| Summary: | <p>Abstract</p> <p>Background</p> <p>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 (<it>C</it><sub>out</sub>) or its reciprocal the outflow resistance (<it>R</it><sub>out</sub>), with or without using the baseline resting pressure, <it>P</it><sub>r</sub>. 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 <it>C</it><sub>out</sub> using two different analyses, with or without <it>P</it><sub>r</sub>, the limitations could be explored. The aim of this study was to compare the <it>C</it><sub>out</sub> estimates, and investigate what effect <it>P</it><sub>r</sub>had on the results.</p> <p>Methods</p> <p>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 (<it>C</it><sub>excl Pr</sub>) and with (<it>C</it><sub>incl Pr</sub>) P<sub>r</sub>. The estimates were compared using Bland-Altman plots and paired sample <it>t</it>-tests (<it>p </it>< 0.05 considered significant).</p> <p>Results</p> <p>Mean <it>C</it><sub>out</sub> for the 63 patients was: <it>C</it><sub>excl Pr </sub>= 7.0 ± 4.0 (mean ± SD) μl/(s kPa) and <it>C</it><sub>incl Pr</sub> = 9.1 ± 4.3 μl/(s kPa) and <it>R</it><sub>out</sub> 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, <it>p </it>< 0.01). The difference, Δ<it>C</it><sub>out</sub>= -2.1 ± 2.7 μl/(s kPa) between methods was significant (<it>p </it>< 0.01) and Δ<it>R</it><sub>out </sub>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 Δ<it>C</it><sub>out </sub>and <it>C</it><sub>out</sub>.</p> <p>Conclusions</p> <p>The difference between <it>C</it><sub>out </sub>estimates, obtained from analyses with or without <it>P</it><sub>r</sub>, 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 <it>C</it><sub>out </sub>as possible causes for the deviation from the CSF absorption model seen in some patients.</p>
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