Fig. 1.

Physiologic model conception of PRx. If cerebrovascular pressure reactivity (CVR) is intact (upper line), small cerebral vessels dilate in response to decreasing ABP, resulting in an increased cerebral blood volume. In regards to the pressure–volume curve of brain [22–24], this causes an increase of ICP, i.e. ABP and ICP are negatively correlated. If CVR is disturbed (lower line), ABP decrease is passively followed by constriction of small vessels. This causes a decrease of cerebral blood volume, and, therefore, causes a decrease of ICP. ABP and ICP are positively correlated. Conversely, in the case of increasing ABP, a negative correlation between ABP and ICP is generally associated with intact CVR, while a positive correlation between both signals indicates impaired CVR