Bolous versus steady-state infusion for determination of CSF outflow resistance

For rapid changes in cerebrospinal fluid volume an exponential relationship was demonstrated between CSF pressure and CSF volume in 15 cats. This relationship was valid over a CSF pressure range from 7 to 50 mm Hg and for acute increases of up to 9% to total CSF volume (approximately 13 ml for humans). Our data agree well with previous reports for the cat. A similar relationship has been shown in the dog and in humans. It has been claimed that, given the equations for CSF bulk flow and the exponential relationship between CSF pressure and CSF volume, one can calculate CSF outflow resistance by observing the decay of CSF pressure after a bolus injection into the CSF space. This claim was evaluated in an additional 18 cats. In these animals CSF outflow resistance calculated by the bolus method was compared with resistance calculated by a steady-state infusion method over the CSF outflow resistance range of 74 to 293 mm Hg/ml min-1. Resistance calculated by the bolus method underestimated resistance calculated by the steady-state method, and this underestimate grew larger with increasing resistance. The bolus technique is therefore not a valid method for determining CSF outflow resistance. The explanation offered for these results is that the decay of CSF pressure after a bolus injection into the CSF space occurs not only because of runoff of the injected volume of CSF but also because of "pressure relaxation" of the brain parenchyma around the CSF space. The phenomenon of pressure relaxation was not considered in developing the equation for calculation of CSF outflow resistance by the bolus technique. The time dependency of pressure relaxation allows for a fundamental element of hysteresis within the CSF space. A method of quantifying this element of hysteresis is suggested.