Heart rate variability has been widely utilized as a non-invasive mean to access the state of auto-nomic activities. However, standard spectral analysis often assumes stationarity and/or linearity of the heart rate signals. In this report we propose a method to examine the validity of these hypotheses implied in the conventional analysis. Further we propose an adaptive method to detect the state changes of the heart rate signal. The method has been applied to the data recorded from a rat as an illustrative example.
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