A method and apparatus for controlling delivery of therapy that includes an emitting portion emitting light at a predetermined emitted light intensity to a volume of tissue at a plurality of wavelengths, and a detecting portion detecting the emitted light scattered by the volume of tissue to generate corresponding detected light intensity output signals. A control module adjusts the detected light intensity output signals for shifts in intensity corresponding to the emitted light intensity, and determines a tissue oxygenation index in response to only the adjusted detected light intensity output signals, and a therapy delivery module controlling therapy in response to the determined tissue oxygenation index.