve During Accelerated Testing

The US FDA Replacement Heart Valve Guidance (1994) requires that new mechanical heart valves (MHV) must be tested in a pulse simulator flow loop for 600 million cycles before premarket approval (PMA) can be considered. Accelerated testing is commonly used by valve manufacturers to meet the requirement. During accelerated testing, MHV components are subjected to excessive hydrodynamic loads, including inertia forces due to the water hammer effect. In this paper, experimental results obtained on a bileaflet MHV (SJM 25 mm) installed in an accelerated tester are documented. During the tests, the maximum flexural stresses in an MHV leaflet were monitored using strain gauges. The testing rate was kept constant for each segment of the experiment, and was step-wise increased from 300 to 1000 bpm at increments of 100 bpm. It was found that stresses corresponding to the bending deformation about the leaflet centerline were dominant. The effects of water hammer and differential transvalvular pressure were also investigated. It was found that at a rate below 550 bpm, the transvalvular pressure difference (Δ P) was the primary dynamic load, while the water hammer effect became outstanding at a rate above 550 bpm. The peak dynamic stresses attributable to water hammer were correlated approximately linearly with the testing rates. On the other hand, little changes were noted in the peak stresses caused by transvalvular pressure difference regardless of testing rates, as long as the transvalvular pressure difference was maintained at 120 mmHg.