Pressure reactivity index (PRx). PRx is a clinical method for assessing cerebral autoregulation based on the correlation of spontaneous variations of arterial blood pressure (ABP) and intracranial pressure (ICP).
In severe traumatic brain injury, cerebral perfusion pressure management based on cerebrovascular pressure reactivity index has the potential to provide a personalized treatment target to improve patient outcomes. So far, the methods have focused on identifying “one” autoregulation-guided cerebral perfusion pressure target-called “cerebral perfusion pressure optimal”.
Pressure-reactivity index has shown prognostic value in adult traumatic brain injury, with one study confirming this in children. Pressure-reactivity index can identify a cerebral perfusion pressure range within which pressure reactivity is optimal. An increasing difference between optimal cerebral perfusion pressure and cerebral perfusion pressure is associated with worse outcome in adult traumatic brain injury.
In pediatric patients with traumatic brain injury, pressure-reactivity index has prognostic value and can identify cerebral perfusion pressure targets that may differ from treatment protocols. The results suggest but to not confirm that cerebral perfusion pressure targeting using pressure-reactivity index as a guide may positively impact on outcome. This question should be addressed by a prospective clinical study 1). Cortical cerebral blood flow (CBF) can be monitored with laser Doppler flowmetry, and CBF plotted against cerebral perfusion pressure (CPP) shows the autoregulatory curve, called Lassen's curve, with clear lower limit of autoregulation (LLA).
However while this is illustrative of the principle, in clinical practice an indicator able to predict the autoregulatory reserve is needed, in order to demonstrate how far the patient is from the LLA.
It is calculated from slow fluctuation of the arterial blood pressure (ABP) of a period between 20 seconds and 3 minutes. The response of the ICP may be passive when the vascular bed is not reactive. PRx, calculated as moving correlation coefficient between ABP and ICP with the pulse wave filtered out, is positive. With an active vascular bed, a rise in ABP produces vasoconstriction and a decrease in ICP. A decrease in ABP produces vasodilatation and an increase in ICP. Therefore, normally positive PRx starts to be negative, indicating negative correlation between slow changes in ABP and ICP.
PRx is a “moving index,” able to be calculated continuously, forming a new variable.
PRx gives similar results to ICP when plotted against mortality rate, but with even steeper threshold. Mortality rate, expressed as a function of CPP, shows distinctive “U shape” curve.
A algorithm was proposed by Steiner in order to trace the PRx/CPP curve. The minimum of the curve was named “optimal CPP.” Steiner, using retrospective data, proved that greater distance between “optimal” and current CPP associates with worse outcome following TBI. A randomized trial is required to see whether “optimization” of CPP may improve outcome after TBI 2).
The optimal index frequency range for prediction of outcome was identified as 0.018-0.067 Hz (oscillations with periods from 55 to 15 s). The index based on this frequency range correlated with GOSe with ρ = -0.46 compared to -0.41 for standard PRx, and reduced the 30-min variation by 23 % 3).
Three ICP derived continuous indices of cerebrovascular reactivity, PRx, PAx and RAC were validated against the LLA within this experimental model of arterial hypotension, with PRx superior 4).