1. A pulse oximeter, comprising: a probe that is adapted to be attached to a body part of a patient to create a signal indicative of an oxygen saturation of blood of the patient; and a processor that is adapted to receive the signal created by the probe, to calculate an SPO2 value based on the signal, to detect a plurality of pattern types of SPO2 indicative of pathophysiologic events, to detect at least one SPO2 shape feature of a rise event of one of the pattern types of SPO2, the shape feature being suggestive of arousal response failure, to calculate an SPO2 severity index responsive to the plurality of pattern types of SPO2, wherein the SPO2 severity index increases in response to detection of a pattern suggestive of arousal response failure, and to produce an output indicative of the SPO2 severity index.