Concepts of Cerebral Perfusion Pressure and Vascular Compression During Intracranial Hypertension

Abstract

Continuous monitoring of intracranial pressure (ICP) has become an increasingly common practice in neurosurgical units. Intracranial hypertension is produced by a variety of pathological states, and the increased pressure rather than the underlying disorder may be primarily responsible for death in patients. There is general agreement that the intracranial hypertension disturbs brain function by reducing cerebral blood flow (CBF) and causing brain herniation with brainstem compression or displacement. This chapter measures ICP continuously in patients with severe head injury, brain tumor, subarachnoid hemorrhage, hydrocephalus, cerebral hypoxia, pseudotumor cerebri, or encephalitis, and regional CBF has been measured in some of these patients using the 133 Xe clearance technique. Several models have been developed in an attempt to simulate clinical conditions in the laboratory animal. This report describes observations in one of these animal models. In recent reports on the effects of increased ICP on CBF increasing emphasis has been placed on the concept that changes in ICP affect CBF through alterations in cerebral perfusion pressure (CPP), and that in this respect, they may be compared with changes in systemic arterial pressure (SAP) by considering CPP to be the difference between SAP and ICP. This chapter discusses the effects of changes in ICP and SAP on CBF in the same animal, with intact and defective autoregulation, as a means of further evaluating several proposed mechanisms of autoregulation; the sensitivity of autoregulation to raised ICP;)and the response of CBF to a concomitant increase in SAP and ICP at a constant CPP, with intact and defective autoregulation.