Complications of Coronary Intervention

Drug-eluting stents (DES) substantially reduce restenosis compared with bare metal stents and represent a significant advance in percutaneous coronary interventions (PCIs). Accordingly, DES have been rapidly adopted into practice and are currently used in the vast majority of PCI procedures. As PCIs for more complicated lesions increase, various complications, such as stent thrombosis, fracture, dissection or perforation, are also increase. For example, PCIs for patients who have chronic total occlusion increase and these patients tend to have more risk factors like diabetes mellitus, hypertension, dyslipidemia, and previous myocardial infarction and also have multi-vessel diseases and have decreased left ventricular ejection fraction. If major procedure-related complications were developed in these high risk patients, it may leads to fatal results. So it is important to understand possible complications of PCIs and eliminate potential risk factors before procedures.


Introduction
Drug-eluting stents (DES) substantially reduce restenosis compared with bare metal stents and represent a significant advance in percutaneous coronary interventions (PCIs). Accordingly, DES have been rapidly adopted into practice and are currently used in the vast majority of PCI procedures. As PCIs for more complicated lesions increase, various complications, such as stent thrombosis, fracture, dissection or perforation, are also increase. For example, PCIs for patients who have chronic total occlusion increase and these patients tend to have more risk factors like diabetes mellitus, hypertension, dyslipidemia, and previous myocardial infarction and also have multi-vessel diseases and have decreased left ventricular ejection fraction. If major procedure-related complications were developed in these high risk patients, it may leads to fatal results. So it is important to understand possible complications of PCIs and eliminate potential risk factors before procedures.

Stent fracture
Drug-eluting stents (DES) have proven very effective in reducing restenosis by suppressing neointimal hyperplasia. However, potentially serious complications such as in-stent restenosis and thrombus still occur. Stent fracture has been identified as a possible contributor to these adverse outcomes. A number of risk factors for the development of stent fracture have been described, although a detailed analysis of the angiographic factors predisposing to stent fracture is lacking.

Incidence and definition
Stent fracture is defined as the cases where the linear or curvilinear connections of stent struts are interrupted and areas of the stented segment are uncovered by stent struts visible on coronary angiography. The incidence of stent fracture is reported in 0.8-7.7% of cases. [1][2][3][4][5][6][7][8] However, because of limited sensitivity of angiography to detect fracture, its true incidence is still unknown. In a recent report analyzed from autopsy findings, stent fracture was observed in 29% of total patients. 9 So, the real incidence of stent fracture is assumed to be a little higher than what has been clinically reported. Stent fracture from patients treated with Cypher stents is more frequently observed than in cases of Taxus stents. The incidence of stent fracture of Cypher stent was 1.3% in the SIRIUS trial, 9 compared to 0.58% incidence with the Taxus stent is in the Taxus IV/V/VI trials. Stent fracture has previously been recognized in noncoronary vessels, especially in the superficial femoral and popliteal arteries

Clinical implications
The complications observed with DES fracture include in-stent restenosis, target lesion revascularization, stent thrombosis, myocardial infarction, stent-related aneurysm, and sudden death. Lee et al. observed binary restenosis in six out of 10 patients (60%) with stent fracture and one patient had stent thrombosis. 5 Local mechanical irritation of the vessel can occur from fractured stent struts, which may result in inflammation and neointimal hyperplasia. Restenosis could also reflect decreased local drug availability secondary to distortion of the stent architecture and polymer coating. 13 Exposure of a free metal strut protruding into the vessel lumen could trigger platelet activation and resultant stent thrombosis. Acute myocardial infarction and sudden death can develop from fracturerelated stent thrombosis.

Diagnosis and treatment
Usually stent fracture can be diagnosed by conventional fluoroscopy or follow-up coronary angiography.

No-reflow phenomenon
The phenomenon of no-reflow is defined as inadequate myocardial perfusion through a given segment of the coronary circulation without angiographic evidence of mechanical vessel obstruction. 15 The underlying cause of no-reflow is microvascular obstruction, which may be produced by various mechanisms. The concept of no-reflow was first described in experimental models in 1966 16 and then in the clinical setting of reperfusion after myocardial infarction in 1985. 17 No-reflow has been documented in ≥30% of patients after thrombolysis 18 or mechanical intervention for acute myocardial infarction. The prevalence is variable, ranging from 5% up to 50%, according to the methods used to assess the phenomenon and to the population under study.
A series of consistent data has clearly shown that no-reflow has a strong negative impact on outcome, negating the potential benefit of primary percutaneous coronary intervention (PCI). 19,20 Indeed, patients with no-reflow exhibit a higher prevalence of: 1) early postinfarction complications (arrhythmias, pericardial effusion, cardiac tamponade, early congestive heart failure); 2) left adverse ventricular remodeling; 3) late repeat hospital stays for heart failure; and 4) mortality.
Therefore, it is important to prevent and effectively treat the no-reflow phenomenon during PCI to achieve an optimal outcome.

