Evaluation Of Subendocardial Strain And Displacement Of Left Ventricle Using Ultrasonic Velocity Vector Imaging: A Canine Model With Acute Myocardial Ischemia

Objective: A regional decrease in myocardial perfusion resulting in regional ischemia, which will affect electrophysiological properties and mechanical pattern of the cardiac cells and induce redistribution of stress and strain. Left Ventricular electrical-mechanical dyssynchrony is related to reduced regional and global cardiac function. Further mechanical pattern study of the left ventricular subendocardium before and after ischemia and its relationships between myocardial motion and cardiac function is necessary. This research is designed to demonstrate the changes of systolic circumferential strain (CS), radial strain (RS) and radial displacement (RD) of the LV at subendocardium in short-axis views and global systolic function using a canine model with acute myocardial ischemia by ultrasonic velocity vector imaging, and to establish the correlationships among these myocardial mechanical parameters, to assess the mechanical dysynchrony of the LV at subendocardium.Methods:Twelve healthy female Beagle dogs,weight 10 to12 kg. The chest was opened via left thoracotomy at the fourth or fifth rib space. The pericardium was opened to form a cradle for the heart. Left anterior descending coronary artery was ligated to induce acute myocardial ischemia. At baseline before ischemia and 20 minutes after ischemia, the two-dimensional dynamic gray-scale images of the standard LV short-axis views at the levels of mitral annulus, papillary muscle and apex, and the apical four chamber, two-chamber and long-axis view of LV were acquired and transferred to VVI workstation for off-line analysis in three consecutive cardiac cycles of the acquired loops. The subendocardial peak systolic CS, RS and Rd at eighteen segments and of each global short-axis view of LV were measured ; The systolic time to peak circumferential strain(Tcs), systolic time to peak radial strain(Trs) and systolic time to peak radial displacement(Trd) were derived from LV short-views. The standard deviation of Tcs, Trs and Trd (Tcs-18SD, Trs-18SD and Trd-18SD) of 18 segments were used as indicators of LV contraction dyssynchrony ; LV ejection fraction and stroke volume were calculated using Simpson's method. The linear correlationships among them were analyzed.Results:(1)Compared with the values at baseline, LVEDV, LVESV increased and LVEF decreased significantly (P<0.05) after ischemia.(2) At baseline, there were no significant differences among the segmental and global systolic peak CS, RS and RD at different short axis levels of LV.(3)After acute ischemia, the systolic peak CS, RS and RD at subendocardial abnormal segments and global papillary muscle and apex views were significantly lower than those at baseline(P<0.05); Compared with the values at baseline, there were no significant differences among the values of Tcs, Trs(except the middle segment of the inferior wall )and Trd(except the apical segment of the anterior septum); Tcs-18SD were significantly higher than those at baseline(P<0.05).(4)the systolic segmental and global peak RD decreased significantly from base to apex after ischemia ;(5)There was a signifcant correlationship between CS and RD of 17 segments (except the middle segment of the lateral wall after ischemia )and global CS in each short axis level of LV before and after ischemia(r=0.662～0.995, P<0.05～0.001); The systolic peak RS of six segments and each global short-axis walls (except the apical level at baseline ) was correlated with RD(r=0.580～0.916, P<0.05～0.001).(6)All the systolic global peak RDs at different LV levels were not correlated with SV and EF statistically.Conclusion:The subendocardial strain and displacement may reflect the changes of regional and global myocardial systolic function induced by acute ischemia sensitively ; The significant correlationship between the left venricular circumferential systolic strain and radial displacement at subendocardium might indicate that circumferential deformation of myocardium result in the changes of the radial displacement mainly. The circumferential mechanical dyssynchrony of the left venricular subendocardium was existed commonly in acute myocardial ischemia. The sum of different direction motion of regional and global myocardium at different LV levels may reflect the changes of cardiac function accurately during acute ischemia.