Interactions Between Volumes, Flows and Pressures in the Brain: Intracranial Pressure, Cerebral Perfusion Pressure, Cerebral Autoregulation and the Concept of Compensatory Reserve

The incidence of traumatic brain injury (TBI) has been increasing globally and is estimated at 27–69 million of new cases per year. It remains the main single cause of death in young people below 40 years of age. The ultimate extent of brain injury and the patient’s outcome result from the degree of primary damage to the central nervous system at the time of impact and the subsequent, pathophysiological sequelae of injury expansion over following hours and days. Clinical management of TBI aims to interrupt the pathological cascade of events and minimise development of secondary damage. Although the entire spectrum of complex, multilevel processes occurring in injured regions of the brain and the peri-lesional penumbra are not fully studied and understood, important part of treatment protocols constitutes invasive monitoring of brain physiology. Intracranial pressure (ICP) measurement plays a key role as a diagnostic tool in critical care of TBI patients. ICP and its derivates are indirect source of data on status of brain autoregulation, intracranial compliance, and compensatory reserve. The pressure reactivity index reflects the condition of cerebrovascular reactivity and indicates an individual, optimal cerebral perfusion pressure. Moreover, the real-time observation of continuous ICP waveform does not only alert clinicians of intracranial hypertension but delivers prognostic information.