ABSTRACT
Figure 22.18 illustrates how the intracranial pressure (ICP) changes as the intracranial volume of brain tissue increases. Initially (A), a relatively large increase in volume (in the order of 25 ml) can be accommodated with little increase in ICP (also called the compensation phase), but as the volume increases a critical point is reached (B) where the pressure-buffering system is exhausted. Small increases in volume (approximately 5 ml) then produce large increases in pressure. Once on the steep part of the curve, increases in ICP can affect the CBF profoundly by reducing cerebral perfusion pressure (CPP) to critical levels (see later). In this situation, the mean arterial pressure (MAP) should be increased in an attempt to maintain the CPP, although this may itself increase the ICP further. Alternatively, strategies aimed at reducing the ICP can be introduced as detailed in Table 22.5. Conversely, if in the presence of a raised ICP the MAP should decrease (e.g. on induction of anaesthesia), active measures must be taken to restore it to appropriate levels.
