For patients with acute traumatic brain injury (TBI), the higher the prehospital systolic blood pressure the better — a finding that challenges the conventional wisdom that there is a clinically meaningful threshold, new research suggests.
The study — the largest to date to look at this issue — found a linear association between lowest prehospital systolic blood pressure and severity-adjusted probability of mortality across the whole range of pressures between 40 and 120 mmHg.
The researchers note these findings, which were in JAMA Surgery, show that for the injured brain, physiologically detrimental hypotension may occur at significantly higher blood pressure levels than current guidelines advise.
The researchers note the current study “is unique in both its size and its access to detailed prehospital data” and that their findings “highlight the need for specific trials comparing various blood pressure treatment thresholds well above the classic 90 mmHg.”
“This data really changes how we think about blood pressure in patients with a serious brain injury in an acute setting. This is a major paradigm shift,” study coauthor, Joshua Gaither, MD, associate professor of emergency medicine, College of Medicine, University of Arizona, Tucson, told Medscape Medical News.
“Historically we have been taught that when considering blood pressure in the setting of acute traumatic brain injury there is a magic number of 90 and as soon as the pressure is below 90 mmHg then damage will occur.
“We show in this study that this is not the case and for patients with systolic pressures anywhere between 40 and 120 mmHg, mortality rises as blood pressure falls. There is no magic cutoff. This suggests we need to treat blood pressure aggressively across a larger spectrum than previously thought,” he added.
“Despite decades of assuming otherwise, it appears that the interaction between prehospital blood pressure and outcome may be physiologically continuous rather than dichotomous across a remarkably wide range. While it is hard to conceive of an approach to managing TBI that doesn’t include some level of blood pressure that requires treatment, it appears that the science that forms the basis for the current guidelines may require an entirely new way of thinking,” the authors write.
Dr Gaither explained that TBI causes two types of damage: (1) the primary injury — the damage to the brain when the head is hit — which is thought to be irreversible, and (2) secondary damage caused by the injured brain being extra susceptible to secondary insults, such as low blood pressure.
He noted that current guidelines, which recommend maintaining blood pressure above 90 mmHg in these patients, are based on several small studies that were not big enough to look at the whole spectrum of blood pressure and so instead focused on comparing two groups above and below the 90-mmHg threshold.
“Our study, however, was much larger — we had prehospital blood pressure readings for many thousands of patients so we could look at it as a continuous variable, and we saw a continuous relationship between blood pressure and mortality — the lower the blood pressure the higher the mortality right up to a systolic of 120mmHg.
“It doesn’t matter whether we are comparing systolic pressures of 120 and 110 or 110 and 100 or 100 and 90 — each of these 10-mmHg reductions was associated with an 18% increase in mortality,” he added.
He cautioned, however, about making definitive recommendations on treatment targets based on this observational study.
“Because this is observational data we cannot say for sure that raising blood pressure will reduce mortality in these patients. But it does show that the lower the blood pressure, the more likely a patient is to die. We really need a randomized trial to investigate whether more aggressive treatment with fluids does make a difference in terms of mortality in patients at all levels of blood pressure.”
In the meantime, Dr Gaither suggests that keeping pressures up as high as possible up to 120 mmHg is a reasonable goal. “At present a paramedic may see a patient with a traumatic brain injury and a blood pressure of 110 mmHg and think that is okay, but our study suggests it will still be beneficial to treat such a patient with IV [intravenous] fluids to keep their pressure up.”
“After this data, we have been advising our paramedics to give IV fluids to any patient whose blood pressure is trending down. If a patient has a systolic pressure of 120 mmHg then a few minutes later it has reduced to 110 mmHg, then we would say that patient is a candidate for aggressive IV fluids.”
For the study, the researchers analyzed information from a large prehospital database established as a part of the Excellence in Prehospital Injury Care (EPIC) Traumatic Brain Injury Study to determine the association between systolic pressure and probability of death, adjusting for significant/important confounders. The study included 3844 patients age 10 years and older with moderate or severe TBI and a prehospital systolic pressure between 40 and 119 mmHg.
Results revealed a linear decreasing association between systolic pressure and adjusted probability of death across the entire blood pressure range. Each 10-point increase of systolic pressure was associated with a decrease in the adjusted odds of death of 18.8%.
The researchers point out that a key reason for evaluating the effect of blood pressure measured before hospital arrival is that the injured brain is so highly sensitive to changes in perfusion and the timeframe during which neuronal damage begins is so short.
“It is well established that secondary brain injury is initiated by even brief periods of compromised blood flow. Thus, decreased perfusion occurring during the prehospital time interval may have a profound effect on outcome,” they write.