Less VTE After Bariatric Surgery With LMW Vs Unfractionated Heparin

November 21, 2012

By James E. Barone MD

NEW YORK (Reuters Health) Nov 21 - Venous thromboembolism (VTE) rates after bariatric surgery were significantly lower, and bleeding rates were similar, when low molecular weight heparin (LMW) was used for clot prevention instead of unfractionated heparin (UF) in a large population-based study.

In an email to Reuters Health, lead author Dr. Nancy Birkmeyer of the University of Michigan said that based on this study, pre- and post-operative LMW prophylaxis, "Is now current practice both at UM and also at almost all bariatric programs in the state."

Venous thromboembolism is the primary cause of postoperative mortality after bariatric surgery. It occurs in almost 1% of patients within the first 30 days.

The new study was conducted by the Michigan Bariatric Surgery Consortium and involved all 32 hospitals where the surgery is performed.

The 24,775 subjects accounted for 79% of all patients who underwent bariatric surgery there. Dr. Birkmeyer said, "Since VTE is so rare, we were only able to evaluate the dominant strategies." The other 21% of patients had various combinations of UF and LMW heparin. Fewer than 2% of patients were managed without any heparin prophylaxis at all.

The primary outcomes were rates of VTE, hemorrhage and serious hemorrhage requiring more than four units of blood or reoperation.

Patients were categorized as being at low-risk (less than 1%) or high-risk (greater than 1%) for VTE. Three different heparin regimens were used-pre- and post-op unfractionated heparin (UF/UF), pre-op UF and post-op LMW (UF/LMW) and LMW both pre- and post-op (LMW/LMW).

In addition, 98% of all patients had sequential venous compression devices in place.

Among the three treatment groups, a number of risk factors for VTE were significantly different: age, body mass index, gender, hypertension, liver disease, operative duration and type of bariatric procedure performed. However, the predicted risk of VTE did not differ for the three cohorts.

In low-risk patients, adjusted rates of VTE were significantly lower with UF/LMW and LMW/LMW than with UF/UF (p=0.03 and p<0.001, respectively). There was no difference in the incidence of VTE among the three treatment strategies for the high-risk patients, however - but the since only 5% of patients overall were high-risk, the authors feel there were too few to allow for a meaningful comparison.

Rates of hemorrhage and serious hemorrhage were similar for all three groups. Dr. Birkmeyer said she was surprised that there was not an increase in bleeding among patients receiving LMW heparin because "This is a very strongly held belief among surgeons." However, when she reviewed the literature she found that most of the prior studies found either no difference in rates of bleeding or that LMW heparin is protective.

About 18% of patients received UF/LMW. Dr. Birkmeyer said that although giving UF/LMW had not been reported elsewhere, some surgeons believed that giving LMW pre-op might cause bleeding. Others used it because they believed that LMW heparin needed to be given several hours before the procedure to be effective intra-operatively. She said, "Our data show that this approach is about as good as LMW/LMW for most patients except for those with the highest risk."

The authors listed a few limitations of their work. As in all observational studies, unmeasured confounding variable might exist. Certain surgeon- or hospital-specific factors might have biased the results. But the authors say they analyzed the data in multiple ways and found no evidence of those issues.

In addition, the doses of heparin used were not included in the data set, and some cases of VTE were diagnosed on clinical grounds only.

An editorial published with the paper this month in Archives of Surgery, by Dr. M. Margaret Knudson at the University of California, San Francisco, notes that the study does not provide level 1 evidence for the use of LMW heparin. Dr. Knudson expressed concerns that the number of patients screened for VTE and the doses and duration of heparin use were unknown.

Costs were not addressed in the paper but Dr. Birkmeyer estimates that LMW is about 10 times more expensive than UF.

Dr. Michel Gagner, clinical professor of bariatric and metabolic surgery at Florida International University, spoke by telephone with Reuters Health. Dr. Gagner, who was not involved with the work, congratulated the authors on such a large study. Still, he said, "I am not convinced that heparin is necessary in all bariatric surgery patients."

Dr. Gagner is the lead author of a paper this month in Surgery for Obesity and Related Diseases, the official journal of the American Society for Bariatric Surgery. He and his coauthors report there that sequential venous compression with or without the use of heparin resulted in similar VTE rates: 0.47% and 0.25% respectively.

His group does not use heparin except in high-risk patients. He said "We have only had two instances of VTE in our last 1000 patients."

Dr. Gagner pointed out that although a randomized trial would be difficult due to the small number of patients who suffer VTE, it would be the only way to definitively answer the question of whether chemoprophylaxis is needed.

SOURCE: http://bit.ly/Ug1QJ5

Arch Surg 2012