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ORIGINAL ARTICLES
Strain Rate Acceleration Yields a Better Index
for Evaluating Left Ventricular Contractile
Function as Compared with Tissue Velocity
Acceleration During Isovolumic Contraction
Time: An In Vivo Study
Xiaokui Li, MD, Michael Jones, MD, Hui-Fang Wang, MD, Crispin H. Davies, MD,
Julia C. Swanson, BS, Ikuo Hashimoto, MD, Rosemary A. Rusk, MD,
Sebastian T. Schindera, MD, Brent J. Barber, MD, and
David J. Sahn, MD, Portland, Oregon, and Bethesda, Maryland
Objective: Our study aimed to investigate whether
strain rate acceleration (SRA) during isovolumic
contraction time (IVCT) could serve as a sensitive
indicator of myocardial function.
Methods: A total of 8 sheep underwent occlusion of
left anterior descending coronary artery or diagonal
branches and 2 sheep underwent left circumflex coronary artery occlusion to create septal, apical, or basal
segment myocardial ischemia 19 to 27 weeks before
the study. Baseline, volume-loading, dobutamine, and
metoprolol infusion were used to produce 4 hemodynamic stages for each sheep. Doppler tissue imaging
was acquired using a 5-MHz probe (GE/VingMed Vivid
Five, GE Medical Systems, Milwaukee, Wis) on openchest animals using the liver as a standoff at the apex.
Using software (EchoPac, GE Medical Systems), SRA
during IVCT was calculated and compared with tissue
velocity acceleration (TVA) during IVCT from areas
Ventricular contractile function in the ischemic
heart has been previously evaluated by ventricular
pressure acceleration (peak dP/dt) during isovolumic contraction time (IVCT) measured during cardiac catheterization.1 Noninvasive 2-dimensional
(2D) echocardiography methods such as spectral
Doppler, 2D color, and color M-mode Doppler2-4
have served as alternative ways of measuring systolic
and diastolic functions. In recent years, Doppler
tissue imaging (DTI), strain, and strain rate (SR)
From the Clinical Care Center for Congenital Heart Disease,
Oregon Health and Science University, Portland, Oregon, and the
National Heart, Lung, and Blood Institute, Bethesda, Maryland
(M.J.).
Reprint requests: David J. Sahn, MD, the Clinical Care Center for
Congenital Heart Disease, Oregon Health and Science University,
3181 SW Sam Jackson Park Rd, L608, Portland, OR 97239-3098
(E-mail: [email protected]).
Copyright 2003 by the American Society of Echocardiography.
0894-7317/2003/$30.00 ⫹ 0
doi:10.1067/j.echo.2003.07.006
located in the normal and ischemic zones. Also, invasively monitored left ventricle dP/dt was measured as
reference contractile function.
Results: Both TVA and SRA during IVCT showed
higher values for normal tissue than for ischemic area
(P < .0001). SRA for normal wall segments changed
significantly during the 4 stages (P ⴝ .01) with corresponding changes on high-fidelity left ventricular
pressure catheters (r ⴝ 0.92). TVA over normal segments showed no significant change (P ⴝ .29) in the 4
hemodynamic stages. Both TVA and SRA of the ischemic segments showed no significant change with
pharmacologic maneuvers or loading conditions.
Conclusions: SRA and TVA during IVCT are both
useful indicators for detecting abnormal heart wall
motion. However, SRA tends to be more sensitive
than TVA for differentiating the response to stress
conditions. (J Am Soc Echocardiogr 2003;16:1211-6.)
imaging (SRI) of myocardial mechanics are new
modalities that have attracted interest as the new
modalities for quantitatively evaluating abnormal
cardiac segmental motion.2,5 Tissue velocity (TV)
reflects the net effect of the contractile, elastic
recoil, and passive filling events in the region of
interest including those caused by tethering from
adjacent segments and it is affected by translational
artifacts, whereas SR is calculated along the echocardiography beam direction as the velocity difference between 2 points on the muscle divided by the
distance between them, which more directly reflects absolute local segmental myocardial deformation and function.6,7
Both pulsed wave Doppler and 2D Doppler-based
myocardial TV methods have been reported to be
useful for assessing wall-motion acceleration time
during IVCT when evaluating ventricular contractile
function, and both have been compared with the
peak left ventricle (LV) first derivative dP/dt.8-10 As
yet, there is no report of the use of SR acceleration
1211
Journal of the American Society of Echocardiography
December 2003
1212 Li et al
(SRA) during IVCT to evaluate ventricular contractile function. We measured SRA and compared it
with TV acceleration (TVA) to investigate whether
SRA alone would be a strong indicator in defining
abnormal ventricular function in ischemic heart
disease.
METHODS
Animal Preparation
Occlusion of left anterior descending coronary artery, or
diagonal or proximal circumflex branches, to create regional chronic myocardial infarction and aneurysm formation was performed 19 to 27 weeks before this study.
