1. A method for early detection of subacute, potentially catastrophic illness in a patient comprising: (a) monitoring the patient's RR intervals; (b) generating a normalized data set of the RR intervals; (c) calculating one or more of (i) moments of the data set selected from the second and higher moments, including the standard deviation, (ii) percentile values of the data set, (iii) sample entropy, and (iv) sample asymmetry, (d) identifying an abnormal heart rate variability associated with the illness based on one or more of the moments and the percentile values, sample entropy and sample asymmetry analysis.