Intracranial pressure-volume curve correlated with blood flow velocities. A relative relationship exists between intracranial compliance, intracranial pressure (ICP), cerebral perfusion pressure (CPP), and the pulsatility index (PI), which is obtained from the blood flow velocity profiles measured by transcranial Doppler (TCD). The normal PI value ranges from about 0.5 to 1.19 and correlates best with normal compliance and ICP ranges (identified on the left column). The PI starts to increase approximately starting from 0.9 to 1.19 with compromised intracranial compliance even when ICP still remain normal (middle column). The PI further increases from about >1.19 with exhausting intracranial compliance, ICP elevation, CPP reduction, and decreasing vascular bed, which eventually leads to circulatory arrest (right column). Characteristic TCD flow velocity waveform changes with increasing PI are represented above the graph. TCD is a helpful and readily available bedside technology to monitor intracranial compliance. Source: Intracranial Pressure: Monitoring and Management, Principles of Critical Care, 4e Citation: Hall JB, Schmidt GA, Kress JP. Principles of Critical Care, 4e; 2015 Available at: http://accessmedicine.mhmedical.com/DownloadImage.aspx?image=/data/books/1340/hall4_ch86_fig-86-06.png&sec=80036218&BookID=1340&ChapterSecID=80036194&imagename= Accessed: January 02, 2018 Copyright © 2018 McGraw-Hill Education. All rights reserved