Weak and strong myocardium in series: implications for segmental dysfunction

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Abstract
The interaction of segments of myocardium of different strengths was modeled as the contraction of strong (normal) and weak (hypoxic) muscles in series, and the motions of the weak segment were studied. Isolated rat trabecular muscles performed physiologically sequenced contractions that approximate the loading of the intact ventricle. A computer stored the force record of a control contraction and subjected the muscle to the same force during 60 min of hypoxia and 30 min of reoxygenation. In effect, the hypoxic and reoxygenated muscle was contracting in series with normal myocardium. Passive muscle stiffness increased during hypoxia to 171% of control and remained significantly higher at the end of reoxygenation (115%, P < 0.01). The pattern of shortening during hypoxia became polyphasic, with 2 shortening phases per contraction: early shortening diminished rapidly with progressive hypoxia, whereas late shortening, which occurred at the time during contraction when imposed force declined, was dominant after 15 min of hypoxia and further exaggerated in the first 10 min of reoxygenation. Analysis of muscle work showed that work performed during late shortening exceeded work performed during early shortening in the 1st minutes of reoxygenation. The motions of the weak segment observed in this study are qualitatively similar to those seen in the intact heart during regional myocardial dysfunction.