Summary
Background: Although left ventricular (LV) contractility in atrial fi brillation (Af) is known to change in a beatto- beat fashion, little is known about the changes in LV compliance in Af.
Material/Methods: We experimentally induced tachycardic Af (average heart rate – 154 beats per minute) in 18 sheep. LV volume and pressure were simultaneously monitored using a conductance catheter. LV enddiastolic volume (VED) and pressure (PED) were plotted in a beat-to-beat fashion and fi tted to the following exponential equation (PED=g × eb × VED) in each animal. A random effects model was constructed to determine if the intercepts and slopes differ.
Results: In all animals, those plots after the induction of Af fi t quite well to the exponential function (r=0.834±0.184) by gating short preceding interval (RR1) beats. By simply taking the natural logarithm
of both sides in the equation, the linear relationship (ln(PED) =a + b × VED, where a = lng) was observed in all animals before (normal sinus rhythm, NSR) and after the induction of Af. Only two of 18 intercepts and four of 18 slopes deviate between NSR and Af. Most interestingly, the random effects model clearly detailed that the average animal had intercepts and slopes that were not discernibly different between NSR and Af.
Conclusions: Unlike LV contractility, myocardial compliance did not change after the acute induction of Af. These interesting results may give us insights into the understanding of the physiology in acute
rapid Af.

Keywords: Atrial Fibrillation - physiopathology, Blood Pressure - physiology, Diastole, Electrocardiography, Heart - physiopathology, Hemodynamics, Myocardial Contraction - physiology, Sheep, Ventricular Function, Left