LEFT-VENTRICULAR BEAT-TO-BEAT PERFORMANCE IN ATRIAL-FIBRILLATION - CONTRIBUTION OF FRANK-STARLING MECHANISM AFTER SHORT RATHER THAN LONG RR INTERVALS

Objectives. This study sought to evaluate control mechanisms of the varying left ventricular performance in atrial fibrillation. Background. Atrial fibrillation is characterized by a randomly irregular ventricular response, resulting in continuous variation in left ventricular beat-to-beat mechanical behavior and hemodynamic variables. Methods. Fourteen patients with chronic nonvalvular atrial fibrillation were studied, using a nonimaging computerized nuclear probe linked to a personal computer. Left ventricular ejection fraction, end-diastolic and end-systolic volume counts, stroke volume counts and filling time were calculated on a beat-to-beat basis during 500 consecutive RR intervals. Multiple regression analysis was used to assess how ejection fraction was predicted by these variables. Results. The preceding RR interval and end-diastolic volume shelved a positive relation, and prepreceding interval and end-systolic volume an inverse relation, with ejection fraction (all p < 0.0001). Sensitivity analysis suggested that the preceding interval and the end-diastolic volume were equally important in predicting ejection fraction. There was a relatively strong interaction between the preceding interval and end-diastolic volume, indicating that the influence of the end-diastolic volume on ejection fraction was diminished after long intervals. A second interaction showed that the effect of end diastolic volume on ejection fraction,vas attenuated after short prepreceding cycles. Conclusions. Cycle length-dependent contractile mechanisms, including postextrasystolic potentiation and mechanical restitution, determine the varying left ventricular systolic performance during atrial fibrillation over the entire range of intervals. Beat-to-beat changes in preload, consistent with the Prank-Starling mechanism, also play a role, but their influence is diminished after long preceding and short prepreceding intervals.