Coronary stents (CS) are expandable tubular metallic devices which are introduced into the coronary arteries that demonstrate stenosis due to an underlying atherosclerosis disease. This revascularization procedure is termed as a percutaneous coronary intervention (PCI) or coronary angioplasty with stent placement. The coronary stent was first developed in the 1980s and has continued to evolve in terms of shape, structure, and the material used within them. In a pre-stent era, balloon angioplasty was the mainstay of coronary revascularization in which an inflatable balloon-tipped catheter was inserted percutaneously through arterial entry site in extremity and advanced into coronary arteries. On reaching the coronaries, the balloon was inflated to compress atherosclerotic plaque against the vascular wall and restore blood flow to the myocardium. The balloon was withdrawn after deflating. This procedure had major drawbacks such as acute vessel closure due to arterial recoil, coronary artery dissection, acute arterial thrombosis, and restenosis due to neointimal hyperplasia. With the introduction of coronary stents, coronary dissection and vascular recoil were eliminated due to the expandable, metallic meshwork of the stent which prevents negative remodeling.
Types of Coronary Stents
DES consists of three components: metallic stent platform, active pharmacological drug agent, and a carrier vehicle. Stainless steel or cobalt chromium is the most common metal and gives long-term mechanical stability to counteract vascular recoil. Commonly used drugs act to block signal transduction and cell cycle progression in different phases thereby blocking smooth muscle cell (SMC) proliferation or intimal hyperplasia in the stented arterial site. Rapamycin agents bind to the intracellular protein, FKBP-12, that inhibit the protein kinase mammalian target of Rapamycin (mTOR). This intracellular complex increases the expression of p27 and blocks progression of the cell cycle from the G1 phase to the S phase (DNA synthesis). Another drug category is taxanes which interfere with microtubule function which is necessary for M phase (mitosis). So cells get arrested in the G2 phase of the cell cycle. To increase surface area, a carrier vehicle matrix or a polymer coating is used to enable sufficient drug loading and release for a long time. Polymer coating consists of repeating units of biodegradable poly-L-lactide, poly–D, and L-lactic-co-glycolic acid in a regular pattern, which are degraded into lactic acid and glycolic acid that ultimately get converted into water and carbon dioxide. First generation DES has sirolimus or paclitaxel coating on a stainless steel base while second generation DES has zotarolimus or everolimus coating on top of biocompatible cobalt-chromium or platinum-chromium platform. The drug release is carried out by diffusion through pores in the polymeric coating.
While DEB has only antiproliferative drug coating without underlying metallic structure of the stent, BRS is also devoid of metallic structure and is resorbed completely in few months after serving their purpose. Other stents like bifurcation stents and covered stents are designed for special circumstances such as lesion over vascular bifurcation or coronary artery perforation respectively.
Atherosclerosis, which is characterized by the formation of luminal plaques, is the underlying cause of coronary artery disease. Risk factors for the development and progression of atherosclerosis have been studied and classified as modifiable or non-modifiable.
Modifiable Risk Factors
Non-modifying Risk Factors
CAD is the leading cause of morbidity and mortality worldwide and is responsible for approximately 20% of all deaths in the United States. CAD is due to underlying atherosclerosis which leads to plaque formation in the lumen of coronary arteries. The actual frequency of atherosclerosis cannot be precisely determined as it is largely asymptomatic until late stages. It starts early in childhood and develops fatty streaks over time. More advanced and complicated lesions are present in individuals in the fifth or sixth decade of life. In the United States, approximately 14 million individuals experience CAD and its complications including post-myocardial infarction heart failure (HF) being the most common one. Approximately 1.5 million Americans have an acute myocardial infarction (AMI) annually, of which one-third result in death.
