Biomedical subjects

Zhaoming He

Publications and source records attributed to Zhaoming He.

14 recordsLinked to original sources

PubMed2006

Effects of annular size, transmitral pressure, and mitral flow rate on the edge-to-edge repair: an in vitro study.

BACKGROUND: Although edge-to-edge repair is an established adjunctive procedure, there is still debate on its long-term durability and efficacy. METHODS: Fifteen porcine mitral valves were studied in a physiologic left heart simulator with a variable size annulus (dilated = 8.22 cm2, normal = 6.86 cm2, contracted = 5.5 cm2). Mitral valves were tested under steady and physiologic pulsatile flow conditions (cardiac outputs: 4 to 6 L/min), at peak transmitral pressures between 100 mm Hg and 140 mm Hg. A miniature force transducer was used to measure the Alfieri stitch force (F(A)). Mitral flow rate (MFR), transmitral pressure, effective orifice area, mitral regurgitation, and F(A) were monitored. RESULTS: The edge-to-edge repair led to a decrease in effective orifice area of 16.55% +/- 8.22%; further reduction in effective orifice area was attained with annular contraction. Mitral regurgitation after the edge-to-edge repair was significantly higher (p <0.05) with annular dilation. In the pulsatile experiments, two peaks in F(A) were observed: one during systole (F(A) = 0.059 +/- 0.024 N) and a second during diastole (F(A) = 0.072 +/- 0.021 N). Multivariate analysis of variance analysis showed that during systole, transmitral pressure and mitral annular area (MAA) had significant effects on F(A) [F(A) = (4.40 x 10(-4)) transmitral pressure (mm Hg) + (5.0 x 10(-3)) MAA (cm2) - 0.05 (R2 = 0.80)], whereas during diastole MFR and MAA had significant effects on F(A) [F(A) = (1.03 x 10(-4)) MFR2 (L/min) - (1.60 x 10(-3)) MAA (cm2) + 0.02 (R2 = 0.90)]. CONCLUSIONS: With annular dilation, mitral regurgitation persisted even after the edge-to-edge repair. The edge-to-edge repair does not cause clinically relevant mitral valve stenosis in a normal size mitral valve. Mitral flow rate and transmitral pressure are the main determinants of F(A) during the cardiac cycle. Increasing annular area increases F(A) during systole but decreases F(A) during diastole. Systolic F(A) may become dominant with increases in MAA or peak transmitral pressure, or both.

Animals↗

PubMed2006

The material properties of the native porcine mitral valve chordae tendineae: an in vitro investigation.

The material properties of the mitral valve chordae tendineae are important for the understanding of leaflet coaptation configuration and chordal pathology. There is limited information about the mechanical properties of the chordae during physiologic loading. Dual camera stereo photogrammetry was used to measure strains of the chordae in vitro under physiologic loading conditions. Two high-speed, high-resolution cameras captured the movement of graphite markers attached to the central section of the chordae. A uniaxial test simulating the same loading conditions was conducted on the same chordae using the same markers. The maximum strain experienced during the cardiac cycle was 4.29% +/- 3.43%. The loading rate was higher at 75.3% +/- 48.6% strain per second than the unloading rate at -54.8% +/- -56.6% strain per second. The anterior lateral strut chordae had a higher maximum strain (5.7% +/- 3.8%) and loading rate (80.5% +/- 51.9% strain per second) than the posterior medial strut chordae (5.5% +/- 2.3% strain and 68.1% +/- 48.3% strain per second). The posterior medial strut chordae had a higher unloading rate (-68.5% +/- -59.1% strain per second) than the anterior lateral strut chordae (-44.9% +/- -57.2% strain per second). Although the anterior lateral and posterior medial strut chordae have a significantly different diameter and length, they experience a similar strain, strain rate, and tension. In conclusion, a non-destructive technique was developed to measure in vitro chordal strain in the mitral valve. This technique allows the investigation of the behavior of biological tissues under physiologic loading conditions.

Animals↗

PubMed2005

Mitral valve function and chordal force distribution using a flexible annulus model: an in vitro study.

