Blood clots pose a medical dichotomy: They save lives when they form on cuts or wounds, protecting underlying tissue and enabling healing; yet they threaten lives when they form in blood vessels, blocking necessary blood flow to vital organs. So while we rely on our blood to clot in circumstances of traumatic injury, we want to guard against the propensity for internal arterial or venous blood clots.

“There are three causes (of blood clots) that are classically given in the literature and ‘doctor books’ when we study this, and they are injury, immobility and hypercoagulability of the blood, meaning the blood is thick,” explains Dr. Ricardo Rao, founder and president of Midwest Vascular and General Surgery and medical director of the Missouri Baptist Medical Center Vascular Laboratory.

Blood that thickens and clots too easily may be related to various risk factors, including obesity, smoking tobacco products or inherited clotting disorders. However, Rao cites immobility as the most common cause. “People are most likely to get a blood clot when they are injured or have some sort of trauma like surgery, and then they’re immobile, lying around,” he says.

That scenario often describes post-operative hospital patients, who are among those at highest risk for blood clots. “Blood-thinning drugs are routinely used to prevent blood clots in patients who are at risk,” says Dr. Suresh Vedantham, a diagnostic and interventional radiologist with Washington University Physicians. “In patients who cannot receive blood-thinning drugs, leg compression devices are used. If their condition permits, patients who are or have recently been hospitalized should try to walk for at least a few minutes several times a day. These measures should be taken in at-risk patients before they develop a blood clot.”

Another common cause for lengthy immobility is travel. Long road trips and, particularly, long flights in which passengers sit in one position for many hours increase risk of developing blood clots deep in the veins of the leg, a condition known as deep vein thrombosis (DVT). These clots may break loose and travel to the lungs, causing a potentially life-threatening condition known as a pulmonary embolism.

“People shouldn’t be that concerned in general because these events are fairly rare,” Rao says. “However, if you’re going to be on a long plane flight, there are two factors that can contribute to blood clots. No. 1 is the pressurization of the airplane. That slows down the blood flow in general, especially in the venous system. No. 2 is you end up sitting with your legs not moving for long periods of time and that goes into the immobility issue.”

The solution to the latter concern is periodic movement. Getting up to walk the aisles occasionally and rotating the ankles can help move blood through the legs’ veins. If, despite precautions, you develop abrupt pain, swelling or cramping, especially in only one leg, seek medical attention as soon as possible.

“If you are diagnosed with a large blood clot, ask your doctor if he/she thinks you might benefit from targeted clot removal treatments delivered by an interventional radiologist,” Vedantham says. “With these treatments, a physician specialist injects a clot-busting drug into the blood clot through a catheter (a thin tube) that is placed in your leg with careful imaging guidance. This enables rapid elimination of the blood clot, which restores blood flow through the vein, with only a tiny skin incision. This may lead to faster relief of leg pain and swelling, and may also prevent long-term complications of the blood clot.”

Rao emphasizes that clot prevention is key. “With a little bit of knowledge and a little bit of forethought, the average person can help prevent most of this. And we in the medical community are also trying to contribute to (prevention) when you’re in the hospital, which is probably the most common place a blood clot occurs.”

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