Pediatric traumatic brain injury (TBI) remains the leading cause of mortality in children younger than 19 years. Most people think that the secondary brain injury following TBI occurs in the setting of elevated intracranial pressure (ICP) and diminished cerebral perfusion pressure (CPP). These physiologic measurements are thought to affect outcomes, so current guidelines recommend treatment of both elevated ICPs and decreased CPPs, although the thresholds for CPPs have not been well established in children. Although a smaller study looked at CPP in children with TBI, this work by Allen et al is the largest study to date that attempts to determine these thresholds.
Using a prospective, observational cohort, the authors examined data from TBI-trac, an online data repository run by the Brain Trauma Foundation. This databank collects information about patients with severe TBI and uses these data to track guideline compliance at 22 different trauma centers and for research. In this report, the authors reviewed the data on patients treated between 2000 and 2008.
In all, the authors studied 2074 records and divided the patients into categories based on age: 0-5 years (55 patients), 6-11 years (65 patients), 12-17 years (197 patients), and 18 years or older (1757 patients). They subsequently defined high and low CPP thresholds for each age group to determine if these thresholds impacted short-term survival. For those in the youngest group, the authors chose 30 mm Hg for a low CPP threshold (CPP-L) and 40 mm Hg for a high threshold (CPP-H). For subjects in the group 6-11 years, the CPP-L was 35 mm Hg and the CPP-H was 50 mm Hg. For the subjects 12 years or older, the CPP-L selected was 50 mm Hg and the CPP-H was 60 mm Hg. CPP values between the high and low thresholds were labeled as CPP-B.
The results of this study affirm the opinion that CPP targets should be age specific, but it appears that the time spent at lower CPPs may be at least as important as the actual values. In only one age group was mortality associated with the number of events below CPP-L. This group was made up of children from the two youngest groups (i.e., 0-11 years), which were combined after the study was completed because of inadequate power. More importantly, when Kaplan-Meier survival plots for total time monitored around the different CPP thresholds were applied, the authors demonstrated significant increases in mortality when the patients had prolonged exposures to CPP-L and CPP-B compared to CPP-H. Additionally, the authors noted that the number of CPP-L events appeared to be related to higher ICP more consistently than hypotension.
This study has a number of limitations, which were addressed by the authors. First, their primary outcome of short-term mortality does not address the myriad of other potential morbidities associated with TBI. Second, the presence of multisystem trauma, which can clearly affect outcomes, was not differentiated. Finally, and more importantly, the study does not address any therapeutic strategies that might have been used to maintain CPPs. Lower CPPs may increase mortality, but it still has not been shown that manipulation of this value using fluids, pressors or inotropes can improve outcomes. In fact, maintenance of CPP above 70 mm Hg in adults may increase the risk of acute respiratory distress syndrome.