Sitting for Long Time Linked to Pulmonary Embolism

July 5, 2011 -- Maybe the couch should come with a warning label.

A flurry of recent research has shown that excessive sitting increases the odds of obesity, heart disease, diabetes, some kinds of cancer, and even premature death. Now a new study is adding another health risk to that list: pulmonary embolism (PE), or a blood clot in the lungs.

The study, of nearly 70,000 female nurses who were followed for 18 years, found that those who spent hours sitting and sedentary in their leisure time were much more likely to have blood clots in their lungs compared to those who were more active.

And the risk remained even after researchers adjusted their data to take into account other factors including body weight, heart disease, smoking, and the use of medications like blood thinners and nonsteroidal anti-inflammatory drugs (NSAIDs).

"Women who sat the most had more than twice the risk of PE compared to women who sat the least," says study researcher Christopher Kabrhel, MD, an attending physician and assistant professor of surgery in the department of emergency medicine at Massachusetts General Hospital, in Boston.

Most of the time, these blood clots, which can cause shortness of breath pain with breathing and increased heart rate, can be treated, but they can also be fatal. Researchers say that makes them a problem to take seriously.

"It's the third most common cause of cardiovascular death in the country. It's about as common as strokes," says Kabrhel.

Though this study wasn't able to definitively prove women in the study got blood clots because they sat so much, experts point out that sluggish blood flow, especially in the legs, is known to contribute to the problem. A piece of a clot in a deep vein, often in a leg, breaks off and travels to the lungs, causing a pulmonary embolism.

"Vein blood clots do occur in people who are not very mobile," says James D. Douketis, MD, an associate professor in the school of medicine at McMaster University in Hamilton, Ontario, Canada.

"The extreme example is someone who's had a stroke and is paralyzed. The risk of having a blood clot there is one in two," says Douketis, who co-authored an editorial that accompanied the study but was not involved in the research. 

In contrast, excessive sitting -- in this case, for nearly six hours each day on average -- increased the annual risk of having a pulmonary embolism from one or two out of every 1,000 adults to one or two in 500 adults.

Douketis points out that the risk from sitting appears to be slightly higher than the risk of blood clots associated with taking birth control pills and about half the risk of getting blood clots that a woman faces during pregnancy.

The study is published in BMJ.

"It's a nice study," says Jack E. Ansell, MD, chairman of medicine at Lenox Hill Hospital in New York City and an expert on thrombosis, or the formation of blood clots.

"There are always limitations to a retrospective review of this nature, but the findings look fairly definite," Ansell tells WebMD. "It provides further, additional evidence that inactivity can be harmful."