Anaes · Applied cardiovascular & respiratory physiology
CSF & intracranial pressure physiology
Also known as Cerebrospinal fluid · CSF circulation · Intracranial pressure · Monro-Kellie doctrine · Cerebral perfusion pressure · Glymphatic system
Cerebrospinal fluid cushions and supports the brain within the rigid skull, and the fixed intracranial volume sets the rules for intracranial pressure. The framework rests on five exam-critical ideas: the intracranial volume is the sum of brain (about 80 percent), blood (about 10 percent) and CSF (about 10 percent), and because the skull is rigid the total is fixed (the Monro-Kellie doctrine), so an increase in one component displaces another; CSF is produced by the choroid plexus at about 500 mL per day, circulates from the ventricles through the foramina to the subarachnoid space, and is absorbed at the arachnoid granulations into the venous sinuses; normal intracranial pressure is about 5 to 15 mmHg and cerebral perfusion pressure equals mean arterial pressure minus intracranial pressure; raised intracranial pressure is at first compensated by CSF and venous displacement, then decompensates steeply (the intracranial volume-pressure curve), producing cerebral ischaemia and the Cushing triad; and the anaesthetist defends cerebral perfusion pressure by controlling mean arterial pressure, intracranial pressure (head position, PaCO2, drugs) and cerebral metabolic rate. Built on the glymphatic-imaging study (Wang 2026), the CSF-circulation-variability review (Engelhard 2026), the hydrocephalus-imaging review (Munir 2026), the ketamine-in-brain-injury review (Haywood 2026), the REBOA cerebral-perfusion study (Bader 2026), and the sodium-ascorbate-ICP study (Bishop 2026).
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- Cerebral perfusion pressure equals mean arterial pressure minus intracranial pressure (CPP = MAP minus ICP); raised ICP erodes cerebral perfusion even at a preserved MAP, so defending MAP and lowering ICP are both essential.
- The Monro-Kellie doctrine: the skull is rigid and total intracranial volume (brain plus blood plus CSF) is fixed, so a rise in one component must displace another — once displacement is exhausted, intracranial pressure rises steeply.
- CSF is produced by the choroid plexus (about 500 mL per day) and absorbed at the arachnoid granulations; obstruction to flow (aqueduct, fourth-ventricle foramina) causes obstructive hydrocephalus, absorption failure (subarachnoid blood, infection) causes communicating hydrocephalus.
- Raised intracranial pressure decompensates on a steep volume-pressure curve — the landmark of decompensation is a falling conscious level, then Cushing triad (hypertension, bradycardia, irregular respiration) heralding brainstem herniation.
- Anaesthetic agents affect intracranial pressure predictably: volatile agents vasodilate cerebral vessels and can raise ICP; propofol and barbiturates reduce cerebral blood flow and metabolic rate and lower ICP; ketamine, once thought to raise ICP dangerously, is safe in modern controlled-ventilation practice.
Meet the patient
A 24-year-old cyclist, no helmet, lands on the left temporal bone. He talks in the ambulance, vomits in resus, then — over fifteen minutes — stops answering questions and develops a dilated left pupil. His blood pressure is climbing while his heart rate is falling.[5]
That march across the resus bay is the whole topic in one patient: an expanding extradural haematoma exhausting intracranial compliance, then herniating. Hold the sequence — falling conscious level first, Cushing response late — and every equation below becomes a tool to keep that brain perfused.[5]
References6ShowHide
- [1]Wang N, et al. Noninvasive whole-brain imaging of glymphatic dynamics Sci Adv, 2026.PMID 42308292
- [2]Engelhard HH, et al. Variability in the circulation of cerebrospinal fluid: causes and clinical implications for intraventricular drug delivery Front Drug Deliv, 2026.PMID 42205472
- [3]Munir A, et al. Imaging Perspectives on Hydrocephalus: CSF Flow, Glymphatic Function, and Clinical Implications in the Diagnosis and Management of Normal Pressure Hydrocephalus Semin Roentgenol, 2026.PMID 42116253
- [4]Haywood S, et al. Ketamine Use in Acute Brain Injury: Debunking the Myth Emerg Med Clin North Am, 2026.PMID 42342307
- [5]Bader SE, et al. Resuscitative endovascular occlusion of the aorta restores cerebral metabolic markers of ischaemia induced by haemorrhagic shock Eur J Trauma Emerg Surg, 2026.PMID 42340407
- [6]Bishop MS, et al. Intravenous megadose sodium ascorbate normalises elevations in intracranial pressure and restores pressor responsiveness to norepinephrine in ovine Gram-negative sepsis Sci Rep, 2026.PMID 42350542