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Previous studies have reported "bulging" of the ischemic zone and reciprocal shortening of the normal zone during the isovolumetric contraction period. This study examines the interaction and relation of these reciprocal strains during the isovolumetric contraction period. Normal zone and ischemic zone segment length data were acquired at 1 ms intervals during acute ischemia in 10 open chest dogs. The relation of ischemic zone and normal zone segment length was inversely linear in the isovolumetric contraction period during steady state ischemia and during preload reduction (mean correlation coefficient 0.92). The slope derived from the regression analysis was the same as that determined from the first and last data points of the isovolumetric contraction period (correlation coefficient between the regression versus two-point slope 0.96). This slope was used to calculate the size of the ischemic area based on the hypothesis that the isovolumetric normal zone shortening quantitatively accounted for the ischemic zone bulging during the isovolumetric contraction period, with the percent risk region serving as a weighing factor. The calculated risk region correlated with the anatomic risk region (r = 0.83, p less than 0.01; n = 9) and was independent of preload. During the isovolumetric contraction period, left ventricular short-axis diameter shortened approximately 0.2%; 80% of the ischemic zone lengthening occurred during this period.(ABSTRACT TRUNCATED AT 250 WORDS)