An implantable cardiac device is provided for adaptive ventricular rate smoothing during atrial fibrillation (AF). When AF is detected, the operation of the device is switched to non-atrial synchronized pacing mode such as DDI, DDIR, VDI, VDIR, VVI, or VVIR. The ventricular escape interval (VEI) is beat-by-beat modulated around a physiological interval zone (PIZ), which is determined by the pre-arrhythmia ventricular rate or the output of rate responsive sensor. The VEI remains unchanged if the preceding ventricular event is sensed and its RR interval is within the PIZ. Otherwise, the VEI is decreased asymptotically toward a lower interval threshold if the preceding RR interval is longer than the upper limit of the PIZ, or the VEI is increased asymptotically toward an upper interval threshold if the preceding RR interval is shorter than the upper limit of the PIZ. The step of incrementing or decrementing the VEI is adaptive to the absolute difference value between the preceding RR interval and the asymptotic interval threshold to ensure fast recovery of deviant RR interval toward the PIZ and to reduce the amount of high rate ventricular paces after a ventricular sense with very short coupling interval.