Low body mass index is a significant predictor of mortality within 30 days of surgery, according to a large analysis based on the the American College of Surgeons’ National Surgical Quality Improvement Program data set.
Low BMI raises postoperative mortality risk even after allowances are made for the type of surgery and for the individual patient’s preoperative expected risk of death, said Florence E. Turrentine, Ph.D., R.N., of the department of surgery, University of Virginia, Charlottesville, and her associates in a report published online in Archives of Surgery. In particular, the one-fifth of patients with the lowest BMI (less than 23.1 kg/m2) “demonstrated a significant increased risk of death, with 40 percent higher odds than the risk of death among patients in the middle range for BMI,” they noted.
The study subjects were 189,533 patients who underwent a major general or vascular surgical procedure in 2005 and 2006 at 183 participating medical centers. All patients were examined preoperatively and were given a 30-day mortality probable risk score based on more than 30 demographic characteristics, comorbidities, and laboratory values.
“The analysis of such a large number of patients allowed us to [examine] individual procedures done by general surgeons to a level of specificity not previously available,” Dr. Turrentine and her colleagues noted (Arch. Surg. 2011 Nov. 21 [doi:10.1001/archsurg.2011.310]).
A total of 3,245 patients (1.7 percent) died within 30 days of their surgery.
Interestingly, compared with normal weight, obesity was associated with lower rather than higher postoperative surgical risk, but not to a statistically significant degree. Somewhat unexpectedly, it was low BMI that raised mortality risk. Patients were divided into quintiles based on their BMI. After the data were adjusted to account for predicted mortality risk and type of surgery, the percentage of deaths among patients in the lowest BMI quintile (2.8 percent) was more than double that among patients in the highest BMI quintile (1.0 percent), which included patients with a BMI of 35.3 or greater.Thirty-day mortality also was significantly higher among patients in the lowest quintile of BMI than among patients in the middle quintiles (1.8 percent, 1.5 percent, and 1.4 percent).
In this analysis, overweight and obese patients in the upper two quintiles had lower 30-day mortality than did normal-weight patients in the middle quintiles, but those differences did not attain statistical significance.
The investigators further analyzed the data by type of surgery, discriminating among 45 different procedures. Laparoscopy, which was selected as the reference procedure, had an overall mortality of 2.0 percent. In comparison with laparoscopy, exploratory laparotomy carried the highest 30-day mortality (13.9 percent).
The effect of BMI on 30-day mortality was found to be quite different for certain operations. High BMI raised 30-day mortality risk for colostomy formation, wound debridement, colorectal resection, hernia repair, and mastectomy.