NIRS Directed Optimal Cerebral Perfusion Pressure in Aneurysmal Subarachnoid Hemorrhage Patients: A Feasibility Study

Overview

Neurologic dysfunction is common in aneurysmal subarachnoid hemorrhage (aSAH) patients. As we know, after the initial injury, secondary insults including poor cerebral perfusion are main contributors to poor outcome and their early detection and amelioration are keystone to neurocritical care. Nonetheless, the guidelines for blood pressure management still recommend a single target blood pressure for critically ill patients: the Brain Trauma Foundation (BTF) guidelines recommend maintaining CPP between 60 and 70 mmHg; Some guidelines recognize that patients with a history of hypertension may require a higher MAP. However, these guidelines do not currently recommend cerebral autoregulation-guided therapy and leave many unanswered questions. Cerebral autoregulation is the mechanism that maintains cerebral blood supply, hence CBF approximately constant despite changes in MAP or, more precisely, despite changes in CPP. Maintaining blood pressure within the cerebral blood flow (CBF) autoregulation range (termed "optimal MAP") is associated with improved outcomes for patients. The observational data suggests that management of patients above or below CPPopt 5mmHg is associated with better outcomes and mortality than the other greater variation range. The most commonly used method for monitoring dynamic cerebrovascular reactivity is the pressure reactivity index (PRx) that uses ICP as a surrogate for CBV. However, assessing the PRx requires invasive ICP monitoring which limits its application in many clinical areas. Alternatively, in the absence of invasive intracranial pressure monitoring to determine CPP, a continuous autoregulation monitoring can be accomplished by the continuous correlation between transcranial Doppler (TCD)-measured CBF velocity of the middle cerebral artery and the mean arterial blood pressure (termed mean velocity index or Mx). Mx is a validated index of cerebral autoregulation based on measures of cerebral perfusion pressure and mean flow velocity on transcranial doppler but is impractical for longer-term monitoring and requires system training, the results are operator-dependent. Near-infrared spectroscopy (NIRS) measurements is another alternative for real-time autoregulation monitoring in the form of a Tissue Oxygenation Index. In contrast to TCD, the NIRS sensors are very easy to apply (the probes attach to the forehead with self-adhesive pads) and do not require frequent calibration making them more suitable for long-term monitoring. Therefore, in this study, Patients in the intervention group will be monitored by continuous NIRS and invasive blood pressure monitoring. The correlation curve between ORI/THx and blood pressure will be obtained through continuous monitoring of 2 hours. According to the correlation curve, the optimal blood pressure which provides the optimal CPP will be determined. Then the optimal blood pressure which CPP is within 5 mmHg of CPPopt will be maintained as the target of blood pressure management for the intervention group during ICU stay.