Modeling intracranial pressure dynamics and measurement of cerebral blood flow changes induced by orthostatic tilt.

摘要

Two studies are presented in this dissertation. The first study involved modifying an isovolumetric model of cerebrospinal fluid dynamics with the purpose of developing a further understanding of the mechanisms of the dynamics of intracranial pressure (ICP) recordings of patients with brain injury during intensive care management. The original model was proposed by Ursino[1,2]. Modifications were made to address the hypothesis that: (1) cyclic extravascular compressional modulation of the capillary and venous bed occurs with positive pressure inhalation; and (2) the degree of modulation is diminished with increasing vascular dilation induced by increasing the level of the partial pressure of carbon dioxide (PCO₂) with the arterial blood.; Modification of the isovolumetric model was accomplished by introducing a cyclic modulation of the resistance of the capillary and venous bed synchronized by ventilation. Both the steady-state value and the magnitude of modulation of resistance over the ventilation cycle were progressively reduced with increased vascular dilation induced by increasing levels of simulated PCO₂. In agreement with reported laboratory results, the simulated model recordings demonstrated a systematic change and the correlation index between arterial blood pressure and ICP progressively increased monotonically as the level of PCO₂ progressively increased from 30 mmHg to 80 mmHg. These results support the general premise that during positive pressure, cyclic extravascular compressional modulation of the capillary and venous bed produces a cyclic variation of ICP and the degree of modulation is dependent on the state of the vascular dilation.; The second study involved the use of Near-infrared spectrometry (NIRS) to non-invasively monitor changes in cerebral blood flow in adult subjects. Low level near-infrared laser light, focused on the subject's skull and reflected internally was monitored, recorded, and converted to measures of cerebral oxygenation. Relative changes in cerebral oxyhemoglobin (ΔHbO₂ ), deoxyhemoglobin (ΔHb), and the NIRS Oxygenation Index, [ΔHbO ₂ - ΔHb], indicative of blood flow, using 750 nm and 810 nm laser diodes were obtained. Six subjects were monitored as each underwent a 20 degree head-down tilt from a horizontal supine position. Because of the autoregulatory response of the cerebrovascular, it was expected that head down tilt would cause a transitory increase in blood flow in the cerebral tissue under interrogation. As a result, the expected [ΔHbO₂ - ΔHb] response in all subjects to head down tilt was expected to demonstrate a transitory increase followed by a return to the original baseline value. The ΔHbO₂ , ΔHb, and [ΔHbO₂ - ΔHb] responses were found to vary from subject to subject and were not consistent with expectations. With head down tilt ΔHbO₂ and [ΔHbO₂ - ΔHb] increased, however Mb sometimes increased and sometimes decreased. Also, with head down tilt, only 59% of the measurements exhibited the autoregulation characteristic of returning to the before tilt value. The conclusion is that by monitoring the [ΔHbO₂ - ΔHb] response induced by head down tilt, changes in cerebral blood flow can be detected. However, the erratic nature of Hb measures reveals that unexplained artifacts exist in 41% of the responses. Thus, use of NIR to detect the autoregulatory response of cerebral blood flow induced by head tilt is limited.