Effect of resting pressure on the estimate of cerebrospinal fluid outflow conductance

BACKGROUND: A lumbar infusion test is commonly used as a predictive test for patients with normal pressure hydrocephalus and for evaluation of cerebrospinal fluid (CSF) shunt function. Different infusion protocols can be used to estimate the outflow conductance (C(out)) or its reciprocal the outflow resistance (R(out)), with or without using the baseline resting pressure, P(r). Both from a basic physiological research and a clinical perspective, it is important to understand the limitations of the model on which infusion tests are based. By estimating C(out) using two different analyses, with or without P(r), the limitations could be explored. The aim of this study was to compare the C(out) estimates, and investigate what effect P(r)had on the results. METHODS: Sixty-three patients that underwent a constant pressure infusion protocol as part of their preoperative evaluation for normal pressure hydrocephalus, were included (age 70.3 ± 10.8 years (mean ± SD)). The analysis was performed without (C(excl Pr)) and with (C(incl Pr)) P(r). The estimates were compared using Bland-Altman plots and paired sample t-tests (p < 0.05 considered significant). RESULTS: Mean C(out) for the 63 patients was: C(excl Pr )= 7.0 ± 4.0 (mean ± SD) μl/(s kPa) and C(incl Pr) = 9.1 ± 4.3 μl/(s kPa) and R(out) was 19.0 ± 9.2 and 17.7 ± 11.3 mmHg/ml/min, respectively. There was a positive correlation between methods (r = 0.79, n = 63, p < 0.01). The difference, ΔC(out)= -2.1 ± 2.7 μl/(s kPa) between methods was significant (p < 0.01) and ΔR(out )was 1.2 ± 8.8 mmHg/ml/min). The Bland-Altman plot visualized that the variation around the mean difference was similar all through the range of measured values and there was no correlation between ΔC(out )and C(out). CONCLUSIONS: The difference between C(out )estimates, obtained from analyses with or without P(r), needs to be taken into consideration when comparing results from studies using different infusion test protocols. The study suggests variation in CSF formation rate, variation in venous pressure or a pressure dependent C(out )as possible causes for the deviation from the CSF absorption model seen in some patients.