What is Comprehensible from the Infusion Test?: Experimental Analysis of the Intracranial Volume-pressure Relationship

In the previous report a new model was proposed for the understanding of the intracranial mutual pressure interactions after a bolus volume loading. In this report the intracranial volume-pressure relationship (VPR) is examined in 15 dogs. They were separated into two groups: the steady-state infusion group (9 dogs) and the epidural balloon inflation group (6 dogs). The former contained 3 dogs in which 50 mg/kg of kaolin emulsion had been injected into the basal cistern a week before the experiment. They were regarded as a model of cerebrospinal fluid (CSF) malabsorption. In comparison with the actual volume-pressure (VP) curves, theoretical curves were displayed which were extracted mathematically by using the pressure-response (p-r) curve of each dog. The plateau level of the theoretical VP curve was always lower than the actual steady-state infusion curve. It was thought that more spatial compensation of the vascular bed occur in the actual plateau of the steady-state infusion than in the theoretical plateau, which was based on a single bolus injection. The r value calculated by the actual steady-state infusion was therefore very high because it demonstrated an exaggerated absorption rate under the pressure buffering effect of the venous system. So the CSF absorption resistance obtained by analyzing the p-r curve of a bolus injection should be closer to the true value than that obtained by steady-state infusion. The shape of VP curves derived from steady-state infusions and balloon compressions were not monoexponential. The latter developed a plateau level under the maximal pressure buffering by the vascular bed. Consequently analytical methods in which the hypothesis that the VPR was exponential was used were proved to be untrustworthy.