Regional wall function before and after acute myocardial infarction; an experimental study in pigs

Abstract

Background: Left ventricular function is altered during and after AMI. Regional function can be determined by cardiac magnetic resonance (CMR) wall thickening, and velocity encoded (VE) strain analysis. The aims of this study were to investigate how regional myocardial wall function, assessed by CMR VE-strain and regional wall thickening, changes after acute myocardial infarction, and to determine if we could differentiate between ischemic, adjacent and remote segments of the left ventricle.

Methods: Ten pigs underwent baseline CMR study for assessment of wall thickening and VE-strain. Ischemia was then induced for 40-minutes by intracoronary balloon inflation in the left anterior descending coronary artery. During occlusion, (99m)Tc tetrofosmin was administered intravenously and myocardial perfusion SPECT (MPS) was performed for determination of the ischemic area, followed by a second CMR study. Based on ischemia seen on MPS, the 17 AHA segments of the left ventricle was divided into 3 different categories (ischemic, adjacent and remote). Regional wall function measured by wall thickening and VE-strain analysis was determined before and after ischemia.

Results: Mean wall thickening decreased significantly in the ischemic (from 2.7 mm to 0.65 mm, p < 0.001) and adjacent segments (from 2.4 to 1.5 mm p < 0.001). In remote segments, wall thickening increased significantly (from 2.4 mm to 2.8 mm, p < 0.01). In ischemic and adjacent segments, both radial and longitudinal strain was significantly decreased after ischemia (p < 0.001). In remote segments there was a significant increase in radial strain (p = 0.002) while there was no difference in longitudinal strain (p = 0.69). ROC analysis was performed to determine thresholds distinguishing between the different regions. Sensitivity for determining ischemic segments ranged from 70-80%, and specificity from 72%-77%. There was a 9% increase in left ventricular mass after ischemia.

Conclusion: Differentiation thresholds for wall thickening and VE-strain could be established to distinguish between ischemic, adjacent and remote segments but will, have limited applicability due to low sensitivity and specificity. There is a slight increase in radial strain in remote segments after ischemia. Edema was present mainly in the ischemic region but also in the combined adjacent and remote segments.

Figure 1

Outlining the left ventricle. A. Cine 4 chamber view of the heart before occlusion of LAD. The left ventricle is delineated in white in end-diastole. B. Velocity-encoded strain image of the heart in end-diastole, where the white line represents the left ventricle delineation as exported from the cine 4 chamber view. C. Illustration of radial and longitudinal strain directions.

Figure 2

Left panel shows a MPS polar plot displaying region of ischemia in black. Middle panel shows AHA 17 segment model of the left ventricle with ischemic region indicated in gray and% of ischemia. Right panel shows colour coded final classification.

Figure 3

Wall thickening in mm in ischemic, adjacent and remote areas of the left ventricle before and after ischemia.

Figure 4

Polar plots of MPS, LGE and CMR absolute wall thickening for each subject. The first column shows the ischemic area by MPS. The second column shows infarct transmurality assessed by LGE. The third column shows cardiac function by CMR absolute wall thickening before ischemia and the fourth column cardiac function by CMR absolute wall thickening after ischemia. The color bars next to the respective polar plots denote the scale.

Figure 5

The upper panel shows radial strain in ischemic, adjacent and remote areas before and after ischemia. The lower panel shows longitudinal strain before and after ischemia.

Figure 6

Mean regional function for all subjects expressed as radial strain (top row) and longitudinal strain (bottom row). Left column shows high regional function before ischemia. Middle column shows decreased regional function in segments supplied by left anterior descending artery. Right column shows difference in regional function before and after ischemia. White means high strain and black low strain.

Figure 7

ROC analysis determining threshold. Upper panel shows results for wall thickening. Middle panel shows results for radial strain. Lower panel shows results for longitudinal stain. Left column shows differentiation between ischemic and non-ischemic areas. Right column shows differentiation between remote and non-remote areas. Red marker shows sensitivity and specificity for optimal thresholds.