What would we discover if we could transport ourselves sci-fi style to the inside of a cerebral arterial lumen to see what goes on when blood pressure is high, low, or just right? Would we see the blood brain barrier doors becoming leaky with hypotension and lack of perfusion? Would we witness arterial shutdown when the ICP overcomes MAP, resulting in inadequate CPP?
Optimal parameters for blood pressure in neurologically injured patients are far from definitive. Last year we saw further proof that pushing blood pressure below systolic 140 was not necessarily beneficial to a patient with intracerebral hemorrhage. (N Eng J Med 2016;75:1033.) Blood pressure goals for acute ischemic stroke depend on the patient, and whether he is “blood pressure dependent.” We generally target a goal of less than systolic 140 for subarachnoid hemorrhage, but the supporting data are not strong.
Randall Chesnut, MD, a professor of neurological surgery at Harborview Medical Center in Seattle, brought us the seminal study in 1993 that showed a single blood pressure below systolic 90 was consistent with a significant increase in mortality. (J Trauma 1993;34:216.) But is this cutoff value our best clinical threshold for blood pressure? Is there a significantly worse outcome below 90 or better outcome above it? It has been nearly universally adopted, although new data and TBI guidelines now advise maintaining SBP >100 for ages 50-69 and >110 for ages 15-49 or over 70. (www.braintrauma.org/coma/guidelines; Injury 2012;43:1833; J Trauma Acute Care Surg 2012;72:1135; J Neurotrauma 2007;24:329.)
It is convenient to have a single numeric cutoff to guide prehospital protocols, but we may be missing opportunities for improvement by allowing or pushing blood pressure to be higher. We witnessed results from a large trial at the end of 2016 that challenges the existence of a “cutoff” value as a benchmark for traumatic brain injury treatment. (JAMASurg 2016; doi:10.1001/jamasurg.2016.4686.)
Spaite, et al., in that study used data from a large multicenter observational study designed to evaluate the effect of prehospital guidelines. They took 3844 patients from the pre-implementation arm of the study who had isolated TBI or multisystem trauma, an initial SBP between 40 and 120, and evaluated for mortality. The data were adjusted for confounders, and using logistic regression, showed that for every increase in 10 points of SBP between 40 and 120, there was an 18.8 percent lower adjusted odds of death. This relationship was linear throughout the range, showing no discreet cutoff for significant change in mortality. Interestingly, the group broadened the analysis to look at blood pressures as low as 60 and as high as 135, and there was still a difference in the odds ratio of mortality at every level with five points of difference in SBP.
The uniqueness of this study lies in its size and the authors' ability to obtain prehospital information. Entering qualified patients into the database triggered an EMS agency to send patient care records matching the patient identifiers. In this way, the data reflect very early SBP information, showing the very early effect of hypotension on the injured brain. The findings are limited by their observational nature, and prospective studies will have to take into account the treatment parameters for mutisystem trauma and the downside of pushing blood pressure higher.
Will this have an effect on prehospital or emergency care of TBI patients? Knowledge of better outcomes with improved blood pressure can be translated into protocol starting in the field. We have taught for years that blood pressure must not drop below systolic 90, but we may change our practice with future prospective studies and become even more aggressive with blood pressure for the TBI patient. Stay tuned.
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