According to 2009 epidemiology data, around 785,000 Americans suffered their first AMI, while 470,000 Americans had a recurrent MI. An estimated 195,000 "silent" heart attacks occur each year. About every 34 seconds, an American will have an AMI. In developed countries, the incidence of AMI is similar to that observed in the United States. The frequency of symptomatic CAD in ethnic immigrant populations in the United States approaches that of the white population which supports the role of environmental factors. In France, the Mediterranean diet with the use of alcohol and its possible HDL-raising benefit, and the predominant use of monosaturated fatty acids (canola oil or olive oil) and omega-3 fatty acids, leads to a lower risk of atherogenesis with the said factors. The reduced incidence of CAD and AMI is a phenomenon named as “French paradox.” The Spanish cohort of the European Prospective Investigation into Cancer and Nutrition suggests that the Mediterranean diet may help lower the risk of the CAD. The frequency of symptomatic CAD is also very high in Great Britain, Scotland, Scandinavia, Finland, and Russia. Symptomatic CAD due to atherosclerosis is rare on the African continent, although recent evidence points to a rapid rise in these statistics due to rapid urbanization and westernization of the rural African populations. The statistics of CAD vary significantly with race. As compared to whites, blacks have higher morbidity and mortality rates of CAD due to higher risk factors such as hypertension, obesity, metabolic syndromes and physical inactivity. Men have a higher prevalence of CAD than women before menopause, but after menopause the rates of CAD for both sexes are fairly similar.
The early pivotal event in atherogenesis is endothelial cell dysfunction (ED) due to the long-term presence of various cardiovascular risk factors, such as dyslipidemia, chronic uncontrolled hypertension causing shear stress, and uncontrolled diabetes mellitus, which cause non-enzymatic glycosylation of surface lipoproteins on endothelial cells. It is believed that ED causes endothelial nitric oxide synthase (ENOS) dysregulation which leads to decreased production of vasodilator nitric oxide (NO) and increased amount of superoxide which is oxidant. This imbalance between vasodilators (nitric oxide and histamine) and vasoconstrictive factors impairs vascular hemostasis which leads to increased turbulence, shear stress, and ED.
ED causes endothelial cell activation which leads to increased expression of adhesion molecules, activation of platelets and inflammatory cells, and increased permeability of endothelial cells; thus it promotes diffusion of lipids and transmigration of the adherent leukocytes in the presence of chemo-attractant cytokines secreted by inflammatory cells. Both innate and adaptive immunity are involved in atheroma formation. Innate immune cells, such as monocytes, and dendritic cells and adaptive immune cells, such as T lymphocytes, present antigen on their surface to dendritic cells in the interstitium and regional lymph nodes. Monocyte turns into macrophage when it reaches the intima. It engulfs the lipid and turns into lipid-laden foam cells. Activated macrophages, platelets, and dysfunctional endothelial cells produce growth factors which modulate early atherogenesis. These factors include transforming growth factors alpha and beta, thrombin, platelet-derived growth factor, insulin-like growth factor, and angiotensin II. The relative deficiency of endothelium-derived NO further potentiates proliferative stage of plaque maturation
The earliest morphological manifestation of atherosclerosis is fatty streaks which consist of lipid-laden macrophages, T lymphocytes, and smooth muscle cells (SMC). Fatty streaks go on to transform into fibrous plaque due to SMC activation. SMC secretes extracellular collagen matrix which forms the fibrous cap and stabilizes plaque. Due to ongoing inflammation, some macrophages die and releases lipids into extracellular space in the intima, resulting in a necrotic core. Matrix metalloproteinases (MMP) are protease enzymes which are secreted by SMC, and they are very crucial for plaque progression due to their role in extracellular matrix destruction. MMP is counteracted by tissue inhibitor of matrix metalloproteinase (TIMP) and statin drugs. The imbalance between MMP and TIMP can lead to the rupture of the fibrous cap due to excessive extracellular matrix destruction and subsequent activation of platelets and acute thrombosis which leads to a critical level of coronary occlusion. This compromises blood flow to the myocardium. It is manifested as angina to acute myocardial infarction, depending upon the level of arterial occlusion. The luminal obstruction must be between 50% to 70 % to cause flow limitation.
