A Review of Cerebral Autoregulation: Assessment and Measurements

Principles of Cerebral Autoregulation Testing Cerebral Autoregulation (CA) is the intrinsic ability of the brain to maintain stable cerebral blood flow (CBF) while mean arterial blood pressure (MAP) and cerebral perfusion pressure (CPP) are changing. It constitutes a regulatory mechanism to provide metabolic substrates under physiological and pathological conditions, for instance after neuro-trauma or spontaneous intra-cranial haemorrhage. Constant CBF is regulated by changing arteriolar diameter, which will alter cerebral blood volume (CBV) and, ultimately, intra-cranial pressure (ICP). On the other hand, ICP is the sum of the partial pressures of brain tissue, cerebrospinal fluid (CSF) and CBV. 3 CA has two components: (a) Metabolic Autoregulation, which is the cerebrovascular response to changes in paCO2, and (b) Pressure Autoregulation, which is the cerebrovascular response to changes in transmural vascular pressure resulting from changes in CPP or MAP. Moreover, Pressure Autoregulation has a slow component, referred to as “static CA” and a fast component, namely “dynamic CA”. CA testing is a process which requires two parameters, a stimulus and a dependent variable. In clinical practice, two stimuli are used for metabolic CA testing: (a) induced hyperor hypoventilation, and (b) administration of acetazolamide, a carbonic anhydrase inhibitor. CA pressure can be challenged by external mechanical or pharmacologic blood pressure changes. Alternatively, intrinsic slow MAP fluctuations can also be used as a physiological constant which provides a consistent, and continuously available, stimulus generator.