Analysis of phases of contraction during graded acute myocardial ischemia.

The quantitative effects of graded ischemia on the phases of contraction in ischemic and nonischemic myocardium were studied in 10 open-chest dogs. During progressive coronary stenosis, an index of the overall severity of ischemia, total myocardial blood flow deficit (TMFD), was measured using microspheres. Segment shortening (% delta L) during the isovolumic and ejection phases of systole in the ischemic and nonischemic zones was obtained with ultrasonic crystals. In the ischemic zone, total systolic and ejection phase % delta L decreased progressively as stenosis increased; isovolumic phase % delta L showed segment elongation (bulging) during mild ischemia, which was unchanged as ischemia became severe. Total % delta L in the nonischemic zone increased progressively with increasing stenosis, reaching 132 +/- 20% of the control value when TMFD was 24.8 +/- 6.5% of total expected left ventricular (LV) myocardial blood flow. The increase in total % delta L was entirely attributable to increased ejection % delta L and correlated with indices of preload (multivariate r = 0.65) and afterload (multivariate r = 0.62). Multiple linear regression analysis revealed that augmented contraction in the nonischemic zone was best described (multivariate r = 0.94) by a combination of four hemodynamic variables estimating LV preload and afterload, which correlated in turn with the overall severity of ischemia. Thus systolic bulging in the ischemic zone is primarily an isovolumic phase phenomenon that develops initially with mild ischemia. Little additional change occurs in the isovolumic phase as ischemic severity increases, although further reductions in shortening occur in the ejection phase.(ABSTRACT TRUNCATED AT 250 WORDS)