The understanding of ICP is based on Monro-Kelly Doctrine which states that after the fontanelles and sutures of the skull close, the cranium acts as a rigid container as its volume cannot change (AK Chaudhary, 2005). The contents of the brain cannot be compressed; therefore a constant amount of venous outflow is required to maintain ICP.There are three contents of cranial cavity: blood (10%), brain (80%) and CSF (10%). Under normal circumstances, several regulatory mechanisms exist to control ICP. The blood flow is autoregulated between 50-150 mmHg and is approximately constant (S Strandgaard, 1984). If the volume of the cranial cavity increases, the blood moves out of the distensible veins (can change their cross-sectional area to increase capacity) and creates space inside the intracranial cavity. The CSF also moves into the arachnoid space and spinal thecal sac. When these mechanisms are functioning, the ICP remains within normal range. But as these mechanisms exhaust, this intracranial cavity is no more a compliant system and small increases in volume can lead to greater changes in pressure.
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