CPP is defined as MAP-ICP (or MAP-CVP, if CVP>ICP). Cerebral perfusion pressure is the major determinant of cerebral blood flow (CBF). At low and high levels of CPP, CBF is linearly related to CPP- illustrated by the well-known plot of CBF vs. CPP published by Lassen in 1959 (see figure). Along the plateau, cerebral autoregulation maintains a constant CBF across a range of perfusion pressures. At a CPP below this plateau, CBF drops linearly with CPP and hypoperfusion injury can occur. At CPP above this plateau, CBF rises linearly with CPP and hyperperfusion injury can occur. This plateau varies between individuals; as well as within an individual amongst different vascular beds (although it is often simplistically portrayed as ranging from MAP ~60 to ~150mmHg). CPP is also affected by other physiologic parameters such as hyper/hypocapnia. Hypercapnia, with resultant cerebral vasodilatation, both narrows and raises the plateau. Hypocapnia has the opposite effect by widening (most likely) and lowering the plateau (see Meng, 2015 for further explanation).
- Meng L, Gelb AW. Regulation of cerebral autoregulation by carbon dioxide. Anesthesiology 2015; 122: 196-205.