Acute Myocardial Infarction: Ventricular Remodeling

Ventricular Volumes, Ejection Fraction, and Mass

LV cavity dilation and systolic dysfunction are the hallmarks of postinfarct remodeling and can be accurately quantified with high spatial and temporal resolution using cine CMR. Early breath-hold cine imaging used fast low-angle shot (FLASH), a spoiled gradient recalled echo (GRE) sequence. Currently, balanced steady-state free precession (bSSFP) is the most commonly used pulse sequence because of faster acquisition times, improved temporal resolution, and optimal contrast between the myocardium and the blood pool. The use of bSSFP facilitates qualitative and quantitative assessment of cardiac anatomy and function mainly by improving endocardial border definition.

Unlike echocardiography, acoustic windows do not limit CMR; therefore images can be acquired in any desired orientation to accurately depict LV morphology and function. A carefully prescribed stack of short-axis slices extending from the cardiac base to the apex provides a three-dimensional (3D) dataset that can be used to make accurate ventricular morphologic measurements at end diastole (ED) and end systole (ES) without the need for geometric assumptions, as is the case with 2D modalities ( Fig. 20.2 ). Ventricular volumes are calculated using the summation of disks method.

CMR derived LV volumes and mass have been validated with phantom, animal and human studies and currently considered the clinical reference standard for other imaging modalities. Although accuracy is important, reliability may be even more important because it dictates the validity of differences between serial examinations. Volumetric measurements by CMR provide high-interstudy, interscan, and interobserver reproducibility in normal hearts and those deformed by prior MI. Interstudy, interobserver, and intraobserver reproducibility of CMR-derived RV volumes is also excellent in healthy controls, patients with HF, ventricular hypertrophy, and congenital heart disease. Because of improved reproducibility compared with echocardiography, CMR facilitates the identification of clinically significant ventricular changes in serial examinations after AMI. The reduced variability of the measurement significantly reduces the sample size needed to detect differences between treatment arms in clinical trials.

Cine CMR has been successfully used to evaluate post-MI remodeling in animal models and humans. Konermann and colleagues studied the reliability of CMR in the evaluation of LV morphology and function in post-MI patients. They compared cine CMR LV volumes and LVEF with 2D echocardiography (2DE), radionuclide, and x-ray left ventriculography in a group of 65 patients who suffered a transmural MI and did not undergo thrombolysis or angioplasty. Good correlation was found between x-ray and radionuclide ventriculography. However, correlation with 2D echocardiography was limited because of inconsistent 2DE image quality. In this study, the investigators also demonstrated good correlation between creatine kinase (CK)-derived infarct size and CMR infarct mass determined from cine images.

In a follow-up publication, Konermann et al. reported the natural history of LV remodeling in the same group of 65 patients at 1, 4, and 26 weeks post-MI. Medical therapy was limited to nitrates, beta-blockers, aspirin, and diuretics. Only those patients with first transmural infarcts were included in the study. Infarct transmurality and size were defined by clinical, electrocardiographic, and enzymatic criteria. The extent of negative remodeling was not only dependent on the enzymatic size but also on the location of the infarct. Small infarcts caused modest but concordant ED and ES dilation, so that LVEF remained stable. Individuals with large anterior infarcts developed an unbalanced, progressive diastolic and systolic dilation that resulted in a marked reduction in LVEF. Infarct region thinning was almost always observed at 6 months except for small posterior infarcts. LV stroke volume index (SVI) was smaller in large anterior MI, but there was no statistically significant difference in LV SVI between 1 and 26 weeks post-MI. Despite a significant LVEF reduction in those with large infarcts, stroke volume was maintained at the expense of ventricular dilation. No clinical difference between the groups was observed when analyzing New York Heart Association (NYHA) symptom classification. Four patients died before the completion of the study and all had large anterior MI.

In a cohort of 26 patients with reperfused anterior AMI, Kramer et al. used cine tissue tagging to evaluate morphologic changes, LVEF, and regional intramyocardial function of myocardium adjacent and remote to the infarct. All patients received either angiotensin-converting enzyme inhibitor (ACEI) or beta-blocker. Most patients received ACEI and about half received both agents. Imaging was done on day 5 ± 2 and week 8 ± 1 after MI. There was a significant increase in LV end-diastolic volume index (EDVI) with stable end-systolic volume index (ESVI). Therefore there was an improvement in global LVEF 8 weeks post-MI. The increased LVEF was mediated by contractile improvement and normalization in the adjacent and remote regions, respectively. Enzymatic infarct size correlated with the degree of diastolic dilation.

Regional Contractility

Although LVEF is the most commonly used global metric of LV systolic function, it does not provide information regarding regional performance. In fact, segmental systolic abnormalities because of AMI may sometimes be concealed by a normal global LVEF. Accordingly, to understand the contributing factors to global functional alterations during the remodeling process, it is important to characterize regional contractile performance in the adjacent and remote noninfarcted myocardium. In clinical practice this is usually done by subjective visual scoring of segmental endocardial motion and thickening using cine CMR images. However, visual assessment can be insensitive to subtle abnormalities and is subject to greater interobserver variability. Thus objective methods to quantify contractile function have been developed. Wall thickening and endocardial motion can be measured from cine CMR images. A more accurate assessment of intramyocardial contractility in the infarcted heart can be determined by measuring strain with the use of myocardial tissue tagging sequences such as spatial modulation of magnetization (SPAMM) and phase-based sequences such as displacement encoding with stimulated echoes (DENSE). More recently, feature tracking analysis has allowed the measurement of circumferential and longitudinal strain from bSSFP cine images.