Reading an ICP waveform before you read the number
The three peaks of a normal ICP trace carry information about compliance that the mean value hides completely.
Most ICP monitoring reduces to a single averaged number on a screen. The waveform underneath it is doing more work than that number admits.
The three peaks
Each cardiac cycle produces a characteristic trace with three components:
- P1 — the percussion wave. Arterial pulsation transmitted from the choroid plexus. Sharp, and normally the tallest of the three.
- P2 — the tidal wave. Reflects the brain’s compliance: how much the intracranial contents give when volume is added.
- P3 — the dicrotic wave. Follows aortic valve closure.
In a compliant brain the peaks descend in order: P1 > P2 > P3.
When P2 rises
As compliance falls, P2 climbs. It first equals, then exceeds P1, and the waveform loses its stepped-down shape and starts to look rounded.
This matters because the change can appear while the mean ICP is still normal. A patient sitting at an ICP of 15 with a P2 towering over P1 has very little reserve left; the next small increase in volume — a coughing fit, a badly positioned head, a rise in PaCO₂ — will move the pressure a long way. The mean value alone gives no warning of that.
Plateau waves
The other pattern worth recognising is Lundberg’s A wave: a sustained rise to 50 mmHg or more, held for several minutes, then a fall back toward baseline. These are not artefact and they are not benign. They indicate exhausted compensatory reserve, and they warrant treatment rather than observation.
The practical habit
Before you act on an ICP number, look at the trace for a few beats:
- Can you identify the three peaks at all?
- Is P1 still the tallest?
- Does the waveform look damped — flattened, or drifting without pulsatility?
A damped trace usually means an equipment problem, and treating a fictional number is worse than having no number at all. Flush, check the connection, and re-zero before you reach for the mannitol.