After recovery, the follow-up study session was scheduled
at which time echocardiography DTI was obtained on
open-chest anesthetized animals. Accordingly, 8 juvenile
sheep, weighing 35 to 47 kg (mean: 40.13 ⫾ 4.22 kg),
underwent repeated thoracotomy under general anesthesia with 2% isoflurane and oxygen. Assisted ventilation
was given by a volume-cycle respirator. In the follow-up
study, after baseline measurement, volume-loading (500
mL blood transfusion), dobutamine (2.5 ␮g/kg/min), and
metoprolol infusion (5 mg) were performed at least 1 hour
apart from each other to produce 4 hemodynamic conditions for each of the sheep. All surgical methods and
animal treatment procedures were approved by the Animal Care and Use Committee of the National Heart, Lung
and Blood Institute in Bethesda, Md.
Hemodynamic and Pressure Measurement
Aortic and ventricular pressures were monitored by manometer-tipped catheters (model SPC-350, Millar Instruments Inc, Houston, Tex) positioned intramurally and
recorded simultaneously on the same physiologic recorder. A hydrostatic standard was used for calibration of
all pressure recordings.11
DTI and Data Analysis
Epicardial 4-chamber views including LV outflow tract and
aortic valve were obtained using a 5-MHz probe controlled
by a scanner (GE/VingMed Vivid Five, GE Medical Systems, Milwaukee, Wis) with simultaneous electrocardiogram monitoring. After 2D imaging was optimized, color
TV imaging (TVI) was obtained with maximal frame rates
(⬎90/s). The temporal and spatial (radial and lateral)
averaging were disabled during acquisition to obtain
better images. An aliasing velocity for DTI was set at 10 to
16 cm/s, depending on wall-motion velocity and depth of
imaging, for each hemodynamic condition. A section of
each animal’s liver was used as the standoff between the
apex and the transducer to enhance apical signal quality;
the fresh liver pieces were saved in a 0.9% sodium solution
ready for use. DTI scan-line data loops, including 3 to 4
heart cycles, were saved both on hard disk and magneticoptical disk for offline data analysis. TV and SR analysis
was performed with software (EchoPac, GE Medical Systems) installed on a computer (G4, Macintosh, Apple,
Cuppertino, Calif). The start and end points of IVCT were
carefully selected on the electrocardiogram (the first
frame after R wave) and confirmed by visualization of
mitral valve closure and subsequent aortic valve opening.
The sampling points for TVI and SRI were selected at 2
regions: an ischemic or aneurysm segment in the apical/
anterior region or placement of inferior wall and a normal
segment. Sampling points on myocardial walls were oriented toward a zone of significant velocity change as the
boundary of the aneurysm or the ischemic segment and
had also been verified visually during the experiment by
looking at the corresponding zone, which was pale and
moving paradoxically. For both segments, we could derive
TVI and SRI traces over the same sampling zones by
changing the quantification mode of the software. Thus,
the velocity and SR tracing were derived for identical
locations. Curves were averaged by software for 3-pixel
smoothing. SRI sample size was 3 to 5 mm. The TVA was
calculated as end-IVCT velocity value minus initial IVCT
velocity value divided by time difference (seconds) between the 2 points (cm/s2 is TVA’s unit) (Figure 1).
TVA ⫽ (V2-V1)/(t2-t1)
(1)
The SRA was calculated as end-IVCT SR value minus the
initial IVCT value divided by time duration of the IVCT
(seconds⫺2 is SRA’s unit) (Figure 1).
SRA ⫽ (SR2-SR1)/(t2-t1)
(2)
LV positive dP/dt was measured from the digitized pressure recordings for these same heart beats as reference
data for global ventricular contractile function.
Statistical Analysis
Quantitative data are presented as mean value ⫾ SD.
Differences in continuous variables such as TV slope, SR
slope, and dP/dt between the 4 stages were analyzed by
1-way analysis of variance. Correlations between TVA,
SRA, or both with high-fidelity LV dP/dt were determined
by simple linear regression method. Statistical significance
was defined as P ⬍ .05.
RESULTS
Absolute values are used for both TV and SR results
during IVCT because the direction of motion in
expansion/contraction sometimes changed.
TVA
TVA (mean) showed significant differences between
normal and infarcted segments as normal 73.75 ⫾
27.97 cm/s2 versus infarcted 36.17 ⫾ 15.76 cm/s2, P
⫽ .03 for baseline; normal 111.69 ⫾ 41.62 cm/s2
versus infarcted 40.79 ⫾ 15.04 cm/s2, P ⬍ .0001 for
blood infusion; normal 98.93 ⫾ 58.44 cm/s2 versus
Journal of the American Society of Echocardiography
Volume 16 Number 12
Li et al 1213
infarcted 36.18 ⫾ 13.16 cm/s2, P ⫽ .001 for dobutamine; and normal 58.22 ⫾ 36.82 cm/s2 versus
infarcted 25.21 ⫾ 11.77 cm/s2, P ⬍ .0001 for
metoprolol (Figure 2 and Table 1). For these 4
hemodynamic stages, however, there was no significant difference between the normal TVA during the
4 stages (P ⬎ .05) or that of the infarcted wall
segment (P ⫽ .29).
