| BackgroundRats are widely used in cardiovascular research as experimental model and the development of cardiac diseases in rats is similar to that in humans. With the development of research in myocardial protective treatment, more and more basic research prepared myocardial infarction model in the rats. After the targeted drug delivery was given, the ventricular global and regional function and metabolism change were observed, making the evaluation of the correlation of ultra-structural characteristic and regional myocardial function to be possible. Echocardiography is the preferred method in assessing the cardiac structure and function in small experimental animal, for its advantages such as non-invasive, economic and convenient for use.The left ventricle moved in a complex and three dimensional patterns. The motion can be divided into longitudinal, radial and circumferential components. An adequate presentation of these components could enhance the understanding of the biomechanics and pathophysiology of ischemia myocardium. Two-dimensional speckle tracking imaging (STI), without angle and frame rate dependent, is a new method to quantify left ventricular global and regional function accurately and comprehensively. STI can track the location of the echo spots in the two-dimensional grey scale image and reestablish the real time movement of the myocardium through frame by frame algorithm to obtain the myocardial deformational information in radial, circumferential and longitudinal direction.Myocardial perfusion reduced immediately after myocardial ischemia occurred, which would lead to change electrophysiological properties of cardiac myocytes and induce redistribution of regional myocardial stress and eventually caused global and regional myocardial dysfunction. Timely and accurately detection of damaged segment after myocardial infarction is essential for guiding clinical treatment and prognosis as well as defining range of infarction and extension of injury.The activity of rennin-angiotensin system and sympathic-adrenergic system increased after myocardial infarction. This would induce a series of responses, including apoptosis of cardiac myocytes, rupture of extracellular matrix and fibrosis of myocardium and finally promoting ventricular reverse remodeling. As necrotic myocardium loss contractile function, left ventricular motion was lack of coordination and ventricular cavity blood could not be drained effectively. Wall stress increased caused by ventricular pressure overload could result hypertrophy of regional myocardium and stretch of myocytes. The left ventricle significantly expanded and ultimately changed the global geometry, clinically manifested as progressive enlargement and dysfunction of the heart. Left ventricular anterior wall infarction, often involving the apex, almost was transmural injury with large necrotic area and abnormal left ventricular shape. Studies demonstrated that anterior wall myocardial infarction with necrotic area over 30% often leaded to significantly ventricular remodeling and heart failure.Previous studies identified ventricular remodeling through the wall thickness and global morphology, function and mass of the left ventricle, while few research focused on the regional myocardial function change during the process of ventricular remodeling. Bogaert et al. demonstrated that patients with first anterior wall myocardial infarction occurred remote myocardial dysfunction early after myocardial reperfusion treatment. Narula et al. demonstrated that over 50% of the severely dysfunctional myocardium in patients with coronary disease had normal blood flow, and proposed this hypocontractile but normally perfused myocardium to be called "remodeling myocardium". Jackson et al. demonstrated that systolic function of segment in peri-infarcted region persisted decreased and remote region was involved. The extentsion of remodeling myocardium could lead to ventricular enlargement and heart failure. The dysfunction of peri-infarcted myocardial region may be the important factor in the left ventricular remodeling processing. Consequently, quantifing function of segments in the myocardial infarcted region, peri-infarcted region and remote region accurately and observing the dynamic course is important for evaluating the ventricular remodeling processing.This study measured the left ventricular segmental peak systolic radial strain(RS), radial strain rate(RSr), circumferential strain(CS), circumferential strain rate(CSr) and left ventricular global circumferential strain(GCS) and global circumferential strain rate(GCSr) of rats model at baseline and after myocardial infarction by STI and aimed to 1.assess the characteristics of left ventricular segmental systolic movement in normal rats and determine the reproducibility of parameters based on STI; 2.explore the value of STI in evaluating left ventricular regional myocardial function in rats after acute myocardial infarction, and select the sensitive parameter to distinguish segment with irreversible dysfunction from segment