Dec. 4, 2009 -- The risk of getting a potentially deadly blood clot after surgery is even higher than experts believed -- and it persists longer than they thought, according to a new study that evaluated nearly a million women in the U.K., including nearly 240,000 who had surgery.
''We already know there is a substantially increased risk of blood clots after major surgery," says researcher Jane Green, MD, PhD, a clinical epidemiologist at the University of Oxford, England. "What is striking about our finding is the scale of the risk. Even more important is the fact that this risk is raised for quite a long time."
"The bulk of the increased risk is up to three months after surgery," she says. "The peak is at three weeks. Then the risk falls off." But some level of risk remains for a year, the researchers say.
The study is published in BMJ Online First.
Blood Clot Risk After Surgery: Study Details
Green's team drew on questionnaire data from the Million Women Study, which recruited 1.3 million women, average age 56, through the National Health Service to quiz them about lifestyle and other characteristics for health research.
For this study, they looked at hospital admission and death records for 947,454 participants, tracking each for an average of 6.2 years.
They tracked women who were diagnosed with venous thromboembolism, an umbrella term for both deep vein thrombosis (a clot in a vein deep within the body) and a pulmonary embolism (blockage of the main artery of the lung). The risk of blood clots is known to be higher after surgery, especially orthopedic surgery, but less is known about long-term risk and about patterns with different surgeries.
Green and her colleagues compared the risk of blood clots for those who didn't have surgery with those who did have surgery. Of the total, 239,614 women had surgery during the follow-up period; 5,419 of those were admitted to the hospital for blood clots, and 270 died of a blood clot.
The researchers excluded from the analysis women who had been in the hospital previously for blood clots or who had a history of blood clot problems, as well as those who had a cancer history or who had surgery the year before the follow-up started.
Blood Clot Risk After Surgery: Study Results
Compared to the risk of women who didn't have surgery, those who had surgery in the hospital were nearly 70 times more likely to be admitted with blood clots during the first six postoperative weeks. Women who had day surgery (including arthroscopy and minor procedures such as biopsies) were nearly 10 times as likely to have a clot as those who had no surgery.
Overall, one in 140 middle-aged women in the U.K. who have surgery in a hospital would be expected to be admitted with a blood clot during the 12 weeks after the surgery.
The numbers vary by surgery, Green found. ''The riskiest are the major operations -- hip replacement, knee replacement, and other major orthopedic operations, such as operations for hip fractures," Green tells WebMD. Her findings:
• 1 in 45 had clots after hip or knee replacement
• 1 in 85 after cancer surgery
• 1 in 815 after day surgery
• 1 in 6,200 women who did not have surgery
Lack of movement is mostly to blame, Green says. "Your legs are immobile, and you don’t have the normal mechanism where muscles are helping blood flow. Some anesthetics increase the risk even further by causing veins in the leg to dilate, so there is more pooling of blood."
Cancer itself may also trigger blood changes that boost blood clot risk, she says.
The study results, Green says, may inspire doctors and policy makers to extend the typical time that preventive measures such as anticoagulants and compression stockings are used. Those times vary, with some ending at hospital discharge and others continuing for up to four weeks, she writes.
Blood Clot Risk Post-Op: Other Opinions
The study is an important one, says James Muntz, MD, clinical professor of medicine at Baylor College of Medicine in Houston. Muntz, an internist, recently published a paper advocating extended-duration therapy to prevent clots after orthopedic surgery. He says he isn't sure the new findings will inspire an immediate change in guidelines for preventing clots. "I think it will at least trigger a change in people's awareness,'' he says. And prolonged preventive measures would be expected to be as good for men as for women, he says.
Another physician in practice, Charles F. Chandler, agrees. ''The study is important because it raises an index of suspicion that this phenomenon can occur after relatively minor procedures and for a long period of time."
Chandler is a general surgeon at Santa Monica-UCLA Medical Center and Orthopaedic Hospital and a clinical professor of surgery at the University of California Los Angeles David Geffen School of Medicine. One limitation of the study, also pointed out by the authors, is that no information was available on whether the women took preventive measures to avoid clots, he says.
Blood Clot Risks: What Can Patients Do?
Surgeons routinely caution their patients to be aware of potential symptoms of blood clots, Green says. Her advice to surgery patients in the wake of her new findings? "To repeat what they will have been told by their surgeon -- the message than an unexplained pain in the calf, or swelling, can be a sign, and not to dismiss it, even if it's several months after the surgery.''
Chandler tells his surgery patients: "Treat it like an injury" and give the body time to heal. ''We want patients to be proactive," he says. "Ask your surgeon: 'What is my risk of a blood clot?'"