In a method of and apparatus for deriving indices characterizing atrial arrhythmias, a temporally isolated portion of a digital representation of a time-domain electrical signal originating from the atria of a patient is frequency analyzed to provide a corresponding frequency spectrum. The obtained frequency spectrum is compared with a template frequency spectrum, selectively adapted dependent on previously obtained frequency spectra from different temporally isolated portions, to determine a frequency shift value and an amplitude value that optimize a correlation between the two spectra. An output representing the two determined values is used, possibly together with parameters describing the template frequency spectrum, to classify an atrial arrhythmia manifest in the digital representation.