Eye on a ticking bomb
Left untreated, an abdominal aneurysm can be fatal. This weak spot in the main artery leading from the heart can rupture, causing a person to bleed to death internally within minutes.
To prevent it from bursting, physicians usually place a stent inside the bulging artery to relieve pressure on its walls. Because about 5% of these repairs eventually fail, periodic checks for slippage or leaks with a CT scan are crucial.
Unfortunately, CT scans have serious limitations, and they can miss potentially lethal problems. Some researchers hope that an experimental wireless pressure sensor may be better at spotting blood pressure changes and leaks, greatly simplifying how these patients are monitored.
“This technology is revolutionary,” says Dr. Barry T. Katzen, an interventional radiologist at Baptist Cardiac & Vascular Institute in Miami who has implanted these devices in several patients. “This is the first time we’ve been able to put something in a patient that can send information without needing batteries or an external power source.”
About 2 million Americans have abdominal aortic aneurysms, which are most common in men older than 60 who smoke, suffer from cardiovascular disease or have a family history of the condition. These aneurysms are commonly caused by arteriosclerosis, or hardening of the arteries, which can weaken the aortic wall.
Tests for aneurysms, which are detected with an ultrasound, aren’t part of routine checkups. Doctors usually discover them accidentally when they’re doing tests for other ailments, and about 90% remain undetected. Still, more than 200,000 Americans are diagnosed every year with the condition, and 15,000 die.
To prevent catastrophic tears, physicians usually treat the aneurysms with stent grafts made of flexible fabric and metal tubes. These internal braces are inserted through small incisions in the patient’s groin and placed snugly inside the bulging artery.
Patients must then have their stents checked at least once a year. But the CT scans used to monitor the stents are expensive, and they sometimes fail to detect small tears that allow blood to escape around the stent. These leaks can lead to a sudden catastrophic failure of the stent. Furthermore, the scans can be done only once every six months because they use radiation, as well as contrast dyes that are toxic to the kidneys.
The experimental sensor, in comparison, monitors the graft continuously and can be checked any time. Because the device doesn’t require a power source, it can be implanted permanently inside the body during the stent procedure.
“Once the stent graft is put in, you really can’t go back in there, so we needed something that was extremely reliable and sturdy,” says Dr. Jay S. Yadav, a vascular surgeon at the Cleveland Clinic Foundation in Ohio who helped devise the sensor.
The thin oval-shaped device, which is about the size of a quarter, contains microchips that sense changes in pressure, and a coil that works like a tiny radio antenna. When doctors want to glean information from the sensor during office visits, they use an electric wand that transmits a signal to the sensor.
“The coil inside the sensor picks up energy in the same way an antenna from a radio would pick up signals from a radio station,” says Mark Allen, an electrical engineer at the Georgia Institute of Technology in Atlanta who helped develop the device.
When the sensor bounces the signal back, the wand uses changes in the electrical signals to determine how much pressure the sensor is under. The sensor can detect very subtle changes -- equivalent to a 1-millimeter shift of the mercury in blood pressure readings -- and if pressure climbs, doctors know something is wrong.
“Because it’s inside the body, the sensor gives us information we never had before,” says Yadav. “This is a life-threatening condition, and the sooner we get information, the better we can treat people.”
The experimental device has been successfully implanted in 50 patients in the U.S. and Brazil, and preliminary data indicate that it is safe, reliable and accurate.
A larger test that will study 100 to 200 patients at several sites around the U.S. is scheduled to begin in early 2005. If all goes well, the sensor could be on the market within the next year.
“We still don’t know if this works better than CT scans,” Katzen says. “But it may be a more sensitive and more effective surveillance mechanism than what we have now.”
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Pressure sensors may soon be used to monitor other medical conditions.
CardioMEMS, the Atlanta-based company that makes the aneurysm sensor, has developed a similar experimental device to monitor pressure changes in the heart for people with congestive heart failure, a condition in which fluids build up inside the heart.
Other companies, among them ISSYS Inc. of Ann Arbor, Mich., are investigating the use of miniature sensors in glaucoma sufferers to detect increased pressure in the eye, which damages the optic nerve and causes blindness. The sensors could also eventually be used in hydrocephalus patients to alert doctors if there’s a dangerous buildup of fluids in the brain.