Factors determining mean ICP in hydrocephalic patients with Hakim-programmable valve: implications of the parallel arrangement of the CSF outflow resistance and shunt.

Abstract

Measurement of CSF pressure is used clinically to test shunt function in vivo in hydrocephalic patients. Criteria for appropriate shunt function have never been validated. Hakim-Programmable valve was tested in a model of CSF circulation with variable resistance to CSF outflow (from 12 to 50 mm Hg/ml/min), increased hydrodynamic compliance (> 1.5 ml/mm Hg), and constant perfusion of a rate of 0.4 ml/min, i.e. conditions typical for hydrocephalus. The main question was how the simulated CSF pressure was influenced by the shunt setting and the residual resistance to CSF outflow. Measured baseline CSF pressure correlated well with shunt operating pressure only when high resistance to CSF outflow (50 mm Hg/(ml/min)) was used. For the medium resistance (20 mm Hg/(ml/min)) operating pressure was strongly affected by system's absorption capacity. For low resistance (12 mm Hg/(ml/min)) operating pressure through the valve was independent on valve's settings and no fluid drainage through the valve was recorded. Patients with moderately elevated resistance to CSF outflow (12-18 mm Hg/(ml/min)) cannot possibly react to changes of the valve's settings above 100 mm H2O. Mean CSF pressure results both from shunt setting and patient's own re-absorption capacity.