Historical overview
The term no-reflow was first used by Majno and colleagues 21 in the setting of vertebral ischemia in 1967. This phenomenon was initially described by Krug et al. 16 during induced myocardial infarction in the canine model in 1966 and again by Kloner et al. 15 in 1974 in which it occurred for 90 min after temporary epicardial coronary artery occlusion followed by reperfusion. Electron microscopic examination showed severe myocardial capillary damage with loss of pinocytonic vesicles in the endothelial cells, endothelial blisters or blebs and endothelial gaps with neutrophil infiltration. Intraluminal capillary plugging by neutrophils and/or microthrombi with myocardial cell swelling was also noted. Galiuto et al. 22 with sequential measurements of myocardial perfusion by myocardial contrast echocardiography, have recently shown that in humans no-refow detected 24 h after successful PCI spontaneously improves over time in approximately 50% of patients. Thus, noreflow can be categorized as sustained or reversible. Sustained no-reflow is probably the result of anatomical irreversible changes of the coronary microcirculation, whereas reversible noreflow is the result of functional and thus, reversible, changes of the microcirculation.
In humans, no-reflow is caused by the variable combination of 4 pathogenetic components: 1) distal atherothrombotic embolization; 2) ischemic injury; 3) reperfusion injury; and 4) susceptibility of the coronary microcirculation to injury.

Distal embolization
Emboli of different sizes can originate from epicardial coronary thrombus and from fissured atherosclerotic plaques, in particular during primary PCI. 23 Large emboli (>200μm diameter) can obstruct pre-arterioles, causing infarctlets. Experimental observations have shown that myocardial blood flow decreases irreversibly when microspheres obstruct more than 50% of coronary capillaries. 24

Ischemia-related injury
Changes in endothelial cells, visible after prolonged ischemia, are represented by endothelial protrusions and membrane-bound bodies, which often fill the capillaries up to luminal obliteration. Furthermore, large endothelial gaps with extravascular erythrocytes are common. 25

Reperfusion-related injury
A massive infiltration of the coronary microcirculation by neutrophils and platelets occurs at the time of reperfusion. 25,26 Reintroduction of neutrophils in post-ischemic myocardium results in their activation, with subsequent adhesion to the endothelial surface and migration in the surrounding tissue. Activated neutrophils, in turn, release oxygen free radicals, proteolytic enzymes, and pro-inflammatory mediators that can directly cause tissue and endothelial damage. Neutrophils also form aggregates with platelets that plug capillaries, thus mechanically blocking flow. 27,28 Finally, vasoconstrictors released by damaged endothelial cells, neutrophils, and platelets contribute to sustained vasoconstriction of the coronary microcirculation. 29 Tumor necrosis factor-alpha expression is induced by reperfusion, and can impair endothelium-dependent coronary flow reserve. 30 Interleukin-1β also has recently been associated with ischemia-reperfusion injury, because interleukin-1β knockout animals exhibit marked reduction of ischemic induced inflammation. 31 Selectin expression on cell surfaces is also important for mechanical plugging of the microcirculation. 32 Finally, the balance between nitric oxide and superoxide is tipped in favor of superoxide within minutes of reperfusion of ischemic tissues, due to increased production of xanthine oxidase by neutrophils, endothelial cells, and cardiac myocytes, which leads to an exacerbation of the inflammatory state. 33 Reperfusion might also cause irreversible injury to myocytes. 34 During ischemia there is an increase of the intracellular sodium (Na + ) content due to accumulation of hydrogen (H + ), which is exchanged by the Na + /H + exchanger. The subsequent exchange of doubly charged positive calcium ion (Ca ++ ) with Na + by the sarcolemmal Na + /Ca ++ exchanger produces a calcium overload that triggers uncontrolled hypercontraction and stimulates opening of the mitochondrial permeability transition pore (m0PTP), which further enhances calcium overload. Furthermore, Na + extrusion trough Na + /potassium (K + ) adenosine triphosphate (ATP)-ase is impaired and together with Ca ++ accumulation leads to myocyte cell swelling, which contributes to subsequent rupture of the cell membrane when the extracellular osmolality is rapidly normalized by reperfusion. Of note, cyclosporine, which blocks the m-PTP, has been recently shown to reduce infarct size by 20% when administered intravenously in patients undergoing primary PCI. 35 Finally, ischemic pre-conditioning might also reduce infarct size by blockade of m-PTP. 36 Natriuretic peptides might modulate ischemia-reperfusion injury. Atrial natriuretic peptide might suppress the rennin-angiotensin-aldosterone system and endothelin (ET)-1 that increase infarct size, microvascular obstruction, and cardiac remodeling. 37