As the plaque progresses, two types of remodeling can occur: positive and negative.
It is the outward growth of plaque due to underlying compensatory arterial dilation. It does not compromise blood flow, but due to the overburden of unstable plaque, it is at risk for plaque rupture and thrombosis which results in acute myocardial infarction. It does not present as stable angina symptoms because luminal diameters remain the same until plaque ruptures suddenly and presents with acute coronary syndromes.
In this case, the plaque grows inwardly towards the vessel lumen thereby decreasing luminal diameter over time because there is no compensatory vascular dilation. As soon as the luminal obstruction is between 50% and 70%, blood flow to the myocardium is limited, and stable angina symptoms are more likely to develop. Plaque, in this case, is still at risk of acute rupture, which can result in myocardial infarction.
According to Stary et al., atherosclerosis is classified in Roman numeral numbers in the sequence of lesion progression.
Type I: (initial) contains scattered isolated lipid-laden macrophage; also called foam cells.
Type II: consists of foam cells mainly in the form of layers; also called fatty streaks.
Type III: is the intermediate stage lesion which contains foam cells in layers and scattered extracellular lipid droplets due to apoptosis of foam cells.
Type IV: consists of an extensive dense accumulation of extracellular lipid which is also termed as lipid core; also called atheroma.
Type Va: These lesions are also termed as fibro-atheroma, and it consists of the fibrous cap on top of atheroma. There is also smooth muscle cells present in an intimal layer which help in the deposition of the extracellular matrix of the fibrous cap.
Type VI : The disruption of the lesion surface leading to thrombosis or hemorrhage into lesion is a hallmark of this complicated lesion. These lesions are associated with high mortality and morbidity. Type VI may be subclassified by the superimposed features.
A major drawback of BMS is in-stent restenosis due to intimal layer injury. Neo-intimal hyperplasia (NIH) is the underlying phenomenon in which intimal injury leads to increase SMC migration and proliferation in the intimal layer thereby leading to restenosis. DES is designed to locally deliver antirestenotic drugs which act to block SMC migration and proliferation at stent site with no systemic adverse drug outcomes. The efficacy of the drug depends upon the diffusion rate, tissue accumulation, distribution, and local vascular toxicity. Thus, a balance should be achieved between adequate drug amount, delivery, and minimal local vascular toxicity. The rate of diffusion depends upon several factors, such as coating thickness, amount of drug loaded, a drug to polymer ratio, and partition coefficient (PC) of the drug. PC is directly proportional to the rate of diffusion.
Drug release has an initial fast phase due to the immediate dissolution of the drug from the outer most layer of the polymer coating. The fast-phase release has first-order kinetic, with the majority of the total available dose-released within the first few days. Later on, a sustained slower-release phase occurs, for the most part, due to slower diffusion-mediated release. The higher loading dose tends to have a greater release during the initial fast-release phase.
The hydrophobic, hydrophilic, or lipophilic nature of a drug also regulates the transport or distribution of the drug in arterial tissue. Hydrophilic drugs mix readily with blood and can distribute into and around local arterial tissue; therefore, there is lower local drug concentration. By contrast, hydrophobic drugs tend to distribute more towards the arterial tissue homogenously, leading to high drug accumulation in arterial tissue and slower clearance. Lipophilic drugs have a slower drug release rate.
Studies have shown that rapamycin and paclitaxel enhance the expression of prothrombotic factors such as tissue factor and plasminogen activator inhibitor-1. In the presence of local drug toxicity in the stent site leading to delayed re-endothelialization, the acidic byproducts (lactic acid and glycolic acid) of polymeric degradation and these prothrombotic factors can cause stent thrombosis due to platelet activation. It is more pronounced when antithrombotic drugs are withdrawn postoperatively. Hypersensitivity towards polymer coating has also been reported which also augments the stent site for chronic inflammation.
