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Research Papers

Role of Mitral Annulus Diastolic Geometry on Intraventricular Filling Dynamics

[+] Author and Article Information
Ikechukwu U. Okafor

School of Chemical and
Biomolecular Engineering,
Georgia Institute of Technology,
311 Ferst Drive NW,
Atlanta, GA 30332-0100
e-mail: iokafor3@gatech.edu

Arvind Santhanakrishnan

Wallace H. Coulter Department of
Biomedical Engineering,
Georgia Institute of Technology
and Emory University,
Atlanta, GA 30313-2412;
School of Mechanical and
Aerospace Engineering,
Oklahoma State University,
218 Engineering North,
Stillwater, OK 74078-5016
e-mail: askrish@okstate.edu

Vrishank S. Raghav

Wallace H. Coulter Department of
Biomedical Engineering,
Georgia Institute of Technology
and Emory University,
Atlanta, GA 30313-2412
e-mail: vrishank@gatech.edu

Ajit P. Yoganathan

School of Chemical and
Biomolecular Engineering,
Georgia Institute of Technology,
311 Ferst Drive NW,
Atlanta, GA 30332-0100;
Wallace H. Coulter Department of
Biomedical Engineering,
Georgia Institute of Technology
and Emory University,
Technology Enterprise Park,
Suite 200, 387 Technology Circle,
Atlanta, GA 30313-2412
e-mail: ajit.yoganathan@bme.gatech.edu

1Corresponding author.

Manuscript received March 20, 2015; final manuscript received October 15, 2015; published online November 3, 2015. Assoc. Editor: Ender A. Finol.

J Biomech Eng 137(12), 121007 (Nov 03, 2015) (9 pages) Paper No: BIO-15-1124; doi: 10.1115/1.4031838 History: Received March 20, 2015; Revised October 15, 2015

The mitral valve (MV) is a bileaflet valve positioned between the left atrium and ventricle of the heart. The annulus of the MV has been observed to undergo geometric changes during the cardiac cycle, transforming from a saddle D-shape during systole to a flat (and less eccentric) D-shape during diastole. Prosthetic MV devices, including heart valves and annuloplasty rings, are designed based on these two configurations, with the circular design of some prosthetic heart valves (PHVs) being an approximation of the less eccentric, flat D-shape. Characterizing the effects of these geometrical variations on the filling efficiency of the left ventricle (LV) is required to understand why the flat D-shaped annulus is observed in the native MV during diastole in addition to optimizing the design of prosthetic devices. We hypothesize that the D-shaped annulus reduces energy loss during ventricular filling. An experimental left heart simulator (LHS) consisting of a flexible-walled LV physical model was used to characterize the filling efficiency of the two mitral annular geometries. The strength of the dominant vortical structure formed and the energy dissipation rate (EDR) of the measured fields, during the diastolic period of the cardiac cycle, were used as metrics to quantify the filling efficiency. Our results indicated that the O-shaped annulus generates a stronger (25% relative to the D-shaped annulus) vortical structure than that of the D-shaped annulus. It was also found that the O-shaped annulus resulted in higher EDR values throughout the diastolic period of the cardiac cycle. The results support the hypothesis that a D-shaped mitral annulus reduces dissipative energy losses in ventricular filling during diastole and in turn suggests that a symmetric stent design does not provide lower filling efficiency than an equivalent asymmetric design.

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References

Figures

Grahic Jump Location
Fig. 1

(a) Schematic of the ellipses fit to patient MRI scan and compliant ventricular geometry created out of liquid silicone. (b) LHS consisting of the flexible LV, atrial, and aortic sections. (c) Schematic of the in vitro LV flow circuit. Flow probes F1 and F2 are used to measure mitral and aortic flow rates, respectively. Pressures were measured in the ventricular and aortic positions. The flow direction through the LV model is indicated using a dashed arrow. PPP stands for programmable piston pump. (d) Mitral annular geometries used for the study. The O-shaped annulus had a diameter of 19.6 mm, while T = 22.10 mm and A = 16.76 mm for the D-annulus. Orifice areas were kept constant at 3.0 cm2.

Grahic Jump Location
Fig. 2

(a) Digital PIV setup with camera, laser, and optics, on the LV physical model. A cylindrical lens was used to convert the laser beam into a light sheet. (b) Planes of data acquisition. The central plane (center of the mitral annulus) was acquired along with two other planes at a distance of d/2 on either side of the central plane, where d (19.6 mm) is the diameter of the O-annulus. (c) Mitral inflow curve also depicting time points where PIV images were acquired. The peak E-wave was 16.5 l/min and A-wave was 13.75 l/min, giving an E/A ratio of 1.2. The flow curves were averaged over 15 cardiac cycles. The x-axis variable, t*, is the time T normalized by the cardiac cycle duration (856 ms).

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Fig. 3

Center plane isovorticity contours overlaid with velocity vectors at select time points((a) peak of E-wave, 0.23; (b) deceleration of E-wave, 0.29; (c) end of E-wave, 0.35; and (d) peak of A-wave, 0.61) during the diastolic period of cardiac cycle for both the D (left column) and the O (right column) shaped mitral annulus

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Fig. 4

Velocity profiles at the center plane at select locations during peak E-wave for D- and O-annulus. d′ is the diameter = 2r normalized by the hydraulic diameter (dH = 4A/P, where A is the cross-sectional area and P is the wetted perimeter) of each annulus, respectively. u is the component of velocity in the x-direction.

Grahic Jump Location
Fig. 5

Two-dimensional kinetic energy per unit mass for center, left, and right planes. The x-axis variable, t*, is the time T normalized by the cardiac cycle duration (856 ms). For the center plane, the x-axis begins at t* = 0.15 because it was at this time that EK started to increase due to the effect of mitral inflow. For the left and right plane, EK begins at t* = 0.2 because at this time, the side walls of the ventricle were not in the same plane as the PIV acquisition plane. This phenomenon is also seen in Fig. 6.

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Fig. 6

Circulation of vortex proximal to the aorta throughout diastole at the center, left, and right planes. The x-axis variable, t*, is the time T normalized by the cardiac cycle duration (856 ms).

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Fig. 7

EDR per unit volume for all the three measured planes, through the diastolic period of the cardiac cycle

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