  A healthy human brain is perfused with blood flowing laminarly through
cerebral vessels, providing brain tissue with substrates such as oxygen and
glucose. Under normal conditions, cerebral blood flow is controlled by
autoregulation as well as metabolic, chemical and neurogenic regulation.
Physiological complexity of these mechanisms invariably leads to a question as
to what are the relations between the statistical properties of arterial and
intracranial pressure fluctuations. To shed new light on cerebral hemodynamics,
we employ a complex continuous wavelet transform to determine the instantaneous
phase difference between the arterial blood pressure (ABP) and intracranial
pressure (ICP) in patients with traumatic brain injuries or spontaneous
cerebral hemorrhage. For patients with mild to moderate injury, the phase
difference slowly evolves in time. However, severe neurological injury with
elevated ICP are herein associated with synchronization of arterial and
intracranial pressure. We use Shannon entropy to quantify the stability of
ABP-ICP phase difference and discuss the clinical applicability of such measure
to assessment of cerebrovascular reactivity and autoregulation integrity.
