Noninvasive optical method of measuring tissue and arterial saturation: an application to absolute pulse oximetry of the brain.

We present a frequency-domain optical method for real-time noninvasive measurement of absolute tissue and arterial saturation. This method is based on quantitative measurement of the tissue absorption spectrum (for tissue saturation) and of the amplitude of the arterial-pulsation-induced absorption oscillations (for arterial saturation) at eight wavelengths in the range 633-841 nm. We report results obtained from readings taken from the forehead of a healthy volunteer, showing baseline saturation values of 74.7+/-0.2% (tissue) and 96.9+/-0.5% (arterial). These values dropped to minimum values of 71.6+/-0.2% and 90.0+/-0.2% , respectively, after 1 min of reduced inspired oxygen concentration [10% (by volume) O(2) from a baseline value of 21% O(2) ].