The next major revolution in the management of blocked heart arteries, they do not contain metal and do not remain indefinitely in the body. By Michael Lim
THE last fortnight marked a major milestone as our team inserted “invisible” stents to open blocked heart arteries for my patients in the hospital. Having “invisible” stents which do not contain metal and do not remain indefinitely in the body is a dream come true for heart patients.
Over the past few decades, the management of blocked heart arteries has gone through three major milestone developments. In 1977, at the University Hospital in Zurich, Andreas Gruentzig was the first doctor to use small plastic tubings with expandable balloons (balloon catheters) that could be inserted into the human heart arteries to expand the narrowed segments, a procedure termed angioplasty. This major historical development resulted in a major shift, so that by the mid 1980s, over 300,000 heart angioplasties were being performed annually, equalling the number of open heart bypass surgeries being performed for blocked heart arteries. However, as the wall of the artery is elastic, there was a natural tendency for the wall of the narrowed segment to spring back or elastically recoil once the expanded balloon was deflated. If the balloon was expanded excessively in a hardened narrowed segment, there was a risk of causing a significant tear in the arterial wall.
Dawn of the heart artery stent
The next major milestone development focused on doing angioplasties successfully with minimal complications by the use of thin cylindrical metal meshes (stents) which are wrapped onto the balloon catheters and when expanded across the narrowed segment will overcome the problem of elastic recoil and tears in the arterial wall. In 1989, the first balloon-expandable heart artery stent, the Palmaz-Schatz stent, was made available. By the end of 10 years, nearly 85 per cent of all heart artery angioplasty procedures involved the use of stents. As the expansion of the stent will cause trauma to the arterial wall, the healing of the stented segment may sometimes result in the formation of “scar” tissue which will grow through the openings in the mesh and cause narrowing of the arterial lumen. In addition, the presence of a metallic scaffold within the heart artery also increased the risk of clot formation within the stent during the first year. These were the major setbacks of these bare metal stents. The incidence of “scar” tissue growing into the stent and causing narrowing of the heart arteries (in-stent restenosis or ISR ) was as high as 30 per cent and for stenting of small arteries in diabetics, the incidence was even higher.
This led to the quest for a solution for ISR. The focus was now on reducing recurrence of disease after initial procedural success. The addition of a drug containing polymer coating on the metallic surfaces of the stents (drug eluting stents or DES) marked another major milestone in the treatment of heart disease. Polymer drug coated stents were then developed to reduce the likelihood of “scar” tissue formation within the stents. These metallic stents were enclosed within a very thin layer of polymer onto which drugs were coated. These drugs are released gradually over a period of time to reduce the likelihood of “scar” tissue formation. In 2002, the first commercially available DES, Cypher stent, containing the drug sirolimus, was launched. The advent of DES meant that ISR was reduced to a few per cent as compared to the 30 per cent for bare metal stents. By 2004, DES was used in the large majority of heart artery angioplasties. While the use of DES reduced the incidence of restenosis significantly, the problem of clot formation within the stent remained.
A new revolution
Today, we stand on the threshold of the next major revolution in the management of blocked heart arteries. The desire to overcome the clot formation associated with DES led to the development of an entirely new stent made of polymers that will be resorbed by the body over time and obviate the need for long term blood thinning therapy. This week, Abbott announced the international launch of Absorb™ Bioresorbable Vascular Scaffold (BVS), a polymer stent that has a radial strength comparable to metallic stents, is able to open a blocked heart artery like a metallic stent but is mostly dissolved into the body after two years, eventually resulting in an artery which is able to return to its natural state and resume its normal functions, and is not constrained in its movement by a permanent metallic scaffold. The BVS is made of polylactide, a naturally biodegradable polymer which has been used in medical implants such as screws and sutures for more than 10 years. In addition, it is also coated with everolimus, a drug that reduces “scar” formation.
The use of a stent that completely dissolves in the body means that there is no need for long term blood thinning agents and this not only reduces the risk of bleeding complications but also results in substantial cost savings. The absence of a permanent metallic scaffold in heart arteries and the restoration of natural vessel function is important for the young and active as the heart artery can respond appropriately to demands of exercise. One of the most important benefits is the ability to visualise the stented heart artery with non-invasive imaging methods such as magnetic resonance imaging which does not require any injection of contrast and has no X-ray radiation. The current polymer stents have a limited range of sizes and will not completely replace DES yet, but as history has shown, in the years to come, it will be increasingly used in a large majority of angioplasty procedures. This will happen not only because of better designed stents but the high price it commands now will drift downwards and make this option more affordable as more competitors enter the market.
The absence of a permanent metallic scaffold in heart arteries and the restoration of natural vessel function is important for the young and active as the heart artery can respond appropriately to demands of exercise.