Interpretation of intracranial pressure waveforms
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.
Intracranial pressure in general
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
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.
Intracranial pressure pulse waveforms (generated by the arterial pulse)
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.
References
AA series of excellent resource were available from the NSW ICU protocols and publications, but the links broke. Thank you Ganesh for identifying this problem. New links have been added (until they break too).
Medtronic have this brochure to describe the bedside use of their DUET drain system.
American Association of Neuroscience Nurses. "Care of the patient undergoing intracranial pressure monitoring/external ventricular drainage or lumbar drainage." Glenview (IL): American Association of Neuroscience Nurses (2011): 1-38.
Raboel, P. H., et al. "Intracranial pressure monitoring: invasive versus non-invasive methods—a review." Critical care research and practice 2012 (2012).
Smith, Martin. "Monitoring intracranial pressure in traumatic brain injury."Anesthesia & Analgesia 106.1 (2008): 240-248.
Catherine J. Kirkness, Pamela H. Mitchell, R bert L. Burr, Karen S. March, David W. Newell Intracranial Pressure Waveform Analysis: Clinical and Research Implications Journal of Neuroscience Nursing, Oct, 2000
Cardoso ER, Rowan JO, Galbraith S. Analysis of the cerebrospinal fluid pulse wave in intracranial pressure. J Neurosurg. 1983 Nov;59(5):817-21.
Brian North, Intracranial Pressure Monitoring. Ch. 10 in Head Injury. Edited by Peter Reilly and Ross Bullock. Published in 1997 by Chapman & Hall, London. ISBN 0 412 58540 5