Since variations in annular motion/shape and papillary muscle displacement have been observed in studies of dilated cardiomyopathy and ischemic mitral regurgitation, the objective of this study was to investigate the effects of annular motion/flexibility and papillary muscle displacement on chordal force and mitral valve function. Six human mitral valves were studied in a left heart simulator using a flexible annular model. Mitral flow, trans-mitral pressure and chordae tendineae tension were monitored online in normal and pathophysiologic papillary muscle positions. The flexible annulus model showed a significant increase in mitral regurgitation volume (p < 0.05) when compared to static annuli models. Furthermore, there was a significant increase of force on the basal chords compared to the force present with the static annuli models. Utilizing the flexible annulus model, papillary muscle displacement significantly increased the force on the anterior strut, posterior intermediate and commissural chords. (1) Papillary muscle displacement increases the tension on the intermediate chords inducing tenting of the leaflets and subsequent regurgitation. (2) The tension on the intermediate and marginal chords is relatively insensitive to annular motion, whereas tension on the basal chords is directly affected by annular motion.

Cadaver↗

PubMed2005

Normal physiological conditions maintain the biological characteristics of porcine aortic heart valves: an ex vivo organ culture study.

The aortic valve functions in a complex mechanical environment which leads to force-dependent cellular and tissue responses. Characterization of these responses provides a fundamental understanding of valve pathogenesis. The aim of this work was to study the biological characteristics of native porcine aortic valves cultured in an ex vivo pulsatile organ culture system capable of maintaining physiological pressures (120/80 mmHg) and cardiac output (4.2 l/min). Collagen, sGAG and elastin contents of the valve leaflets were measured and cusp morphology, cell phenotype, cell proliferation and apoptosis were examined. Presence of endothelial cells (ECs) on the leaflet surface was also evaluated. The differences in collagen, sGAG and elastin contents were not significant (p > 0.05) between the cultured and fresh valve leaflets. The cultured valves maintained the native ECM composition of the leaflets while preserving the morphology and cell phenotype. Cell phenotype in leaflets incubated statically under atmospheric conditions decreased compared to fresh and cultured valve leaflets, indicating the importance of mechanical forces in maintaining the natural biology of the valve leaflets. ECs were retained on the surfaces of cultured leaflets with no remodeling of the leaflets. The number of apoptotic cells in the cultured leaflets was significantly (p < 0.05) less than in the statically incubated leaflets and comparable to fresh leaflets. The sterile ex vivo organ culture system thus maintained the viability and native biological characteristics of the aortic valves that were cultured under dynamic conditions for a period of 48 h.

Animals↗

PubMed2005

In vitro dynamic strain behavior of the mitral valve posterior leaflet.

Knowledge of mitral valve (MV) mechanics is essential for the understanding of normal MV function, and the design and evaluation of new surgical repair procedures. In the present study, we extended our investigation of MV dynamic strain behavior to quantify the dynamic strain on the central region of the posterior leaflet. Native porcine MVs were mounted in an in-vitro physiologic flow loop. The papillary muscle (PM) positions were set to the normal, taut, and slack states to simulate physiological and pathological PM positions. Leaflet deformation was measured by tracking the displacements of 16 small markers placed in the central region of the posterior leaflet. Local leaflet tissue strain and strain rates were calculated from the measured displacements under dynamic loading conditions. A total of 18 mitral valves were studied. Our findings indicated the following: (1) There was a rapid rise in posterior leaflet strain during valve closure followed by a plateau where no additional strain (i.e., no creep) occurred. (2) The strain field was highly anisotropic with larger stretches and stretch rates in the radial direction. There were negligible stretches, or even compression (stretch < 1) in the circumferential direction at the beginning of valve closure. (3) The areal strain curves were similar to the stretches in the trends. The posterior leaflet showed no significant differences in either peak stretches or stretch rates during valve closure between the normal, taut, and slack PM positions. (4) As compared with the anterior leaflet, the posterior leaflet demonstrated overall lower stretch rates in the normal PM position. However, the slack and taut PM positions did not demonstrate the significant difference in the stretch rates and areal strain rates between the posterior leaflet and the anterior leaflet. The MV posterior leaflet exhibited pronounced mechanically anisotropic behavior Loading rates of the MV posterior leaflet were very high. The PM positions influenced neither peak stretch nor stretch rates in the central area of the posterior leaflet. The stretch rates and areal strain rates were significantly lower in the posterior leaflet than those measured in the anterior leaflet in the normal PM position. However, the slack and taut PM positions did not demonstrate the significant differences between the posterior leaflet and the anterior leaflet. We conclude that PM positions may influence the posterior strain in a different way as compared to the anterior leaflet.

Animals↗

PubMed2005

Design of a sterile organ culture system for the ex vivo study of aortic heart valves.

