Doctors often prescribe unfractionated heparin to obese patients with venous thromboembolism below the recommended doses based on body weight, according to a new study. As a result, some patients in the study took longer than 24 hours to reach therapeutic anticoagulation — possibly putting them at risk for VTE recurrence, already a worry because of their weight.

“We’re seeing larger and larger people,” Dr. Jeffrey Weitz, director of Hamilton, Ontario’s Henderson Research Centre at McMaster University who co-wrote the current guidelines for heparin dosing in patients with VTE, told Reuters Health. When they do dosing calculations, “physicians are frightened when they see these large doses of anticoagulants that need to be prescribed for people of these very heavy weights.”

The guidelines, from the American College of Chest Physicians, recommend a dose of 80 units/kg of body weight for unfractionated heparin, with an initial infusion rate of 18 units/kg/hr.

The current study’s authors, led by Dr. Adam Hurewitz of Winthrop-University Hospital in Mineola, New York, recorded the cases of 84 obese patients treated for VTE with unfractionated heparin over the course of three years at Winthrop. The patients received a mean heparin bolus of 58 units/kg and an initial continuous infusion of 13 units/kg/hr. Median time to therapeutic anticoagulation was 18.5 hours (range: 4 to 96 hours.) Time to therapeutic anticoagulation exceeded 24 hours in 29% of patients.

While there was not a significant correlation between prescribed dose per kg and time the therapeutic anticoagulation, initial dose in units/kg/hr was correlated with time to therapeutic anticoagulation (p < 0.02). According to the results, published in the Journal of General Internal Medicine, for every decrease in 1 unit/kg/hr of the initial heparin dose, therapeutic anticoagulation was delayed by about 45 minutes to 1.5 hours. The study authors did not track whether patients that took longer to reach therapeutic anticoagulation were more at risk of VTE recurrence, but previous studies have confirmed this to be the case. The gap between recommended and actual initial dose was highest among the most obese patients, widening by about 100 units/hr for every 10kg body weight increase. Internal medicine residents interviewed by the authors said that they may have deviated from the dosing guidelines because they worried about bleeding risks with high doses of heparin. That may stem from the common risk of bleeding seen in cardiac patients taking unfractionated heparin — patients who have often undergone invasive procedures, said Dr. Maritza Groth, one of the study’s authors, also at Winthrop-University Hospital. But, she told Reuters Health, “The patients with VTE usually have not undergone (invasive) procedures, so we don’t see many complications in those patients.“ And supratherapeutic partial thromboplastin times have not shown to increase the risk of bleeding complications, the authors say. Dr. Bruce Davidson, who studies treatment of VTE the University of Washington School of Medicine and was not involved with the research, agrees. “Early on the risk of an important bleed in a patient who doesn’t have risk factors for bleeding is quite low,” he told Reuters Health. But, because they are cautioned to “First, do no harm” and are wary of lawsuits, doctors may err on the side of lower dosing, he said. Physicians, Dr. Weitz said, “are just scared by the high doses. They also need reassurance that these doses are necessary in order to achieve a therapeutic level of anticoagulation.” He worries that dosing problems may become a bigger issue as hospitals switch from unfractionated heparin to low molecular weight heparin. When hospitals fill up syringes of low molecular weight heparin to give to obese patients, the most concentrated solution available isn’t concentrated enough to fit in one syringe, Dr. Weitz said. “You can’t even treat them (only) once a day, and that also scares people,” he said. A computerized ordering system that calculates heparin doses for doctors could cut down on underdosing, Dr. Weitz said — but that alone may not be enough. “I think there’s really some system changes that need to happen in the hospital, but (also) some education that needs to happen around the fact that we have a population that’s getting bigger and the doses of anticoagulants that we’re all used to are going to go up,” he said. Dr. Davidson agrees that reminders to doctors about the evidence in favor of weight-based dosing will help change practice. “You just have to raise awareness (that) there is an authoritative answer” about the best dosing, he said, “and then it becomes part of the dogma.” Reference:
Dosing of Unfractionated Heparin in Obese Patients with Venous Thromboembolism

J Gen Intern Med, online 15 Dec 2010.