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08.05.2020 | Review Article - Brain trauma

Alternative continuous intracranial pressure-derived cerebrovascular reactivity metrics in traumatic brain injury: a scoping overview

Acta Neurochirurgica
Mohammed Hasen, Alwyn Gomez, Logan Froese, Joshua Dian, Rahul Raj, Eric P. Thelin, Frederick A. Zeiler
Wichtige Hinweise

Electronic supplementary material

The online version of this article (https://​doi.​org/​10.​1007/​s00701-020-04378-7) contains supplementary material, which is available to authorized users.
This article is part of the Topical Collection on Brain trauma


Traumatic brain injury (TBI) affects millions of people each year and the most severe cases are treated in highly specialized neuro-intensive care units using multimodal neuromonitoring. Various invasive/non-invasive measurements are integrated to aid real-time assessment of brain physiology and guide therapeutic interventions to prevent secondary brain injury. The pressure reactivity index (PRx) has been proposed as a metric to continuously monitor cerebrovascular reactivity, but other similar indices with different advantages have been put forward. In this scoping review, the authors summarize the scientific evidence of non-PRx (ICP based) indices of cerebrovascular reactivity in adult TBI. As concluded in the paper, the literature is limited and primarily consists of retrospective investigations from a few centers. Currently, PRx-based optimization of cerebral perfusion pressure is investigated in a clinical feasibility trial (COGiTATE), but much further research is needed (including investigations in animal models) before any of the other indices can be put into clinical testing.
Alexander Lilja-Cyron
Copenhagen, Denmark

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Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.



Pressure reactivity index (PRx) has emerged as a means to continuously monitor cerebrovascular reactivity in traumatic brain injury (TBI). However, other intracranial pressure (ICP)-based continuous metrics exist, and may have advantages over PRx. The goal of this study was to perform a scoping overview of the literature on non-PRx ICP-based continuous cerebrovascular reactivity metrics in adult TBI.


We searched MEDLINE, BIOSIS, EMBASE, Global Health, SCOPUS, and Cochrane Library from inception to December 2019. Using a two-stage filtering of title/abstract, and then full manuscript, we identified pertinent articles. Data was abstracted to tables and each technique summarized, including pulse amplitude index (PAx), correlation between pulse amplitude of ICP and cerebral perfusion pressure (RAC), PRx55-15, and low-resolution metrics LAx and L-PRx.


A total of 23 articles met the inclusion criteria, with the vast majority being retrospective in nature and based out of European centers. Sixteen articles focused on high-resolution metrics PAx, RAC, and PRx55-15, with 6 articles focusing on LAx and L-PRx. PAx may have a role in low ICP situations, where it appears to perform superior to PRx. RAC displays similar behavior to PRx, with a trend to stronger associations with favorable/unfavorable outcome at 6 months, and stronger parabolic relationship with CPP. PRx55-15 provides a focused assessment on the vasogenic frequency range associated with cerebral autoregulation, with preliminary data supporting a strong association with outcome in TBI. LAx and L-PRx display varying associations with 6-month outcome in TBI, depending on the window length of calculation, with shorter windows demonstrating stronger correlations with classical PRx.


Non-PRx continuous ICP-based cerebrovascular reactivity metrics can be split into high-resolution and low-resolution measures. High-resolution indices include PAx, RAC, and PRx55-15, while low-resolution indices include L-PRx and LAx. The true role for these metrics beyond classic PRx remains unclear. Each displays situations where it may prove superior over PRx, given limitations with this currently widely accepted measure. Much future investigation into each of these alternative metrics is required prior to adoption into the clinical monitoring armamentarium in adult TBI.

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