Updated July 2026 · 9 min read
Part of the ICU Devices Hub — browse every related guide in one place.
An external ventricular drain is one of the highest-stakes devices a nurse manages. Set the level wrong or leave it open during a transfer and you can over-drain the brain; leave it clamped when it should be open and pressure climbs unseen. The physics are simple once they click, but the consequences of a small error are large. This is a guide to the numbers, the mechanics, and the herniation signs that turn a routine neuro patient into a code.
Intracranial pressure is the pressure inside the skull, a closed box holding brain, blood, and CSF. When one component swells, the others must give way; once they cannot, pressure rises steeply. But ICP alone is only half the story. The brain lives on perfusion, and cerebral perfusion pressure is the driving number:
| Value | Meaning / target |
|---|---|
| ICP | Normal ~7–15 mmHg; treat sustained >20–22 mmHg |
| CPP = MAP − ICP | Goal typically ~60–70 mmHg; too low starves the brain, too high can worsen edema |
| Implication | A rising ICP or a falling MAP both drop CPP — you can protect perfusion by lowering ICP or supporting pressure |
This is why a neuro ICU nurse watches the MAP and the ICP together: the patient can be perfusing poorly from either direction, and the fix depends on which one moved.
An EVD reads pressure and drains fluid relative to a reference point, so the reference point must be correct and consistent. The drain's stopcock/zero is leveled to a fixed anatomic landmark — commonly the external auditory meatus (tragus), which approximates the foramen of Monro. Get this wrong and everything downstream is wrong:
| If the reference is set too... | Result |
|---|---|
| Too low | The drain sits below the brain → CSF over-drains → risk of collapse/siphoning, subdural bleed |
| Too high | Under-drains → CSF backs up → ICP rises unnoticed |
Re-level and re-zero after any change in bed height or patient position, and confirm the ordered drainage height (for example, "level at 15 cm above the tragus"). Document the level, the character and hourly volume of CSF output, and the ICP waveform each check.
The fluid itself is data. Note the color (clear, bloody, xanthochromic, cloudy), the hourly volume against the ordered range, and any sudden change — a drain that stops draining may be occluded, mispositioned, or the ventricles may have collapsed. A drain that suddenly pours out may be over-draining. Cloudy CSF or fever raises concern for ventriculitis, a serious EVD infection, so the closed system is kept sterile and undisturbed.
First-line nursing maneuvers to lower ICP while you get help: keep the head of bed elevated (commonly ~30 degrees) with the neck midline so venous drainage is not kinked, ensure oxygenation and normal CO2 (hypercapnia dilates cerebral vessels and raises ICP), treat pain and agitation, avoid clustering stimulating care, and open the EVD to drain per order. Medical escalation includes hyperosmolar therapy (mannitol or hypertonic saline), controlled ventilation, and sometimes surgical decompression.
Respect the drain and watch two numbers. Level and zero the EVD to a fixed landmark, re-check it after every position change, and clamp before you move the patient so you never siphon CSF. Track ICP and CPP together, keep the head up and midline and the CO2 normal, and treat any new exam change or a sustained ICP over target as an emergency. Cushing's triad is the brain's last warning — the goal is to act on the neuro exam long before it appears.
Related: Mannitol & hypertonic saline for ICP · Subarachnoid hemorrhage & vasospasm · Status epilepticus · Cerebral salt wasting vs SIADH
Educational content for licensed clinicians. Always follow your facility's protocol and provider orders. Not medical advice.
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