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A Review of Electrical Equivalence Models for the Evaluation of Intracranial Pulsatility.

Arterial blood entering the rigid skull is pulsatile. The brain requires a homeostatic environment, and a number of disease processes are now being linked to excessive pulsatility in the brain. Given this fact there must be some mechanism in normal brains for diverting the pulsatility away from the brain, providing a smooth flow of blood. Any such system would be constrained by the Monro-Kellie doctrine, which tells us that the sum of volumes entering the rigid skull must equal the sum of volumes exiting the rigid skull for all instances of time. A further constraint exists whereby the normal brain is considered to be of constant volume. Consequently, the sum of the blood and cerebrospinal fluid (CSF) volumes in the skull is constant at any given time. Since the venous outflow does not exactly match the arterial inflow at all times, this must be compensated for by bulk movement of CSF from the skull. The production and absorption mechanisms of CSF are far too slow to be responsible for this. Fortunately there exists a compliant spinal subarachnoid space which is able to accommodate the superfluous CSF, during systole, and return it during diastole. It may be possible for the exact nature of the intracranial fluid dynamics to be described quantitatively via modelling with an equivalent electrical circuit. In this document we describe the relevant biological structures and discuss this possibility. The Anatomy of the Ventricular System The ventricular system (Fig.1) consists of four ventricles; two lateral ventricles, between and below which sits the third ventricle, which is connected by the cerebral aqueduct to the fourth ventricle [1]. Figure 1: A) Anterior view of the ventricular system B) Posterior view of the ventricular system. Each of the lateral ventricles sits in one of the cerebral hemispheres such that they become mirror images of one another. They form a shape that approximates the letter C, but with a tail, as if it was to be joined up to a previous letter. They are virtually independent, but a connection exists between them, known as the interventricular foramen, which also connects the lateral ventricles to the third ventricle. The third ventricle is thin in one dimension so as to slot in between the two cerebral hemispheres, but has a relatively large area in the other two dimensions. A tubular structure with diameter approximately equal to the thickness of the