Normal Intracranial Pressure: Values, Thresholds and How to Read the Waveform

Normal intracranial pressure is between which values?

In a supine adult, normal intracranial pressure is between about 7 and 15 mmHg. In children the usual range is lower, and in infants it is lower still, so paediatric thresholds must come from your unit's protocol rather than from adult tables. Values vary with posture: ICP falls when the head is raised and rises with coughing, straining or a Valsalva manoeuvre.

Always interpret the figure together with the mean arterial pressure, because cerebral perfusion pressure is the difference between the two. A reading that looks acceptable can still hide inadequate perfusion if blood pressure is low. Local guidelines define the exact treatment thresholds; many units use a sustained value above 20 to 22 mmHg as the trigger for escalation.

High ICP vs low ICP

High ICP is the familiar problem: swelling, haemorrhage, hydrocephalus or a mass raise the pressure inside a fixed skull. Typical clinical clues include a falling level of consciousness, headache, vomiting and pupil changes, but the monitor is what quantifies the trend. Sudden rises or sustained plateaus need immediate bedside review.

Low ICP is less common and usually follows overdrainage of cerebrospinal fluid, a leak, or a change in posture with an external drain. It can cause headache when upright. When an invasive monitor is used, check the drain height, the stopcock position and the zero reference before concluding that the pressure is truly low. See how to measure intracranial pressure for the available methods.

What does negative ICP mean?

A negative ICP reading is often a technical or positional artefact rather than a physiological finding. The most common causes are a transducer positioned above the reference level, zero drift, an air bubble or loose connection in a fluid-coupled system, or a patient who is sitting upright with an external drain.

True subatmospheric pressure can occur briefly in the upright position, but it should never be accepted without first re-levelling and re-zeroing. The checks are described in our article on ICP transducer zeroing and calibration.

Types of ICP waveforms: P1, P2 and P3

A normal pulsatile ICP trace has three peaks. P1 is the percussion wave, transmitted from arterial pulsation through the choroid plexus. P2 is the tidal wave and reflects brain compliance. P3 is the dicrotic wave, linked to aortic valve closure. In a compliant brain the peaks descend in height from P1 to P3.

A non compliant ICP waveform shows P2 equal to or higher than P1, and the trace becomes rounded, which suggests that the brain has little reserve for added volume. This is an early warning that a further small increase in volume may produce a large rise in pressure.

• P1 above P2 above P3: normal compliance
• P2 at or above P1: reduced compliance, watch closely
• Loss of pulsatility or a damped trace: check the line before drawing conclusions

Slow waves and good monitoring practice

Beyond the pulse waves, longer patterns matter. Lundberg A waves, or plateau waves, are sudden rises to very high levels lasting several minutes, and indicate exhausted compliance. B waves are slower rhythmic oscillations seen over minutes. Document the trend rather than single readings.

Good practice includes recording the zero point, the reference level and the transducer type at each shift handover. For choosing between catheter types, see the ICP monitoring devices overview. If you are equipping a neuro-ICU, you can contact Beckfinity for guidance on critical care monitoring.