Intracranial pressure: cerebrospinal fluid dynamics and pressure-volume relations.

Continuous measurement of the intracranial pressure (ICP) is routine in todays evaluation of various intracranial diseases and increased ICP is a common therapeutical problem in neurosurgical patients. Still, very little is known about the patho-physiological and biomechanic events that lead to increased ICP. ICP is governed by 1) the resistance to absorption of cerebrospinal fluid (Rout), 2) the production rate of CSF (If) (taken together Rout and If are referred to as the "CSF dynamics"), and 3) the pressure in the Sagittal Sinus (Pss) in accordance with the equation: ICP = If X Rout + Pss. When an intracranial mass grows the cranio-spinal volume buffering capacity is exhausted and the ICP subsequently rises. This event has been imitated in experiments and is described by the classical exponential pressure-volume curve. In a semilogarithmic coordinate system the curve will be linear and if one exchanges the abscissa and ordinate (x = log ICP, y = volume) the slope is the pressure-volume index (PVI). In normal adults PVI = 25 ml and defines the volume that theoretically will increase the ICP tenfold when injected into the CSF space. The main goal of the present study was to analyse the ICP in accordance with the above mentioned principles by measurements of Rout and the PVI. Furthermore, to evaluate the PVI method (synonymous with the "bolus injection" method) described by Marmarou and coworkers. By this method a bolus of a few milliliters of fluid is injected into the ventricles via an intraventricular cannula. PVI is computed based on the immidate ICP rise. The following slowlier ICP decrement defines the Rout. Another goal was to analyse whether measurements of the ICP pulse amplitude, which cancels the need of manipulations of the CSF space, could replace PVI measurements. Finally, to evaluate whether or not CT of the brain depicts pressure-volume relations and Rout in adult patients with hydrocephalus. The study comprised 63 patients with subarachnoid haemorrhage, cranio-cerebral injury or so-called normal-pressure hydrocephalus. The following variables were measured: 1) ICP, 2) pulseamplitude, 3) PVI and 4) Rout. The latter was measured by means of the PVI method and in some instances for reasons of comparison with the constant rate infusion technique and "controlled withdrawal". The main conclusions of the studies were: 1) For estimates of PVI the bolus injection technique was applicable. For Rout measurements the method was only safe at relatively low ICP levels.(ABSTRACT TRUNCATED AT 400 WORDS)