The purpose of this study was to use MR myocardial tagging to assess regional cardiac function after myocardial infarction (MI) in mice. Eight mice were imaged before and 1 day after MI. MRI included cine imaging, myocardial tagging, and contrast-enhanced imaging. Regional percent circumferential shortening (%CS) was measured from the tagged images, and the region of hyperenhancement on the contrast-enhanced images was used to determine the infarcted, adjacent, and remote zones. Ejection fraction (EF) fell from 59% ؎ 6% at baseline to 32% ؎ 6% after MI (P < 0.01). At baseline, %CS was 14.5% ؎ 3.4%. After MI, %CS was 0.7% ؎ 4.4% in the infarcted zone, 7.4% ؎ 4.4% in the adjacent zone, and 11.8% ؎ 4.2% in the remote zone. %CS was statistically different for all comparisons between the infarcted, adjacent, remote, and baseline groups (P < 0.01 Transgenic and knockout mice can be used to study the genes and basic mechanisms involved in cardiovascular disease, and have therefore assumed a central role in modern cardiovascular research. Over the past 3 years, MRI has emerged as an accurate, noninvasive imaging modality for measuring murine cardiac structure and function in vivo (1-3). Examples demonstrating cardiac MRI in transgenic mouse research include studies of myocardial infarction (MI) (4) and left ventricular (LV) remodeling (5). To date, such studies have used MRI to measure myocardial mass, end-diastolic volume (EDV), end-systolic volume (ESV), and ejection fraction (EF). While such measures of global cardiac function are very useful, in the setting of coronary heart disease (where contractile dysfunction is generally localized to specific coronary territories) it is critically important to evaluate myocardial function on a regional basis. In the current study, we sought to demonstrate that myocardial tagging (6,7) could be used to measure regional contractile function in a mouse model of reperfused MI.
METHODS
Animal PreparationNine C57BL/6 mice were used in this study, which was approved by the University of Virginia Animal Care and Use Committee. MI was surgically induced as described previously (8). Briefly, mice were anesthetized, the upper portion of the trachea was exposed, and an endotracheal tube was inserted orally. Artificial respiration was maintained with an SAR-830/P ventilator (FiO2 of 0.80, rate of 100 strokes/min, stroke volume 2.0 -2.5 ml). After intubation, an incision was made to open the left pleural cavity. A piece of 7-0 silk suture was passed underneath the left anterior descending coronary artery (LAD) at the level of the lower left atrium and myocardial ischemia was induced by tying down the suture over PE-10 tubing. After 60 min of LAD occlusion, reperfusion was achieved by loosening the suture. Significant ECG changes (widening of the QRS wave and ST segment elevation as monitored with a PowerLab data recorder) and blanching within the region at risk were used to confirm coronary occlusion. A volume of 1-1.5 ml 5% dextrose was given intraperitoneally (i.p.) to replace fluids. Body...