Fig. 4.  Left : Time course of oxyhemoglobin saturation in the superior sagittal sinus (SSS) of eight anesthetized sheep in which Fio²was progressively decreased and then, if the initial hypoxemia was well tolerated, increased. Several sheep had two or more cycles of decreasing and increasing Fio². At intervals during each experiment, oxyhemoglobin in blood withdrawn from the SSS was measured by a co-oximeter (  triangles, dashed lines ) and compared with the optoacoustic signals from the SSS (  circles, solid lines ) calculated with the first algorithm.  Right : Linear correlation of the optoacoustic signals compared with oxyhemoglobin saturation in the SSS.