The biological response of valves to mechanical forces is not well understood. The aim of this study was to design a pulsatile system to enable the ex vivo study of aortic valves when subjected to various hemodynamic conditions. A bioreactor was designed to subject porcine aortic valves to physiological and pathophysiological pressure and flow conditions, while maintaining viability and sterility. Pressure and flow rate could be independently controlled to produce clinically relevant mechanical conditions. The oxygen transfer rate was characterized and sterile operation was achieved over 96 hours. The oxygenation capabilities ensure sufficient oxygen transport to valves, allowing operation for extended periods.

Animals↗

PubMed2005

Effects of papillary muscle position on chordal force distribution: an in-vitro study.

BACKGROUND AND AIM OF THE STUDY: Mitral insufficiency, a common and morbid pathology, has been related to topological changes in the left ventricle. These changes may affect mitral leaflet coaptation by displacing the tips of the papillary muscles (PMs), subsequently changing the tension distribution on the chordae tendineae. Therefore, further understanding of the effects of PM displacement on chordal force distribution is required. METHODS: Six human and five porcine mitral valves were studied in a physiological left heart simulator. Cardiac output and transmitral pressure were recorded online and maintained within physiological ranges. Force transducers were placed on six chordae tendineae to measure chordal force distribution. Tension on individual chordae tendineae was recorded online during the cardiac cycle. The experiment was conducted for eight different PM positions, which were constructed from 5-mm vectorial displacements from the normal PM position. RESULTS: The anterior strut chord showed significant (p <0.05) variations in peak systolic tension (PST) for those positions associated with apical motion of the PMs. The posterior intermediate chord also showed significant variations in PST for positions associated with apical displacement of the PMs, whereas posterior displacement of the PMs resulted in a reduction in tension. In contrast, both the anterior marginal and posterior marginal chords showed a relatively uniform PST for the eight different PM positions. The posterior basal and commissural chords were the most sensitive to tension variations due to PM displacement. These chords showed relatively large and significant (p <0.05) variations in PST for most of the different PM displacements. CONCLUSION: The effects of PM relocation on chordal tension depended on chordal type. Chords which insert closer to the annulus were more sensitive to PM displacement, whereas those further from the annulus, the marginal chords, were the least sensitive to PM displacement.

Animals↗

PubMed2004

Effects of constant static pressure on the biological properties of porcine aortic valve leaflets.

An understanding of how mechanical forces impact cells within valve leaflets would greatly benefit the development of a tissue-engineered heart valve. In this study, the effect of constant ambient pressure on the biological properties of heart valve leaflets was evaluated using a custom-designed pressure system. Native porcine aortic valve leaflets were exposed to static pressures of 100, 140, or 170 mmHg for 48 h. Collagen synthesis, DNA synthesis, sulfated glycoaminoglycan (sGAG) synthesis, alpha-SMC actin expression, and extracellular matrix (ECM) structure were examined. Results showed that elevated pressure caused an increase in collagen synthesis. This increase was not statistically significant at 100 mmHg, but at 140 mmHg and 170 mmHg collagen synthesis increased by 37.5 and 90%, respectively. No significant difference in DNA or sGAG synthesis was observed at elevated pressures, with the exception that DNA synthesis at 100 mmHg decreased. A notable decline in alpha-SMC actin was observed over the course of the experiments although no significant difference was observed between the pressure and control groups. It was concluded that elevated pressure caused a proportional increase in collagen synthesis of porcine aortic valve leaflets, but was unable to preserve alpha-SMC actin immunoreactive cells.

Actins↗

PubMed2004

Cyclic pressure affects the biological properties of porcine aortic valve leaflets in a magnitude and frequency dependent manner.

