Temporal Dynamics of Cerebral Autoregulation after Aneurysmal Subarachnoid Hemorrhage: Insights from Pressure Reactivity Index and Non-injured Controls

Aneurysmal subarachnoid hemorrhage (aSAH) disrupts cerebral autoregulation, contributing to secondary brain injury and poor outcomes. The low-frequency pressure reactivity index (LPRx) is an indirect index of autoregulatory function derived from invasive intracranial pressure (ICP) and arterial blood pressure (ABP) monitoring; however, its clinical dynamics in aSAH remain underexplored. This study aimed to characterize the temporal behavior of the LPRx in patients with aSAH, assess its response to standard interventions, and compare it with that in non-injured neurosurgical controls. This retrospective study included 30 patients with aSAH and 28 controls who underwent tumor resection via external ventricular drain placement. The LPRx was calculated from continuous ICP and ABP recordings over the first 7 days post-admission. Outcomes included survival, functional status (Glasgow Outcome Scale-Extended), and responses to cerebrospinal fluid (CSF) diversion and osmotherapy. The LPRx patterns were compared across groups and analyzed relative to the ICP and optimal cerebral perfusion pressure (CPPopt). The LPRx was significantly higher in patients with aSAH than in controls, particularly among non-survivors. The LPRx showed a consistent U-shaped relationship with ICP, with optimal autoregulation at 5–15 mmHg. No significant changes in the LPRx were observed after CSF diversion, osmotherapy, or different treatment modalities. CPPopt showed no clear association with the LPRx. The LPRx is a feasible and physiologically relevant marker of autoregulatory integrity in aSAH. Its consistent elevation and non-linearity with ICP highlight its potential for individualized ICP management. Conventional interventions do not significantly restore autoregulation, highlighting the need for LPRx-informed treatment strategies. Aneurysmal subarachnoid hemorrhage (aSAH) is a type of bleeding around the brain that can lead to serious brain damage. One important function of the brain is its ability to maintain steady blood flow even when blood pressure changes, referred to as “cerebral autoregulation.” After aSAH, this protective mechanism often does not work properly, increasing the risk of further brain injury. In this study, we measured a marker called the low-frequency pressure reactivity index (LPRx), which can indicate how well the brain is regulating its blood flow, in patients with aSAH. We compared these measurements with those from patients undergoing elective brain tumor surgery, who served as a reference group for near-normal physiology, while recognizing that tumor pathology and surgical intervention may still influence intracranial dynamics. We found that patients with aSAH had higher LPRx values, indicating worse autoregulation, and that these higher values were linked to poorer outcomes. We also discovered that common treatments, such as draining fluid from the spine or using certain medications to lower brain pressure, did not significantly improve autoregulation. Interestingly, we noticed that the brain’s ability to regulate blood flow was best when the pressure in the brain was within a certain range. These findings suggest that using LPRx to monitor and guide treatment could help doctors better protect the brain after aSAH.