The relationship of segmental left ventricular (LV) wall motion abnormalities to LV function 2-6 days after acute transmural myocardial infarction (MI) was investigated in 45 patients by quantitative contrast ventriculography. Patients were divided into four classes according to the MIRU criteria. Segmental wall motion was assessed by determining the percentage of systolic shortening (AS) along nine hemiaxes and the extent of akinetic or dyskinetic abnormally contracting segments (% ACS) expressed as a percentage of end-diastolic perimeter.
When compared with that in 17 normal control subjects, the LV end-diastolic volume was increased only in patients in class III and class IV; the LV end-systolic volume increased progressively from normal through class IV. Ejection fraction had a negative linear correlation with %ACS (r = 0.97). The size of ACS was larger in anterior (34 ± 14%) than in inferior MIs (23 ± 7%), resulting in greater LV dysfunction. However, for a comparable size of ACS, infarct location alone did not influence LV function parameters. In the noninfarcted zone, AS was increased when the size of ACS was < 25% and reduced when the size of ACS was > 25%.
Thus, the size of ACS is a major determinant of LV dysfunction in acute MI. The compensatory mechanisms operate either through an augmented mechanical function of residual myocardium when the infarct is small, or through the Frank-Starling mechanism when the infarct is large.