Optoacoustic, noninvasive, real-time, continuous monitoring of cerebral blood oxygenation: An in vivo study in sheep

Background: Current, invasive cerebral oxygenation monitors require either retrograde jugular venous bulb cannulation or intraparenchymal probe insertion. There is no accurate, noninvasive, continuous monitor of cerebral blood oxygenation. Methods: The authors designed, built, and tested novel optoacoustic instrumentation that continuously measures blood oxygenation in the superior sagittal sinus (SSS) in vivo in 12 anesthetized sheep. In this technique, laser pulses generate acoustic signals, the amplitudes and slopes of which are proportional to oxyhemoglobin saturation in the SSS. Optoacoustic signals from the SSS measured through the scalp and cranium were compared with directly measured oxyhemoglobin saturation in blood withdrawn from the cannulated SSS. Results: In the first experiments (feasibility), FIO₂ changes produced rapid corresponding changes in optoacoustic signals and arterial oxygen saturation. In the second experiments (validation), the authors correlated oxyhemoglobin saturation in the SSS with optoacoustic signals and developed quantifying algorithms. In eight of nine validation experiments, the authors quantified optoacoustic signals by subtracting the temporal profile at low FIO₂ (0.08-0.1) from profiles at higher FIO₂ and integrating those signals in the range from 3 to 5 μs. In each validation experiment, optoacoustic signals showed tight temporal association and good linear correlation with measured oxyhemoglobin saturation (r² 0.75 to 0.99 for eight individual experiments). Conclusions: The optoacoustic system detects signals induced in the SSS and optoacoustic signals from the SSS linearly correlate with oxyhemoglobin saturation. The data suggest that the optoacoustic technique merits clinical evaluation.