Zhaoming He

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.

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.

Mechanics of the Mitral Valve Strut Chordae Insertion Region

Interest in developing durable mitral valve repair methods is growing, underscoring the need to better understand the native mitral valve mechanics. In this study, the authors investigate the dynamic deformation of the mitral valve strut chordae-to-anterior leaflet transition zone using a novel stretch mapping method and report the complex mechanics of this region for the first time. Eight structurally normal porcine mitral valves were studied in a pulsatile left heart simulator under physiological hemodynamic conditions -120 mm peak transvalvular pressure, 5 l/min cardiac output at 70 bpm. The chordal insertion region was marked with a structured array of 31 miniature markers, and their motions throughout the cardiac cycle were tracked using two high speed cameras. 3D marker coordinates were calculated using direct linear transformation, and a second order continuous surface was fit to the marker cloud at each time frame. Average areal stretch, principal stretch magnitudes and directions, and stretch rates were computed, and temporal changes in each parameter were mapped over the insertion region. Stretch distribution was heterogeneous over the entire strut chordae insertion region, with the highest magnitudes along the edges of the chordal insertion region and the least along the axis of the strut chordae. At early systole, radial stretch was predominant, but by mid systole, significant stretch was observed in both radial and circumferential directions. The compressive stretches measured during systole indicate a strong coupling between the two principal directions, explaining the small magnitude of the systolic areal stretch. This study for the first time provides the dynamic kinematics of the strut chordae insertion region in the functioning mitral valve. A heterogeneous stretch pattern was measured, with the mechanics of this region governed by the complex underlying collagen architecture. The insertion region seemed to be under stretch during both systole and diastole, indicating a transfer of forces from the leaflets to the chordae and vice versa throughout the cardiac cycle, and demonstrating its role in optimal valve function.

Annulus tension of the prolapsed mitral valve corrected by edge-to-edge repair

BACKGROUND Mitral valve (MV) performance after edge-to-edge repair (ETER) without ring annuloplasty is suboptimal. ETER efficacy needs to be evaluated from annulus tension (AT) of a prolapsed MV corrected by ETER to understand annular dilatation. METHODS Ten porcine MVs were harvested and mounted on a MV closure test rig. The MV annulus tissue rested on top of a saddle-shaped plastic ring on which the annulus could slide freely. The annulus was held by strings in the periphery during MV closure under a hydrostatic trans-mitral pressure. String tensions were measured and further divided by string spacing to obtain AT. The MVs were then prolapsed by shifting split papillary muscles to simulate mono-leaflet prolapse due to elongation of chords, which insert into a single leaflet. Last, MV prolapse was corrected by ETER applied in the central leaflet region and AT was measured. RESULTS AT in both anterior and posterior leaflet prolapse corrected by ETER was less than that of normal MVs. AT in the anterior leaflet prolapse corrected by ETER was less than that in the posterior leaflet prolapse corrected by ETER. CONCLUSION ETER does not restore the normal AT and therefore leads potential of annular dilatation. The anterior leaflet prolapse has a greater potential of annular dilatation than the posterior leaflet prolapse after ETER. Annuloplasty is recommended to maintain long-term ETER efficacy.

Papillary muscle and annulus size effect on anterior and posterior annulus tension of the mitral valve: An insight into annulus dilatation

Mitral valve (MV) annulus mechanics and its effect on annulus dilatation are not well understood. The objective of the current study was to understand annulus tension (AT) during valve closure. A porcine MV rested on top of annulus rings with papillary muscles (PMs) held at slack, normal and taut conditions. The annulus was held by strings in the periphery during valve closure under static trans-mitral pressures. String tensions were measured and further used to calculate the anterior and posterior ATs. Three rings of different sizes were used to simulate normal and dilatated annuli. Fourteen MVs were tested. The anterior ATs were 37.21+/-11.03, 53.86+/-14.98 and 58.87+/-15.72N/m, respectively, at the slack, normal and taut PM positions in the normal annulus at the trans-mitral pressure of 16.3kPa (122mmHg). The posterior ATs were 24.52+/-5.68, 36.29+/-8.89 and 42.32+/-11.82N/m, respectively, at the slack, normal and taut PM positions in the normal annulus at the trans-mitral pressure of 16.3kPa (122mmHg). AT increased as the PM changed from slack to normal, then to taut PM positions. The AT increases with the increase of annulus area and linearly with the increase of trans-mitral pressure. The AT increases with the increases of apical PM displacement and dilatated annulus area, and reduces the potential of annulus dilatation. Low trans-mitral pressure due to existent mitral regurgitation, and MV prolapse increase the potential of annulus dilatation.

A novel method to measure mitral valve chordal tension.

Proper leaflet coaptation of the mitral valve is vital for a healthy functioning heart. Chordal tension directly affects leaflet coaptation. The C-shaped transducer used previously to measure chordal tension was too big for tension measurement of multiple chordae and their branches. A new method is needed to measure chordal tension with minimum interference with chord and leaflet motion. The method was to extrapolate longitudinal chordal tension from transverse chordal fibril force measured by inserting a small elliptical AIFP4 sensor from MicroStrain Inc. (Williston, VT) through a chord. Sensitivity of the method has been tested with the sensor implanted in chordae, and error of the method has been estimated at various sensor deviation angles. Intact porcine and ovine hearts were used to measure mitral valve strut and marginal chordal tensions at static transmitral pressures of 120 mm Hg and 160 mm Hg under an in vitro condition. The results obtained from the AIFP4 sensor were similar to the results obtained previously by C-shaped transducers in the porcine mitral valves. The sensor output errors increased with the increase in sensor deviation angle in the chord at a peak systolic tension. Strut chordal tensions of four ovine mitral valves of Edwards ring size M 28 were 0.29+/-0.06 N at the transmitral pressure of 120 mm Hg. The tension of 18 porcine strut chordae of porcine mitral valves of Edwards ring size M 32 was 1.00+/-0.42 N at the transmitral pressures of 120 mm Hg. The tension of 22 anterior leaflet marginal chordae from porcine mitral valves of Edwards ring size M 32 was 0.10+/-0.04 N at the transmitral pressure of 120 mm Hg. A new method using an AIFP4 miniature force sensor to measure mitral valve chordal tension indirectly is successfully developed. This force sensor works well in measuring mitral valve chordal tension at an in vitro hydrostatic transmitral pressure. The size and simple fixation of the sensor make it favorable for chordal tension measurement of multiple chordae and their branches under in vitro or in vivo conditions with minimal interference with chordal geometry and dynamics.

