Near-infrared monitoring of the cerebral circulation

Abstract

Near-infrared spectroscopy is a noninvasive bedside technique for monitoring hemoglobin saturation (HbO₂%) in brain vasculature. The method linearly relates the optical signal detected from the surface of the head to HbO₂%. To do so, the method relies on constant transcranial optical pathlength and light scattering as well as minimal interference by tissues overlying the brain. This study examined these premises. Optical signals from a dual-wavelength, near-infrared spectrometer were correlated with sagittal sinus HbO₂% in 7 anesthetized piglets subjected to 7 different physiological conditions: normoxia, moderate and severe hypoxia, hyperoxia, hypocapnia, hypercapnic hyperoxia, and hypotension. These conditions were induced by varying the inspired O₂ concentration (7–100%),k ventilatory rate (5–35 breaths/min), and blood pressure (phlebotomy 20 ml/kg) to force HbO₂% over a wide range (5–93%). To evaluate interference by tissues overlying the brain, correlations were repeated after the scalp and skull were rendered ischemic. Transcranial optical pathlength was measured by phase-modulated spectroscopy. Linear relationships between optical signals and sagittal sinus HbO₂% were found with correlation coefficients ranging from −0.89 to −0.99 (p2% in the clinical setting, further study is recommended before the method can be used to manage patients.