從過去的研究指出，造成機械心瓣發生穴蝕的機制包括水錘效應(water hammer effect)、擠壓流效應(squeeze flow effect)、渦流效應(vortex effect)與文氏效應(Venturi effect)。由於加速狀態下的穴蝕現象較為明顯，同時過去尚未有學者針對加速狀態下的機械心瓣穴蝕成因做探討，因此本研究首先利用不同設計的機械心瓣在加速狀態下研究穴蝕的成因。然而加速狀態下無法說明真實體內的情況，因此本研究隨後在生理條件下針對包括直接量測造成穴蝕發生時的擠壓流與關閉後渦流效應與穴蝕的關係進行研究。機械心瓣在關閉的瞬間包含所有可能造成穴蝕的效應都可能會發生，因此難以了解這些效應何者為最主要的效應以及這些效應是否單獨能造成穴蝕的發生，因此本研究設計一個能模擬單葉片機械心瓣關閉的裝置，同時能將上述可能造成穴蝕的成因各自分離與合併進行研究。本研究中主要利用strobe lighting technique的方法觀察穴蝕汽泡影像、利用高頻之壓力計針對穴蝕汽泡的高頻壓力進行、利用高感度之CCD雷射位移計量測心瓣葉片的關閉速度、利用雷射都卜勒流速儀與質點影像流速儀來量測流場的速度與渦流現象。
綜合上述各個研究的結果指出單獨水錘效應、擠壓流效應均可造成穴蝕的發生，但是單獨水錘效應需要較大之關閉速度才能造成。而當兩個效應伴隨產生時，此時所需要的關閉速度較低，此說明了擠壓流的效應應為機械心瓣穴蝕發生的主因，而水錘效應則有加成的效果。本研究中所直接量測心瓣關閉瞬間之擠壓流流速遠小於利用數值模擬計算出來之流速，造成此結果可能是因為流體加速度項所造成，然而此加速度項不易進行量測。本研究中量測心瓣關閉後渦流中心的壓降僅約25 mmHg，此不足以單獨造成穴蝕的發生，因此渦流效應應不是主因，但在穴蝕的發生上具有加成的效果。

Prior research shows that the factors which induce cavitation include water hammer, squeeze flow, vortex and Venturi effects. Because the MHV cavitation in accelerated condition is more intense than in physiologic condition, and investigators never studied the mechanisms of cavitation formation in accelerated condition in past years. Therefore, in this study, we utilized the different design of MHVs testing under accelerated condition and studied the mechanisms related to cavitation at first. However, accelerated testing may not accurately reflect in vivo performance. Hence, we tried to measure the squeeze flow velocity directly and calculate the pressure drop during MHV closure related to MHV cavitation in physiologic condition later. During the MHV closure, it may combine several mechanisms to induce MHV cavitation formation. Because it is very difficult to study one of the mechanisms mentioned above alone, we designed a simplified model to simulate the closing behavior of a monoleaflet valve. This model allowed us to modify various parameters and measure flow velocities and pressure changes by separating or combining the various flow effects mentioned above. In this study, the strobe lighting technique (SLT) was used to capture the image of cavitation bubbles, the high-frequency pressure transducer was mounted very close to the leaflet surface to obtain the transient pressure signal, and the laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) were used to measure the flow velocity and vortex phenomenon.
Results showed that both squeeze flow and water hammer individually are sufficient to generate cavitation bubbles, with the latter requiring higher velocities. When both squeeze flow and water hammer effects are compounded, cavitation occurs at lower closing velocity. This indicated that the dominate mechanism which induce MHV cavitation inception is the squeeze flow effect, and the water hammer effect may play a minor role in MHV cavitation. The maximum pressure drop in the vortex center is roughly 25 mmHg. Since cavitation formation requires the local pressure to drop below vapor pressure, our results clearly showed that vortex formation cannot provide significant contribution to MHV cavitation.

