A visualization of a blood clot.

Jonathan Halevy, MD, is a medicine chief resident at the University of Vermont Medical Center and plans to continue his medical training with a fellowship in cardiovascular disease at the University of Vermont Medical Center.

Jonathan Halevy, MD, is a fellow in cardiovascular medicine at the University of Vermont Medical Center.

Abnormal blood clots can form in your veins (blood vessels returning blood to the heart) or arteries (blood vessels taking blood away from the heart). In this article we will talk about clots in the veins, also called “venous thrombosis.”

About 1% of hospitalizations in the United States are for venous blood clots making them the third most common blood vessel disease (after heart attacks and strokes). These clots usually start in the leg veins and can then travel to the lungs. Every year in the United States there are up to 900,000 cases of lung clots (pulmonary embolism) and deep vein clots (deep vein thrombosis). As many as 300,000 people die each year from this. Up to eighty percent of these cases are associated with risk factors.

More than 100 years ago, German physician Rudolf Virchow described 3 factors contributing to blood clots: abnormality of blood coagulation (“hypercoagulability,” or increased tendency to clot), interrupted blood flow (“stasis,” or blood staying still), and blood vessel injury. To this day, these three mechanisms are considered the major risk factors for venous clots.


The human body has evolved complex systems to stop bleeding. Small molecules called coagulation factors and blood cells called platelets work together to clot off blood and keep us from bleeding out when we cut ourselves. Too much clotting, however, or clotting at the wrong place or the wrong time can be very harmful. This can happen because people are born with genetic conditions (Factor V Leiden, Protein C deficiency), or because they develop diseases such as cancer or kidney disease. Other factors that can make you clot more include being overweight, taking birth control pills, and taking estrogen replacement for menopause. We don’t understand every cause of hypercoagulability. For example, we often see families where several people have had blood clots, suggesting a genetic condition, but we can’t determine the genetic condition based on current medical knowledge. At the University of Vermont we are working on increasing scientific knowledge about this.


Just like running water in the winter that freezes when the water slows down, when blood flow slows down it can clot. Leg clots are more common in the left leg than the right leg because blood going back to the heart flows slower on that side due to compression of a vein in your thigh. Clots are also more common in pregnancy because the growing fetus in the uterus can push on the veins that connect the legs to the body. Slower blood flow in the body can also be caused by common situations such as sitting for a long time (like on an airplane or long car trip), being in a cast, and even minor injuries that limit your ability to move. Any time these situations occur, you are at higher risk for venous blood clots. Recent research at UVM showed that elderly patients who are immobile in their homes are at increased risk of venous clots. Marked immobility (defined as greater than 4 days of bedrest) significantly increases the risk of blood clots. For this reason, patients who are sick in the hospital are often given medications called blood thinners to decrease their risk of blood clots. At the University of Vermont we are studying how to best prevent clots in the hospital.

Blood vessel injury

Any time you injure your body you are at a higher risk of getting a blood clot, particularly with surgeries or major injuries. One study showed that more than half of patients in a trauma unit in the hospital had a leg blood clot. Even minor injuries not requiring surgery or bedrest can increase the risks of blood clots. Drug users who inject drugs into their veins also have high rates of vein clots due to irritation and trauma to veins from injection. There is also an association between having an intravenous line (IV), especially big ones, and blood clots in the arm. Our recent research showed that about half of all vein clots occurring as a complication of hospital stay on medical services at the UVM Medical Center were vein clots of the arm related to specialized IV lines.

How can I prevent venous blood clots?

If you think you are at higher risk of getting a blood clot, you can limit your risk with some of these few key steps:

  • If you are overweight or obese, work on healthy nutrition and lifestyle changes to help lose weight. Try to exercise regularly.
  • Avoid bedrest or prolonged immobility. For example, go for walks on your breaks at work if you’re sitting all day, or walk the aisles during a long plane flight.
  • Know if you have a family history of vein clots. If a close relative has had this, be sure your doctor knows about it.
  • If you are hospitalized, experience immobility related to illness or injury, or are planning to undergo surgery, ask your provider how they plan to prevent blood clots in your case.
  • Be aware of the symptoms of a possible blood clot, and what to do if you have these symptoms. Your doctor’s office can be a great resource for more information and guidance.

Jonathan Halevy, MD, is a fellow in cardiovascular medicine at the University of Vermont Medical Center.

Mary Cushman, M.D. MSc, is a hematologist at the University of Vermont Medical Center where she is Medical Director for the Thrombosis and Hemostasis Program. She is Professor of Medicine at the Larner College of Medicine at UVM.

For more information, please visit UVMHealth.org/CancerCenter or call (802) 847-0000. Click here to learn more about the Thrombosis and Hemostasis Program at the UVM Medical Center. 

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