Porcine Mitral Valve Chordae Tendineae: An In Vitro Investigation
1
Jennifer Ritchie1*, Jorge Jimenez1, Zhaoming He1, Michael Sacks2, and Ajit Yoganathan1
Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, Georgia and
2
Department of Bioengineering, University of Pittsburgh, Pittsburgh, Pennsylvania
School of Biomedical Engineering
Suite 1126, IBB Building
315 Ferst Drive, Georgia Institute of Technology
Atlanta, GA 30332-0535
Abstract
The material properties of the chordae tendineae of the mitral valve are important for the understanding of leaflet coaptation
configuration and chordal rupture; however, 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 tendineae in vitro under
normal physiologic loading conditions. Two high speed (500 fps), high resolution (1280 X 1023) cameras were set up to capture
the movement of graphite markers attached to the central area of the chordae using autotracking software developed in house. A
uniaxial test with the same loading conditions as the left heart simulator was completed on the same chordae using the same
markers. The load/strain curve was obtained and compared with the strain/time curve from the flow loop measurements. The
final outcome, strain/time and load/time curves, were compared to the pressure distribution obtained from the flow loop
measurements to determine the behavior of the chordae tendineae during the cardiac cycle. A non-invasive technique was
developed to measure chordal force distribution of the mitral valve. This technique allows further understanding of the behavior of
a complex biomaterial under physiologic loading.
Introduction
Mitral valve prolapse (MVP), a condition in which the mitral valve leaflets prolapse upward into the left atrium rather than forming
a line of coaptation as the heart contracts, is the most common heart valve abnormality in the United States. Recent studies
estimate that approximately 2 to 6% of the adult population in the United States have MVP[1]. The most common cause of chronic
mitral valve regurgitation (MR) is MVP. The mitral valve has a finely tuned closing mechanism composed of the mitral leaflets,
chordae tendineae, and papillary muscles that work together to ensure proper valve closure; hence, preventing mitral valve
regurgitation.
Cardiac surgeons currently perform mitral valve replacement procedures instead of mitral valve repair. A complete understanding
of the mitral valve complex is needed before surgeons can successfully complete mitral repair. Surgeons are using Goretex®,
ePTFE, and polypropelene sutures to replace chordae which have become elongated or shortened due to chordae pathologies or
[2]
other heart pathologies such as ischemia and rheumatic fever which can cause alterations of the mitral apparatus . In vivo
studies need to be conducted on which the chordae are replaced with these sutures. The mechanical properties of these sutures
also need to be compared with the properties of the chordae tendineae before surgeons can use these materials as
replacements.
The material properties of the chordae tendineae of the mitral valve are important for the understanding of leaflet coaptation
configuration and chordal rupture; however, there is limited information about the mechanical properties of the chordae during
physiologic loading. Few studies have been completed that describe the material properties of the chordae; however, these
studies do not test the chordae under physiologic conditions, but rather at strain rates which do not reflect in vivo strains. Also, no
studies have been completed in vitro that describe the strain relationship of the chordae. Therefore, the objective of this study
was to design an experimental method in order to determine the strain exhibited on the chordae during physiological conditions
utilizing an in vitro model.
Methods
For this study, fresh porcine hearts were attained from a local abattoir; the mitral valves were extracted, preserving the chordae
tendineae and papillary muscles, and stored in 0.9% saline solution. Mitral valves with similar chordae insertion patterns and
similar orifice size were used. After valve extraction, the papillary muscles were wrapped in Dacron cloth and attached to the
papillary muscle holding disks. The mitral valve was then sutured to a planar D-shaped annular board using 3-0 sutures (braided
silk, Ethicon, NJ, USA). Four graphite markers, approximately 0.368 mm in diameter, were affixed using glue to the central region
of the desired chordae tendineae where no branching was observed.
The mitral valve was inserted into the modified Georgia Tech left heart simulator[3,4,5] and was run under physiologic flow and
[3,5]
(cardiac output: 5 L/min, peak trans-mitral pressure: 120 mmHg, heart
pressure conditions with the valve in the normal position
rate: 70 BPM, systolic duration: 300 ms). Flow and pressure curves were saved on a laptop computer using a Labview based
program for offline processing.
Resistance
High Speed Cameras
Flow Direction
Atrial
Reservoir
Compliance
PM holder
Flow
Probe
Mitral Valve
Compressed Air
Bladder Pump
Pressure
Transducer
Data Acquisition System
Figure 1: Schematic of the Georgia Tech Left Heart Simulator
The left ventricle chamber is an octagon shaped acrylic chamber designed to allow optical access to all chordae of the mitral
valve. The sides of the model are arranged at 60° angles to each other such that the cameras could be arranged perpendicular to
the surface of the model in order to reduce the effects of refraction. The front section of the chamber, which holds the aortic
valve, forms a 135° angle with the frontal face of the chamber to ensure geometrical positioning of the mitral and aortic valves. A
25 mm tilting disk mechanical valve (Omnicarbon 3523, MedicalCV Incorporated, USA) was used in the aortic position.
Dual camera stereophotogrammetry was used to obtain pictures of the chordae tendineae during the cardiac cycle. Two high
speed, high resolution cameras (Basler a504k, Basler Vision Technologies, PA) were positioned on tripods approximately 0.5 m
from the ventricle chamber and approximately 30° relative to each other. Both cameras were positioned such that they were
centered on the desired chordae, and light and lens were adjusted to achieve maximum contrast between the markers and
background. Images were recorded simultaneously at 500 frames per second with an 8 bit gray-scale and a resolution of 1280 ×
1023 pixels as shown in Figure 2. A trigger signal was sent to a TTL module to ensure that the cameras and pressure data were
recorded simultaneously.
