In accordance with an aspect of the present invention, a system and method allows for the assessment of health and mortality based on dynamic nonlinear calculations of self-similar fluctuation patterns in a time series of QT intervals and of other physiological signals, such as RR intervals, temperature, blood pressure, respiration, saturation of peripheral oxygen, intracardiac pressures, and electroencephalogram. In order to nonlinearly determine health and mortality, time series of QT intervals and of other physiological signals (e.g., RR intervals) are simultaneously obtained, and entropy values are calculated for each signal over the same temporal interval. “EntropyX” is calculated from relative changes between moments and entropy of QT intervals and those of other physiological signals over seconds to days. The absolute and relative entropy values at a specific time point and/or subsequent changes in entropy over future time points can be used to determine a treatment plan for the subject.