Intracranial Pressure in Traumatic Brain Injury

Licentiate thesis, 2009

Traumatic Brain Injury (TBI) continues to be a major problem worldwide. Today, intensive care of patients with TBI is mainely focused on preventing and treating secondary brain injuries. High pressure inside the intracranial cavity (ICP) has been found to be an important feature of disturbed cerebral dynamics and secondary injuries. Thus, invasive measurement of ICP has been turned into a well established routine in modern neuro-intensive care. ICP can be regarded as a response of the cerebrospinal system to different stimuli. Within this system, parameters are affected by each other, often through complex nonlinear relationships. An evaluation of the brain state is often performed using the time average of the ICP signal. Although this feature provides important information, it is far too simple to grasp and reflect the complete picture. The development of more informative features through ICP signal processing has been the center of attention in many TBI-related investigations as well as in this thesis. This study considers the ICP signal as a composed wave, which contains contributions by circulatory and respiratory mechanisms. By this we avail ourself of studying the cerebrospinal system through its dynamic response upon circulatory and respiratory excitations. Such study is conducted through two different methods. First, arterial blood pressure (ABP) and ICP is defined as the cerebrospinal system input and output, respectively. The associated transfer function is empirically estimated and its amplitude at the fundamental cardiac frequency is compared to the index of compensatory reserve. Linear correlation between two estimates is approximately 0.82, suggesting that the transfer function between ABP and ICP conveys the same sort of information as the index of compensatory reserve. The second method is more concerned with morphology aspects of the ICP. A harmonic model is proposed to decompose the pressure signals of human body to the cardiac and respiratory components. Both linear and nonlinear approaches to the problem are discussed. Results then are compared to the tidal component of ICP, tracked over time. In this study, the fundamental cardiac component is found to have the most significant contribution in the construction of the tidal peak, implying that time evolution of this component can be tracked by tracking the fundamental cardiac component in ICP.