Coronary atherosclerosis is an underlying pathology in ischemic heart disease. Coronary atherosclerosis is predominantly an asymptomatic condition. It starts at an early age, such as in the 20s, as fatty streaks, but it is clinically silent. It only becomes symptomatic once 50% to 70% of the luminal diameter is obstructed due to growing fibrous plaque, leading to end-organ ischemia or infarction. Clinically, a chronic stenotic lesion (stable CAD) maifests with clinical symptoms of exertional angina while plaque rupture may results in symptoms of acute myocardial infarction.
Patients with symptomatic coronary artery disease may present with the following signs and symptoms:
The most common symptom associated with coronary artery disease is dull, squeezing, or pressure-like chest pain located in the sub-sternal or left side of the chest and radiating to the left arm, neck, or jaw. It may be associated with dyspnea and increased sweating. In stable angina, it is exertional in nature and relieved on resting for a while or ingestion of nitrates, while pain associated with unstable or AMI (STEMI and NSTEMI) occurs at rest. The pain of AMI is partially relieved with nitrates.
The following signs can be noted in a patient with AMI.
On physical exam, tachycardia is common in a patient with AMI. While bradycardia can occur due to blockage of the right coronary artery branch which supplies sino-atrial node. Arrhythmias of different types also can be seen secondary to ischemia of myocardium. Acute ischemia or infarction causes hypoxia and depletion of intracellular adenosine triphosphate which shifts the cell to anaerobic glycolysis and lactic acid is accumulated inside the cell. This intracellular acidic environment lowers the pH and activates Na+/H+ and Na+/Ca++ ion exchange channels to dump H+ ion out of the cell and raise intracellular PH. This is accompanied by intracellular calcium overload and increased late sodium current. A lower resting membrane potential is generated in this manner which turns infarcted tissues more excitable and leads to arrhythmias. There are many types of ventricular tachyarrhythmias (VA) present after AMI such as non-sustained ventricular tachycardia (NSVT), sustained ventricular tachycardia (VT), or ventricular fibrillation (VFIB). VA is the most common cause of sudden cardiac death, occurring within 1 hour of AMI. Thrombolysis and percutaneous coronary intervention have reduced the incidence of VA and reduced the burden of SCD in over the past decades. The incidence of VA is directly proportional to the size of an infarct and inversely related to the left ventricular ejection fraction after AMI.
In this section, we define and discuss different types of coronary stents in details used for the treatment of symptomatic CAD.
Grüntzig developed the concept of percutaneous treatment of obstructive coronary lesions by using balloon dilatation in 1977. This was a revolutionary approach, as it avoided open thoracotomy and also became a preferred therapeutic measure in the setting of acute emergencies such as myocardial infarction. The long-term results of balloon angioplasty were however limited by the risk of acute vessel recoil, restenosis and also abrupt vessel closure during early post-intervention period. To overcome the limitations of the balloon angioplasty, Puel and Sigwart developed bare metal stents (BMS) in 1986. BMS reduced the risk of abrupt vessel closure resulting from local dissections. In addition, BMS also eliminated vascular wall elastic recoil and constrictive remodelling. Further optimisation of stent implantation techniques along with use of dual antiplatelet therapy yielded better results supporting the widespread use of BMS. By late 1990's, BMS use approached almost 80% in all PCIs. Stent implantation, however, causes an arterial injury which leads to vascular smooth musclecell activation and proliferation, resulting in neointimal hyperplasia which if excessive can lead to in-stent restenosis.
Long-term follow-up of BMS patients showed that about 30% patients experienced neointimal hyperplasia and in-stent restenosis. This appeared to be a major limitation of BMS. New strategies were thus proposed to address this issue which included systemic administration of anti-inflammatory agents, antiproliferative agents and stent surface coatings. This ultimately led to the development of drug-eluting stents (DES).
