Investigation of the changes in cerebral tissue oxygenation measured with near infrared spectroscopy in response to moderate hypercapnia

Introduction: The arterial partial pressure of carbon dioxide (PaCO2) has a major influence on cerebral blood flow (CBF). Cranial near-infrared spectroscopy (NIRS) can monitor the resulting changes in cerebral haemodynamics and recent technical advances enable absolute measurements of tissue oxygenation using spatially resolved spectroscopy (SRS). This mixed (arterial, venous) oxygenation signal has been termed the tissue oxygenation index (TOI). TOI can be related to the arterial saturation (SaO2), oxygen consumption (CMRO2 in ml of O2/min), CBF (in ml/min) and arterial (Va), venous (Vv) blood volume per unit mass of tissue (in l/g) by Eq. 1: