**4. Cardiac remodeling following myocardial infarction and progression to heart failure**

Myocardial infarction (MI) reduces the amount of ventricular contractile myocardium, which in turn reduces the left ventricular (LV) contractile capacity for pumping out adequate cardiac output (CO). As MI extends, it progresses into heart failure (HF). A manifestation of MI and its decreased contractile capacity is the inability of the LV to retain its compact systolic curved shape. In other words, in HF resulting from MI, the left ventricle undergoes remodeling and its curvedness index decreases. As MI progresses into HF, LV size increases, LV function deteriorates, and symptoms of HF become evident. Thus, cardiac remodeling (expressed in terms of regional curvedness of the LV) constitutes a measure of the progression of HF after MI, and interventions causing some reversal of this LV remodeling process have been shown to improve mortality in patients with HF. Therefore, reversal of LV remodeling (through medical or surgical treatment) has been emerging as a therapeutic target in HF of all etiologies. The challenge is to develop specific measures of LV remodeling that can be incorporated into the clinical management pathway. For patients with HF after MI, the LV shape is more spherical in terms of the global sphericity index. However, focus on global sphericity index may be misleading, since the simple plane ratio reflects a linear alteration in the two axes of the LV chamber. Hence, regional curvedness index, determined from 3D magnetic resonance imaging (MRI)-based LV model, is proposed as a measure of LV shape. The curvedness value describes the magnitude of the curvature at a surface point, i.e., a measure of degree of curvature a point; the regional curvedness describes the curvedness of the segment of the LV. In MI patients, the regional curvedness index value does not increase significantly from end-diastole to end-systole (due to decreased LV contractile capacity), as in the case of normal patients. In the case of patients with HF following MI, the LV cannot generate adequate contractile force, as explained earlier in section 1. As a result, the overall LV contractility index of maximal rate-of-change of normalized systolic wall stress dσ\*/dtmax is decreased, and the LV ejection force is also diminished. Now, LV regional wall stress is proportional to the wall surface radius-of-curvature and inversely proportional to the wall thickness. Hence, for patients with HF after MI, the inability of the remaining viable myocardium to compensate for the increased wall stress associated with LV dilatation and thinning is a

### trigger for LV enlargement.

784 Biomedical Science, Engineering and Technology

For patients whose echo-texture analysis showed presence of HREs, it was found that the echocardiographic intensities of the HREs from these patients intensities were distinctly

Myocardial tissue pixels having echo-intensity values greater than 200 are generally noted to be infarcted. This region's echo-intensity values can remain unaffected by administration of a myocardial perfusing agent. This infarcted sub-region is seen to be surrounded by an ischemic sub-region whose pixels are noted to have echo intensity values between 100 and 200. This region's echo-intensity can be reduced by the administration of a myocardial perfusing agent. The surrounding healthy tissue has echo intensity less than 100. Figure 2(a) depicts an echo image of an infant with visible scars regions 1 and 2, while figure 2(b) depicts printouts of the echo intensities from these two regions, wherein the infarcted segments are depicted in dark

Fig. 2. (a) Long axis view of a pediatric patient's heart showing HRE regions 1 and 2 and a

Fig. 2. (b) Pixel values correcponding to highly reflectile echo region 1. The central region

having echo-intensity values greater than 200 is infarcted, while its immediately

surrounding region shown in lighter shade is ischemic.

healthy region 3 [1].

higher than the echo intensity range of normal tissue (as depicted in Table 1).

colour and the surrounding ischemic segments are depicted in a lighter shade.