Figure 2: Pictures from cameras A and B
After collecting the data, a calibration cube 10 mm on edge was positioned in the area of the markers such that seven corners of
[6,7]
the cube could be clearly seen with both cameras. This calibration technique has been described in previous publications .
The images of the cube were taken with both cameras, digitized, and used later for direct linear transformation (DLT). The
pictures were processed using an in-house, autotracking algorithm based on Matlab[8].
A calibrated standard direct linear transformation (DLT) method was used to transform the 2D pixel coordinates to 3D spatial
coordinates[6,7]. The DLT parameters were obtained through discretization of the seven corners of the 10 mm calibration cube.
The reference frame for the strain calculations was determined during the cube calibration. With no pressure on the valve, the
valve was in a relaxed state during calibration. This relaxed state was used as the zero strain reference. A wire of known spatial
length was used as a ruler, and positioned parallel to the maker placements on the chordae during the calibration to obtain
pictures of the chordae in the relaxed state. Pictures were obtained for both cameras and subsequently imported into Sigma
Scan Pro (Jandel Scientific, San Rafael, CA, USA), and the distance between each centroid of the markers was recorded.
Uniaxial Tests
The uniaxial tests were performed no more than 24 hours after the in vitro flow loop experiments. The same chordae and the
same markers were utilized for both experiments. The chordae were preloaded at a 40% strain per second rate up to 2 N force
for 25 cycles. This force was chosen since previous studies have shown that the strut chordae withstand a force of approximately
[3]
1.5 N during the cardiac cycle . After preloading, the chordae were then immediately loaded at 40% strain per second up to 2 N
for five cycles.
The pictures of the uniaxial test were processed using the same protocol as the in vitro flow loop picture files. The uniaxial data
was processed using the autotracking code to track the same markers tracked in the flow loop. Using the calibration picture taken
before the test, the pixel coordinates were transferred to spatial coordinates. The initial length used for the flow loop data was
used as the initial length for the uniaxial data.
After data collection from the in vitro flow loop and the uniaxial test, load versus strain curves can be generated. The data from
the uniaxial test must be curved fitted in order to generate a relationship between the tension and strain. Due to the shape of the
uniaxial data, a linear and exponential curve fit is necessary. The section of the curve up to the “toe” region was fit with a linear
curve. The section of the curve after the “toe” region was fit with an exponential curve.
Results
Flow Loop
A total of six valves and eleven strut chordae were examined during this study. Figure 3 depicts the strain experienced during the
cardiac cycle under physiologic flow conditions. The valves were tested with an average peak trans-mitral pressure of 106.1 ± 7.7
[3,5]
mmHg and an average flow rate of 5.41 ± 0.35 L/min. When tested under the defined normal position , the valves coapted well
with no regurgitation.
The strain exhibited on the strut chordae during the cardiac cycle is described in Figure 3. The valve closes between 0 and 70
msec, experiences a strain plateau during valve closure between 70 and 360 msec, and opens between 360 to 432 msec.
Between 432 and 508 msec, the valve is opening to its maximum.
After 508 msec, the valve remains open until 860 msec when the next cycle begins. Notice the small oscillations in the chordae
when the valve is open and the fluid is flowing from the atrial chamber, through the mitral valve, and into the left ventricle
chamber. The maximum amount of strain experienced during the cardiac cycle was approximately 6.95% and was experienced
at 270 msec during valve closure.
8
120
Chordae Strain
Trans-mitral Pressure
90
4
60
2
30
0
0
0
0.2
0.4
0.6
0.8
1
-2
-30
-4
-60
-6
Pressure, mmHg
%Strain
6
-90
Time, sec
Figure 3: An example of strain versus time relationship for the in vitro measurements of strain using dual camera stereo
photogrammetry
Uniaxial
Preliminary experiments were conducted to determine if there was a significant error when using the cross head displacement or
the camera system to measure strains in a localized region. Figure 4 shows two curves, the cross head displacement strain, and
the strain determined using the camera system.
4.5
DDL
4
Camera
3.5
3
Load, N
2.5
2
1.5
1
0.5
0
0
2
4
6
8
10
12
-0.5
% Strain
Figure 4: Uniaxial test results comparing the camera system and the cross head displacement strain.
The uniaxial tests described the amount of hysteresis inherent in the chordae. The two curves in Figure 5 show the loading and
unloading behavior of the chordae. The loading and unloading curves were fit with two different curves. The first section of the
curve can be described with a simple linear fit. The second section of the curve can be described with an exponential fit. The
“toe” region between A and B is the physiological range in which the tissue normally functions. The section of B to C is
considered the reserve strength of the chordae.
2.5
C
2
Load, N
1.5
B
1
0.5
A
0
0
1
2
3
4
5
6
-0.5
%Strain
Figure 5: Uniaxial test of the strut chordae of the mitral valve
Summary
In this study, dual camera stereo photogrammetry was used to measure in vitro strains experienced by the chordae tendineae of
the mitral valve under physiologic function. This technique has been used in previous studies to measure strains on the surface
[9]
of the anterior leaflet of the mitral valve , but has never been utilized to measure strains on the chordae tendineae. This is the
first known technique to measure strains on the chordae in vitro during normal physiologic function. The results presented here
show the functionality of the system to measure in vitro strains on the chordae in a dynamic environment.
Acknowledgments
This research was supported by a grant from the National Heart Blood and Lung Institute, Grant Number HL52009
Holifield Slaughterhouse
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