Fig. 2

Intracranial pressure-volume diagram. RAP: correlation coefficient between the change in pulse amplitude and the mean ICP value. The pulsations of CBV are shown in red, and the pulsatile response of ICP is in grey. System elastance is proportional to the dp/dV. RAP = 0 → Good Compensatory Reserve: At RAP = 0, the body has sufficient compensatory mechanisms in place, such as vasoconstriction and increased cardiac output, to maintain adequate tissue perfusion. The system is in a linear phase, meaning small changes in RAP do not lead to dramatic changes in venous return or cardiac output. RAP < 1 → Start of Depletion of Compensatory Reserve. As RAP decreases below 1 (but remains linear), the body begins to deplete its compensatory mechanisms. Although still linear, the system’s reserve capacity is gradually being exhausted. Transition at RAP = 1: This appears to represent a critical point where compensatory reserve is nearly or fully exhausted. Beyond this point, small changes in RAP lead to steep declines in venous return and cardiac output, signaling the onset of physiological decompensation