Physiology, Cerebral Autoregulation

Cerebral autoregulation is the ability of the cerebral vasculature to maintain stable blood flow despite changes in blood pressure (or, more accurately, cerebral perfusion pressure). Under normal circumstances, cerebral blood flow is regulated through changes in arteriolar diameter, which, in turn, drive changes in cerebrovascular resistance following the Hagen-Poiseuille equation. Although decades of research have illuminated some underpinning mechanisms, the exact molecular means underlying autoregulation remain elusive. Various processes, including myogenic, neurogenic, endothelial, and metabolic responses, have mediated cerebral vasomotor reactions. See  Physiology of Cerebral Autoregulation. Still, it is essential to differentiate carbon dioxide reactivity and flow-metabolism coupling from cerebral autoregulation. Carbon dioxide reactivity describes vascular reactions in response to changes in the partial pressure of arterial carbon dioxide (PaCO) but does not consider reactions to pressure changes. Flow-metabolism coupling, in comparison, involves regulating cerebral blood flow relative to local cellular demand, for example, as a consequence of neural activation during cognitive tasks. Similar to PaCO reactivity, flow-metabolism coupling, and the neurovascular unit function irrespective of fluctuations in cerebral perfusion pressure. With a working definition of autoregulation and an understanding of what it is not, researchers have developed technology that now boasts the ability to measure autoregulatory function in real-time, which may lead to fine-tuning long-established guidelines. Updated guidelines may ameliorate clinical and functional outcomes after acute brain injury by individualizing cerebral perfusion pressure targets based on patients' unique hemodynamic physiology. Autoregulation is assessable by examining changes in cerebral blood flow, or its surrogates, in response to changes in cerebral perfusion pressure or mean arterial pressure as its surrogate. Individualization of autoregulatory pressure ranges and the developing concept of an optimum mean arterial pressure landscape for the injured brain represent a novel and innovative application of autoregulation neuromonitoring. This topic is further discussed in the concluding section of this review.