Abbreviations
List of Figures
Chapter 1 - General Introduction  1	
1.1  Human Heart and Cardiac Valves  1
1.2  The Prosthetic Heart Valves (PHV)  2	
1.3  Cavitaion Phenomena  4
1.4  Cavitaion in Mechanical Heart Valves  5
1.4.1  Parameters of MHV Cavitation Intensity  7
1.4.1.1  MHV Cavitation Threshold(Loading Rate)  7
1.4.1.2	Occluder Closing Velocity of MHVs  9
1.4.1.3	High Frequency Pressure Oscillations at MHV Closure  10
1.4.2  Causes of MHV Cavitation  12 
1.4.2.1  Water Hammer Effect  12
1.4.2.2	Squeeze Flow Effect  13
1.4.2.3	Vortex Effect  15
1.4.2.4	Venturi Effect  17
1.5  The Objectives of this Dissertation  17
References  19
Figure Legends  24

Chapter 2 - Cavitation Behavior Observed in Three Monoleaflet MHVs under Accelerated Testing Conditions  27
2.1  Introduction  27
2.2  Materials and methods  30
2.3  Results  33
2.4  Discussions  37
2.5  Conclusions  42
References  44
Figure Legends  47
Table Legends  56

Chapter 3 - Squeeze Flow Measurements in Mechanical Heart Valves  57
3.1  Introduction  58
3.2  Materials and methods  61
3.3  Results  64
3.4  Discussions  65
3.5  Conclusions  69
References  71
Figure Legends  74
Table Legends  78

Chapter 4 - Quantitative Characterization of Vortices in the MHV Regurgitant Flow Field  79
4.1  Introduction  79
4.2  Materials and methods  81
4.3  Results  83
4.4  Discussions  84
4.5  Conclusions  87
References  88
Figure Legends  90

Chapter 5 - A Physical Model Investigation of Cavitation Phenomena in Mechanical Heart Valves  95
5.1  Introduction  96
5.2  Materials and methods  99
5.3  Results and Discussions  101
5.4  Conclusions  108
References  110
Figure Legends  113
Table Legends  121

Chapter 6 - Conclusions  122
6.1  Conclusions  122
6.2  Recommendations for Further Study  124

List of Figures
Figure 1-1 Schematic illustration of human heart. (From: Texas Heart Institute website:
http://www.texasheart.org/HIC/Anatomy/anatomy2.cfm) 24
Figure 1-2 Cardiac cycle. (From Guyton AC, Text book of Medical Physiology, 8th Edition, Philadelphia, W.B. Saunders Company, 1991 24
Figure 1-3 Examples of some Bioprosthetic Heart Valves – porcine valve. 25
Figure 1-4 Examples of some Mechanical Heart Valves. 25
Figure 1-5 Cavitation pitting on leaflet (arrow 1) corresponding to precracked-housing damage at seating lip (arrow 2) and HSV cavitation location (arrow 3), 120 exercise hours in 16 months. (Ovine) (Ref: Ralph Kafesjian et al., JHVD,1994 s2-7) 26
Figure 1-6 Schematic of a suddenly stopped piston in a flow. 26
Figure 1-7 Schematic of the squeeze flow formation during MHV closure. 26

Figure 2-1a. Schematic diagram of the accelerated test system. 47
Figure 2-1b. Pictures of Medtronic Hall Standard (MHS) 29mm, Medtronic Hall D-16 (MHD) 29mm, and OmniCarbon (OC) 29mm, and schematic diagrams of different stop designs. 47
Figure 2-2. Illustration of stroboscopic lighting system used for cavitation visualization. 48
Figure 2-3. Schematic setup for the valve closing velocity measurement. 48
Figure 2-4. Images of cavitation bubbles on the inflow side of Medtronic Hall Standard MHV at different time delays. 49
Figure 2-5. Images of cavitation bubbles on the inflow side of Medtronic Hall D-16 MHV at different time delays. 49
Figure 2-6. Images of cavitation bubbles on the inflow side of Omni Carbon MHV at different time delays. 50
Figure 2-7. Examples of the recorded pressure signals measured on the outflow and inflow sides (top); low-pass signal (75kHz) of outflow/inflow sides and LVP (middle); and band-pass signal (75-200k Hz; bottom) of MHS. (1)5µs (2)45µs (3)145µs (4)225µs (5)265µs (6)345µs. 51
Figure 2-8. Examples of the recorded pressure signals measured on the outflow and inflow sides (top); low-pass signal (75kHz) of outflow/inflow sides and LVP (middle); and band-pass signal (75-200k Hz; bottom) of MHD. (1)5µs (2)105µs (3)205µs (4)325µs. 52
Figure 2-9. Examples of the recorded pressure signals measured on the outflow and inflow sides (top); low-pass signal (75kHz) of outflow/inflow sides and LVP (middle); and band-pass signal (75-200k Hz; bottom) of OC. (1)5µs (2)185µs (3)225µs (4)265µs (5)325µs. 53
Figure 2-10. The positive peak pressure (PPP), negative peak pressure (NPP), and root mean square (RMS) in relation to the valve closing velocity of two different MHVs. 54
Figure 2-11. Typical spectra of the total energy, deterministic energy and non-deterministic energy at 600 bpm of MHS valve. 55
Figure 2-12. Comparison of the RMS value of the high-pass filtered pressure data to the non-deterministic signal energy of MHS, MHD and OC valves at different heart rates.55