Individual predisposition of coronary microcirculation to injury
In humans, no-reflow is occasionally observed during elective procedures, 38 whereas it can be absent after primary PCI in patients with acute myocardial infarction. In particular, diabetes and hypercholesterolemia has been associated with impaired microvascular reperfusion by enhancing endothelial oxidative stress. 39,40 Pre-conditioning by using nicorandil seems to have a beneficial effect on microvascular function. 41

Coronary angiography
Reduced coronary flow after primary PCI (TIMI flow 0 to 2) is associated with worse outcome than normal (TIMI 3) flow, even when no significant epicardial obstruction remains. 42 More sensitive markers of tissue perfusion have now been identified and provide prognostic information beyond that of TIMI flow grade. The TIMI frame count assesses the number of angiographic frames required for the contrast medium to reach standardized distal landmarks of the coronary tree, and the myocardial blush grade (MBG) is a quantitative assessment of myocardial contrast density. The MBG is scored on a scale of 0 to 3, with higher scores indicating better perfusion. An MBG 0 to 1, suggestive of no-reflow, is o b s e r v e d i n a s h i g h a s 5 0 % o f p a t i e n t s w i t h T I M I f l o w g r a d e 3 . 43 Taken together, angiographic no-reflow can be defined as a TIMI flow grade <3 or 3 with an MBG 0 to 1.

Electrocardiography
Rapid ST-segment resolution defined as a reduction of ≥50% in the ST-segment elevation index is highly specific (91%) for myocardial reperfusion (or the absence of no-reflow on myocardial contrast echocardiography) although less sensitive (77%). 44

Myocardial contrast echocardiography (MCE)
Lack of intramyocardial contrast opacification is due to microvascular obstruction; thus, it represents the extent of no-reflow. 45 In the AMICI study, the extent of no-reflow at MCE was demonstrated to be the best predictor of adverse left ventricular remodeling after acute myocardial infarction, being superior to ST-segment resolution and to MBG among patients exhibiting TIMI flow grade 3. 20

Cardiac magnetic resonance imaging
No-reflow can be diagnosed as: 1) lack of gadolinium enhancement during first pass; and 2) lack of gadolinium enhancement within a necrotic region, identified by late gadolinium hyperenhancement. 46

Distal embolization
No specific technique is currently recommended in guidelines to prevent distal embolization during primary PCI. Direct stent implantation, by avoiding balloon-induced thrombus fragmentation and by entrapping the atherothrombus under the stent struts, has been suggested as a possible technique to reduce distal embolization in a specific subset of patients i.e. those with good distal visualization of the infarct-related artery after guidewire passage. 47 A more promising technique is the use of thrombectomy and distal filter devices. Although distal filter devices did not improve early or late prognosis compared with standard primary PCI, thrombectomy performed with a simple manual aspiration catheter revealed improved myocardial reperfusion and significantly reduced no-reflow. 48 A recent large trial www.intechopen.com

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by Svilaas et al. 49 confirmed the improvement of reperfusion associated with manual thrombus-aspiration as compared with standard primary PCI showing a strikingly lower mortality at 12-month follow-up. 50 So, it is suggested that manual thrombus aspiration should be used in the setting of primary PCI, particularly in patients with a high thrombus burden. 51

Ischemia-related injury
Strategies aimed at reducing pain onset-to-balloon time might reduce no-reflow by decreasing total ischemic time. Drugs known to reduce myocardial oxygen consumption and consequently the severity of ischemia and improve myocardial perfusion include carvedilol, fosinopril, and valsartan. 52,53

