A method and apparatus for determining an index indicative of a subjects response to an oxygen desaturation condition is provided. The method includes the steps of: a) providing a NIRS tissue sensor, a pulse oximetry sensor, and a processor in communication with the NIRS tissue sensor and the pulse oximetry sensor b) sensing the subjects tissue using the NIRS tissue sensor and producing first signals c) sensing the subjects tissue using the pulse oximetry sensor and producing second signals d) processing the first signals to determine a change in tissue oxygen saturation values, processing the second signals to determine a change in arterial oxygen saturation values and e) determining the index indicative of the subjects response to the oxygen desaturation condition using the change in tissue oxygen saturation values and the change in arterial oxygen saturation values.
