MIT-Developed Device Helps Patients with Blocked Arteries
Just a few weeks ago, Bernard Davis started experiencing chest pains when he played tennis or golf. Soon thereafter, his pain would bother him even during periods of inactivity.
This prompted Davis to seek medical attention, and an angiogram, which is a picture of a person’s blood vessels, revealed that atherosclerotic plaque had severely blocked one of his coronary arteries.
Dr. Dean Kereiakes, Davis’ cardiologist, determined that his patient qualified for an investigational study involving a new stent, the Multilink DUET, which was designed by Advanced Cardiovascular Systems, Inc. with considerable help from Dr. Elazer Edelman, a MIT professor and director of the Harvard-MIT Biomedical Engineering Center.
A stent is a prosthetic device made of surgical stainless steel and looks much like a spring. When expanded it is generally between three to five millimeters in diameter and is implanted permanently into obstructed blood vessels to increase luminal size.
The coronary stenting procedure, like the one that Kereiakes eventually performed on Davis, takes only about 25 minutes. It involves maneuvering a stent crimped on a deflated cylindrical balloon to the site of obstruction. Then, the balloon is inflated at high pressure to deploy the stent. As it expands, the stent pushes the obstructing plaque away from the interior and against the arterial wall. Finally, after the stent has been implanted inside the artery, the balloon is deflated and withdrawn.
Bernard Davis is the first person in the U.S. to have had the DUET implanted in him. This stent is expected to remain within him for the rest of his life, serving as a scaffold in holding open his diseased artery.
The DUET, according to Kereiakes, is technologically superior to its predecessors. It is skinnier and sleeker, allowing it to pass through arteries more easily. Furthermore, its greater flexibility enables it to travel more efficiently around the bends of tortuous blood vessels.
The DUET is also stronger than the stents with which Kereiakes has previously worked. Strength is important because when the artery has been expanded with such high pressure, it tends to recoil or snap back with a great deal of force; this force of recoil has often deformed weak stents. “The DUET, however, is one of the strongest stents around and holds up well against arterial recoil,” says Kereiakes.
Another advantage of the DUET is that it can be seen more easily on the fluoroscope -- an instrument that lets the cardiologist monitor the stent’s progression as he maneuvers it through his patient’s vascular system. “A more visible stent allows you to better fine-tune its position before inflating the balloon,” explains Kereiakes. “This is important because once the stent has been deployed it cannot be repositioned.”
Despite its merits, the DUET is not perfect. Its stainless steel composition initiates some thrombogenic activity within the patient. “However, stent thrombosis can be circumvented if patients receive aspirin and ticlopidine following stent deployment,” says Kereiakes.
The thrombogenic nature of stainless steel stents has prompted the search for a more suitable material. Currently, studies are underway with stents made of tantalum, nitinol, and biodegradable polymers like polylactic acid.
Although coronary stenting has helped many patients like Bernard Davis, it cannot be performed on everyone with blocked arteries.
“Patients with vessels too small to accommodate a stent undergo balloon angioplasty [PTCA] rather than stenting,” says Kereiakes. In addition, doctors generally choose to perform PTCA on patients exhibiting high risk factors for stent thrombosis.
PTCA is similar to stenting except that no stent is used. A balloon is steered to the site of occlusion and inflated at high pressure to force open the blockage. After the luminal diameter has been increased and sufficient blood is able to flow, the balloon is deflated and withdrawn.
According to experts, the long-term success of PTCA and stenting is limited by restenosis -- a problem in which the artery becomes reoccluded at the site of the interventional procedure.
Restenosis is caused in part by vessel recoil. In addition, the inflammatory response evoked by vessel injury during PTCA or stenting is suspected to cause vascular smooth muscle cells to proliferate and protrude into the lumen, thereby further decreasing luminal size.
Although stenting is better able to resist vessel recoil, it may, in fact, cause greater smooth muscle cell proliferation than PTCA. However, the larger luminal size achieved by stenting initially is often more than sufficient to offset restenosis by cell proliferation.
“A metal stent tends to hold the artery open better than balloon [angioplasty] alone,” contends Kereiakes. “In general, a stent reduces the likelihood of a recurrent blockage within six to nine months by 30 to 50 percent.”
Despite their respective merits and flaws, coronary stenting and balloon angioplasty represent a more attractive alternative to traditional heart bypass surgery. These two procedures are not only much less expensive than surgery but they are also much less invasive. Consequently, they allow patients to return from hospital and resume their lives more quickly.
Kereiakes adds, “Patients we previously might have referred to bypass surgery, we might now be able to help [with coronary stenting]. These new smaller and sleeker stents can now go places where they couldn’t before. Therefore, we can get at blockages which we might not have been able to get at previously.”
In addition to unclogging Davis’ diseased artery with coronary stenting, Kereiakes has convinced his patient to eliminate those habits that can lead to more blockages. Today, Davis has made numerous changes in his lifestyle, including abstaining from smoking, exercising more frequently, and consuming a diet low in fat and cholesterol. In addition, he takes medications that help lower his blood pressure and blood cholesterol level.