Phase synchronization in cerebral hemodynamics
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.