What are we measuring? A refined look at the process of disrupted autoregulation and the limitations of cerebral perfusion pressure in preventing secondary injury after traumatic brain injury

The cerebral perfusion pressure (CPP) and its relationship between intracranial pressure and mean arterial pressure is a concept ubiquitous in caring for the critically ill patient. CPP is often used as a surrogate measure for cerebral blood flow (CBF); however, this view fails to account for changes in cerebral vascular resistance (CVR). Changes in CVR occur due to cerebral autoregulation, which has classically been taught on a sigma shaped curve with a decline and increase at either end of a plateau. Historically, the conceptualized regulation maintains careful homeostatic levels despite external or internal dynamic changes; however, moderate and severe traumatic brain injury (TBI) has been postulated to bring about cerebral autoregulation dysfunction. We review the current application of CPP is limited by the dynamic changes in cerebral autoregulation after TBI. This review highlights CPP's role as a surrogate measure for CBF and the inherent limitations of current clinical management, due to the lack of monitoring capable of capture continuous variables to assist real-time decision making. This review evaluates the known literature and introduces topics for discussion that warrant further investigation via pre-clinical and clinical experimentation.