Objectives: The purpose of this study was to explore the physiology underlying the beat-to-beat variations of ventricular function during atrial fibrillation (AF).
Methods: Left ventricular pressure, and its first derivative (LVdP/dtmax, an index of contractility, and aortic blood velocity (and its integral AVI, an ejection index), were recorded using cathetermounted transducers in 15 patients with AF during cardiac catheterisation. Transfer function modelling was used to examine the influence of preceding intervals on LVdP/dtmax, and of LVdP/dtmax on AVI. The technique also allowed simulation of the behaviour of LVdP/dtmax in response to specific manipulations of interval.
Results: The variations in LVdP/dtmax recorded from the AF patients were shown to be dependent on up to six preceding intervals; a maximum of 91% of the variation was explicable in this way. The influences of mechanical restitution (MR, the relationship between preceding interval and contractility), postextrasystolic potentiation (PESP, the inverse relationship between pre-preceding interval and contractility) and the decay of that potentiation were all demonstrated. These influences collectively appeared to be powerful determinants of AVI. Simulations of LVdP/dtmax, following single interval perturbations, were entirely consistent with these interval force effects.
Conclusions: The cardiac interval force relationship in man is an important determinant of the beat-to-beat variations of contractile and ejection function during AF: the beat-to-beat variations in contractile (or inotropic) function are independent of changes in ventricular filling or fibre-length.