The graded hypoxia method is a modified version of previously described methods.10,20–23 This calibration method assumes that if changes in arterial hemoglobin saturation () are brief, small, and gradual, there will be no accompanying changes in cerebral metabolic rate for oxygen (), cerebral blood flow (CBF), and CBV.24 Numerous studies have validated this assumption, showing that CBV, , and CBF stayed constant during moderate hypoxemia and changed only under conditions of extreme arterial deficiency when dropped below 0.75.20,25,26 The derivation of the calibration method is shown below and is detailed in a previous study.14 NIRS is sensitive to largely to microvessel hemoglobin. Tissue hemoglobin oxygen saturation () measured by NIRS is composed of , capillary hemoglobin saturation, and venous saturation (). It has been generally accepted that the contribution to blood volume is 20% arterial, 10% capillary, and 70% venous. Assuming the capillary contains equal amounts of arterial and venous blood, can be determined by Eq. (1):27Display Formula
(1)where is the tissue hemoglobin oxygen saturation, is the arterial oxygen saturation, and is the venous oxygen saturation.