LOW BP CUT-OFF IN BRAIN TRAUMA?

Is systolic blood pressure (SBP) of 90 the magic threshold in traumatic brain injury (TBI)? This observational paper goes through a large prehospital database, and found an association between lower systolic BP and higher mortality. This has created a lot of discussion on current goals for BP in TBI – but it needs to be pointed out the article is observational – we don’t yet know if manipulating these blood pressures higher will be good for TBI patients.

SBP below 90
It’s long been a rule of thumb that TBI has worse outcome with even a single drop in saturation below 90% or a single measured systolic blood pressure (SBP) below 90, as discussed here, and has still been held as truth, recently in articles in Annnals of EM.

No clear cut-off?
This JAMA surg study is a retrospective obervational/post-hoc ananlysis of TBI-data. They looked at the lowest recorded pre-hospital SBP, and measured that against in-hospital mortality in >3000 pts. The interesting part was the very linear relationship between lower blood pressure and higher mortality. It’s safe to assume that a SBP of 40 in a TBI pt is very bad, but surprising that there is no steep drop-off when going down towards 40. The relationship stays linear. Same goes for SBP 90. There is no inflection of the curve here. And survival increases all the way up to SBP of 135, the highest recorded SBP.

Observational data
The downside is that this is observational data. It shows an association between lower SBP and mortality. And as expected, together with SBP, hypoxia, Injury Severity Score and Head Region Severity Score were all strongly associated with mortality. This points to SBP as just a marker of severity. And there was no intervention, so we have no data supporting the notion that increasing these blood pressures artificially with fluids or pressors will move the patient into a lower mortality category.

They try to isolate the effect of SBP on mortality with some statistical voodoo, creating an impressive inverse linear relationship between SBP and mortality. The unadjusted mortality seems to have a linear relationship going into a flatter slope around SBP 105-110, fitting better with the current tradition of SBP>90.

And although mortality is a hard endpoint, they did not specify mortality caused by cerebral diagnoses. also, in the survivors, good or bad neurologic outcome would have been an equally interesting outcome, as we’re interested in SBP with regards to saving brain.

What can we take home?
This study seems to affirm the notion that the lowest SBP reading has impact on mortality. The study also seems to offer two new insights: 1) there is no clear mortality cut-off around 90mmHg. The good thing is that your pt’s brain isn’t fried if you get a reading of SBP 85. 2) On the other hand, it might be that even higher SBP offers even better chances of survival. The bad thing is we don’t know if BP intervention will help. On the other hand, this article provides the same observational ‘evidence’ as our current rule of keeping TBI >90 SBP.

When reading post-hoc studies, it’s important to remember this article, studying data from the 6S multi center sepsis study, finding significantly better survival from sepsis for pts born in the sign of Pisces.

Mortality and Prehospital Blood Pressure in Patients With Major Traumatic Brain Injury, JAMA Surg, 2017.

The Effect of Combined Out-of-Hospital Hypotension and Hypoxia on Mortality in Major Traumatic Brain Injury, Ann Emerg Med, 2017.

Association of Out-of-Hospital Hypotension Depth and Duration With Traumatic Brain Injury Mortality, Ann Emerg Med, 2017.

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2 Responses to LOW BP CUT-OFF IN BRAIN TRAUMA?

  1. Mads Astvad says:

    Or the many great examples of spurious correlations here:

    http://www.tylervigen.com/spurious-correlations

    • Thomas D says:

      Even though it’s just an association, the correlation between BP and brain injury seems more plausible than most of the examples in your link ;-D

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