An understanding of how mechanical forces impact cells within valve leaflets would greatly benefit the development of a tissue-engineered heart valve. Previous studies by this group have shown that exposure to constant static pressure leads to enhanced collagen synthesis in porcine aortic valve leaflets. In this study, the effect of cyclic pressure was evaluated using a custom-designed pressure system. Different pressure magnitudes (100, 140, and 170 mmHg) as well as pulse frequencies (0.5, 1.167, and 2 Hz) were studied. Collagen synthesis, cell proliferation, sGAG synthesis, alpha-SMC actin expression, and extracellular matrix (ECM) structure were chosen as markers for valvular biological responses. Results showed that aortic valve leaflets responded to cyclic pressure in a magnitude and frequency-dependent manner. Increases in pressure magnitude (with the frequency fixed at 1.167 Hz) resulted in significant increases in both collagen and sGAG synthesis, while DNA synthesis remained unchanged. Responses to pulse frequency (with the magnitude fixed at 100 mmHg) were more complex. Collagen and sGAG synthesis were increased by 25 and 14% respectively at 0.5 Hz; but were not affected at 1.167 and 2 Hz. In contrast, DNA synthesis increased by 72% at 2 Hz, but not at 0.5 and 1.167 Hz. Under extreme pressure conditions (170 mmHg, 2 Hz), collagen and sGAG synthesis were increased but to a lesser degree than at 170 mmHg, and 1.167 Hz. Cell proliferation was not affected. A notable decline in a-SMC actin was observed over the course of the experiments, although no significant difference was observed between the cyclic pressure and control groups. It was concluded that cyclic pressure affected biosynthetic activity of aortic valve leaflets in a magnitude and frequency dependent manner. Collagen and sGAG synthesis were positively correlated and more responsive to pressure magnitude than pulse frequency. DNA synthesis was more responsive to pulse frequency than pressure magnitude. However, when combined, pressure magnitude and pulse frequency appeared to have an attenuating effect on each other. The number of alpha-SMC actin positive cells did not vary with cyclic pressure, regardless of pulse frequency and pressure magnitude.

Animals↗

PubMed2004

Fluid mechanics of heart valves.

Valvular heart disease is a life-threatening disease that afflicts millions of people worldwide and leads to approximately 250,000 valve repairs and/or replacements each year. Malfunction of a native valve impairs its efficient fluid mechanic/hemodynamic performance. Artificial heart valves have been used since 1960 to replace diseased native valves and have saved millions of lives. Unfortunately, despite four decades of use, these devices are less than ideal and lead to many complications. Many of these complications/problems are directly related to the fluid mechanics associated with the various mechanical and bioprosthetic valve designs. This review focuses on the state-of-the-art experimental and computational fluid mechanics of native and prosthetic heart valves in current clinical use. The fluid dynamic performance characteristics of caged-ball, tilting-disc, bileaflet mechanical valves and porcine and pericardial stented and nonstented bioprostheic valves are reviewed. Other issues related to heart valve performance, such as biomaterials, solid mechanics, tissue mechanics, and durability, are not addressed in this review.

Animals↗

PubMed2003

Effects of a saddle shaped annulus on mitral valve function and chordal force distribution: an in vitro study.

Studies have concluded that the shape of the human mitral valve annulus is a three-dimensional saddle. The objective of this study was to investigate the effects of a saddle shaped annulus on chordal force distribution and mitral valve function. Eleven human mitral valves were studied in a physiological left heart simulator with a variable shaped annulus (flat versus saddle). Cardiac output and transmitral pressure were analyzed to determine mitral regurgitation volume. In six experiments, force transducers were placed on six chordae tendineae to measure chordal force distribution. Valves were tested in normal and pathophysiologic papillary muscle positions. When comparing the flat and saddle shaped configurations, there was no significant difference in mitral regurgitation volume 11.2% +/- 24.7% (p = 0.17). In the saddle shaped configuration, the tension on the anterior strut chord was reduced 18.5% +/- 16.1% (p < 0.02), the tension on the posterior intermediate chord increased 22.3% +/- 17.1% (p < 0.03), and the tension of the commissural chord increased 59.0% +/- 32.2% (p < 0.01). Annular shape also altered the tensions on the remaining chords. Annular shape alone does not significantly affect mitral regurgitation caused by papillary muscle displacement. A saddle shaped annulus redistributes the forces on the chords by altering coaptation geometry, leading to an optimally balanced anatomic/physiologic configuration.

Blood Flow Velocity↗

PubMed2003

Mitral leaflet geometry perturbations with papillary muscle displacement and annular dilatation: an in-vitro study of ischemic mitral regurgitation.

BACKGROUND AND AIM OF THE STUDY: Perturbations of leaflet geometry are the final end point through which left ventricular (LV) ischemia causes incomplete mitral leaflet closure and resultant mitral regurgitation (MR). Geometric inconsistencies observed with valvular or subvalvular structural alterations raise several questions. METHODS: A new in-vitro LV flexible bag model was developed in order to visualize and analyze leaflet geometric changes under simulated pathological ischemic MR conditions. RESULTS: Papillary muscle (PM) displacement and annular dilatation decreased leaflet coaptation length, leading to significant MR. Symmetrical PM displacement shifted the coaptation line towards the leaflet edges and created central gaps along this line. Asymmetric PM displacement generated diametrically uneven coaptation with a tent-shaped leaflet at the tethered PM side, while the leaflet bulged at the opposite side towards the left atrium. CONCLUSION: Leaflet geometry during systole is affected by subvalvular structures. Asymmetric PM displacement, which may occur in regional or acute myocardial infarction, induces irregular deformation of the leaflet's coaptation line and, as a result, MR at the tethered side. Direct visualization of leaflet perturbation under these simulated pathological conditions may promote understanding of mechanisms present in ischemic MR.

