The CPP has traditionally been calculated as the difference between MAP and ICP or JVP and thus may easily be derived when ICP is monitored. Although not a frequent occurrence, regional compartmentalization of ICP may result in regional variation in CPP, and hence in CBF. Moreover, when venous pressure is elevated or higher than ICP, both the arterial inflow from the systemic driving force (MAP) and the venous outflow need to be considered in order to optimize CPP.
On the basis of studies of other organs and the peripheral circulation, it can be demonstrated that the critical closing pressure (CCP), located at the arteriolar level, determines the downstream pressure in the vasculature of that organ. The cerebral CCP is defined as the arterial pressure threshold below which arterial vessels collapse, and is determined by both the ICP and the cerebral arterial tone. Thus CCP is the minimum perfusion pressure necessary to keep the cerebral vessels open. This is influenced by the surrounding extrinsic pressure (ICP) as well as by any distal resistance (e.g. jugular venous obstruction). This is not an easily quantifiable variable, but it represents the overall downstream resistance of the cerebral vasculature, and may be a more important determinant of CPP than ICP (i.e. CPP = MAP - CCP) . Complex relationships therefore exist between ICP, systemic blood pressure, cerebral metabolism and CBF, all of which must be taken into consideration in the intensive care management of neurosurgical patients. These principles apply whether the patients had suffered traumatic brain injury, subarachnoid hemorrhage or ischemic stroke. Recent advances in monitoring modalities, which will be discussed later in the chapter, have made it theoretically possible to manipulate 'real-time' information to optimize treatment of these patients.
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