Pressure reactivity index: journey through the past 20 years

Abstract

Autoregulation after traumatic brain injury can be monitored continuously using simple signal processing of intracranial pressure and arterial blood pressure. The pressure reactivity index (PRx) showed several benefits when it was applied to continuous brain monitoring. Among them, a positive and strong correlation with the outcome and possibility of calculation of 'optimal cerebral perfusion pressure' have been listed. For this methodology, prospective clinical trials are missing-few of them are planned in the near future.Keywords Traumatic brain injury . Intracranial pressure . Arterial blood pressure . Optimal cerebral perfusion pressureThe pressure-reactivity index was first described in 1995. Originally, we were inspired by the presentation of Erhard Lang and Randal Chestnut during the ICRAN conference at Gold Coast, Australia (1994). They reported that they could read a state of cerebral autoregulation from relative changes in mean arterial blood pressure (ABP) and intracranial pressure (ICP). They told us that it was enough to ask a research nurse to sit in front of a bedside monitor and instruct her to observe trends of ABP and ICP. The nurse needed to report when the values were changing in the same or opposite directions. When we arrived home, we programmed our computers, running ICM (intensive care monitor) software [7], to calculate a moving correlation coefficient from 30 consecutive 10-s averages of ICP and ABP waveforms. We called this the PRx index (pressure reactivity index) [6].The rationale for averaging ICP and ABP waveforms over 10 s was that only slow waves, of frequencies lower than 0.05 Hz, can carry information about autoregulation [11]. Simple averaging is a sufficient method of filtration of all faster components (mainly respiratory and pulsatile), which do not contain or contain only a little of any autoregulationrelated signatures.The rationale for calculating the correlation coefficient from 5-min-long buffers (30 samples of 10 s produce correlation window of 5 min) is that longer periods (e.g., 30 or 60 min) may include too many reactions to drugs, nursingrelated variations, metabolic reactions, etc. They are all not related to cerebral autoregulation and would produce distortion of the PRx.When a few years later a postgraduate student from S w i t ze r l an d , L uz i u s S t e i n er ( no w P r o f e s s o r o f Anaesthesiology at University of Basel), came to our Laboratory of Brain Physics in Cambridge, asking whether he could be given a PhD project on PRx, we first took it for a joke. However, over the next 3 years we were proven very wrong. Works of Luzius and other clinical neuroscientists who followed in his footsteps brought to light perhaps the most important advantage of PRx, its ability to guide the management of cerebral perfusion pressure. The concept was so simple to understand and so appealing in the clinical setting that many more people than we initially anticipated embraced it enthusiastically and developed it further.After 20 years, summarizing the mile...

Intracranial Pressure Monitoring Signals After Traumatic Brain Injury: A Narrative Overview and Conceptual Data Science Framework

“…Pressure reactivity index (PRx) is another form of correlation analysis between time domain features and has been particularly well-studied. The PRx is calculated as a moving average correlation coefficient between the 10-s mean of the arterial blood pressure (ABP) signal and the mean ICP signal over a 5-min time window (60). PRx quantifies cerebrovascular reactivity and approximate global cerebral autoregulatory reserve by observing the responses of each signal to slow spontaneous changes.…”

Novel index for predicting mortality during the first 24 hours after traumatic brain injury

“…5,8,15 Identifying the "Lethal Dose" of Cerebral Hypoperfusion Currently, the PRx remains the most widely validated surrogate measure of cerebral autoregulation. 9,17 However, the PRx relies on an important assumption that a change in cerebral blood volume would be reflected as a change in ICP. 4 Nonetheless, when the pressure-volume relationship is stable (i.e., the pressure-volume curve is flat, e.g., low ICP), the transmission of volume to pressure is weakened.…”

“…In practice, the status of cerebral autoregulation can be indirectly assessed by measuring the cerebrovascular pressure reactivity. 9 Defined as the moving correlation coefficient between arterial blood pressure (ABP) and intracranial pressure (ICP), the pressure-reactivity index (PRx) has been considered as a de facto surrogate measure of cerebral autoregulation. 17,30,36 Furthermore, in recent years, the PRx has been highlighted for its efficacy in detecting the optimal level of CPP; when plotted against the PRx, a specific point of CPP with the lowest PRx would be identified, which is supposed to be the optimal CPP (CPPopt) with the strongest autoregulatory ability.…”

Fluid management concepts for severe neurological illness

“…Functional autoregulation is inferred when an arterial BP (ABP) change is associated with an anticorrelated change in ICP, representing a reduction of flow and hence cerebral blood volume. The correlation coefficient between ABP and ICP is the pressure reactivity index (PRx); positive values indicate a passive response to ABP, associated with loss of autoregulation, while negative values suggest intact autoregulation (for a very concise expert review, refer to [40]). The CPP optimization strategy takes advantage of the ‘U’-shaped relationship between PRx and CPP, in which the minimum PRx represents an optimal CPP (CPPopt).…”