Hydraulic model of the cerebrovascular bed: an aid to understanding the volume-pressure test.

A hydraulic model of the cerebrovascular bed is presented. The model consists of a Starling resistor in series with an upstream resistance. Volume-pressure tests were performed on the model by injecting fluid into the rigid shell of the Starling resistor. An exponential pressure response to the increase in fluid volume was observed, which supports the hypothesis that the origin of the in vivo exponential pressure response to a transient increase in cerebrospinal fluid (CSF) volume can be attributed to compression of the cerebral vessels, most probably the veins. Mathematical expressions for the dependence of pressure on volume change were derived from the model and applied to in vivo volume-pressure data. The correlation between the model and in vivo experiments suggests that the CSF pressure is coupled to cerebral venous pressure and that the volume-pressure test is an indirect measure of the cerebral venous volume and is not a measure of intracranial elastance. The physical basis for the volume-pressure test is clarified, and expressions are derived to improve the utility of the test.