with reversible dysfunction; 3.assess the change of systolic function of segments in left ventricular infarcted region, peri-infarcted region and remote region. This study included three parts as following:Echocardiography was performed in 20 normal adult male Wistar rats. High frame rate of two-dimensional images were recorded from the left ventricular short-axis views at the papillary muscle level. Peak systolic radial strain(RS), radial strain rate(RSr), circumferential strain(CS) and circumferential strain (CSr) of each segment and left ventricular peak systolic global circumferential strain (GCS) and global circumferential strain rate(GCSr) were measured at EchoPAC work station. Left ventricular internal diameter at diastole (LVIDd), systole (LVIDs), fractional shortening (FS) and ejection fraction (EF) were measured with anatomical M-model echocardiography. The correlations between EF and GCS, EF and GCSr were analyzed respectively. For analysis of intraobserver and interobserver variability, all measurements based on STI were repeated by two observers. Results①Regional RS, as well as RSr, was similar in all segments of mid-ventricular in short-axis view, the difference had no statistics significance (P>0.05).②CS of each segment, as well as CSr, showed heterogeneity, while the anteroseptal and anterior wall showing the largest value and the inferior wall showing the lowest value, the difference had statistics significance (P<0.05).③The left ventricular global systolic function measured by STI and anatomical M-mode echocardiography correlated significantly, while the correlation coefficient between EF and GCS was 0.84 and the one between EF and GCSr was 0.92 (P<0.05).④The parameters based on STI showed good intraobserver and interobserver reproducibility.Twenty-five Wistar rats were randomly divided into acute myocardial infarction group (n=15) and sham-operation group (n=10). The left anterior descending coronary of the acute myocardial infarction group was occlusion by suture for 45-60 minutes, while the sham-operation group underwent an identical operation but the suture was not tired. Echocardiography was performed at baseline and 24 h after reperfusion. High frame rate two-dimensional images were recorded from the left ventricular short-axis views at the papillary muscle level. Peak systolic radial strain (RS), radial strain rate (RSr), circumferential strain (CS) and circumferential strain (CSr) of each segment and left ventricular peak systolic global circumferential strain (GCS) and global circumferential strain rate(GCSr) were measured at EchoPAC work station. Left ventricular internal diameter at diastole (LVIDd) and systole (LVIDs), fractional shortening (FS), ejection fraction (EF), wall thickening rate (TR) were measured with anatomical M-model echocardiography. After echocardiography was performed, the rats were sacrificed. The heart was excised and the left ventricular short-axis slice at the papillary muscle level was stained by triphenyl tetrazolium chloride (TTC). Necrosis area ratio (NAR) of each segment in the slice and global necrosis area ratio (GNAR) of the slice were measured after staining. Based on TTC staining, ROC curve was used to analyze the accuracy of RS, RSr, CS, CSr and TR in identifying segment with irreversible dysfunction. Results①Compared with acute myocardial infarction group at baseline and sham-operation group after operation, LVIDd and LVIDs of acute myocardial infarction group after operation increased significantly respectively (P<0.05), while FS, EF and TR of anteroseptal wall reduced significantly (P<0.05).②Compared with acute myocardial infarction group at baseline and sham-operation group after operation, RS, RSr, CS, CSr decreased significantly in anteroseptal, anterior, lateral, posterior and septal wall of the acute myocardial infarction group after operation (P<0.05), In the acute myocardial infarction group after operation, RS, RSr, CS, CSr of anterosepetal, anterior and lateral wall were significantly lower than the other segments (P<0.05).③RS, RSr, CS, CSr of anterosepetal, anterior and lateral wall measured by STI were significantly correlated with corresponding NAR respectively (P<0.001), while GCS and GCSr were significantly correlated with GNAR respectively (P<0.001). EF and AWTR measured by anatomic M-mode echocardiography were significantly correlated with GNAR and corresponding segmental NAR respectively (P<0.001).