PREVIOUS BEAT CONTRACTION HISTORY ALTERS MECHANICAL RESTITUTION IN THE ISOLATED LEFT-VENTRICLE

Objective: The aim was to test directly the hypothesis that the magnit ude of previous beat contraction history will be greatest for short pu lse intervals, will become smaller as pulse interval is lengthened, an d will vanish when pulse interval is long enough to allow complete res titution. Methods: Experiments were performed in isolated rabbit and f erret left ventricles in which pressure and volume were controlled wit h a servo-motor system. Two restitution curves were generated, each co nstructed from isovolumetric beats that were preceded by a beat with d iffering amounts of ejection: one curve was constructed from isovolume tric beats preceded by non-ejecting beats, while the other was constru cted from isovolumetric beats preceded by a beats that had a high leve l of ejection (ejection fraction similar to 50%). Results: When the is ovolumetric beats used to construct a restitution curve were preceded by ejecting beats, the restitution curve was shifted upward (that is, higher activation at a given interval between beats) when compared to a restitution curve constructed from isovolumetric beats that were pre ceded by non-ejecting beats. Conclusions: Mechanical restitution is af fected by the mechanical events of preceding contractions, implying th at previous beat contraction history and restitution share a common me chanism and that restitution sets the stage for previous beat contract ion history. Hence restitution is not simply a useful tool for assessi ng calcium cycling or interval dependent behaviour and should be viewe d more broadly as a fundamental process in the beat to beat regulation of cardiac contraction.