S Cirovic1, C Walsh, W D Fraser. 1. Institute for Aerospace Studies, University of Toronto, ON, Canada. cirovics@mie.utoronto.ca
Abstract
BACKGROUND: High positive Gz may result in inadequate blood supply to the brain even if the central blood pressures are maintained at normal levels. We use a mechanical model to simulate the influence of sustained +Gz on cerebral circulation. METHODS: The model consists of ascending and descending tubes representing the extracranial arteries and veins, respectively, and a cranium in which the tubes are enclosed within water-filled rigid container to account for the skull and the cerebrospinal fluid. A thick-walled Tygon tube and a thin-walled surgical drain tube were used for the arteries and veins, respectively. The flow of water was driven by a pressure difference at the model ends, and the change in the gravitational vector was accomplished by tilting the model. RESULTS: The flow drops with an increasing tilt angle only if the descending arm collapses. However, when the pressures at the model ends are sufficiently elevated, the flow is restored to normal value. In the cranium model, the pressure in the water surrounding the tubes always stays close to the pressure in the surgical tubing. Consequently, the tubes in the container do not collapse. CONCLUSIONS: The principal effect of Gz on flow through the model occurs via changes in the resistance of the collapsed descending arm. As the pressures at the model ends are elevated, the descending arm opens and the flow increases. The pressure in the cranium model is dictated by the condition that the volume of the container has to remain constant.
BACKGROUND: High positive Gz may result in inadequate blood supply to the brain even if the central blood pressures are maintained at normal levels. We use a mechanical model to simulate the influence of sustained +Gz on cerebral circulation. METHODS: The model consists of ascending and descending tubes representing the extracranial arteries and veins, respectively, and a cranium in which the tubes are enclosed within water-filled rigid container to account for the skull and the cerebrospinal fluid. A thick-walled Tygon tube and a thin-walled surgical drain tube were used for the arteries and veins, respectively. The flow of water was driven by a pressure difference at the model ends, and the change in the gravitational vector was accomplished by tilting the model. RESULTS: The flow drops with an increasing tilt angle only if the descending arm collapses. However, when the pressures at the model ends are sufficiently elevated, the flow is restored to normal value. In the cranium model, the pressure in the water surrounding the tubes always stays close to the pressure in the surgical tubing. Consequently, the tubes in the container do not collapse. CONCLUSIONS: The principal effect of Gz on flow through the model occurs via changes in the resistance of the collapsed descending arm. As the pressures at the model ends are elevated, the descending arm opens and the flow increases. The pressure in the cranium model is dictated by the condition that the volume of the container has to remain constant.