④Based on TTC staining, ROC analysis showed that the parameters based on STI, such as RS, RSr, CS, CSr, had larger area under curve(AUC) than that in TR which was measured by anatomic M-mode echocardiography. CS had the best ability to identify segment with irreversible dysfunction as defined by NAR>50%, as CS had the largest AUC. Using a cut-off of-6.14%, CS had a sensitivity of 93.75% and specificity of 90.91% for distinguishing segment with irreversible dysfunction from segment with reversible dysfunction.Twenty-five Wistar rats were randomly divided into acute myocardial infarction group (n=15) and sham-operation group (n=10). The left anterior descending coronary of the acute myocardial infarction group was occlusion by suture for 45-60 minutes, while the sham-operation group underwent an identical operation but the suture was not tired. Echocardiography was performed at baseline and 1 week,4 weeks and 8 weeks after operation. High frame rate two-dimensional images were recorded in the left ventricular short-axis views at the papillary muscle level. Peak systolic radial strain(RS), radial strain rate(RSr), circumferential strain(CS) and circumferential strain (CSr) of each segment and left ventricular peak systolic global circumferential strain (GCS) and global circumferential strain rate(GCSr) were measured at EchoPAC work station. Left ventricular internal diameter at diastole (LVIDd) and systole (LVIDs), fractional shortening (FS) and ejection fraction (EF) were measured by anatomical M-model echocardiography. After echocadrdiography was performed, the rats were sacrificed. The heart was excised and the left ventricular short-axis slice at the papillary muscle level was stained by triphenyl tetrazolium chloride (TTC). Necrotic area ratio (NAR) of each segment in the slice was measured after staining. Fibrosis of left ventricle myocardium was displayed using Van Gieson stain. The echocardiographic parameters were compared in the interior-group and inter-group. Results①Based on the TTC findings, the left ventricle of the acute myocardial infarction group was divided into three regions:infarcted myocardial region (anteroseptal, anterior, lateral wall), peri-infarcted myocardial region (posterior and septal wall) and remote myocardial region (inferior wall).②Compared with baseline and sham-operated group, segmental RS, RSr, CS, CSr in the infarcted, peri-infarcted and remote region of the acute myocardial infarction group significantly decreased at 1 week after operation (P<0.05). Compared with the acute myocardial infarction group at 1 week after operation, RS, RSr, CS, CSr of some segments in infarcted region (anterior and lateral wall) and peri-infarcted region (posterior wall) significantly decreased at 4 weeks after operation (P<0.05), while RS, RSr, CS, CSr of the other segments at 4 weeks after operation showed no significant reduction (P>0.05) but were still lower than baseline significantly (P<0.05). Compared with the acute myocardial infarction group at 4 week after operation, RS, RSr, CS, CSr of each segment in infarcted, peri-infarcted and remote region at 8 weeks after operation showed no significant reduction (P>0.05).③Compared with baseline and sham-operated group, GCS and GCSr of the myocardial infarction group decreased at 1,4,8 weeks after operation significantly (P<0.05). Compared with the acute myocardial infarction group at 1 week after operation, GCS and GCSr decreased significantly at 4 week after operation (P<0.05). GCS and GCSr of the acute myocardial infarction group at 8 week after operation showed no significant reduction compared with those at 8 week after operation (P>0.05).④Compared with baseline and sham-operated group, LVIDd and LVIDs of acute myocardial infarction group increased significantly at 1 week after operation, and FS and EF decreased significantly (P<0.05). Compared with the acute myocardial infarction group at 1 week after operation, LVIDd and LVIDs increased significantly at 4 week after operation (P<0.05), while FS and EF showed no significant reduction but were still lower than baseline significantly (P<0.05). Compared with the acute myocardial infarction group at 4 week after operation, LVIDd, LVIDs, FS and EF at 8 week after operation showed no significant change but were still lower than baseline significantly (P<0.05).⑤Microscopically, after VG staining the collagenous fibre stained red, whereas the muscle, neurospongium, cytoplasm and erythrocyte stained yellow and nucleolus stained black blue or brown. Analysis of the histologic sections from the infarcted, peri-infarcted and remote myocardial region demonstrated fibrosis in various degrees. The infarcted region showed large platy fibrosis. The peri-infarcted and remote region showed patchy and focal fibrosis respectively.Conclusions1. The circumferential strain and strain rate of segment in left ventricular short-axis views at the papillary muscle level showed significant heterogeneity, while the radial strain and strain rate were similar in all the segments.2. Varying damages were presented in left ventricular regional myocardium in rat model of acute myocardial infarction. CS based on two-dimensional speckle tracking imaging is the best parameter for distinguishing segment with irreversible dysfunction from segment with reversible dysfunction. 3. The myocardial peri-infarcted and remote region with normal perfusion in rat model of acute myocardial infarction revealed dysfunction at lweek after operation, and persisted to 8 weeks.4. Two-dimensional speckle tracking imaging is a sensitive method in evaluating left ventricular global and regional myocardial function in rat model of acute myocardial infarction. |
