Relationship between beat to beat interval and left ventricular function in patients with atrial fibrillation

In atrial fibrillation, the relation between the rhythm and volume of the pulse has long been of interest. However, changes in preload in this condition have not been fully addressed since beat to beat measurement of filling volume have been difficult until recently. In the present study, we evaluated left ventricular outflow and inflow velocity using pulsed Doppler echocardiography and correlated these results with the R-R interval in the individual patient. The study population consisted of 12 patients with atrial fibrillation, aged 36 to 69 years (mean 54 years). The etiology of atrial fibrillation was idiopathic in 10 and 2 patients had dilated cardiomyopathy. Stroke and filling volume were calculated as a pruduct of the flow velocity integral of left ventricular outflow and inflow velocity, and the cross-sectional area of aortic and mitral annulus, respectively. In 10 patients with idiopathic atrial fibrillation, significant positive correlations were observed between the preceding R-R interval and both the stroke volume and the filling volume of the preceding beat when the R-R interval was shorter than 600 msec. Stroke volume and filling volume of the preceding beat were almost constant, independent of the preceding R-R interval when the preceding R-R interval was longer than 600 msec, the interval necessary for the completion of the preceding rapid filling. In the same preceding R-R interval, a larger stroke volume was observed in a shorter pre-preceding R-R interval. In 2 patients with dilated cardiomyopathy no relationship could be observed between the preceding R-R interval and the filling volume of the preceding beat or the stroke volume. In patients with a normally functioning left ventricle (idiopathic atrial fibrillation), reduced cycle length and filling volume in the preceding cardiac cycle appear to be the underlying cause of the regulation of stroke volume, dependent on Starling's law. However, in patients with dilated cardiomyopathy no significant correlation was observed between the preceding R-R interval and both the filling volume of the preceding beat and the stroke volume. In these patients the left ventricle may have limited contractile reserve and altered diastolic re-coil forces possibly due to degenerative changes of myocardium. Pulsed Doppler echocardiography provides a non-invasive method of evaluating the instantaneous changes in left ventricular flow dynamics caused by atrial fibrillation and understanding its fundamental mechanism.