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Department of Physiology, Faculty of Medicine, University of Tokyo

Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, Tokyo, Japan

1. The individual effects of stroke volume and speed of ejection on the duration of the isovolumic relaxation phase were analysed in the canine left ventricle with a constant end-systolic residual volume.

2. A new technique was employed to maintain the ventricular end-systolic residual volume at a desired constant value regardless of wide changes in stroke volume and speed of ejection in a given inotropic background.

3. The duration of isovolumic relaxation, which was defined to be the time taken for ventricular pressure to fall from its end-systolic level to its 75, 50 and 25% levels, markedly decreased with increases in stroke volume. The reduction amounted to as much as 30-50% when stroke volume was increased from zero (isovolumic) to 20-25 ml.

4. The degree of shortening of the duration of isovolumic relaxation was largely independent of changes in speed of ejection which ranged from about 100-800 ml./sec at a constant stroke volume of 15 ml.

5. It was therefore concluded that stroke volume itself could be a major determinant of the duration of isovolumic relaxation.

6. It was speculated that the mechanism of the observed phenomenon might be a manifestation of the uncoupling effect of muscle shortening on contractile state.

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