Variations in myocardial contraction sequence under various hypoxic conditions.

Hypokinetic myocardial segment motion is observed in various pathophysiologic conditions. The aim of this study was to clarify the mechanisms involved in differences in segment motion of hypokinesis. Nineteen open-chest dogs were studied with regard to myocardial segment length, left ventricular pressure, and internal minor-axis diameter. Sequential instantaneous myocardial elastance [alpha(t) curve] was calculated under 4 different hypoxic conditions: complete coronary occlusion and reperfusion, partial coronary occlusion, coronary microembolization, and anoxic perfusion. The alpha(t) curve peaked at end-systole in the case of normal contraction; but it was almost totally flat when complete bulging occurred. The hypokinesis which occurred during development of the complete systolic bulge immediately after complete coronary occlusion had an earlier alpha(t) peak curve than the hypokinesis resulting from partial coronary stenosis (209.5 +/- 35.6 ms after end-diastole vs. 261.9 +/- 18.2 ms; p less than 0.02), microsphere injection into the coronary artery (243.2 +/- 24.5 ms vs. 289.3 +/- 15.4 ms; p less than 0.05), or anoxic perfusion (213.4 +/- 40.2 vs. 275.6 +/- 28.3 ms; p less than 0.05). The early alpha(t) peak resulted in a late-systolic bulge in segment length motion. In conclusion, hypokinetic segment motion differed depending on whether the coronary blood flow was present or not. A late-systolic bulge only developed immediately after complete coronary occlusion, and resulted from an abrupt decrease in myocardial stiffness during the cardiac cycle, which is closely related to the abrupt cessation of coronary blood flow.