Mitral apparatus assessment by delayed enhancement CMR: relative impact of infarct distribution on mitral regurgitation

Abstract

Objectives: This study sought to assess patterns and functional consequences of mitral apparatus infarction after acute myocardial infarction (AMI).

Background: The mitral apparatus contains 2 myocardial components: papillary muscles and the adjacent left ventricular (LV) wall. Delayed-enhancement cardiac magnetic resonance (DE-CMR) enables in vivo study of inter-relationships and potential contributions of LV wall and papillary muscle infarction (PMI) to mitral regurgitation (MR).

Methods: Multimodality imaging was performed: CMR was used to assess mitral geometry and infarct pattern, including 3D DE-CMR for PMI. Echocardiography was used to measure MR. Imaging occurred 27 ± 8 days after AMI (CMR, echocardiography within 1 day).

Results: A total of 153 patients with first AMI were studied; PMI was present in 30% (n = 46 [72% posteromedial, 39% anterolateral]). When stratified by angiographic culprit vessel, PMI occurred in 65% of patients with left circumflex, 48% with right coronary, and only 14% of patients with left anterior descending infarctions (p <0.001). Patients with PMI had more advanced remodeling as measured by LV size and mitral annular diameter (p <0.05). Increased extent of PMI was accompanied by a stepwise increase in mean infarct transmurality within regional LV segments underlying each papillary muscle (p <0.001). Prevalence of lateral wall infarction was 3-fold higher among patients with PMI compared to patients without PMI (65% vs. 22%, p <0.001). Infarct distribution also impacted MR, with greater MR among patients with lateral wall infarction (p = 0.002). Conversely, MR severity did not differ on the basis of presence (p = 0.19) or extent (p = 0.12) of PMI, or by angiographic culprit vessel. In multivariable analysis, lateral wall infarct size (odds ratio 1.20/% LV myocardium [95% confidence interval: 1.05 to 1.39], p = 0.01) was independently associated with substantial (moderate or greater) MR even after controlling for mitral annular (odds ratio 1.22/mm [1.04 to 1.43], p = 0.01), and LV end-diastolic diameter (odds ratio 1.11/mm [0.99 to 1.23], p = 0.056).

Conclusions: Papillary muscle infarction is common after AMI, affecting nearly one-third of patients. Extent of PMI parallels adjacent LV wall injury, with lateral infarction-rather than PMI-associated with increased severity of post-AMI MR.

Trial registration: ClinicalTrials.gov NCT00539045.

Figure 1. Representative Examples

Typical examples of complete (1A) and partial (1B) PMI detected by DE-CMR (each example comprised of two short axis images within the affected papillary muscle). As shown, complete PMI was often associated with transmural infarction of the adjacent LV wall whereas partial PMI was associated with subendocardial infarction. Upper right shows bilateral, complete PMI with transmural infarction of the inferior and lateral walls.

Figure 2. Infarct Distribution

Bullseye plots illustrating segments assigned to each regional infarct category (anterior = anterior/anteroseptal, lateral = anterolateral/inferolateral, inferior = inferior/inferoseptal, segments). Each category was comprised of five segments (highlighted in respective bullseye plots) such that total myocardium subtended by each was equivalent.

Figure 3. Infarct Size and Coronary Anatomy

(3A) Infarct size (mean±SD) stratified by PMI among patients with RCA and LCX culprit vessels. (3B) Stratification of RCA and LCX infarcts by presence of PMI (upper row) and coronary dominance pattern (lower row).

Figure 4. PMI Location and Type

Stratification of PMI by location (upper row) and type (lower row). Analysis based on total PMI (n=51) among 46 patients (n=5 with bilateral PMI).

Figure 5. PMI in Relation to Left Ventricular Injury

Infarct transmurality (5A) and contractile dysfunction (5B) stratified by PMI. Bar graphs based on mean (left) and maximum (right) infarct scores in LV segments adjacent to each papillary muscle (posteromedial = mid inferior/inferolateral, anterolateral = mid anterior/anterolateral walls). Note parallels between PMI extent and severity of injury to adjacent LV segments, whether assessed by infarct transmurality (5A) or contractile dysfunction (5B) (all p<0.001 for trend).

Figure 6. Mitral Regurgitation Severity

Graded severity of echo-quantified MR stratified by PMI (top) and LV infarct distribution (bottom). Corresponding table provides a breakdown of MR severity in relation to PMI and LV infarction.