In vitro stretches of the mitral valve anterior leaflet under edge-to-edge repair condition.

Mitral valve edge-to-edge repair (ETER) alters valve mechanics, which may impact efficacy and durability of the repair. The objective of this paper was to quantify stretches in the central region of the anterior leaflet of the mitral valve after ETER with a single suture and 6 mm suture. Sixteen markers, forming a 4x4 array, were attached onto the central region of the mitral valve anterior leaflet. The mitral valve was subjected to ETER with a single suture and 6 mm suture, and mounted in an in vitro flow loop simulating physiological conditions. Images of the marker array were used to calculate marker displacement and stretch. A total of 9 mitral valves were tested. Two peak stretches were observed during a cardiac cycle, one in systole and the other in diastole under mitral valve edge-to-edge repair condition. The major principal (radial) stretch during systole was significantly greater than that during diastole. However, there was no significant difference between the minor principal (circumferential) stretch during diastole and that during systole. In addition, there were no significant differences in the radial and circumferential, or areal stretches and stretch rates during diastole between the single suture and 6 mm suture. The ETER subjects the mitral valve leaflets to double frequency of loading and unloading. Minor change in suture length may not result in a significant load difference in the central region of the anterior leaflet during diastole.

Tension to passively cinch the mitral annulus through coronary sinus access: an ex vivo study in ovine model

INTRODUCTION The transcatheter mitral valve repair (TMVR) technique utilizes a stent to cinch a segment of the mitral annulus (MA) and reduces mitral regurgitation. The cinching mechanism results in reduction of the septal-lateral distance. However, the mechanism has not been characterized completely. In this study, a method was developed to quantify the relation between cinching tension and MA area in an ex vivo ovine model. METHOD The cinching tension was measured from a suture inserted within the coronary sinus (CS) vessel with one end tied to the distal end of the vessel and the other end exited to the CS ostium where it was attached to a force transducer on a linear stage. The cinching tension, MA area, septal-lateral (S-L) and commissure-commissure (C-C) diameters and leakage was simultaneously measured in normal and dilated condition, under a hydrostatic left ventricular pressure of 90 mm Hg. RESULTS The MA area was increased up to 22.8% after MA dilation. A mean tension of 2.1 ± 0.5 N reduced the MA area by 21.3 ± 5.6% and S-L diameter by 24.2 ± 5.3%. Thus, leakage was improved by 51.7 ± 16.2% following restoration of normal MA geometry. CONCLUSION The cinching tension generated by the suture acts as a compensation force in MA reduction, implying the maximum tension needed to be generated by annuloplasty device to restore normal annular size. The relationship between cinching tension and the corresponding MA geometry will contribute to the development of future TMVR devices and understanding of myocardial contraction function.

Characterization of biomechanical properties of aged human and ovine mitral valve chordae tendineae.

The mitral valve (MV) is a highly complex cardiac valve consisting of an annulus, anterior and posterior leaflets, chordae tendineae (chords) and two papillary muscles. The chordae tendineae mechanics play a pivotal role in proper MV function: the chords help maintain proper leaflet coaptation and rupture of the chordae tendineae due to disease or aging can lead to mitral valve insufficiency. Therefore, the aim of this study was to characterize the mechanical properties of aged human and ovine mitral chordae tendineae. The human and ovine chordal specimens were categorized by insertion location (i.e., marginal, basal and strut) and leaflet type (i.e., anterior and posterior). The results show that human and ovine chords of differing types vary largely in size but do not have significantly different elastic and failure properties. The excess fibrous tissue layers surrounding the central core of human chords added thickness to the chords but did not contribute to the overall strength of the chords. In general, the thinner marginal chords were stiffer than the thicker basal and strut chords, and the anterior chords were stiffer and weaker than the posterior chords. The human chords of all types were significantly stiffer than the corresponding ovine chords and exhibited much lower failure strains. These findings can be explained by the diminished crimp pattern of collagen fibers of the human mitral chords observed histologically. Moreover, the mechanical testing data was modeled with the nonlinear hyperelastic Ogden strain energy function to facilitate accurate computational modeling of the human MV.

Numerical Simulations of High-Frequency Respiratory Flows in 2D and 3D Lung Bifurcation Models

To better understand the human pulmonary system and optimize the high-frequency oscillatory ventilation (HFOV) design, numerical simulations were conducted under normal breathing frequency and HFOV condition using a CFD code Ansys Fluent and its user-defined C programs. 2D and 3D double bifurcating lung models were created, and the geometry corresponds to fifth to seventh generations of airways with the dimensions based on the Weibels pulmonary model. Computations were carried out for different Reynolds numbers (Re = 400 and 1000) and Womersley numbers (α = 4 and 16) to study the air flow fields, gas transportation, and wall shear stresses in the lung airways. Flow structure was compared with experimental results. Both 2D and 3D numerical models successfully reproduced many results observed in the experiment. The oxygen concentration distribution in the lung model was investigated to analyze the influence of flow oscillation on gas transport inside the lung model.