Figure 3-1. Images of the modified SJM (left), original MHS (center), and modified OC (right) valves. 74
Figure 3-2. Schematic diagram of the measurement regions of SJM, MHS and OC MHVs. 74
Figure 3-3. Schematic diagram of the pulsatile mock circulatory loop system. 75
Figure 3-4. The physiologic pressure waveforms of SJM, MHS, and OC MHVs at 70 bpm. 75
Figure 3-5. Images of cavitation bubbles on the inflow side of (a) SJM, (b) MHS, and (c) OC MHVs at the instant of closure. 76
Figure 3-6. Samples of instantaneous squeeze flow velocity data; (a) SJM, (b) MHS, and (c) OC. 76
Figure 3-7.  Negative peak pressure signal trace and corresponding squeeze flow velocities of St. Jude Medical (SJM ) at 70bpm (V1-V4: four different cycles). 77

Figure 4-1 Schematic diagram of the pulsatile mock circulatory loop. 90
Figure 4-2 Schematic view of SJM 29mm PIV measurements planes. I and II designate the two viewing windows. 90
Figure 4-3 Example of the (a) PIV data and (b) centerline velocity profile of a vortex. 91
Figure 4-4 A typical PIV instantaneous velocity vector field of (a) SJM(I) and (b) SJM(II); I and II designate the two viewing windows. 91
Figure 4-5 The magnitude of (a) the vortex core radius, R; (b) the maximum tangential velocity, Vt; (c) the velocity circulation, C; and (d) the pressure drop, dP, at the peripheral orifice of SJM. Error bars represent the standard deviation. 92
Figure 4-6 The magnitude of (a) the vortex core radius, R; (b) the maximum tangential velocity, Vt; (c) the velocity circulation, C; and (d) the pressure drop, dP, of the upper vortex at the central B-datum orifice of SJM. Error bars represent the standard deviation. 93
Figure 4-7 The magnitude of (a) the vortex core radius, R; (b) the maximum tangential velocity, Vt; (c) the velocity circulation, C; and (d) the pressure drop, dP, of the lower vortex at the central B-datum orifice of SJM. Error bars represent the standard deviation. 94

Figure 5-1(a) Schematic of experiment setup (a) acrylic water tank, (b) pneumatic actuator, and (c) the setup of various impinging rods consisting of one movable and one stationary rod. 113
Figure 5-1(b) Schematic of the impinging rods system. 113
Figure 5-1(c) Schematic of the impinging piston system. 114
Figure 5-2 Schematic of the experimental setup that allows direct cavitation imaging, velocity measurement of the moving rod, flow field velocity and pressure changes at the instant of rod impact. 114
Figure 5-3 The images of cavitation bubbles in (a) 5mm and (b) 10mm impinging rods system at moving velocity were 1.31±0.03 and 0.89±0.03 m/s, respectively. 115
Figure 5-4 The images of cavitation bubbles in 10mm impinging rods system at moving velocity was 1.32 m/s. 115
Figure 5-5 The images of cavitation bubbles in 10mm impinging rods system at different moving velocities. 116
Figure 5-6 Images of cavitation bubbles on the 10mm rods impinging system at different time delays. 116
Figure 5-7 Visualization of 10mm rods impinging system at moving velocity was 1.1±0.03 m/s. 117
Figure 5-8 The images of cavitation bubbles in (a) 5mm-24mm and (b) 10mm-24mm impinging rods system at moving velocity were 1.30±0.02 and 0.84±0.02 m/s, respectively. 118
Figure 5-9 Images of cavitation bubbles on the 10-24mm rods impinging system at different time delays. 118
Figure 5-10 The image of cavitation bubbles at piston-cylinder system. 119
Figure 5-11 The images of cavitation bubbles in 24mm piston-cylinder system upon impact with the 5mm stationary rod at (a) 0.56±0.01 and (b) 0.89±0.02m/s, respectively. 119
Figure 5-12 The high-frequency pressure fluctuations when the 10mm movable rod impacts the same sized stationary rod at the critical velocity of 1.03±0.02 m/s. 120

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