According to the present invention, a method and apparatus for non-invasively determining the blood oxygen saturation level within a subjects tissue is provided. The method comprises the steps of: 1) providing a near infrared spectrophotometric sensor operable to transmit light along a plurality of wavelengths into the subjects tissue 2) sensing the light transmitted into the subjects tissue using the sensor, and producing signal data representative of the light sensed from the subjects tissue 3) processing the signal data to account for physical characteristics of the subject and 4) determining the blood oxygen saturation level within the subjects tissue using a difference in attenuation between the wavelengths. The apparatus includes a sensor having a light source and at least one light detector, which sensor is operably connected to a processor. The sensor is operable to transmit light along a plurality of wavelengths into the subjects tissue, and produce signal data representative of the light sensed from the subjects tissue. The algorithm is operable to process the signal data to account for the physical characteristics of the subject being sensed.
