The cerebral pressure reactivity index (PRx), through intracranial pressure (ICP) measurements, informs clinicians about the cerebral autoregulation (CA) status in adult sedated traumatic brain injury (TBI) patients. Using PRx in clinical practice is currently limited by variability over shorter monitoring periods. We applied an innovative method to reduce the PRx variability by ventilator-induced slow (one minute) positive end-expiratory pressures (PEEP) oscillations. We hypothesized that, as seen in a previous animal model, the PRx variability would be reduced by inducing slow arterial blood pressure (ABP) and ICP oscillations and without other clinically relevant physiological changes.TBI patients were ventilated with a static PEEP for thirty minutes (PRx period) followed by a thirty-minute period of slow (1/minute (0.0167 Hz)) +5 cmH2O PEEP oscillations (induced (ᵅ6;PRx period). Ten TBI patients were included. No clinical monitoring was discontinued and no additional interventions were required during the ᵅ6;PRx period. The PRx variability (measured as the standard deviation (SD) of PRx) decreased significantly during the ᵅ6;PRx period from 0.25 (0.22 - 0.30) to 0.14 (0.09 - 0.17) (p = 0.006). There was a power increase around the induced frequency (1/minute) for both ABP and ICP (p = 0.002). In conclusion, 1/minute PEEP-induced oscillations reduced the PRx variability in TBI patients with ICP levels <22 mmHg. No other clinical relevant physiological changes were observed. Reduced PRx variability might improve CA-guided perfusion management by reducing the time to find 'optimal' perfusion pressure targets. Larger studies with prolonged periods of PEEP-induced oscillations are required to take it to routine use.

Impairments in cerebral autoregulation (CA) are related to poor clinical outcome. Near infrared spectroscopy (NIRS) is a non-invasive technique applied to estimate CA. Our general purpose was to study the clinical feasibility of a previously published ‘NIRS-only’ CA methodology in a critically ill intensive care unit (ICU) population and determine its relationship with clinical outcome. Bilateral NIRS measurements were performed for 1–2 h. Data segments of ten-minutes were used to calculate transfer function analyses (TFA) CA estimates between high frequency oxyhemoglobin (oxyHb) and deoxyhemoglobin (deoxyHb) signals. The phase shift was corrected for serial time shifts. Criteria were defined to select TFA phase plot segments (segments) with ‘high-pass filter’ characteristics. In 54 patients, 490 out of 729 segments were automatically selected (67%). In 34 primary neurology patients the median (q1–q3) low frequency (LF) phase shift was higher in 19 survivors compared to 15 non-survivors (13° (6.3–35) versus 0.83° (−2.8–13), p = 0.0167). CA estimation using the NIRS-only methodology seems feasible in an ICU population using segment selection for more robust and consistent CA estimations. The ‘NIRS-only’ methodology needs further validation, but has the advantage of being non-invasive without the need for arterial blood pressure monitoring.