MYOCARDIAL-CONTRACTILITY IS NOT CONSTANT DURING SPONTANEOUS ATRIAL-FIBRILLATION IN PATIENTS

Background-The variation in stroke volume and pulse pressure character istic of atrial fibrillation is usually ascribed to time-dependent ven tricular filling, implying a single positive relationship between end- systolic pressure and volume, which defines a single state of myocardi al contractility. We tested the hypothesis that contractility also var ies. Methods and Results-We measured the left ventricular pressure and volume continuously with a conductance catheter with catheter-tip mic romanometer introduced retrogradely into the left ventricle. The end-s ystolic pressure-volume relationship was determined in 6 patients in a trial fibrillation undergoing cardiac catheterization for diagnostic p urposes and 4 control patients in sinus rhythm undergoing coronary art ery bypass graft surgery. The normal positive relationship between end -systolic pressure and volume was found in the control patients, but n o such positive relationship was found in any patient in atrial fibril lation, In the latter, the slopes of the linear regressions were eithe r not significantly different from zero or significantly negative (r v alues <0.08), both results indicating a change in contractility from b eat to beat. Significantly negative relationships were found between e nd-systolic volume and preceding R-R interval (-0.82<r<-0.24), indicat ing the presence of mechanical restitution. Significantly positive rel ationships were found between end-systolic volume and the R-R interval before the preceding R-R interval (0.35<r<0.74), indicating the prese nce of postextrasystolic potentiation. Conclusions-Myocardial contract ility is constantly changing from beat to beat in atrial fibrillation because of the influence of the force-interval relationships.