Influence of the velocity of changes in end-diastolic volume on the starling mechanism of isolated left ventricles.

1. In this study the relationships between active developed systolic pressure, end-diastolic pressure and different diastolic volumes are studied in Tyrode perfused isolated rabbit left ventricles. Contractions were isovolumic. 2. Rapid diastolic volume changes were imposed on top of different preset basic diastolic volumes. These volume changes are shown to produce systolic and diastolic pressure values that cannot be explained by assuming a single pressure-volume relation during systole and diastole. The changes in pressure are in the same direction but higher than is expected on the basis of the increase or decrease of the ventricular end-diastolic volume alone. 3. The variation of the diastolic pressure-volume relation cannot be explained by assuming variations of the heart's passive elasticity or viscous effects within its wall. During diastole the effect is completely reversible without concomitant systolic effects. No velocity dependent effect of the quick volume change could be observed if the time duration was varied between 10 and 65 ms. The results are in keeping with the hypothesis that active force generating mechanisms may be present during the diastolic pause. 4. The effects observed during systole suggest the possibility of length dependent activation of the myocardial cells. This results in different inotropic conditions of the heart at identical volumes, depending on how these volumes were installed. These volumes may be considered to affect intrinsic properties of the muscle cells on a beat to beat basis.