SRA
SRA showed even more significant changes between
normal and infarcted segments where normal 63.07
⫾ 16.58 seconds⫺2 versus infarcted 15.12 ⫾ 5.07
seconds⫺2 for baseline; normal 71.47 ⫾ 14.01 seconds⫺2 versus infarcted 18.16 ⫾ 6.52 seconds⫺2 for
blood infusion; normal 95.05 ⫾ 24.87 seconds⫺2
versus infarcted 24.57 ⫾ 8.53 seconds⫺2 for dobutamine; and normal 66.49 ⫾ 13.84 seconds⫺2 versus
infarcted 16.99 ⫾ 6.56 seconds⫺2 for metoprolol
(Figure 2) for all 4 pairs (P ⬍ .0001) (Table 1). For
the normal wall segment, unlike the TVA, SRA
showed significant differences between the 4 hemodynamic stages (P ⫽ .01) whereas SRA for the
infarcted wall segments still showed no significant
change between the 4 stages (P ⬎ .05).
Comparison of LV dP/dt with TVA and SRA in
Normal Segments
The reference cardiac output (CO) data during this
study session were significantly different for the 4
hemodynamic stages. The mean baseline CO was
1.61 ⫾ 0.46 L/min; mean CO after blood loading was
2.2 ⫾ 0.47 L/min; mean CO after dobutamine infusion was 2.0 ⫾ 0.48 L/min; and CO after metoprolol
was 0.91 ⫾ 0.38 L/min (P ⬍ .05). Heart rate also
increased on dobutamine infusion from 100 ⫾ 12
bpm to 142 ⫾ 25 bpm. LV positive dP/dt for the 4
stages showed changes concordant with reference
CO that were significantly different from each other:
dP/dt was 1322.96 ⫾ 668.07 mm Hg/s for baseline;
2028.93 ⫾ 798.54 mm Hg/s for blood infusion;
2074.6 ⫾ 1191.30 mm Hg/s for dobutamine; and
648.15 ⫾ 206.94 mm Hg/s for metoprolol (P ⫽ .01)
(Figure 3). For all 4 stages, linear regression showed
that both SRA (r ⫽ 0.81) and TVA (r ⫽ 0.79) have
good correlation with positive dP/dt (Figure 4).
DISCUSSION
Most clinical patients with heart disease are routinely evaluated for ventricular function by echocardiography. In addition to the visual assessment of
wall motion, quantitative parameters such as ventricular ejection fraction are regularly used to quantify LV function, as are end-diastolic volume, stroke
volume, and fractional shortening. Most of the parameters mentioned above reflect a complex inter-
Figure 1 Comparison of tissue velocity (TV) and strain rate
(SR) acceleration from aneurysm (green curve) and normal
(yellow curve) myocardium. Slopes within isovolumic contraction time (IVCT) (red lines) are measured as TV/SR
acceleration. ECG, Electrocardiogram.
action of preload and afterload conditions. There are
some other methods independent from the loading
conditions that have been used such as LV dP/dt,
preload recruitable stroke work, and peak elastance.
However, LV dP/dt, preload recruitable stroke work,
and elastance are all invasive and cumbersome,
especially preload recruitable stroke work and elastance, which are obtained by determination of serial
pressure area relationships that are often limited by
the extrapolation of LV volumes from cross-sectional
areas.12
Color-encoded DTI is a recently developed technique for myocardial velocity measurement.13-16
This quantitative method addresses many of the
inherent limitations encountered by previous methods describing myocardial wall motion. It color
encodes all the velocity components within the
myocardial wall and allows display in real time. A
number of recent studies6,17 have been performed,
however, that demonstrated that cardiac translation
and tethering by adjacent tissue have limited the
ability of DTI to provide quantitative data on regional myocardial function because these effects
confound myocardial tissue motion. For this reason,
some researchers have tried to measure the TVA
during IVCT to minimize the influence of tethering.18-21 We measured TVA in this study, which
provided good data to clearly differentiate the normal or the ischemia/aneurysm segments. However,
TVA failed to detect the normal segment wallmotion differences between the 4 hemodynamic
states in our study.
