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Intravascular
Ultrasound (IVUS) Imaging Technology May Help Lower Rates
of Late Stent Thrombosis |
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November 27, 2006 -- Imagine you're building a house. The hole for a window has been cut out and now the expensive state-of-the-art energy-saving multipane insulated glass window is placed in the cut-out...and it doesn't quite fill the space. There's more than an inch of air around the frame. Ultimately, the contractor may make this work out in any number of ways, but none will make up for the initial error and you'll always be concerned that the seal or filler might fail. A not dissimilar process happens when an interventional cardiologist places a stent. Using angiography, the physician images the blocked area of the coronary artery and measures it on the screen. The image is an x-ray, a shadow image, but it's close enough to pick the correct diameter stent; perhaps this artery will take a 2.5mm, maybe a 2.75mm. The cardiologist threads the stent (on a balloon) to the blockage and carefully expands the stent. But the cardiologist must be careful not to overexpand it, which could result in a tear, or dissection, of the arterial wall. Given these circumstances, it is not hard to imagine that a number of stents may not be expanded as fully as they might be. Unfortunately, it is sometimes difficult to determine whether a stent is fully expanded when viewing it via standard coronary angiography. This is not a new concern. When stents were first introduced in the mid-90's, Antonio Colombo in Milan showed that the some of the suboptimal results being seen were to a large extent due to under-expansion of the devices in the arteries. He demonstrated his findings using intravascular ultrasound or IVUS (see a video clip of IVUS pioneer Dr. Paul Yock explaining this). IVUS utilizes a tiny ultrasound transducer on the tip of a catheter which can be advanced into the coronary artery -- so IVUS is able to see the artery from the inside-out. The IVUS image can very precisely show if there is a gap between the stent and the arterial wall, called mal-apposition, literally "bad positioning". Fast forward to 2006 and the concerns about increased risk of late stent thrombosis in drug-eluting stents. While there are a number of causes that have been identified, one of the factors most associated with late stent thrombosis is mal-apposition of the drug-eluting stent. The small space between the arterial wall and a stent that is not fully expanded against that wall is a prime area for blood clotting to take place, especially with the current generation of drug-eluting stents which several cardiologists have noted are "more thrombogenic" or likely to clot. A recent presentation by Dr. William Wijns of Aalst, Belgium at last month's TCT meeting showed that in 77% of the late stent thrombosis cases (clotting one year after implantation) the stent showed incomplete apposition (mal-apposition was seen in only 12% of cases where there was no stent thrombosis). Other presentations showed similar observations. IVUS systems are currently manufactured by only two companies, Volcano Corporation and Boston Scientific. According to reports, the market is split almost 50-50, with Volcano gaining share (the company currently partners with GE Medical and is in discussions with the other major cath lab manufacturers, Philips, Siemens and Toshiba). Being able to interpret the images takes special training and there are, of course, costs involved. But using this imaging technology may reduce the incidence of late stent thrombosis by helping the cardiologist achieve a "snugger fit". Some cardiologists IVUS every stent patient to be sure the stent is placed optimally: more cost up front, but they feel less complications (and costs) later on. However, the IVUS technology is certainly under-utilized. In a recent interview with Julie Steenhuysen of Reuters, Volcano CEO Scott Huennekens stated:
Another solution to this problem is offered by a Vancouver-based company, Medical Ventures (a.k.a. Angiometrx). It is a balloon catheter (named MetriCath) that is inserted in the artery and inflated gently. The catheter then transmits precise measurements to a small box on the cath lab table, allowing the cardiologist to gain significant information about the diameter of the artery, both pre and post-stenting. While it does not have the wide-ranging imaging capabilities of an IVUS system, it may also be helpful in gaining more precise fits of stent-to-artery, and at a much lower cost. The modern drug coated stent is a state-of-the-art device that has been shown to significantly reduce restenosis of coronary arteries. But, like the state-of-the-art energy-saving insulated glass window, if it is not fit precisely and maintained correctly, it may not function as intended.
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