Variations in pressure-volume index and CSF outflow resistance at different locations in the feline craniospinal axis

Pressure volume index (PVI) and cerebrospinal fluid (CSF) outflow resistance (Ro) were estimated in spontaneously breathing anestheized cats by the bolus injection test under normal conditions and also under abnormal conditions produced by slow infusion of saline into the CSF space. Bolus injections were made separately into the lumbar subarachnoid space, cisterna magna, and lateral ventricle. The mean PVI values in the lumbar sac, cisterna magna, and lateral ventricle under normal conditions were 0.70, 0.71, and 0.64 ml, respectively; not significantly different from each other. Saline infusion lowered PVI significantly at every site; PVI values in the lumbar sac, cisterna magna, and lateral ventricle were 0.54, 0.52, and 0.53 ml, respectively; not significantly different from each other. Indirect values of PVI were calculated from the pressure responses observed at sites other than where the bolus had been injected. These indirect PVI values were always greater than PVI at the injection site under normal conditions, but differences between direct and indirect PVI values were abolished during saline loading of the CSF space. The Ro was estimated under normal conditions in the lumbar sac, cisterna magna, and lateral ventricle to be 81.6, 85.6, and 110.3 mm Hg/ml/min, respectively. The lateral ventricle Ro was significantly higher than at other places. These findings suggest that, when there is no blockage in the craniospinal axis, the pressure response to a bolus change in CSF volume is freely transmitted, direct measurements of PVI are independent of location, and indirect measurements are larger because of "buffering" in the CSF space. When PVI is lowered and buffering capacity is exhausted, these differences between direct and indirect PVI values disappear.