• cerebral autoregulation
• ICP
• pressure reactivity index
• PRx
• reliability
• variability

The pressure reactivity index (PRx) is the most used index for continuous monitoring of dynamic cerebral autoregulation at the bedside. It is a simple correlation coefficient between arterial blood pressure (ABP) and intracranial pressure (ICP). It showed several benefits among them positive and strong correlation with outcome and the possibility of calculation of optimal cerebral perfusion pressure (CPP), therefore the possibility to provide a dynamic and personalised treatment target. Apart from its good points PRx suffers from several weaknesses which should be kept in mind to interpret this index correctly. It relies on the assumption that the only determinant of ICP variability is an extracranial source (ABP) nevertheless there could be intracranial factors that could affect ICP and that could reduce the reliability of PRx. In fact, it has been demonstrated that the influence of cyclic positive end-expiratory pressure (PEEP) waves in ABP, lowers the variability of PRx. Conversely, it has also been reported that periodic adjustments of an anti-decubitus-mattress may have a confounding effect on the metric, as it seems to affect directly also ICP. We scrutinized a regular waves pattern noticed during our neuromonitoring practice in TBI patients. Patients showed different values of PRx, exhibiting low variability. During our neuromonitoring practice we noticed a sudden onset of nice and regular oscillations in ABP and ICP signals in one TBI patient. We started to scrutinise our recording to explain the nature of the waves in ABP and ICP and test the reliability and variability of PRx. We applied spectral-analysis, looked at the phase-shift and coherence between ABP and ICP, and calculated PRx variability: hourly-average of standard-deviation (SD). The analysis shows a very slow mean dominant frequency of oscillations: 0.008Hz, we confirmed a reduced variability of PRx during the oscillations period when compared to the peri-oscillations periods and the coherence was higher during the oscillations period because the oscillations are transmitted from ABP to ICP. We excluded that these oscillations might be caused by an anti-decubitus mattress or other external devices. We understand that the oscillations we analysed didn’t invalidate the PRx but rather made it more reliable. Further studies are required to explore the nature of the oscillations in ABP and scrutinized the variability and reliability of PRx.