Pressure reactivity index for early neuroprognostication in poor-grade subarachnoid hemorrhage

Cerebral autoregulation, spreading depolarization, and implications for targeted therapy in brain injury and ischemia

Cerebral autoregulation is an intrinsic myogenic response of cerebral vasculature that allows for preservation of stable cerebral blood flow levels in response to changing systemic blood pressure. It is effective across a broad range of blood pressure levels through precapillary vasoconstriction and dilation. Autoregulation is difficult to directly measure and methods to indirectly ascertain cerebral autoregulation status inherently require certain assumptions. Patients with impaired cerebral autoregulation may be at risk of brain ischemia. One of the central mechanisms of ischemia in patients with metabolically compromised states is likely the triggering of spreading depolarization (SD) events and ultimately, terminal (or anoxic) depolarization. Cerebral autoregulation and SD are therefore linked when considering the risk of ischemia. In this scoping review, we will discuss the range of methods to measure cerebral autoregulation, their theoretical strengths and weaknesses, and the available clinical evidence to support their utility. We will then discuss the emerging link between impaired cerebral autoregulation and the occurrence of SD events. Such an approach offers the opportunity to better understand an individual patient's physiology and provide targeted treatments.

Time Thresholds for Using Pressure Reactivity Index in Neuroprognostication for Patients With Severe Traumatic Brain Injury

BACKGROUND AND OBJECTIVES

Severe traumatic brain injury (sTBI) represents a diffuse, heterogeneous disease where therapeutic targets for optimizing clinical outcome remain unclear. Mean pressure reactivity index (PRx) values have demonstrated associations with clinical outcome in sTBI. However, the retrospective derivation of a mean value diminishes its bedside significance. We evaluated PRx temporal profiles for patients with sTBI and identified time thresholds suggesting optimal neuroprognostication.

METHODS

Patients with sTBI and continuous bolt intracranial pressure monitoring were identified. Outcomes were dichotomized by disposition status ("good outcome" was denoted by home and acute rehabilitation). PRx values were obtained every minute by taking moving correlation coefficients of intracranial pressures and mean arterial pressures. Average PRx trajectories for good and poor outcome groups were calculated by extending the last daily averaged PRx value to day 18. Each patient also had smoothed PRx trajectories that were used to generate "candidate features." These "candidate features" included daily average PRx's, cumulative first-order changes in PRx and cumulative second-order changes in PRx. Changes in sensitivity over time for predicting poor outcome was then evaluated by generating penalized logistic regression models that were derived from the "candidate features" and maximized specificity.

RESULTS

Among 33 patients with sTBI, 18 patients achieved good outcome and 15 patients had poor outcome. Average PRx trajectories for the good and poor outcome groups started on day 6 and consistently diverged at day 9. When targeting a specificity >83.3%, an 85% maximum sensitivity for determining poor outcome was achieved at hospital day 6. Subsequent days of PRx monitoring showed diminishing sensitivities.

CONCLUSION

Our findings suggest that in a population of sTBI, PRx sensitivities for predicting poor outcome was maximized at hospital day 6. Additional study is warranted to validate this model in larger populations.