The waveforms described below are "normal" - that is to say, one expects to see these waveform morphologies in somebody with normal cerebral compliance and intracranial pressure.
Why such a person has an EVD is another matter.
The Brain Trauma Foundation Guidelines tend to favour a threshold of 20 mmHg.
It has been found that anything higher than 22mmHg for a prolonged period of time is associated with a poorer outcome. Thus, the BTF recommends that action be taken to lower ICP at anything above this point.
Intracranial pressure waveforms are trifid: there are three distinct peaks, provided your transducer has a high enough sampling rate. This waveform is synchronous with the arterial pulse.
When plotted over several seconds, respiratory variation in intracranial pressure can also be observed.
The respiratory wave is synchronous with alterations in central venous pressure, and it reflects changes in intrathoracic pressure with respiration. This respiratory variation in ICP diminishes and eventually disappears altogether as intracranial pressure increases.
These correlate to the arterial pressure.
The P1 wave, also known as the percussion wave, correlates with the arterial pulse transmitted through the choroid plexus into the CSF, and via a column of fluid into the EVD transducer. It will lag slightly behind the arterial transducer.
The P2 wave, also known as the tidal wave, represents cerebral compliance, it can be thought of as a "reflection" of the arterial pulse wave bouncing off the springy brain parenchyma. In truth, I have yet to find a good explanation of what exactly causes these waves.
The P3 wave, also known as the dicrotic wave, correlates with the closure of the aortic valve, which makes the trough prior to P3 the equivalent of the dicrotic notch.
All these waves are rarely more than 4mmHg in amplitude, or 10-30% of the mean ICP.