In a non-expansile, non-contractible (fixed), freely communicating space the pressure of the fluid contents is everywhere equal and directly proportional to the fluid volume. Intracranial pressure results from contributions from intracranial fluid compartments: 1. brain tissue(more than 80% fluid), 2. intra-arterial blood, and 3. cerebrospinal fluid (CSF).
In the Monro-Kellie model total intracranial intracranial pressure is divided into a contribution from each of 3 intracranial volumes: brain parenchyma, cerebrospinal fluid CSF, and cerebral arteries. The contribution of each compartment to the pressure is equal to the ratio of each compartment to the whole intracranial volume. As long as total intracranial volume stays the same the intracranial pressure remains the same. If the volume of one compartment increases or decreases the volume of one or both other compartment must decrease or increase in order for the total volume (and therefore, ICP) to remain the same.
If the volume of any compartment is decreased while that of the other two volumes remains the same the total intracranial pressure must go down. The Monro Kellie model is the conceptual basis for current physical intracranial pressure management strategies and protocols.
Pie chart diagram of Monro-Kellie model relating intracranial pressure (ICP) and volume. The total ICP is the summation of contributions from three volume compartments: 1. Parenchyma = brain / 2. Cerebrospinal fluid = CSF, and 3. Arterial volume = vessels