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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. REFERENCES 1. 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