Assessment of cerebrospinal fluid outflow conductance using constant-pressure infusion—a method with real time estimation of reliability

The outflow conductance (C_out) of the cerebrospinal fluid (CSF) system is a parameter considered to be predictive in selection for hydrocephalus surgery. C_out can be determined through an infusion test. A new apparatus for performing infusion tests in a standardized and automated way was developed. The objective was to evaluate repetitiveness as well as to propose and evaluate a method for real time estimation of the reliability of individual C_out investigations. Repeated investigations were performed on an experimental model simulating the CSF system, and on 14 patients with hydrocephalus. ΔC_out, calculated as the 95% confidence interval of C_out, was introduced as an estimate of the reliability of individual C_out investigations. On the model, no significant difference was found between ΔC_out and the actual C_out variation in repeated investigations (p = 0.135). The correlation between the first and the second patient investigation was high (R = 0.99, p < 0.05), although there was a significant difference between the investigations (p < 0.05). The standard deviation of difference was 2.60 µl (s kPa)⁻¹. The repetitiveness of C_out with the new apparatus was high, and ΔC_out reflected the reliability of each investigation. This feature has to be taken into account in every individual case, before making a decision or performing research based on measurements of C_out in the future.