6.6.1 interpret surrogate ICP waveform

A satisfactory waveform reading may present any of the typical ICP shape characteristics below according to the patient’s clinical condition (from normal to pathological)

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ICP  waveforms. Source: Nucci CG, et al. Acta Neurochir (Wien). 2016. Intracranial pressure wave morphological classification: automated analysis and clinical validation.

As a suggestive reference, the waveforms below are non invasive ICP waveforms that can be shown with the noninvasive ICP sensor.

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examples of ICP waveforms in most common situations

The monitoring session should be as long as required to assess the ICP waveform. Typical sessions are of 15+ minutes but the final duration is ultimately defined by a trained professional. The session should be finalized upon patient request due to discomfort or decision by the primary caregiver.

The ICP pulse waveform (Figure 6.6-9) is an alternative to assess certain ICP characteristics, irrespective of the absolute values that are typically captured by an invasive method .

The ICP waveform contains characteristics that can be mathematically analyzed such as:

• Wave peaks: P1 (percussion wave, due to arterial pressure being transmitted from the choroid plexus to the brain ventricles), P2 (tidal wave, related to brain compliance) and P3 (dicrotic wave);

• Absolute amplitude of wave peaks: dP1 (relative amplitude of percussion wave), dP2 (relative amplitude of tidal wave), dP3 (relative amplitude of dicrotic wave);

• Absolute lags in milliseconds of wave peaks: LT (considered the initial timestamp of a pulse), L1 (lag for percussion wave), L2 (lag for tidal wave), L3 (lag for dicrotic wave), Lx (variable lag);

• Absolute curvature of wave peaks: Curv1 (percussion wave curvature), Curv2 (tidal wave curvature), Curv3 (dicrotic wave curvature).

Under normal ICP conditions, the relative amplitudes of the three peaks are related as follows: P1 > P2 > P3 (Box A in Figure above). In conditions with decreased brain compliance and rising ICP, the pulse waveform morphology gradually changes and certain indicators, such as the estimated P2/P1 ratio eventually increases.