The ability of the cerebral vascular bed to undergo constriction or dilation in response to various stimuli is termed vascular reactivity or cerebral autoregulation. When the stimulus is the change in cerebral perfusion pressure (CPP) the vascular response is termed cerebrovascular pressure reactivity. When the stimulus is the change in PaCO2 the vascular response is termed carbon dioxide reactivity. The transcranial Doppler (TCD) provides information about changes in flow velocities secondary to changes in CPP or PaCO2.
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