Low Blood and Pulse Pressures Linked to Post-op Mortality

Author: Michael Vlessides

Anesthesiology News

A database analysis by a team of researchers has concluded that both low preoperative mean arterial pressure (MAP) and pulse pressure are associated with increased postoperative mortality in patients undergoing major noncardiac surgery. Future research, they said, should explore the mechanisms underlying this association and possibly test interventions.

“Previous research has shown unclear associations between preoperative blood pressure profiles and postoperative outcomes,” said Ashwin Sankar, MD, an anesthesiology resident at the University of Toronto. “Blood pressure is cheap to measure and readily available, and if we can show consistently increased risk for patients of certain blood pressure profiles, it may offer us a potential avenue for interventions. As such, our objective was to define the prognostic impact of blood pressure profiles based on their association with postoperative mortality.”

Dr. Sankar and his colleagues studied the records of 40,289 patients (median age, 61 years; 51% female; median body mass index, 28 kg/m²) undergoing major, elective noncardiac surgery at Toronto’s University Health Network between 2008 and 2015. The institution’s databases link preoperative, surgical and outcomes details through electronic health records.

“The distribution of comorbidities reflected the quaternary care nature of our institution,” Dr. Sankar explained. “Patients underwent a variety of surgical procedures, with a median duration of two hours.”

The study began with an unadjusted analysis, in which the association of MAP and pulse pressure with 30-day mortality was graphically examined, thereby establishing exposure thresholds associated with 30-day mortality. Next, a multivariable logistic regression model was implemented that adjusted for demographics, body mass index, comorbidities and surgical procedure (Figure).

Hypertension Has Been the Primary Focus

Median MAP among patients in the cohort was 97 mm Hg (interquartile range [IQR], 89-105 mm Hg), and median pulse pressure was 52 mm Hg (IQR, 44-63 mm Hg). In total, 1.7% of patients (n=673) died within 30 days of surgery.

After adjustment, MAP less than 90 mm Hg (OR, 1.16 per 5-unit decrease; 95% CI, 1.09-1.23) and pulse pressure less than 40 mm Hg (OR, 1.26 per 5-unit decrease; 95% CI, 1.11-1.42) were both found to be associated with increased odds of death.

“Since we included both MAP and pulse pressure in our models, we’re only able to show the adjusted association of MAP at a fixed pulse pressure, and vice versa,” Dr. Sankar said. “However, when we evaluated the interaction between MAP and pulse pressure in models, it was not significant [P=0.14], suggesting that at whatever MAP we chose, the association between pulse pressure and mortality would be roughly the same. The same relationship existed for pulse pressure as well.”

“We looked globally at the entire cohort, but not within subsets for comorbidity status,” Dr. Sankar replied. “We limited our analysis because we didn’t necessarily want to overadjust.”

For Bobbie Jean Sweitzer, MD, a professor of anesthesiology at Northwestern University’s Feinberg School of Medicine, in Chicago, the analysis yielded interesting insights into the balance between hypotension and hypertension. “We seem to be more obsessed with hypertension in the preoperative clinic and in general,” she said. “Nevertheless, it doesn’t appear that you have shown these signals, with hypotension being the problem instead.”

“That was one of the interesting things that came out of this study,” Dr. Sankar replied. “It appeared that the risk following adjustment was higher in the low-MAP and low–pulse pressure groups, and not among patients with high MAP, such as patients with hypertension.”