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The cerebrospinal fluid (CSF) formation rate (Vf) has been extensively studied by the ventriculocisternal perfusion, a method still regarded as the most precise one. This method as well as the equation for the calculation of the CSF formation rate (Vf) was established by Heisey et al on indicator dilution in perfusate. They assumed that the dilution of the indicator substance in perfusion is a consequence of newly formed CSF i.e. a higher CSF formation rate would result in a higher degree of dilution of the indicator substance. Therefore, such method is indirect and any mistake in the interpretation of the degree of indicator dilution would lead to questionable and often contradictory results regarding CSF formation rates. According to Heisey's equation, Vf shoud not depend on the rate of ventriculocisternal perfusion. However it has been shown that Vf is perfusion dependt value, and also that during perfusion the indicator substance is partially absorbed into surrounding tissue. It is possible that obtained Vf dependence on perfusion rate was caused by observed absorption of indicator substances. For that reason, in anaesthetised cats ventriculocisternal perfusion was performed at higher (252.0 microL/min) and at lower perfusion rate (65.5 microL/min) and Vf was calculated at both experimental and corrected (just for absorbed amount) values of indicator substance. Since (inspite of the correction) the difference of 12.4 microL/min between lower (15.0 microL/min) and higher perfusion rate (27.4 microL/min) was obtained, it is obvious that ventriculocisternal perfusion method cannot be considered reliable for measuring CSF formation rate.

It is unknown which factors determine the changes in cerebrospinal fluid (CSF) pressure inside the craniospinal system during the changes of the body position. To test this, we have developed a new model of the CSF system, which by its biophysical characteristics and dimensions imitates the CSF system in cats. The results obtained on a model were compared to those in animals observed during changes of body position. A new model was constructed from two parts with different physical characteristics. The "cranial" part is developed from a plastic tube with unchangeable volume, while the "spinal" part is made of a rubber baloon, with modulus of elasticity similar to that of animal spinal dura. In upright position, in the "cranial" part of the model the negative pressure appears without any measurable changes in the fluid volume, while in "spinal" part the fluid pressure is positive. All of the observed changes are in accordance to the law of the fluid mechanics. Alterations of the CSF pressure in cats during the changes of the body position are not significantly different compared to those observed on our new model. This suggests that the CSF pressure changes are related to the fluid mechanics, and do not depend on CSF secretion and circulation. It seems that in all body positions the cranial volume of blood and CSF remains constant, which enables a good blood brain perfusion.

The effect of cerebrospinal fluid (CSF) osmolarity on the CSF volume has been studied on different CSF/brain tissue contact areas. It has been shown, on anesthetized cats under normal CSF pressure, that the perfusion of CSF system (12.96 μl/min) by hyperosmolar CSF (400 mOsml/l) leads to significantly higher outflow volume (μl/min) during ventriculo-cisternal perfusion (29.36 ± 1.17 and 33.50 ± 2.78) than the ventriculo-aqueductal perfusion (19.58 ± 1.57 and 22.10 ± 2.31) in experimental period of 30 or 60 min. Both of these hyperosmolar perfusions resulted in significantly higher outflow volume than the perfusions by isoosmolar artificial CSF (12.86 ± 0.96 and 13.58 ± 1.64). These results suggest that the volume of the CSF depends on both the CSF osmolarity and the size of the contact area between CSF system and surrounding tissue exposed to hyperosmolar CSF. However, all of these facts imply that the control of the CSF volume is not in accordance with the classical hypothesis of cerebrospinal fluid hydrodynamic. According to this hypothesis, the CSF volume should be regulated by active formation of CSF (secretion) inside the brain ventricles and passive CSF absorption outside of the brain. Obtained results correspond to the new hypothesis which claims that the volume of CSF depends on the gradients of hydrostatic and osmotic forces between the blood on one side and extracellular fluid and CSF on the other. The CSF exchange between the entire CSF system and the surrounding tissue should, therefore, be determined by (patho)physiological conditions that predominate within those compartments., (Copyright © 2010 Elsevier Ireland Ltd. All rights reserved.)

The aim of the study was to evaluate whether or not cerebrospinal fluid formation rate (Vf) calculated according to the equation of Heisey et al., truly show the produced cerebrospinal fluid. For this reason Vf was simulated (40.6 microL/min) by an infusion pump in a plastic cylinder and the evaluation was done by comparing the results obtained between the calculated Vf and the simulated one. In both cases the result should be the same (40.6 micro/min). Other types of experiments were carried out by ventriculocisternal perfusion (92.4 microL/min) on anaesthetized and sacrificed cats. If the equation is correct, the calculated Vf for sacrificed animals should be zero, because there is no Vf in dead animals. The fact that the calculated Vf (46.5 microL/min) in the plastic cylinder was different (p < 0.0001) from the simulated one (40.6 microL/min) and that Vf was calculated even for dead animals (3-5 microL/min) clearly shows the that perfusion method may not be an accurate method for determination of Vf.