A stent is a tube inserted into a body passage to open it and/or keep it open, allowing normal flow of contents. A vascular stent is one placed into a blood vessel anywhere in the body.
Vascular stents may be large or very small, for example 3 mm, and are usually expandable. Some stents are coated with slow-release drugs.
Why and where are they used?
Blood vessels (usually arteries) of any size are affected by various diseases, and may eventually become narrowed, or partly or completely blocked. This prevents normal blood flow, with oxygen starvation of the tissues whose blood supply has been blocked. The patient develops symptoms such as angina or intermittent claudication. If the blockage is not relieved, it may progress to complete occlusion, and cause a heart attack, stroke or gangrene of a limb. Opening the artery before this critical stage is reached is clearly beneficial.
Stents are most commonly used in the management of
- coronary artery disease
- carotid artery narrowing - may cause strokes
- renal (kidney) artery narrowing - associated with hypertension
- abdominal aortic aneurysms - often used for patients who are a high risk for anaesthesia and conventional surgery
- peripheral vascular disease - arteries of the legs may be narrowed
- haemodialysis patients - a stent may be inserted for use as a temporary access point
- diabetes - stents are now being used with success for the smaller arteries of the leg narrowed by diabetes
The vast majority of patients are thus those with severe atherosclerosis.
How are stents inserted?
Special theatres are equipped with the catheters and X-ray monitoring needed for stent insertion. Depending on the patient, light general anaesthesia may be used, or sedation plus local anaesthetic of the skin.
An area of skin over a blood vessel (often in the groin) is anaesthetised. A guide-wire and special catheter is inserted into the artery, and advanced until X-rays confirm that it is in the correct position - at the site of narrowing. This may be in
- a peripheral artery
- the abdominal aorta
- a coronary artery
- a carotid artery
- any other accessible artery causing problems
The special catheter has at its tip an elongated balloon, which is then inflated to force open the narrowed part of the artery. Once this is achieved, a small collapsible mesh tube is positioned at the same site, and opened up to support the artery walls and keep it open. This mesh stent may be made of bare metal, or may be coated in drugs which are released at the site, to try to slow down whatever disease has caused the narrowing in the first place. The catheter is then withdrawn, the access hole in the artery is sealed off, and the patient is usually sent home the next day.
Stents are placed permanently, and cannot be removed once inserted.
Vascular stenting is an invasive procedure, and mostly done in patients who have underlying cardiovascular disease, with all its problems and risks.
Anaesthetic problems may occur, especially in patients who smoke or who have heart failure or rhythm disturbances.
Perforation or dissection of the artery is possible - if this happens to a coronary artery, it is a surgical emergency, and the patient may need urgent heart surgery.
Ischaemia - lack of adequate oxygenated blood - may be prolonged during the procedure, and may lead to a heart attack or stroke.
Emboli - bits of atheroma may be dislodged when the stent is inserted. They are carried in the blood, and become lodged in an artery, completely blocking it, like a plug. Tissues normally dependent on that artery are thus deprived of their blood supply, and die. If this happens to heart muscle, it is called a heart attack, and in the brain, it is called a stroke.
Excessive bleeding at the puncture site used to insert the catheter may cause major blood loss, even causing the patient to need a transfusion. If much blood is trapped under the skin, surgery is needed to remove the huge clot.
Provided no complications occur during the procedure, the immediate outcome of stenting is very good. Pain is relieved, and full blood flow is re-established.
Long-term outcomes vary:
- Smaller vessels - especially heart arteries - tend to re-narrow or even close off entirely, sometimes within a few months
- Large vessels - such as the aorta - remain open, but have a higher risk of other complications, such as bleeding
- Stents coated with certain drugs have a greater risk of failure
- Narrowing can recur
- within the stent
- in the artery just before and after the stent, giving the "candy-wrapper" effect.
Patient selection - comments
Vascular stents have a role in the management of vascular disease, but care must be taken in the selection of patients with heart disease. Some patients have a better prognosis with surgery, where a bypass graft is more likely to stay open and healthy for many years. Unfortunately, stenting which is done indiscriminately can be to the patient’s disadvantage. If narrowing recurs, more and more stents are inserted until there is no place left in the artery for another one. If the patient still has pain, he is then referred to a surgeon. But if there is no place for a stent, there is also no place for a graft, so the patient may be inoperable. If this patient has had a bypass in the beginning, his problems would have been solved for many years.