This study compares 2 commonly used definitions of hypotension in children with severe TBI, and subsequently uses the better of these hypotension definitions to describe the relationship between location of hypotension and outcome. Our main findings are that: 1) the better definition of hypotension was SBP < 5th percentile for age and 2) early hypotension, documented in the field and/or ED, predicted poor outcome more so than PICU hypotension. This study provides new information regarding the measure of hypotension and the influence of early versus late hypotension on outcome in children with severe TBI.
The adverse relationship between hypotension and poor outcome in children with moderate and severe TBI has been previously reported, but each study used a different definition of hypotension (3
). In this study, we have shown that the incidence of hypotension varies according to the definition of hypotension used (SBP < 5th
percentile for age or SBP < 90mmHg). While this may not be surprising, the comparison of definitions is particularly important for young children in whom SBP 90mmHg may be higher than the SBP 5th
percentile for age (9
). This means that while using SBP < 90mmHg to define hypotension might be appropriate in older children, using SBP < 90mmHg to define hypotension in younger children might overestimate the true incidence of hypotension. To understand the value of one definition over the other, we compared the different definitions of hypotension in the same patients and found evidence supporting the Brain Trauma Foundation’s (BTF) recommendation that an age-related blood pressure definition (SBP < 5th
percentile for age) be used to define hypotension in children with severe pediatric TBI.
The design of the current study allowed us to compare the influence of hypotension occurring early versus late in the course of medical care on outcome. While previous studies have separately described the relationship between ED and PICU hypotension on outcome, none serially compared the relative importance of the early period to the PICU period in the same population. Studies examining the relationship between PICU hypotension and poor outcome did not consider field and/or ED hypotension in their analysis (13
). Our finding that field hypotension predicted ED hypotension may reflect the fact that therapeutic regimens to maintain or restore hemodynamics can be limited in the field. Although not significant, the high incidence of persistent hypotension in the PICU in children who were hypotensive in the field or ED may be due to severity of injury, suggesting that more aggressive measures may be needed to screen and treat children with severe TBI and early hypotension (15
). In addition to describing the risk of persistent and new onset hypotension in the ED and PICU based on field hypotension, we report a greater strength of association between hypotension occurring in either the field and/or ED than hypotension in the PICU. This is important since both field and/or ED represent and early period following severe TBI.
Studies of adults have linked early hypotension following TBI with poor outcome (2
). Hypotension occurring at any point from the time of injury through resuscitation has been shown to double the mortality rate (2
). Field hypotension has been correlated with fatal outcomes in adults with polytrauma including TBI (16
), and minimization of hypotensive episodes in the field has been shown to improve outcome (17
). Observations have also been made in experimental TBI where the incidence of hypotension is higher in younger compared to older rat pup (19
). These data suggest that there is a need for early identification and treatment of hypotension in children with severe TBI.
The initial management of TBI includes protecting the injured brain from secondary insults when cerebral blood flow (CBF) may be compromised and the incidence of impaired cerebral autoregulation may approximate 37% (21
). When cerebral autoregulation is impaired, cerebral ischemia may ensue if cerebral perfusion pressure (CPP) is low. This is important because adult TBI data suggest that persistently impaired autoregulation is associated with poor outcome (25
The limitations of our study merit discussion. The retrospective nature of this study confined our data collection to what was recorded in the charts, and may contain unavoidable bias /residual confounders. It would have been preferable to have long-term outcome data such as 3, 6 and 12 month GOS or the Fiser scores (26
) to assess the impact of hypotension. Unfortunately, there is currently no large or national pediatric database that contains serial hemodynamic data from the field through PICU discharge for a set of patients, making the present relatively small study valuable. As current BTF recommendations consider evidence from studies that use both these definitions of hypotension, it makes the current comparative analysis of the 2 most commonly used definitions of hypotension necessary and important. Another possible limitation is the generalizability of our findings to all children with severe TBI since the data are only from one center and since we had a large number of young patients in our cohort. However, since age < 4 years is associated with worse outcomes, and published studies on TBI involve mainly children of older ages, the information in this study may be important for identification of young children with TBI at risk for poor outcome.
While we did not have data from the field, and PICU in all patients, data are missing only from a small number of patients, making our results valid. While it may be more physiologic to use low CPP to define hypotension, we used systolic blood pressure instead of mean arterial pressure (MAP) or CPP data in this study, because of obvious logistic constraints on measuring CPP in the field or possibly ED. Although our data suggest that hypotension burden may adversely impact outcome, given the retrospective nature of this data, the small number of children with hypotension in the field and ED, and the lack of detailed information regarding hypotension severity or duration in each setting, we only analyzed hypotension as a single event. While we have identified a definition of hypotension that predicts poor outcome, we cannot comment on the age-related blood pressure needed to achieve the best outcomes. Finally, we cannot, from this retrospective study, distinguish between the effect of hypotension vs. severe TBI on outcome.