In early 2000s, DES rapidly became the favored choice due to their improved antirestenotic efficacy. DES have three components: a. a metal stent backbone; b. an antiproliferative agent; and c. drug carrier (often a polymer coating). Although these devices offer similar skeletal framework as BMS, they yield additional site-specific release of antiproliferative agents that suppress neointimal hyperplasia and stenosis. Although delayed restenosis is prevented by DES, early stent thrombosis is a more prevalent adverse event observed with DES due to a delay in the healing of the stented arterial segment/endothelialization with DES. This risk is mitigated by the use of dual antiplatelet therapy (DAPT) with aspirinand a thienopyridine in the early period after stent implantation. Evidence from systematic reviews and meta-analyses demonstrates a very small risk of delayed stent thrombosis in the early-generation CYPHER sirolimus eluting stents and TAXUS paclitaxel-eluting stents beyond one year of stent implantation. New-generation DES have thus emerged to counter-act this limitation which are built using the novel metallic alloys (such as cobalt-chromium and platinum-chromium) allowing thinner strut stent platforms and use of new drug carriers offering improved antirestenotic properties.
Early-generation DES used a stainless steel platform with strut thickness of 130-150 μm while new-generation DES now usually have stent struts measuring 50-90 μm. Thinner stent struts (<100 μm) however improve haemodynamic flow and drug distribution, and also cause less vessel injury at implantation site leading to a lesser chance of restenosis in comparison with stents with thicker struts. Thinner struts also have a lower degree of thrombogenicity. Newer-generation DES also have a more biocompatible durable polymer coatings such as the vinylidene-fluoride hexafluoropropylene copolymer or biodegradable polymer coatings, composed of lactic or glycolic acids that fully resorb by hydrolysis after completion of drug release. Some new DES use a technology which is known as 'polymer-free coating', which release antiproliferative agents directly from the stent surface without use of a polymer coating. Older DES used paclitaxel and sirolimus drug platforms. Paclitaxel interferes with microtubule dynamics during mitosis by binding to the microtubular β-tubulin subunit. Sirolimus, however, blocks protein synthesis, cell cycle progression, and cell migration by inhibiting the target of mammlian rapamycin. The antirestenotic efficacy of sirolimus is higher than the paclitaxel. Thus, new-generation DES use sirolimus or its analogues such as everolimus, zotarolimus, biolimus, and novolimus, which offer better therapeutic profile for the above-mentioned reason. XIENCE/Promus polymer-based everolimus drug eluting stent (DP-EES) has perhaps the largest body of evidence from trials.
Chest pain is the most common symptom of symptomatic coronary artery disease. Others causes of chest pain should also be ruled out while making the correct diagnosis.
Two pivotal randomised controlled trials named the Belgium Netherlands Stent Arterial Revascularization Therapies Study (BENESTENT) and the North American Stent RestenosisStudy (STRESS) – showed a significant decrease in the incidence of restenosis and repeat revascularisation procedures with use of BMS when compared to balloon angioplasty in patients with stable CAD.
CYPHER® sirolimus-eluting stents (now Cardinal Health, Milpitas, CA, USA) demonstrated a reduced risk of restenosis as compared to BMS in the RAVEL trial. TAXUS™ paclitaxel-eluting stents (Boston Scientific, Marlborough, MA) demonstrated a reduced risk of restenosis as compared to BMS in the TAXUS-IV trial. A collaborative network meta-analysis which included 38 randomised trials with over 18,000 patients showed an overwhelming benefit of DES (CYPHER and TAXUS stents) when compared to BMS in terms of the risk of target lesion revascularisation. A comprehensive pairwise meta-analysis including 22 randomised trials and 34 observational studies with at least one year of follow-up showed similar findings and demonstrated a significant risk reduction for target vessel revascularisation (HR 0.45, 95% CI: 0.37-0.54 in randomized studies; and HR 0.54, 95% CI: 0.48-0.61 in observational studies).
