Potentially fatal blood clots after surgery are a much greater risk than has previously been thought, a British study finds.
"What is most striking is that not only is the risk higher, but that it lasts much longer than people have thought," said Dr. Jane Green, a clinical epidemiologist at the University of Oxford and a leader of the team reporting the findings in the Dec. 4 online edition of BMJ.
Blood clots in the deep veins and the lungs, formally called venous thromboembolism, have long been known as a possible complication after any form of surgery. The new study, using data on nearly 1 million women in the United Kingdom who were tracked for an average of 6.2 years after surgery, outlines the risk in precise detail.
One of every 140 women who had surgery that required a hospital stay was readmitted for venous thromboembolism within 12 weeks of the operation. The rate was highest for hip or knee replacement surgery at one in 45, and was one in 85 after cancer surgery. The incidence after operations not requiring a hospital stay was one in 815, the researchers found.
By contrast, the incidence of venous thrombosis during a 12-week period for women who did not have surgery was one in 6,200.
And the risk of such a blood clot remained high for at least 12 weeks after surgery, the study found.
"The extent of the problem is bigger than we thought," said Dr. Alexander Cohen, an honorary consultant vascular physician at King's College Hospital in London, who wrote an accompanying editorial.
The study punctures a belief that surgery done through small incisions -- called keyhole surgery in England, minimally invasive or laparoscopic in the United States -- reduces the risk of venous thromboembolism, Cohen said. "A lot more surgery is being done by keyhole surgery, and they are not getting treatment to prevent thromboembolism," he said.
That treatment is the use of clot-preventing drugs such as warfarin (Coumadin), and "the implications of this study is that treatment is required for the longer term, weeks or months after surgery," Green said.
Coumadin is not an easy drug to manage, requiring frequent blood tests, and it is less widely used in Europe than in the United States, Cohen said. Several new clot-preventing drugs are starting to enter medical practice in Europe, and their use after surgery should be encouraged, he noted.
"It is the surgeon's primary responsibility to make sure not only that the surgery is effective, but also to make sure that the complication rate of surgery is minimized as much as possible," Cohen said.
"This is a really great study, based on its scale and rigor," said Dr. Daniel J. Brotman, associate professor of medicine and director of the hospitalist program at Johns Hopkins Hospital, who has studied venous thrombosis. "It gives more specific data than ever before on the different types of surgery and the time line on thrombosis risk after surgery."
One insight provided by the study is that the time of increased risk for thrombosis is higher after cancer surgery than for other operations, such as hip replacement, Brotman noted. "Also, it shows a measurable increased risk even after outpatient surgery, which typically is not managed with prophylaxis [preventive therapy]," he said.
"This suggests we should be thinking about sending patients out of the hospital with prophylaxis extended for weeks," Brotman said. The longest previous studies of venous thrombosis prevention after surgery have been for no more than four weeks, he added.
"This warrants a controlled study of the potential benefit of extended prophylaxis for a longer time, particularly with cancer patients," Brotman said.
