These special filters protect against pulmonary embolism but also carry risks.
A common complication after surgery, trauma, or a prolonged period of bed rest is the formation of a blood clot in a vein deep inside the legs. Doctors call this a deep vein thrombosis, or DVT. If the clot breaks free of the vessel wall, it can travel through the bloodstream and lodge in a lung artery, causing a catastrophic stoppage of blood flow. This consequence, known as a pulmonary embolism, can be deadly.
An alternative to anti-clotting drugs
Blood thinners such as warfarin are widely used as a hedge against clot formation. However, some people can’t take these drugs because they could worsen conditions such as a hemorrhagic (bleeding) stroke or bleeding in the gastrointestinal tract, says Dr. Piotr Sobieszczyk, an interventional cardiologist at Harvard-affiliated Brigham and Women’s Hospital. “In the old days, one of the treatments for these people would be to clamp or staple the inferior vena cava, the main vein running through the abdomen that carries blood from the lower body back to the heart. If large clots from the legs did break up, they wouldn’t be able get past that point and reach the lungs. Clearly, it was not a great idea, but that’s what they had available,” explains Dr. Sobieszczyk.
In the 1960s, development of a cage-like device called the vena cava filter brought a more elegant solution. It’s like an umbrella without the cloth—just the ribs—that is placed in the inferior vena cava (see figure). All the blood from the lower body passes through the inferior vena cava, so the filter can catch big clots without interrupting blood flow.
The first of these devices had only limited use. “A vena cava filter is not a treatment for DVT,” cautions Dr. Sobieszczyk. “It does not treat the clot and works only as a plan B to prevent it from going to the lungs. Also, the early filters were hard to place because they required a very large catheter to be inserted into the vein.”
The number of vena cava filter placements began to grow sharply in the early 2000s with the advent of sleeker designs that were more easily inserted and features that enabled the device to be retrieved rather than left in place permanently. Increasingly, doctors recognized that some people required the temporary protection of a filter for a few weeks or months, and could then safely return to blood-thinning medication. Similarly, for people undergoing brain surgery or another procedure where excess bleeding would be especially dangerous, the filter could be deployed for the period immediately before and after the operation and removed later.
Placing a vena cava filter
Using x-ray imaging, a doctor inserts a catheter through the skin into a large vein in the neck or leg and threads it into the inferior vena cava, most often below the kidneys and above the common iliac veins. The filter is then passed through the catheter and released into the vena cava where it attaches to the vessel walls. Many filters have a small hook at the tip so that it can be snared with a catheter and withdrawn from the body when it is no longer needed.
Illustration: Scott Leighton
But as the device has been more widely adopted, a growing number of safety complications have surfaced. Over time, the filters can fracture or migrate, or they can become blocked by blood clots. “We began noticing that even when the filters are placed with the intention of temporary use, the vast majority of these patients do not come back and do not have these filters retrieved,” says Dr. Sobieszczyk. The situation prompted the FDA to issue a safety communication in 2010, with a reminder in 2014, that filters inserted for temporary protection should be removed as soon as the danger of pulmonary embolism subsides.
Retrieving a vena cava filter is not difficult for someone familiar with the procedure, although the process becomes trickier the longer the device is in place, says Dr. Sobieszczyk. The optimal window for removing a filter is usually several months. “The biggest obstacle is lack of follow-up,” he says. To that end, many hospitals have instituted automated reminder systems that prompt physicians to bring people back in for follow-up and retrieval. Ongoing technical advances are also making the devices easier to retrieve and less likely to cause complications. “Every year we have a new design or improvement to these filters,” he says.