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Neuroanaesthesia and intracranial pressure
Cerebral perfusion pressure is MAP minus ICP; in a patient with raised intracranial pressure blood pressure, oxygen and CO2 are kept normal, the head is raised and drugs that lower cerebral metabolism are used.
Why it matters in the exam
The exam asks about the effect of CO2 and oxygen on cerebral blood flow, the effect of anaesthetic drugs (halogenated agents dilate cerebral vessels, propofol and thiopental lower flow and consumption), the treatment of raised intracranial pressure, and air embolism during sitting-position surgery. It also asks why steroids are not given in head injury and how that differs from a brain tumour.
Cerebral blood flow stays stable over a wide range of blood pressure (autoregulation), but it depends directly on PaCO2: each 1 mmHg rise increases flow by about 2%–4%. Hypoxaemia (PaO2 below 50–60 mmHg) increases flow sharply. Halogenated agents lower oxygen consumption but dilate the vessels — uncoupling consumption from flow — and above 1 MAC they impair autoregulation. N2O raises consumption and flow, so it is avoided when ICP is high.
In a head-injured patient even a single episode of hypotension or hypoxaemia worsens the outcome, so blood pressure and oxygenation are guarded closely during anaesthesia and transfer. Normal glucose and temperature are also maintained and seizures are prevented. In spine and brain surgery intraoperative nerve monitoring is sometimes used, and then paralysis and high concentrations of volatile agents that interfere with the signals are avoided.
Key points
- CPP = MAP − ICP; target 60–70; treat ICP above 22.
- CO2 raises flow; hyperventilation only temporarily.
- Propofol and thiopental lower flow and consumption; halogenated agents dilate; N2O — avoid.
- Head up 30 degrees, mannitol or hypertonic saline.
- Steroids — yes in a tumour, no in trauma.
Summary table
| Issue | Target or rule | Exam point |
|---|---|---|
| ICP and CPP | Normal ICP 5–15 mmHg; treat above 22; CPP of 60–70 | CPP = MAP − ICP |
| CO2 | PaCO2 of 35–40; hyperventilation to 30–35 only temporarily for herniation | Prolonged hyperventilation causes ischaemia |
| Position and fluids | Head up 30 degrees, neck straight; NaCl 0.9% | Avoid hypotonic solutions and glucose |
| Osmotic therapy | Mannitol 0.25–1 g/kg or hypertonic saline | Mannitol causes diuresis and may lower blood pressure |
| Steroids | Dexamethasone for oedema around a tumour | Forbidden in traumatic brain injury (CRASH trial) |
A free exam question
מוגבר לאחר החייאה INTRACRANIAL PRESSURE -איזו מהפעולות הבאות יש לבצע בחשד ל אצל מטופל עם המדדים הבאים? לחץ דם דופק סטורציה 93% 110 86/58 mmHg (the question as examined, in Hebrew)
- א. LEVETIRACETAM מתן
- ב. מניעת כיפוף הצוואר לצדדים
- ג. מנות חוזרות של .MORPHINE
- ד. ניקוז CSF באמצעות דיקור מותני
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Sources
- Brain Trauma Foundation: Guidelines for the Management of Severe Traumatic Brain Injury, 4th edition (2016) — ICP and CPP thresholds, hyperventilation, hyperosmolar therapy
- Miller's Anesthesia, 9th edition (2020) — Cerebral Physiology and the Effects of Anesthetic Drugs; Anesthesia for Neurologic Surgery
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.