Very Large Database Finds:
Anesthesiologists are working harder to better understand preoperative risks that may influence perioperative care. According to one intriguing study of surgical risks, preoperative low blood pressure—as opposed to high blood pressure—is a risk factor for death.
The study, conducted by researchers in the United States and United Kingdom, looked at data from 250,000 patients to investigate the relationship between preoperative blood pressure and 30-day mortality after noncardiac surgery. They looked for hypertension as a risk factor, and were interested in understanding how the change in risk relates to increasing and decreasing blood pressure values. They expected to see drops in blood pressure as a risk factor but didn’t think the effects would present as strongly as they did.
“While we expected the extremes of blood pressure to be associated with increased risk, the lack of effect of hypertension was surprising. We were additionally surprised that only small drops in blood pressure exerted an effect, especially as these small changes survived the risk-factor adjustment,” said Robert Sanders, MBBS, PhD, FRCA, assistant professor of anesthesiology at the University of Wisconsin–Madison, who worked with Puja Myles, PhD, MPH, and Sudhir Venkatesan, MPH, at The University of Nottingham, in England, and other collaborators at the University of Southampton, Oxford University and University College London, also in England. “We think that lower blood pressure is underrecognized as a factor.”
Sample Size Allows Precise Measurements
The large sample size gave them the statistical power to detect small differences, Dr. Sanders said. The study’s design prioritized a large number so that the researchers wouldn’t have arbitrary thresholds for blood pressure numbers. “To identify blood pressure thresholds for the change in postoperative risk, we focused on a data-driven approach to avoid biases,” Dr. Sanders said. Findings from the study were released at the Euroanaesthesia 2015 conference.
The group took two statistical approaches to the data, which analyzed 252,278 patients from the U.K. Clinical Practice Research Datalink. First, they identified 29 risk factors, such as age, sex, race, comorbidities, medications and surgical risk score, and adjusted for them in various regression models. Second, they adjusted for significant confounders to combat overadjusting for risk factors, which can lead to a loss of statistical power.
After the adjustment, the effects of preoperative systolic and diastolic hypertension were no longer associated with increased postoperative mortality. However, preoperative hypotension was associated with statistically significant increases. For example, the likelihood of death increased by 40% for patients with a systolic number below 100 mm Hg. For those with a diastolic number under 40 mm Hg, the likelihood of death increased 2.5 times. Values below 100/40 mm Hg showed the greatest risk.
At the Euroanaesthesia 2015 conference, other researchers were intrigued and surprised as well, Dr. Sanders said. His research team is developing a full manuscript to report on additional findings in the large database.
“Our working hypothesis is that patients with low blood pressure drop below a critical limit during the perioperative period and become vulnerable,” Dr. Sanders said. “The question is, what do we do about this?”
In the future, the team hopes to study how to modify risk to improve patient outcomes. Dr. Sanders and others want to amass a continuum of data to combine what they know about preoperative and perioperative states to understand the differences for patients who are injured. Then, the next step is to move into preoperative hemodynamics.
“My caution is that these are observational data, and it can’t prove causation,” he said. “But it’s a step toward causality, and it seems reasonable to me to consider these patients at an increased risk.”
Dr. Sanders and his colleagues don’t yet have recommendations or intervention guidelines for how this should be managed, but he noted that anesthesiologists at the bedside should be wary about patients who have low blood pressure before surgery.
“These patients are already potentially at a threshold where further drops could be problematic,” Dr. Sanders said. “Control of blood pressure remains important.”