Reperfusion-related injury
Intracoronary nitroglycerin is usually suggested as the first-line agent, mainly to reverse epicardial vessel spasm, even if the blood pressure is reduced. Theoretically, nitroglycerin should have little impact on arteriolar tone and hence on no-reflow since physiologically it produces little effect in the microvasculature.
Patients at high risk of no-reflow can be treated with drugs such as glycoprotein IIb/IIIa antagonists, adenosine, nicorandil, and nitroprusside aimed at counteracting endothelial, platelet, and neutrophil activation. Among glycoprotein IIb/IIIa antagonists, abciximab has been found to improve myocardial perfusion when started during primary PCI and infused for 12 h thereafter. Interestingly, intracoronary abciximab has been proven to be superior to intravenous abciximab in patients treated by primary PCI. 54 Adenosine is an endogenous purine nucleoside that decreases arteriolar resistance and activates intracellular cardioprotective signaling pathways. Its mechanism of action may involve opening ATP-sensitive potassium channels (K ATP ), inhibition of neutrophil migration, prevention of superoxide generation, or blockade of coronary endothelin release. Nitroprusside is a nitric oxide donor that does not depend on intracellular metabolism to derive nitric oxide, with potent vasodilator properties. Nicorandil is a hybrid of a K ATP opener and nitrate and may prevent reperfusion injury by blocking the mitochondrial permeability transition pore. Verapamil is a calcium-channel blocker that has several beneficial effects in the setting of no-reflow in addition to attenuation of microvascular spasm. Varapamil may also inhibit platelet aggregation and thrombus formation in the microvasculature and may have a direct effect on calcium flux across the sarcolemmal membrane or within intracellular compartment that could protect reversibly injuried myocytes.

Coronary artery perforation and cardiac tamponade
Coronary artery perforation complicating percutaneous coronary intervention (PCI) occurs in 0.1-3.0% of cases. Perforation with balloon angioplasty or stenting is rare, occurring in 0.1% of cases. However, when ablation devices, such as rotablator, directional coronary atherectomy, transluminal extraction catheter, and excimer laser, are used, the frequency is substantially higher than with balloon angioplasty or stents, occurring up to 3.0%. 55 Recently, the use of ablation devices in PCI has tended to decline. Instead, procedures for more complex lesions including calcification, severe angulation, and chronic total occlusion have increased. To treat these complex lesions, stiffer and hydrophilic wires are necessary and high pressure for balloon dilatation is needed. Increasing numbers of procedures using glycoprotein IIb/IIIa inhibitors is also contributing to the fact that coronary artery perforation still occurs.

Classification
The Ellis classification depends on angiographic findings is most widely used (

Risk factors
Many factors are involved in coronary perforation during PCI. Related risk factors can be divided into patient-related, vessel-related, and procedure-related factors. 56 Patients-related factors are old age, hypertension, PCI for unstable angina or non-ST segment elevation myocardial infarction. 57 Vessel-related factors are ACC/AHA type C lesions, calcified lesions, and chronic total occlusion lesions. 3 Procedure-related factors include use of stiff hydrophilic wires, device oversizing, use of atheroablative devices. One observational study reports that 87% of perforation due to guidewires is attributed to hydrophilic wires. 58 In case of rotational atherectomy use, if lesions are eccentric, lesion length are >10cm, or lesions are very tortuous, the risk of perforation is high.

Symptoms and signs
If coronary perforation is develop, usually patients feel severe chest pain. In addition, nausea, dizziness, and vomiting can occur. The heart rate can rise suddenly, blood pressure can drop and if cardiac tamponade happens, an increase of central venous pressure with neck vein engorgement develops. Sustained ST-segment elevation or depression can be observed even though coronary balloons are deflated.

Diagnosis
The diagnosis of coronary artery perforation is not difficult by coronary angiography. If cardiac tamponade is suspected, then echocardiography is very useful. Sometimes perforation can occur 12-48 h later after PCI, if vital signs become unstable and serum hemoglobin and hematocrit levels decrease, cardiac tamponade must be suspected and urgent echocardiography should be performed.

Clinical outcomes and prognosis
Complications related to coronary artery perforation are diverse and depend on the degree of perforation. It has been reported that in cases of perforation, myocardial infarction can occurs in 13-34%, emergency coronary artery bypass graft in 11-39%, cardiac tamponade in 12-31%, and mortality in 7.6-19%. [59][60][61] The degree of perforation is a important marker to predict the late prognosis. Ellis classification type I perforation has a clinically good prognosis in 60% and it is rare that type II perforation has a poor prognosis. However, type III perforation reveals high major adverse events rate. 55,62

Treatment
Generally, guidewire-related perforation does not cause grave results except with concomitant use of glycoprotein IIb/IIIa inhibitors. However, perforation due to balloon, atherectomy devices, or laser can produce hemopericardium or hemodynamic collapse.

Prolonged balloon inflation
The most important thing to stop bleeding is prolonged balloon inflation at the perforation site at least for 10-15 min at 2-6 atm. If bleeding does not stop, a perfusion balloon catheter can be used for 15-45 min inflated at low pressure. This prolonged balloon inflation and timely pericardiocentesis can avoid surgical treatment in patients with Ellis type I perforation.

