When ICP is monitored directly (via an external ventricular drain or parenchymal bolt), the pressure waveform contains information beyond the mean ICP value. The normal ICP waveform has three components: P1 (the percussion wave, reflecting arterial pulsation transmitted to the CSF), P2 (the tidal wave, reflecting intracranial compliance), and P3 (the dicrotic wave, reflecting the aortic valve closure). In a patient with normal compliance, P1 > P2 > P3. When intracranial compliance is reduced, P2 rises and may equal or exceed P1 — this is a sensitive indicator of deteriorating compliance before ICP itself rises to dangerous levels.

Lundberg A waves (plateau waves) are periodic rises in ICP to 50–100 mmHg lasting 5–20 minutes, followed by spontaneous return to baseline. They represent periods of critical cerebral hyperaemia occurring in patients with severely impaired compliance and indicate that the patient is at high risk of permanent ischaemic injury. Their occurrence is an indication for urgent escalation of ICP-lowering treatment.