| Coronary stent | |
|---|---|
| Intervention | |
An example of a coronary stent. This Taxus stent is labeled as a drug-eluting stent. |
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| ICD-9-CM | 36.06 |
A coronary stent is a tube placed in the coronary arteries that supply the heart, to keep the arteries open in the treatment of coronary heart disease. It is used in a procedure called percutaneous coronary intervention (PCI). Stents reduce chest pain and have been shown to improve survivability in the event of an acute myocardial infarction.[1]
Similar stents and procedures are used in non-coronary vessels e.g. in the legs in peripheral artery disease.
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The first type, developed by John Robert Dugan of Shelbyville, IN, were bare-metal stents. More recent are drug-eluting stents.
In development are stents with biocompatible surface coatings which do not elute drugs, and also absorbable stents (metal or polymer).
Treating a blocked ("stenosed") coronary artery with a stent follows the same steps as other angioplasty procedures with a few important differences. The interventional cardiologist uses angiography to assess the location and estimate the size of the blockage ("lesion") by injecting a contrast medium through the guide catheter and viewing the flow of blood through the downstream coronary arteries. Intravascular ultrasound (IVUS) may be used to assess the lesion's thickness and hardness ("calcification"). The cardiologist uses this information to decide whether to treat the lesion with a stent, and if so, what kind and size. Drug eluting stents are most often sold as a unit, with the stent in its collapsed form attached onto the outside of a balloon catheter. Outside the US, physicians may perform "direct stenting" where the stent is threaded through the lesion and expanded. Common practice in the US is to predilate the blockage before delivering the stent. Predilation is accomplished by threading the lesion with an ordinary balloon catheter and expanding it to the vessel's original diameter. The physician withdraws this catheter and threads the stent on its balloon catheter through the lesion. The physician expands the balloon which deforms the metal stent to its expanded size. The cardiologist may "customize" the fit of the stent to match the blood vessel's shape, using IVUS to guide the work.[2] It is critically important that the framework of the stent be in direct contact with the walls of the vessel to minimize potential complications such as blood clot formation. Very long lesions may require more than one stent—this result of this treatment is sometimes referred to as a "full metal jacket".[3]
The procedure itself is performed in a catheterization clinic ("cath lab"). Barring complications, patients undergoing catheterizations are kept at least overnight for observation.[4]
Dealing with lesions near branches in the coronary arteries presents additional challenges and requires additional techniques.[5]
Coronary artery stents, typically a metal framework, can be placed inside the artery to help keep it open. However, as the stent is a foreign object (not native to the body), it incites an immune response. This may cause scar tissue (cell proliferation) to rapidly grow over the stent. In addition, there is a strong tendency for clots to form at the site where the stent damages the arterial wall. Since platelets are involved in the clotting process, patients must take dual antiplatelet therapy afterwards, usually clopidogrel and aspirin for one year and aspirin indefinitely.[6] In order to reduce the treatment, new generation of stent has been developed with biodegradable polymer.
However, the dual antiplatelet therapy may be insufficient to fully prevent clots that may result in stent thrombosis; these and the cell proliferation may cause the standard (“bare-metal”) stents to become blocked (restenosis). Drug-eluting stents were designed to lessen this problem; by releasing an antiproliferative drug (drugs typically used against cancer or as immunosuppressants), they can help avoid this in-stent restenosis (re-narrowing).
The value of stenting in rescuing someone having a heart attack (by immediately alleviating an obstruction) is clearly defined in multiple studies, but studies have failed to find reduction in hard endpoints for stents vs. medical therapy in stable angina patients (see below). The artery-opening stent can temporarily alleviate chest pain, but do not contribute to longevity. The "vast majority of heart attacks do not originate with obstructions that narrow arteries."[7]
A more permanent and successful way to prevent heart attacks in patients at high risk is to give up smoking, exercise, and take "drugs to get blood pressure under control, drive cholesterol levels down and prevent blood clotting".[7]
Some cardiologists believe that stents are over-used; however, in certain patient groups, such as the elderly, GRACE and other studies have found evidence of underuse. Guidelines recommend a stress test before implanting stents, but most patients do not receive a stress test.[8]
While revascularisation (by stenting or bypass surgery) is of clear benefit in reducing mortality and morbidity in patients with acute symptoms (acute coronary syndromes) including myocardial infarction, their benefit is less marked in stable patients. Clinical trials have failed to demonstrate that coronary stents improve survival over best medical treatment.
Several other clinical trials have been performed to examine the efficacy of coronary stenting and compare with other treatment options. A consensus of the medical community does not exist.
One of the drawbacks of vascular stents is the potential for restenosis via the development of a thick smooth muscle tissue inside the lumen, the so-called neointima. Development of a neointima is variable but can at times be so severe as to re-occlude the vessel lumen (restenosis), especially in the case of smaller diameter vessels, which often results in reintervention. Consequently, current research focuses on the reduction of neointima after stent placement. Considerable improvements have been made, including the use of more bio-compatible materials, anti-inflammatory drug-eluting stents, resorbable stents, and others. Restenosis can be treated with a reintervention using the same method.