A method and apparatus for monitoring a blood oxygen saturation level within a subjects tissue is provided. The method includes the steps of: a) sensing the subjects tissue using a near infrared spectrophotometric oximeter, and producing a value representative of the oxygen saturation level within the sensed tissue b) comparing the determined saturation value to a predetermined threshold oxygen saturation value and c) determining at least one of a time under threshold (TUT) value or an area under threshold (AUT) value using the determined saturation value and the threshold value.
