MECHANICAL DYSFUNCTION IN THE BORDER ZONE OF AN OVINE MODEL OF LEFT-VENTRICULAR ANEURYSM

Background. The pathophysiology of regional mechanical dysfunction in the border zone (BZ) region of left ventricular aneurysm was studied i n an ovine model using magnetic resonance imaging tissue-tagging and r egional deformation analysis. Methods. Transmural infarcts were create d in adult Dorsett sheep (n = 8) by ligation of the distal homonymous coronary artery and were allowed to mature into left ventricular aneur ysms for 8 to 12 weeks. Animals were imaged subsequently using double oblique magnetic resonance imaging with radiofrequency tissue tagging. Short axis slices were selected for analysis that included predominan tly the septal component of the aneurysm as well as adjacent BZ region s in the anterior and posterior ventricular walls. Dark grid patterns of magnetic presaturations were placed on the myocardium and tracked a s they deformed during the diastolic, isovolumic systolic, and systoli c ejection phases of the cardiac cycle. Regional ventricular wall stra ins were calculated in BZ regions and regions remote from the aneurysm and compared with strains measured in corresponding regions from norm al control sheep (n = 6). Results. Diastolic midwall circumferential s trains (fiber extensions) were relatively preserved, but abnormal circ umferential lengthening strains were observed in the BZ regions during isovolumic systole. Peak circumferential strains ranged from 0.04 to 0.07 in the BZ regions but averaged -0.05 in the normal hearts (p = 0. 002 for the anterior BZ and p = 0.001 for the posterior BZ). Midwall e nd-systolic fiber strains were depressed in the anterior BZ (-0.03 to -0.09 for the BZ versus -0.11 for the normal heart, p < 0.0001) but no t in the posterior BZ (p = 0.19). Conclusions. Our data support the th eory that the stretching of BZ fibers during isovolumic systole contri buted to a reduction in fiber shortening during systolic ejection and thus reduced the overall contribution of these fibers to forward ventr icular output.