Dynamic changes of edema and late gadolinium enhancement after acute myocardial infarction and their relationship to functional recovery and salvage index

Abstract

Background: Changes in the myocardium in acute ischemia are dynamic and complex, and the characteristics of myocardial tissue on cardiovascular magnetic resonance in the acute setting are not fully defined. We investigated changes in edema and late gadolinium enhancement (LGE) with serial imaging early after acute myocardial infarction, relating these to global and segmental myocardial function at 6 months.

Methods and results: Cardiovascular magnetic resonance scans were performed on 30 patients with ST-elevation--myocardial infarction treated by primary percutaneous coronary intervention at each of 4 time points: 12 to 48 hours; 5 to 7 days; 14 to 17 days; and 6 months. All patients showed edema at 24 hours. The mean volume of edema (% left ventricle) was 37±16 at 24 hours and 39±17 at 1 week, with a reduction to 24±13 (P<0.01) by 2 weeks. Myocardial segments with edema also had increased signal on LGE at 24 hours (κ=0.77; P<0.001). The volume of LGE decreased significantly between 24 hours and 6 months (27±15% versus 22±12%; P=0.002). Of segments showing LGE at 24 hours, 50% showed resolution by 6 months. In segments with such a reduction in LGE, 65% also showed improved wall motion (P<0.0001). The area of LGE measured at 6 months correlated more strongly with troponin at 48 hours (r=0.9; P<0.01) than LGE at 24 hours (r=0.7). The difference in LGE between 24 hours and 6 months had profound effects on the calculation of salvage index (26±21% at 24 hours versus 42±23% at 6 months; P=0.02).

Conclusions: Myocardial edema is maximal and constant over the first week after myocardial infarction, providing a stable window for the retrospective evaluation of area at risk. By contrast, myocardial areas with high signal intensity in LGE images recede over time with corresponding recovery of function, indicating that acutely detected LGE does not necessarily equate with irreversible injury and may severely underestimate salvaged myocardium.

Figure 1. Study protocol

Imaging time points (TP) in relation to the acute myocardial infarction (MI) and primary percutaneous coronary intervention (PPCI). For each TP, the CMR data acquired is given. CMR = cardiovascular magnetic resonance; LGE = late gadolinium enhancement.

Figure 2. Time course of edema

A Mean percentage of LV volume positive for myocardial edema at each time point. The volume of edema remained stable in the first week post event with a significant decrease at 15 -17 days with near resolution by 6 months. B. The time course of edema and resolution is given for each patient. There was a large range of LV % volume of edema (0 to 60%). This analysis on an individual level confirms the constancy of edema measured in the first 5-7 days, that is suggested in Panel A, with marked variation thereafter. Using the objective thresholding methods described in the text, a small number of patients had a substantial volume of apparent residual edema at 6 months.

Figure 3. Impaired segments (% affected) as a function of the extent of segment positive for (A) edema and (B) LGE at 24H

Segments with acute myocardial injury as identified either by presence of late gadolinium (LGE) and/or edema. For both edema and LGE, the probability of segmental dysfunction increased with the fraction of segment affected. For unaffected segments, as expected, a large majority showed normal function.

Figure 4. (A) Myocardial LGE early (12-48hrs) vs. late (6 months) and (B) wall motion improvement abnormalities

A. The myocardial volume (mean % ± SD) positive for LGE, decreased significantly from 12 - 48 hrs to 6 months. The change in extent of LGE is also shown for each patient. Patients with unchanged LGE at 6 months are shown in red while those with decreased LGE in black. Eleven patients out of 24 (46%) who underwent CMR at 6 months time, showed a reduction in LGE volume (of 38 ± 14%). In the remaining 13 patients, no reduction in size of the LGE volume was identified (23% ± 14% vs. 23% ± 14% respectively). B. Segments with improved function at six months (n = 68) broken down by extent of LGE within that segment LGE (none; 1-75% = partial thickness and 76-100% = full thickness) at both 24H and 6M. Across categories, the extent of LGE, measured at 12 - 48 hours (dark bars) was a poor predictor of functional recovery. Significantly, even segments showing full thickness LGE were associated with functional recovery. By contrast, LGE extent at 6 months (light bars) was strongly inversely correlated with improved function in that segment.

Figure 5. Representative CMR images

Edema images (left column), acute LGE images (center), chronic LGE (right column) are displayed. Three separate patients (rows A to C) representing alternative patterns of acute vs. chronic CMR features are shown. A. Edema is present in inferior wall; the appearance of LGE is unambiguous as high signal zone that corresponds to the area of edema, but there is no LGE present at 6 months. B. T2W image, on the left, shows edema in the anterior wall; the acute LGE shows compact enhancement, which is reduced in size by 6 months. C. The edema imaging confirms acute injury. In this example LGE present early persists without significant alteration to the 6 month time point.

Figure 6. Relationship between the extent of late gadolinium (LGE) and the Troponin I at 48 hrs

The correlation between the LV % of LGE (% LV_score) assessed at 6 months and troponin I assessed at 48 hrs (r = 0.9; P < 0.01) is shown in comparison to the equivalent relationship with LGE measured at 12 - 48 hours (r = 0.7, P < 0.01).