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 been implicated in the mediation of cerebral vasomotor reactions. 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 (PaCO2) but does not take into consideration reactions to pressure changes. Flow-metabolism coupling, in comparison, involves the regulation of cerebral blood flow relative to local cellular demand, for example, as a consequence of neural activation during cognitive tasks. Similar to PaCO2 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 the fine-tuning of long-established guidelines. By individualizing cerebral perfusion pressure targets based on patients' unique hemodynamic physiology, updated guidelines may ameliorate clinical and functional outcomes after acute brain injury. 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, together with 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 will be further discussed in the concluding section of this review.