Cerebral venous blood outflow: a theoretical model based on laboratory simulation.

OBJECTIVE: The cerebrovascular bed and cerebrospinal fluid circulation have been modeled extensively except for the cerebral venous outflow, which is the object of this study. METHODS: A hydraulic experiment was designed for perfusion of a collapsible tube in a pressurized chamber to simulate the venous outflow from the cranial cavity. CONCEPT: The laboratory measurements demonstrate that the majority of change in venous flow can be attributed to either inflow pressure when the outflow is open, or the upstream transmural pressure when outflow is collapsed. On this basis, we propose a mathematical model for pressure distribution along the venous outflow pathway depending on cerebral blood flow and intracranial pressure. The model explains the physiological strong coupling between intracranial pressure and venous pressure in the bridging veins, and we discuss the limits of applicability of the Starling resistor formula to the venous flow rates. The model provides a complementary explanation for ventricular collapse and origin of subdural hematomas resulting from overshunting in hydrocephalus. The noncontinuous pressure flow characteristic of the venous outflow is pinpointed as a possible source of the spontaneous generation of intracranial slow waves. CONCLUSION: A new conceptual mathematical model can be used to explain the relationship between pressures and flow at the venous outflow from the cranium.