Surgical patients with a body mass index at the lower end of the normal range were more likely to die within 30 days of the procedure than those in the moderately overweight range, researchers found.

Compared with patients with a BMI of 26.3 to 29.6, those with a value below 23.1 had a significantly higher risk of death (adjusted OR 1.40, 95% CI 1.25 to 1.58), according to an analysis of 189,533 surgeries performed in 2005 and 2006 and recorded in the National Surgical Quality Improvement Program (NSQIP) database.

Patients with higher BMI values above 23.1 -- including the morbidly obese -- had about the same risk of 30-day mortality as the moderately overweight, George J. Stukenborg, PhD, of the University of Virginia in Charlottesville, and colleagues, reported online in Archives of Surgery.

Action Points Explain that surgical patients with a BMI at the lower end of the normal range were more likely to die within 30 days of the procedure than those in the moderately overweight range.

Point out that some procedures -- for example, colorectal resection, colostomy formation, cholecystectomy, hernia repair, mastectomy, and wound debridement -- were associated with higher mortality risk related to increasing BMI.

But for some individual types of surgeries, obesity was associated with increased mortality, the researchers found.

"These individual types of procedures include procedures with which the general surgeon should have definite experience: colorectal resection, colostomy formation, cholecystectomy, hernia repair, mastectomy, and wound debridement," Stukenborg and colleagues wrote.

NSQIP data in the study were extracted from medical records at 183 participating hospitals. Those with low volumes reported all cases performed each year whereas high-volume hospitals reported the first 40 consecutive cases for 42 eight-day cycles each year.

Patients in the study were categorized into BMI quintiles, with values of less than 23.1 being the lowest and those above 35.2 being the highest. For the overall 30-day mortality risk calculation, the middle quintile -- 26.3 to 29.6 -- served as the reference.

BMI values of 20 to 25 are considered normal. A value of 30 is the standard threshold separating overweight from frank obesity.

Odds ratios for 30-day mortality in the two lowest and two highest quintiles were adjusted for procedure type and baseline mortality risk. The latter is a standard part of the NSQIP data and is calculated from more than 30 patient variables including sociodemographic factors, comorbidities, and preoperative laboratory values such as serum albumin and white blood cell count.

Only the lowest quintile showed a significant difference from the middle, reference quintile in the adjusted overall odds ratio for 30-day mortality:

Quintile 1: 1.40 (95% CI 1.25 to 1.58)

Quintile 2: 1.11 (95% CI 0.98 to 1.26)

Quintile 4: 1.02 (95% CI 0.89 to 1.17)

Quintile 5: 0.91 (95% CI 0.78 to 1.06)

Stukenborg and colleagues also evaluated 30-day mortality for 45 individual types of surgeries recorded as the "principal procedures" in the NSQIP data.

With laparoscopy as the reference, adjusted odds ratios for mortality ranged from 0.03 for breast lumpectomy to 2.47 for noncardiac vascular bypass and shunt procedures, before accounting for BMI.

The researchers found that mortality for a few procedures bucked the overall trend toward higher risk for low-BMI patients. For these, there were statistically significant increases in mortality associated with higher BMI, relative to laparoscopy.

These included temporary and permanent colostomies, ileostomies and other enterostomies, wound debridement, colorectal resection, therapeutic musculoskeletal procedures, endarterectomies in the head and neck, upper GI therapeutic procedures, cholecystectomies, hernia repairs, and mastectomies.

Limitations to the study included lack of data on nonfatal complications, use of hospital resources, or deaths beyond the 30-day mark. The possibility of inaccurate or incomplete data in the NSQIP database also cannot be excluded.