Previous beat contraction history alters mechanical restitution in the isolated left ventricle

The aim was to test directly the hypothesis that the magnitude of previous beat contraction history will be greatest for short pulse intervals, will become smaller as pulse interval is lengthened, and will vanish when pulse interval is long enough to allow complete restitution.

Experiments were performed in isolated rabbit and ferret left ventricles in which pressure and volume were controlled with a servo-motor system. Two restitution curves were generated, each constructed from isovolumetric beats that were preceded by a beat with differing amounts of ejection: one curve was constructed from isovolumetric beats preceded by non-ejecting beats, while the other was constructed from isovolumetric beats preceded by a beats that had a high level of ejection (ejection fraction approximately 50%).

When the isovolumetric 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 preceded by non-ejecting beats.

Mechanical restitution is affected by the mechanical events of preceding contractions, implying that previous beat contraction history and restitution share a common mechanism and that restitution sets the stage for previous beat contraction history. Hence restitution is not simply a useful tool for assessing calcium cycling or interval dependent behaviour and should be viewed more broadly as a fundamental process in the beat to beat regulation of cardiac contraction.