Infarct involution and improved function during healing of acute myocardial infarction: The role of microvascular obstruction

Delayed contrast-enhanced cardiac magnetic resonance imaging (ceCMR) delineates infarct size. The presence of hypoenhancement consistent with microvascular obstruction (MO) signifies larger infarcts with a worse prognosis. We hypothesized that the size of the contrast defect (CD) on ceCMR in acutely infarcted myocardium may change during infarct healing and depend upon the presence of MO. Twenty-five patients underwent CMR on weeks 1 and 8 after reperfused myocardial infarction. After short-axis cine CMR was performed, gadolinium was infused and ceCMR images and matched tagged cine MR images were obtained in the three most dysfunctional short-axis slices on cine CMR. The area and transmural extent of hyperenhancement (HE) with or without MO representing total CD size were planimetered. Between week 1 and week 8, the CD area fell from 1729 970 mm 2 at week 1 to 1270 +/- 706 mm(2) (p<0.001), as did the transmural extent of infarction (71 +/- 22% to 63 +/- 24%, p<0.001). The decline in CD trended to be higher in patients 2 with MO (840 +/- 807 mm(2)) than in HE (312 +/- 485 rum, p<0.07). In the patient group as a whole, ejection fraction (EF) improved (56 +/- 9% to 60 +/- 10%, p = 0.002) between weeks 1 and 8, but patients with MO showed no increase in EF. Segments with some HE demonstrated partial functional improvement whereas no improvement was seen in HE+MO segments. In patients 8 weeks after reperfused myocardial infarction (MI), the size of infarction by ceCMR decreases compared to week 1 post-MI, especially in those with microvascular obstruction in whom there is little improvement in regional or global function.