Future of neurocritical care: Integrating neurophysics, multimodal monitoring, and machine learning

Table 2 Near infrared spectroscopy-derived indices and clinical utility for the neurointensivist

NIRS derived indices	Utility
COx = correlation coefficient (rSO2, MAP)	COx: This index is derived by correlating rSO2 with MAP. It is used to assess cerebral autoregulation, with a positive correlation indicating impaired autoregulation
TOx = correlation coefficient (TOI, MAP)	TOx: Similar to COx, TOx is calculated by correlating the TOI with MAP. It provides information on the balance between oxygen supply and demand at the tissue level
HVx = correlation coefficient (rTHb, MAP)	HVx: HVx correlates the rTHb, as measured by NIRS, with MAP. It is used to evaluate cerebral blood volume changes in response to blood pressure fluctuations.
BFAx = correlation coefficient (rCBFi, MAP)	BFAx: BFAx correlates the rCBFi with MAP. A positive correlation indicates that cerebral blood flow is more passive to pressure, suggesting impaired autoregulation
CMRO2 = cerebral blood flow × (arterial O2 content - venous O2 content)	Estimation of the CMRO2: Although the direct calculation of CMRO2 using NIRS is complex, NIRS data combined with other parameters can estimate CMRO2. This provides information on the metabolic state of the brain

BFAx: Blood flow autoregulation index; COx: Cerebral oximetry index; CMRO₂: Cerebral metabolic rate of oxygen consumption; HVx: Hemoglobin volume index; MAP: Mean arterial pressure; NIRS: Near infrared spectroscopy; O₂: Oxygen; rCBFi: Regional cerebral blood flow index; rSO₂: Regional cerebral oxygen saturation; rTHb: Relative total tissue hemoglobin concentration; TOI: Tissue oxygenation index; TOx: Tissue oxygenation index.