Comparison between 3 infusion methods to measure cerebrospinal fluid outflow conductance

Clinical article

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Object

There are several infusion methods available to estimate the outflow conductance (Cout) or outflow resistance (Rout = 1/Cout) of the CSF system. It has been stated that for unknown reasons, the bolus infusion method estimates a higher Cout than steady-state infusion methods. The aim of this study was to compare different infusion methods for estimation of Cout.

Methods

The following 3 different infusion methods were used: the bolus infusion method (Cout bol); the constant flow infusion method, both static (Cout stat) and dynamic (Cout dyn) analyses; and the constant pressure infusion method (Cout cpi). Repeated investigations were performed on an experimental model with well-known characteristics, with and without physiological pressure variations (B-waves, breathing, and so on). All 3 methods were also performed in a randomized order during the same investigation in 20 patients with probable or possible idiopathic normal-pressure hydrocephalus; 6 of these patients had a shunt and 14 did not.

Results

Without the presence of physiological pressure variations, the concordance in the experimental model was good between all methods. When they were added, the repeatability was better for the steady-state methods and a significantly higher Cout was found with the bolus method in the region of clinically relevant Cout (p < 0.05). The visual fit for the bolus infusion was dependent on subjective assessment by the operator. This experimental finding was confirmed by the clinical results, where significant differences were found in the investigations in patients without shunts between Cout of the visual bolus method and Cout stat, Cout dyn, and Cout cpi (4.58, 4.18, and 6.12 μl/[second × kPa], respectively).

Conclusions

This study emphasized the necessity for standardization of Cout measurements. An experienced operator could partly compensate for difficulties in correctly estimating the pressure parameters for the bolus infusion method, but for the general user this study suggests a steady-state method for estimating Cout.

Abbreviations used in this paper: Cout = outflow conductance; ICP = intracranial pressure; INPH = idiopathic normal-pressure hydrocephalus.

Article Information

Address correspondence to: Kennet Andersson, M.Sc., Department of Biomedical Engineering and Informatics, Umeå University Hospital, S-901 85 Umeå, Sweden. email: kennet.andersson@radfys.umu.se.

Please include this information when citing this paper: published online September 10, 2010; DOI: 10.3171/2010.8.JNS10157.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Time scheme for the performance of 3 different infusion tests in a patient during the same session. The order of the tests is as follows: constant pressure infusion, bolus infusion, and constant flow infusion. At the beginning of the investigation the resting pressure of the patient was measured, and a sample of CSF was taken. The black line shows ICP as a function of time, and the gray line shows the net infused volume of artificial CSF.

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    Schematic picture of ICP before, during, and after a bolus infusion, accompanied by a fitted, exponentially declining curve. The different pressures used in the analysis—Pstart, Pp, and Pt—are indicated in the figure. The gray box shows the duration of the bolus infusion.

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    Upper: Cout for each pipe as measured by the 5 different methods when no physiological pressure variations were present. For the first 3 pipes, ICP did not reach a plateau pressure during constant flow infusion. Lower: Cout for each pipe as measured by the 5 different methods when physiological pressure variations were present. The error bars show SEM (6 measurements per pipe). For the first 3 pipes, ICP did not reach a plateau pressure during constant flow infusion.

  • View in gallery

    Cout for each of the 20 patients. Patients in Cases 1–14 underwent preoperative investigations and those in Cases 15–20 underwent shunt test follow-up. In general, and specifically for the constant pressure infusion method, Cout was higher for the patients with shunts than for those without.

  • View in gallery

    Mean Cout for all patients (excluding patients with shunts) where a proper value from analysis was attained for each method (8 patients). The error bars indicate SEM. Corresponding values for Rout (from left to right) are 16.0, 13.4, 12.6, 9.2, and 10.4 mm Hg/ml/min.

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