← Anatomy & Physiology

Intracranial compliance

Compliance is the change in volume tolerated per unit change in pressure. Plotted out, the intracranial pressure–volume relationship is not a straight line but a hockey stick: nearly flat while the buffers hold, then turning sharply upward once they are exhausted.

This explains an otherwise puzzling bedside observation — a patient can sit at an unremarkable pressure for hours and then deteriorate over minutes. Nothing about the rate of the underlying process changed. The patient simply moved from the flat part of the curve to the steep part.

At the bedside: a normal mean ICP does not establish that a patient has reserve. The waveform tells you more — as compliance falls, the P2 (tidal) peak rises to meet and then exceed P1, often while the mean value still looks acceptable.