Assessment Of Regional Myocardial Function In Dogs With Acute Subendocardial Ischemia By Using Quantitative Tissue Velocity Imaging Combined With Real Time Myocardial Contrast Echocardiography

PART I. Studies on Preparation of Acute Subendocardial Ischemia and Infarction in Closed-chest Dogs(1) Detection of Perfusion Territory of Left Circumflex Coronary Artery in Dogs with Intracoronary Myocardial Contrast EchocardiographyObjective To assess the feasibility of intracoronary myocardial contrast echocardiography in closed-chest domestic mongrel dogs with less weight, and to determine the left circumflex coronary artery (LCX) distribution.Methods 5 dogs weighing less than 15 kg involved in this study. The tip of suitable 6F Judkins catheter for left coronary artery was first introduced to the aortic root parallel to the orifice of left main coronary artery (LCMA) by using standard coronary catheterization technique under echocardiography guidance, and then 3F micro-catheter was advanced to LCMA or the proximal LCX via the Judkins catheter. Myocardial contrast echocardiography (MCE) was performed during the intracoronary infusion of self-made microbubbles through that micro-catheter.Results Modified Judkins left catheters with the second curvature of 3 cm or 2.5 cm were suitable for implementation of intracoronary myocardial echocardiography in those smaller domestic dogs. After the intra-LCX injection of microbubbles, the lateral, posterior, inferior wall of left ventricle and ventricular septum were opacificated simultaneously before the beginning of right ventricle opacification.Conclusions Intracoronary MCE can be performed successfully in closed-chest domestic dogs, and left ventricle are dominantly supplied by left coronary artery.(2) Preparation of Acute Subendocardial Ischemia and Infarction in Closed-chest Dogs under Echocardiography Guidance Objective To develop a new closed-chest animal model of acute subendocardial ischemia/infarction.Methods In 6 closed-chest anesthetized dogs, polyvinyl alcohol (PVA) microspheres of 200～300μm in diameter were repetitively injected into the proximal left circumflex coronary artery (LCX) via catheter until subendocardial perfusion abnormality was detected by intracoronary real time myocardial contrast echocardiography (MCE) with self-made microbubbles. The extent of myocardial infarct was determined with triphenyl tetrazolium chloride in the end.Results Subendocardial opacification abnormalities were detected in 18 left ventricular myocardial segments on real-time MCE after several intracoronary injection of microspheres. The total dose of microsphere injected into the LCX of every dog was (3.43±0.38)×10⁴. Finally, subendocardial infarction confirmed by TTC staining occurred in 16 segments of these abnormal perfusion segments.Conclusion Acute subendocardial ischemia/infarction in dogs could be successfully induced by intracoronary injection of PVA microspheres with standard catheterization technique under ultrasound guidance.PART II. Quantitative Assessment of Regional Myocardial Perfusion in Dogs with Subendocardial Ischemia/Infarction by Using Real-time Myocardial Contrast EchocardiographyObjective To explore the value of real-time myocardial contrast echocardiography in detection of experimental subendocardial ischemia and infarction.Methods Polyvinyl alcohol microspheres of 200 to 300μm in diameter, with the amount of 3.5×10⁴ particles, were injected into the proximal portion of left circumflex coronary artery in 9 closed-chest dogs via micro-catheters, to induce myocardial ischemia by embolization. Real-time myocardial contrast echocardiography (RT-MCE) with self-made contrast agents were performed at baseline and 10, 60, 120 minutes after embolization, respectively. The regional myocardial contrast intensity versus refilling time plots were fitted to an exponential function: y=A·(1-exp^-k·t)+B, the product of A and k represented myocardial blood flow. The extent of myocardial infarct was determined with triphenyl tetrazolium chloride (TTC) staining 4 hours after intracoronary embolization.Results RT-MCE showed that normalized A·k in the subendocardial myocardium decreased severely (P<0.05) in 19 left ventricular myocardial segments after embolization, including 8 middle segments of lateral wall, 7 middle segments of posterior wall and 4 middle segments of inferior wall. However, there was no statistically difference in epicardial myocardium before and after embolization. The normalized A·k values at 60 minutes and 120 minutes after embolization improved somewhat compared with those at 10 minutes (P<0.05). Moreover, the decrement of A·k at 60 minutes correlated better with the increment of ultimate myocardial infarct area determined by TTC staining than 10 minutes. However, the systolic wall thickening in these ischemia segments did not decrease significantly after embolization.Conclusions Real time MCE-derived A·k value can be a sensitive index for early detection of subendocardial ischemia and an effective predictor of transmural extent of infarct.PART III. Assessment of Regional Myocardial Function in Dogs with Acute Subendocardial Ischemia by Using Quantitative Tissue Velocity ImagingObjective To explore the value of quantitative tissue velocity imaging (QTVI) in assessing regional myocardial systolic and diastolic functions in dogs with acute subendocardial ischemia.Methods Microsphere suspensions of 200 to 300μm in diameter, with the amount of 3.5×10⁴ microspheres, were injected into the proximal portion of left circumflex coronary artery in 11 closed-dogs via micro-catheters, to induce subendocardial ischemia by embolization. Before and after embolization, 2D echocardiography, QTVI and real time myocardial contrast echocardiography (MCE) by intravenous infusion of self-made microbubbles were performed separately. The wall thickening and subendocardial myocardial longitudinal velocities of risk segments before and after embolization were compared by paired t analysis separately. The regional myocardial contrast intensity versus time plots were fitted to an exponential function: y =A·(1-exp-k·t)+B, the product of A andβrepresented myocardial blood flow.Results MCE showed that normalized A·k decreased severely (P<0.05) in 28 left ventricular (LV) myocardial segments in the subendocardial myocardium after embolization, including 6 basal and 9 middle segments of lateral wall (LW), 8 middle segments of posterior wall (PW) and 5 middle segments of inferior wall (IW). However, there was no statistically difference in subepicardial myocardium before and after embolization. Although the systolic wall thickening did not change 5 minutes after embolization in these ischemia segments (30±3% vs 32±4%, P>0.05), the longitudinal peak systolic velocities (Vs) and early-diastolic peak velocities (Ve) recorded by QTVI declined significantly (P<0.05). Moreover, the subendocardial myocardial velocity curves during isovolumic relaxation predominantly showed positive waves, whereas they mainly showed negative waves before embolization.Conclusions Longitudinal peak systolic velocity, peak velocity during isovolumic relaxation and early-diastolic velocity are sensitive indices for the detection of subendocardial ischemia.