Validity of measurements of cerebrospinal fluid outflow resistance estimated by the bolus injection method.

Estimation of cerebrospinal fluid (CSF) outflow resistance (Ro) using a bolus injection method was carried out under normal conditions and in a state of artificially increased elastance produced by saline loading into the CSF space. These values of Ro were compared with Ro estimated by the steady state infusion method. Saline loading into the subarachnoid space decreased the buffering capacity significantly, as represented by the pressure-volume index (PVI), which was lowered from 0.72 ml under normal conditions of 0.56 ml during saline loading. The estimated values of Ro under normal conditions and during saline loading were 90.6 and 133.6 mm Hg/ml/minute, respectively. The steady state infusion method yielded an Ro value of 166.3 mm Hg/ml/minute. Ro was therefore considerably underestimated by the bolus method when used under normal conditions of resting intracranial pressure. Moreover, its error was bigger when Ro was higher. On the other hand, during the course of saline loading, the estimated value of Ro using the bolus method was not significantly different from that estimated by the steady state infusion method, even in the higher range of Ro. Estimation of Ro by the bolus injection method was more reliable when the buffering capacity of the craniospinal space was reduced.