Abstract. This paper deals with the problem of estimating atrial activity during atrial fibrillation periods in the electrocardiogram (ECG). Since the signal of interest differs in kurtosis sign from the dominant sources in the ECG, we propose an independent component analysis method for source extraction based on the different kurtosis sign and extend it with a constraint of spectral concentration in the 3-12Hz frequency band. Results show that we are able to estimate the atrial fibrillation with a single algorithm having low computational complexity (O(7n-7)T).