A PPG system includes a processor configured to receive a PPG signal from a PPG sensor, the PPG signal having a plurality of pulses each representing a heartbeat of a patient. The processor is also configured to determine an amplitude variance of the plurality of pulses over a time period, determine a pulse period variance of the PPG signal over the time period, determine a cardiac stability based on a ratio of the amplitude variance and the pulse period variance, and provide an indication of the cardiac stability via a display. The amplitude variance includes an average of squared differences of an amplitude of a peak of each pulse of the plurality of pulses from a mean amplitude of the plurality of pulses over the time period, and the mean amplitude includes an average of respective midpoints of each of the plurality of pulses over the time period.