Adaptation, Physiological↗

PubMed2003

Effects of papillary muscle position on in-vitro dynamic strain on the porcine mitral valve.

BACKGROUND AND AIM OF THE STUDY: Improved knowledge of mitral valve (MV) mechanics is essential to understand normal MV function and design and evaluate new surgical repair procedures. Initially, the dynamic deformation of the central region of the porcine MV anterior leaflet was quantified under simulated physiological conditions to explore the effects of varying papillary muscle (PM) position. METHODS: Fresh porcine MVs were mounted in an in-vitro physiological flow loop. The PM positions were set to normal, taut, and slack states to simulate physiological and pathological PM positions. Leaflet deformation was measured by tracking the displacement of 16 markers (approximately 250 microm diameter) placed in a 5 x 5 mm section of the central region of the anterior leaflet, approximately 5 mm from the annulus and coaptation line. Local leaflet tissue strain and strain rates were calculated from measured displacements under dynamic loading conditions. A total of 11 mitral valves was studied. RESULTS: Under the normal PM positional state, the average areal strain rate during valve closure was typically approximately 620% per second. While not measurably altering the maximal areal strain rate, the slack PM position led to a delay in complete valve closure and more rapid leaflet loading (defined as delta transvalvular pressure/delta areal strain) in late systole compared to the other two PM positions. CONCLUSION: It was shown that PM position influenced the MV loading process in the central area of the anterior leaflet. The slack PM position led to a delay in complete valve closure and more rapid leaflet loading in late systole. This increase in loading rate may have long-term consequences in MV function.

Animals↗

PubMed2002

Microflow fields in the hinge region of the CarboMedics bileaflet mechanical heart valve design.

OBJECTIVE: The design of bileaflet mechanical heart valves includes some degree of leakage flow on valve closure for the reverse flow to wash the hinge and pivot region of the valve. It is believed that this reverse flow helps to prevent areas of stasis and inhibit microthrombus formation. However, the magnitude of this retrograde flow may also give rise to unacceptable levels of blood element damage and lead to platelet activation or hemolysis as a result of the increased flow velocities through the hinge region. The purpose of this study was to evaluate the hinge flow dynamics of a 23-mm CarboMedics bileaflet mechanical valve (Sulzer CarboMedics Inc, Austin, Tex) and then to compare the results with those of the St Jude Medical 23-mm Regent (St Jude Medical Inc, Minneapolis, Minn) and Medtronic Parallel (Medtronic, Inc, Minneapolis, Minn) valves studied earlier. This comparison allows new insight into the microflow fields within the hinge region of the CarboMedics bileaflet mechanical valve, which have not been previously assessed during its clinical history. METHODS: Two-dimensional laser Doppler velocimetry was used to measure the velocity and turbulent shear stress fields in the hinge regions. To conduct these measurements, exact dimensional models of the bileaflet hinge regions were cast or machined from transparent plastic materials. The experiment was conducted in a pulsatile flow loop with measurements taken at different levels within the pivot and hinge regions. RESULTS: In the 23-mm CarboMedics valve hinge, the phase-averaged forward velocity obtained at the flat level and levels of 190 microm and 390 microm above flat and 1 mm below flat were 0.54 m/s, 0.77 m/s, 0.3 m/s, and 1.0 m/s, respectively. Corresponding values of the peak phase-averaged leakage velocities were 3.17 m/s, 2.91 m/s, 2.52 m/s, and 0.5 m/s, respectively. Corresponding turbulent shear stresses were 5510 dyne/cm(2), 5640 dyne/cm(2), 4380 dyne/cm(2), and 4810 dyne/cm(2), respectively. CONCLUSIONS: The hinge flow dynamics of the CarboMedics bileaflet design lie somewhere in between those of the St Jude Medical and the Medtronic Parallel valve designs. The fluid dynamics of the investigated valve were found to be similar to those of the St Jude Medical valves, although with slightly higher leakage velocities and turbulent shear stresses. This discrepancy may be a result of the sharper corners associated with the hinge design of the CarboMedics valve. It could also be due to the incremental enlargement of the internal orifice area of the St Jude Medical Regent design.

Adult↗