Doppler-derived strain/SRI on the basis of TVI was
projected as being more specific for evaluating
segmental motion than TVI.6,7 Because velocity data
alone can only detect wall motion including tether-
Journal of the American Society of Echocardiography
December 2003
1214 Li et al
Table 1. Comparison of TV and SR acceleration at normal and aneurysmal segments during the 4 stages
Normal
Stage 1
Stage 2
Stage 3
Stage 4
TV Acceleration (cm/s2)
Aneurysm
⫺73.75 ⫾ 27.97
⫺111.69 ⫾ 41.62
⫺98.93 ⫾ 58.44
⫺58.22 ⫾ 36.82
36.17 ⫾ 15.76
40.79 ⫾ 15.04
36.18 ⫾ 13.16
25.21 ⫾ 11.77
P value
Normal
.03
⬍.0001
.001
⬍.0001
⫺63.07 ⫾ 16.58
⫺71.47 ⫾ 14.01
⫺95.05 ⫾ 24.87
⫺66.49 ⫾ 13.84
SR Acceleration (s2)
Aneurysm
15.12 ⫾ 5.07
18.16 ⫾ 6.25
24.57 ⫾ 8.53
16.99 ⫾ 6.56
P value
⬍.0001
⬍.0001
⬍.0001
⬍.0001
TV, Tissue velocity; SR, strain rate.
Figure 2 Bar graph of averaged tissue velocity acceleration (TVA) and strain rate acceleration (SRA); both
showed significant change between aneurysmal and normal myocardium. Only SRA showed significant
change between each hemodynamic stage. Stage 1, Baseline; stage 2, volume loading; stage 3, dobutamine;
stage 4, beta block.
Figure 3 Bar graph for peak positive dP/dt in 4 dynamic
stages. There is significant change (P ⫽ .01) between 4
stages. LV, Left ventricular; stage 1, baseline; stage 2, volume loading; stage 3, dobutamine; stage 4, beta block.
ing-induced movement rather than local deformation, they may be influenced by overall heart motion. Strain and SR is obtained from 2 points of the
myocardium; the velocity difference between the 2
points reflects the tissue deformation without confounding factors such as tethering or translational
problem. Thus, a significant change in SRA may
reflect a better sensitivity to changes in regional
function. This study focused on SRA, which indicates the acceleration of SR during isovolumic contraction. Some colleagues have indicated that SR
curve is sometimes noisier than TV curve, which
limits the reproducibility of SR amplitude.6,22,23
However, other researchers reported that SR amplitude has a good correlation with peak dP/dt obtained by LV pressure measurement that reflects LV
contractility.24,25 Abraham et al23 suggested using
timing of phasic changes of SR for evaluating LV
relaxation.
Passive motion is a possible reason for some fairly
high TVA from apical aneurysmal segments on TVI.
The SRA values would be expected to be more
accurate as they are less affected by translational
motion. Also, strain and SRI analysis by the method
we used detects the tissue deformation only when it
occurs along the direction of the echocardiography
beam, thus, position of the wall and the Doppler
angle could affect the SR curve and its interpretation.6
We used the longitudinal apical-based scanning in
our study because it best differentiates the motion
Journal of the American Society of Echocardiography
Volume 16 Number 12
Li et al 1215
Figure 4 Linear regression of peak dP/dt versus tissue velocity acceleration (TVA) and strain rate
acceleration (SRA); both showed good correlation with peak positive dP/dt value. LV, Left ventricular.
and function of normal and abnormal wall segments.26,27 Radial strain, however, is reported to
show the radial myocardial deformation in a shortaxis view, which shows opposite waveforms from
the long-axis view and has demonstrated more complicated patterns than the longitudinal strain.28,29
Our data were expressed as absolute values because
our goal was to detect the differences of acceleration between normal and abnormal myocardial segment motion or deformation during IVCT regardless
of their vectorial direction.
Limitations
Only 8 sheep were examined in this study; further
study is needed for verification of SR slope as a
reproducible and standardized numeric index of
evaluation of ventricular contractile function. Openchest study allowed us to have the best-quality
echocardiographic view of the heart possible. Transthoracic and transesophageal studies for the same
purpose would have the potential limitation of echo
windows. Measuring an acceleration during a short
time interval like IVCT requires enough samples to
clearly delineate the start and end points. At the
frame rates we used, we were getting 4 to 6 samples
from the start to the end of this period. Standard 2D
imaging usually allows us to image only 1 plane at a
time and we did not calculate SRA other than at the
2 positions in the same plane (infarction/aneurysm
and basal and inferior). A 3-dimensional SR reconstruction method, however, is currently under investigation in our laboratory, which would provide a
more robust sampling of myocardial segments. Although presently cumbersome, 3-dimensional imaging has the potential of quantifying myocardial function from any desired view.
Conclusions
Our study suggests that myocardial SRA and TVA
both are helpful in evaluating abnormal ventricular
contractile function. However, SRA is more sensitive
than TVA to detect and differentiate between normal, ischemic, or aneurysmal segments.
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