Modeling cyclic variation of intracranial pressure

To test the theoretical feasibility that low frequency baseline changes of the intracranial pressure (ICP) recording during mechanical ventilation are due to cyclic extravascular compressional modulation primarily of the cerebral venous bed, an established isovolumetric model of cerebrospinal fluid dynamics proposed by M. Ursino (1988) was modified. These modifications were made to address the hypothesis that: (1) cyclic extravascular compressional modulation of the cerebral venous bed occurs during 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) within the arterial blood. Modification of the isovolumetric model was accomplished by introducing a cyclic modulation of the resistance of the cerebral venous bed synchronized with ventilation. Simulated model recordings demonstrated that the correlation index between arterial blood pressure and ICP progressively increased monotonically as the level of PCO₂ increased from 30 mmHg to 80 mmHg. These results support the premise that during positive pressure ventilation, cyclic extravascular compressional modulation of primarily the cerebral venous bed produces a cyclic variation of ICP and the degree of modulation is dependent on the state of vascular dilation.