CVR and autoregulation define the physiological compensatory mechanisms when flow is impaired. Identification of exhausted CVR can be a good indication for high risk for stroke.
CVR can be assessed with TCD under two monitoring conditions: hypercapnia or CO2 reactivity, and with intravenous injections of Acetazolamide or the Diamox test.
Typically, an increase in peak velocities above 40% after the perturbation reflects intact CVR, whereas smaller increase reflect different degrees of impaired CVR. Absence of flow increase reflects a complete exhaustion of the autoregulatory capacity.