Measures of cerebral autoregulation

Pressure reactivity (PR)
This measure is calculated as the slope of the regression line relating MAP to ICP for an individual patient over one hour. One study of TBI patients showed that a value of ≥0.13 suggests pressure-passive (i.e. loss of autoregulation) and <0.13 suggests pressure active patients. In pressure active patients (autoregulation intact), targeting CPP of 70 mmHg led to better outcomes, while in pressure passive patients targeting ICP <20 mmHg led to better outcomes.

Pressure Reactivity Index (PRx)
While pressure reactivity measures correlations over a 1 hour period, PRx is a moving correlation coefficient between 40 consecutive 5-second averages of ICP and ABP, moving in a 5 second window. A positive value means a positive slope (y-axis MAP, x-axis ICP) indicating loss of autoregulation, while negative reflects intact autoregulation. In a study of 82 TBI patients, a value < -0.2 correlated strongly with favorable outcome, while PRx >0.2 correlated strongly with unfavorable outcomes. This may be more predictive of outcome when a filter is used to limit the analysis to oscillations with periods of 15-55 seconds, which has been dubbed PRx55-15.

In TBI patients, the percentage of time with PRx > 0.25 is associated with progression of pericontusional cerebral edema.

Pressure amplitude index (PAx)
This measure utilizes the pulse amplitude of ICP (AMP), and is the moving correlation between MAP and AMP. In TBI patients, the percentage of time with PAx > 0.25 is associated with progression of pericontusional cerebral edema.