Article
Resistance to outflow decreases unsteadily with increasing intracranial pressure
Die Resistance to outflow nimmt nicht stetig mit steigendem Hirndruck ab
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Authors
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Michael Kiefer
- Neurochirurgische Klinik, Universitätskliniken des Saarlandes, Homburg-Saar
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R. Eymann
- Neurochirurgische Klinik, Universitätskliniken des Saarlandes, Homburg-Saar
| Published: | April 23, 2004 |
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Outline
Text
Objective
According to the literature resistance to outflow (ROF) decreases with increasing intracranial pressure (ICP). However, from a theoretical viewpoint, this is questionable. The aim was to evaluate the influence of ICP on the ROF level.
Methods
In 5 hydrocephalic and 10 non-hydrocephalic H-Tx rats and in 125 men suffering from chronic hydrocephalus, a dynamic infusion test was performed. The resulting ROF was averaged for each ICP level between resting pressure and peak pressure.
Results
In rats ROF is high at a low ICP level, decreasing steadily to an ICP level of 25 (non-hydrocephalic) or 29 mm Hg (hydrocephalic). From there ROF increases steadily with rising ICP. The highest ROF values in hydrocephalic rats were 4500 - 5000 mm Hg x ml /min and the lowest ~ 2000 mm Hg x ml / min. In non-hydrocephalic the highest value was ~ 2500 mm Hg x ml / min and the lowest value ~ 900 mm Hg x ml / min. So the highest ROF is more than twice as high as the lowest depending on the ICP. The U-shape slope of the curve representing the dependency of ROF on ICP, which was found in rats, could also be found in men. ROF seems to be generally extremely dependent on the ICP level at which it is measured. Of tremendous influence is the ICP level between 25 - 34 mm Hg. While the lowest ROF values may be found at 25 mm Hg, more than two fold higher values are measured at an ICP level of 34 mm Hg in men and animals.
Conclusions
This enormous influence of ICP on ROF must be considered when interpreting the results of an infusion test. Using the Katzmann, test it is more the level of plateau-pressure than the intracranial pathology that determines ROF and accordingly the decision to shunt or not. Accordingly the Katzmann test seems to be obsolete. The usage of infusion tests, which calculates ROF in dependency on ICP, is obligatory. The increase of ROF at high ICP levels may be a consequence of a reduced outer CSF space during the infusion test. Earlier findings of a linear decrease of ROF with increasing ICP are only true for low ICP levels.
