1. A device for assessing the condition of a mammalian heart, comprising: a lead comprising two sensors, a processor coupled to the lead, the processor configured to: (i) obtain a signal from the lead, and compute a waveform corresponding to the signal, the waveform being characterized by a plurality of QRS complexes; (ii) compute a plurality of values of a waveform feature from the waveform representing the values of the waveform feature over time; (iii) apply a first filter to the plurality of values of the waveform feature, thereby deriving a first time series filtered waveform; (iv) derive a differenced waveform from the filtered waveform defining a second time series waveform where the differenced waveform is an absolute difference of at least one waveform feature taken in consecutive time intervals within a time segment; (v) compute from the second time series differenced waveform, a measure of the rate of change in the waveform feature over said time segment where the measure of the rate of change of said waveform feature is calculated as a function of the summation of the absolute difference of said second time series waveform feature taken in consecutive time intervals over said time segment, thereby deriving a change metric; and (vi) apply a test to detect a pathological heart condition, wherein the test is based on the change metric.