The physiological basis of intracranial pressure change with progressive epidural brain compression

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Abstract
Sequential CSF pressure-volume studies were carried out in 7 cats during the expansion at a constant rate of an epidural balloon. The same studies were performed in 3 control cats. Beginning after 20 min of inflation and continuing to the point of pupillary dilatation, there was a progressive increase in the pressure-volume index (volume required to change intracranial pressure (ICP) by 10-fold). During the course of balloon inflation there was also a progressive increase in CSF elastance (instantaneous ICP change per unit change in CSF volume). At the point of pupillary dilatation there was a marked, abrupt increase in the pressure-volume index and an equally dramatic decrease in CSF elastance. The CSF outflow resistance increased to a variable extent during balloon inflation. The plot of the CSF pressure vs. balloon volume (the mass lesion pressure-volume curve) was of the classical configuration with an initial relatively flat segment and a final steep segment. An hypothesis was presented that interprets the shape of the mass lesion pressure-volume curve in terms of changes occurring in the elastic properties of the tissues surrounding the CSF space and the volume of the CSF space. This hypothesis will explain most of the commonly observed variations in CSF pressure. Confusion regarding the ICP-volume relationships resulted from lack of specificity regarding which anatomical spaces are being perturbed.