6 Intracranial Pressure and Elastance

Head injury is a common form of trauma. For example, in the UK head injury occurs in more than 500 000 persons per annum of which about 10% are diagnosed as severe, 15% moderate and the remainder as minor head injury (Miller et al., 1992; Pickard and Czosnyka, 1993). Head trauma is a significant cause of death and disability, especially in young males (median age < 30) and is associated with raised intracranial pressure (ICP). Raised ICP is defined as pressure greater than 20 mmHg and appears most commonly in about 50–75% of patients with severe head injury who remain comatose after resuscitation. Over the past 50 years there has been an active and wide-ranging research into the causes and consequences of raised ICP which, to date, has been the subject of nine international symposia embracing such diverse disciplines as neurosurgery, anesthesia, radiology, biophysics, electronic and mechanical engineering, mathematics and computer science. In particular, the introduction during the 1970s of the continuous monitoring of ICP has led to renewed activity in both clinical and experimental research into the physiology and pathophysiology of maintaining craniospinal volume and pressure. This interest has not just been in monitoring pressure alone but also in using information derived from pressure monitoring to help both predict raised ICP and determine the underlying cause. ICP is a reflection of the relationship between alterations in craniospinal volume and the ability of the craniospinal axis to accommodate added volume. The craniospinal axis is essentially a partially closed box with container properties including both viscous and elastic elements. The elastic or its inverse, the compliant, properties of the container will determine what added volume can be absorbed before intracranial pressure begins to rise. So an understanding of raised ICP encompasses an analysis of both intracranial volume and craniospinal compliance. This chapter reviews the relationship of raised ICP to outcome and its significance as part of the development of the primary injury and as a superimposed secondary insult. This is followed by a review of both the historical and current concepts underlying our present understanding of the physiology and pathophysiology of maintaining intracranial pressure and volume.