Individualizing Cerebral Perfusion Pressure Targets
The authors comment in the discussion about the fact that “patients spent a significant period of time with CPP above current recommendations” with mean CPPs between 75 and 80 mm Hg. Although the 2017 Brain Trauma Foundation (BTF) guideline suggests to maintain CPP between 60 and 70 mm Hg (2), it is surprising that the guidelines still do not comment at all on level at which blood pressure should be monitored. In the present study, the arterial catheter was zeroed at the level of the heart, but many institutions level it at the foramen of Monroe (ear). This difference in height might translate into a decrease in CPP of 10–15 mm Hg depending on the elevation of the head of the bed (15–30° usually). Therefore, absolute values observed in the study must be interpreted accordingly to insure proper external validation. In our opinion, there is an urgent need to study regional variation in arterial catheter zeroing level when calculating CPP, and published studies should always mention this level. We feel that positioning on the matter by scientific societies would be helpful.
Second, the concept of optimal CPP (CPPopt) is attractive since it seems to delineate the width and limits of the plateau of the cerebral autoregulation curve, allowing the clinician to target specific and safe values. Although methodology of computation and generation of the pressure reactivity index (PRx) curve has improved the capacity to find CPPopt in 93% of their patients, the ability to identify lower and upper limit of autoregulation is not described in the results. Extrapolations were used when a nice U-shaped curve could not be found. How often did this occurred? Also, although a CPPopt could be found, the absolute PRx value associated with it might still indicate poor autoregulation (e.g., PRx above 0, 1). How many patients had a complete loss of autoregulation? This might have an important impact on treatment and outcome (3, 4).
Finally, the results support the concept of brain ischemia since time spent with CPP below the lower limit of autoregulation is associated with mortality and unfavorable outcome, but time spent above the upper limit of autoregulation was only associated with unfavorable outcome. This is possibly due to the fact that patient with intact autoregulation tolerate higher CPP level as suggested in the 2007 BTF guidelines. But, elevated blood pressure might still create edema in areas of the brain with blood brain barrier disruption (5).