Systematic review by the Task Force of the European Society of Cardiology (ESC) on coronary stent evaluation and the European Association for Percutaneous Cardiovascular Interventions (EAPCI) analyzed a total of 158 randomised trials. It showed that the early-generation DES were associated with a lower risk of target lesion revascularisationwhen compared with BMS, while new-generation DES provided a further risk reduction when compared to early-generation DES (median rates per 100 person-years at 12 months: BMS 12.3%, early-generation DES 4.3%, new-generation DES 2.9%).
NORSTENT trial directly compared DES with BMS in 9,013 patients. DES were associated with a lower risk of stent restenosis when compared to BMS (0.8% vs. 1.2%; HR 0.64, 95% CI: 0.41-1.00, p=0.0498) at six-year follow-up. It did not howver show any significant difference between DES (mostly new-generation) and BMS for the composite primary endpoint of death and myocardial infarction on a six year follow-up (16.6% vs. 17.1%; HR 0.98, 95% CI: 0.88-1.09, p=0.66).
EXCEL trial demonstrated a non-inferiority of PCI with new-generation drug eluting stents DP-EES as compared to CABG in patients with low-to-moderate anatomical complexity of CAD (that is SYNTAX score <33) with respect to the major adverse cardiovascular events with composite of death, stroke, and MI at three-year follow-up. NOBLE trial, however failed to show non-inferiority between PCI with biodegradable polymer-based biolimus-eluting stents and CABG in patients with left main disease, irrespective of anatomical complexity of CAD. It showed that the composite of death, MI, stroke, and repeat revascularisation, were lower in CABG at five years of follow-up.
CAD remains the number one cause of death for men and women worldwide. In the United States, around 1.5 million Americans suffer from AMI annually, and one-third of these die. Survivors of AMI have a poor prognosis with a 1.5 to 15 times greater risk of mortality and morbidity than the rest of the population. Historical figures among survivors of AMI show that 25% of the men and 38% of women die within 1 year after having an AMI, 18% of men and 34% of women have a second MI within 6 years of AMI, and 22% of men and 46% of women are diagnosed with CHF due to ischemic cardiomyopathy.
The prognosis depends on the following factors:
Coronary artery disease has following complications.
Consultation with the following personnel is mandatory to manage CAD effectively.
Ischemic heart disease (IHD) is the most common cause of death globally. The most cost-effective and clinically significant way of reducing the burden of IHD is primary prevention. Education of the general population regarding a healthy lifestyle, dietary habits, and regular exercise has been shown to reduce the prevalence of IHD. A recent study suggests that education based interventions may improve health-related quality of life. Patients with IHD should be counseled on compliance with medications and lifestyle modifications to alter their risk factors' impact on cardiac health. The patient also should be encouraged to avoid cigarette smoking, eat a low-fat diet that is rich in fruits and vegetables, and exercise regularly. The patient should be counseled to avoid over-exertion. They can resume sexual activity 4 weeks after an AMI event but should not have angina symptoms during sexual activity. Patients should be counseled to join support groups. According to a recent study, women are more likely to benefit from women-only groups while men may prefer to have a mixed-gender group.
Even though there are several treatments for people with ischemic heart disease, the primary care provider and nurse practitioner should emphasize prevention. The most cost-effective and clinically significant way of reducing the burden of IHD is primary prevention. Education of the general population regarding a healthy lifestyle, dietary habits, and regular exercise has been shown to reduce the prevalence of IHD. A recent study suggests that education-based interventions may improve health-related quality of life. Patients with IHD should be counseled by the clinicians and nurses on compliance with medications and lifestyle modifications to alter their risk factors' impact on cardiac health. The patient also should be encouraged to avoid cigarette smoking, eat a low-fat diet that is rich in fruits and vegetables, and exercise regularly. The patient should be counseled to avoid over-exertion. They can resume sexual activity 4 weeks after an AMI event but should not have angina symptoms during sexual activity. Patients should be counseled to join support groups. According to a recent study, women are more likely to benefit from women-only groups while men may prefer to have a mixed-gender group.
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