The Physiological Basis of Cerebrovascular Reactivity Measurements

The brain requires a continuous supply of oxygen, which in normal environments is directly related to cerebral blood flow. Accordingly, a number of mechanisms are in place to maintain blood supply in the face of challenges such as variations in arterial blood pressure, hypoxemia, and vascular occlusions. These include the recruitment of collateral flow such as via the circle of Willis, the pial networks, and anastomoses between penetrating arterioles and capillaries. In addition, physiological mechanisms adjust the distribution of flow: (1) autoregulation to maintain flow during supply pressure changes, (2) neurovascular coupling to increase flow in regions of neuronal activity, and (3) hypoxia-induced vasodilatation. The common effector pathway for physiological mechanisms is the adjustment of vascular diameter. In the absence of steno-occlusive vascular disease, the flow response to a vasodilatory challenge, cerebrovascular reactivity (CVR), interrogates the physiological response. In the presence of steno-occlusive disease, CVR reflects both the health of the physiological regulators and the availability of collateral flow. Even in health, CVR varies between anatomical regions. As a result, CVR must be normalized for the region before it can be interpreted.