BACKGROUND Beat-to-beat variations in the pulse during atrial fibrillation (AF) have conventionally been attributed to time-dependent changes in filling. We have explored the possibility that they are dependent on the intrinsic myocardial interval force relation. RESULTS Left ventricular (LV) contractility (maximum rate of rise of pressure, LV dP/dtmax) and ascending aortic blood velocity were measured during cardiac catheterization in 15 patients with AF. Beats preceded by an interval of less than 500 msec were excluded from analysis to reduce the confounding influence of incomplete mechanical restitution. The LV dP/dtmax was then related to the prepreceding interval. An inverse relation consistent with postextrasystolic potentiation was obtained in all 15 patients (Spearman's rank correlations, -0.56 to -0.86; p less than or equal to 0.0001). This relation was confirmed in three patients during pacing that overrode the AF and introduced single-interval variations into steady-state pacing. The ECG sequences from six of the AF patients were used to drive isometrically contracting guinea pig papillary muscle and human right ventricular tissue (n = 7); the same inverse relation was demonstrated. On a beat-by-beat basis, the maximum rate of rise of force in the isolated muscle correlated well with LV dP/dtmax in the patients (r = 0.50-0.86, p less than or equal to 0.0001). The relation of the integral of aortic velocity (AVI, proportional to stroke volume) to prepreceding interval was also inverse, whereas important correlations were demonstrated between LV dP/dtmax and AVI (Spearman's rank correlations, 0.27-0.95; p less than or equal to 0.0001). CONCLUSIONS This study demonstrates that postextrasystolic potentiation contributes to the characteristic beat-to-beat variation of the pulse in AF.