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)

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

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.