Stent
In some cases, a polytetrafluoroethylene(PTFE)-covered stent is effective. Rarely, one or more conventional stent (bare metal or drug-eluting stent) implantation can be considered.

Pericardiocentesis
If perforation is suspected, echocardiography should be performed and if hemopericardium is confirmed, pericardiocentesis should be performed immediately. An indwelling pericardial catheter should be maintained for 6-24 h and echocardiography should repeated every 6-12 h.

Management of anticoagulation
When perforation occurs, anticoagulation should be maintained to prevent thrombus formation. However, if perforation happens after use of atherectomy or laser, it is recommended that protamine sulfate should be administrated intravenously in order to partially reverse the effect of heparin. If contrast leakage is sustained despite prolonged balloon inflation, repeated balloon inflation should be performed; meanwhile the dosage of protamine sulfate should increase under activated clotting time monitoring.
Glycoprotein IIb/IIIa inhibitors must be stopped. The effect of abciximab can be reversed after platelet transfusion of 6-10 units, but there are not any known antidotes for eptifibatide or tirofiban.

Embolization
In case of small vessel size or distal location, limited involved myocardium, chronic total occlusion, or situation where surgery is unavailable, embolization using coils or gelfoam can successfully occlude the perforation.

Surgical treatment
If the perforation is severe, with hemodynamic instability or perforation is sustained despite nonsurgical management, emergency surgery is necessary.

Coronary dissection and acute closure 4.1 Angiographic definition
Dissection is defined as disruption of an arterial wall resulting in splitting and separation of the intimal (or subintimal) layers.

Feature Definition Abrupt
Obstruction of contrast flow (TIMI 0 or 1) in a dilated segment with closure previously documented anterograde flow Ectasia A lesion diameter greater than the reference diameter in one or more areas Luminal irregularities Arterial contour that has a saw-toothed pattern consisting of opacification but not fulfilling the criteria for dissection or intracoronary thrombus Intimal flap A discrete filling defect in apparent continuity with the arterial wall

Pathogenesis and incidence of coronary dissection
Intima-media cracks and medial dissection can be developed by balloon injury and if dissection involves the adventitia layer, narrowing of the lumen can occur. 65 In NHLBI classification, coronary dissection occurs in 32-41% of total balloon procedures. If the lumen narrows >50% or the length of dissection is >10mm, the risk of abrupt vessel closure increases.
In the modern PCI era, where coronary dissection can be promptly resolved by stent implantation, clinically significant dissection is reported only 1.7%. Residual dissection increases the risk of post procedure MI, emergency CABG, and stent thrombosis and mortality also increases threefold. 66

Pathogenesis and incidence of abrupt vessel closure
Coronary balloon dilatation leads to endothelial denudation, intimal fissuring, and medial penetration and extensive damage causes obstructive dissection or intramural hematoma. When subintimal structures are exposed to the blood, then activation of platelets and thrombin formation occur. Obstructive thrombus can be formed with or without medial dissection.
An autopsy finding of patients who experienced abrupt closure within 30 days after balloon angioplasty revealed that over 50% have intimal/medial dissection flaps with or without thrombi. Cases presenting with pure thrombi without dissection was very rare.
It has been reported that the incidence of abrupt closure due to balloon angioplasty or atherectomy is 2-13.5%. About two thirds of cases of abrupt closure arise inside the catheterization laboratory and the majority occurs within the first 6 h after angioplasty. After routine stent implantation had replaced balloon angioplasty, the incidence of abrupt closure was dramatically decreased. Proper deployment technique and supportive drugs such as dual antiplatelet therapy and heparin also contribute toward the lower incidence.

Clinical manifestations of abrupt closure
Before the stent era, the incidence of abrupt closure-related mortality, myocardial infarction, and emergency CABG were 5%, 45%, and 55% respectively. However, since bailout stenting had been introduced, the incidence of emergency CABG owing to abrupt closure was reported to be 0.8%. Long term prognosis of abrupt closure in related to the increase of restenosis, 2-year mortality, myocardial infarction, and CABG. Well known predictors of abrupt closure are in Table 4.

Initial management
When acute chest pain is recognized, nitroglycerin 100-200mcg should be injected into the coronary artery. If activated clotting time is less than 250 sec, additional heparin should be injected. Before the stent era, a size-matched balloon was inflated for at least 5 min and sometimes perfusion balloon was used. Currently, bailout stenting is considered as the most effective tool for abrupt or threatened coronary closure. Additional balloon inflation can recover blood flow to normal. Because the effect of rescue abciximab has been reported in many clinical trials and since it reduces subacute thrombus formation, administration of glycoprotein IIb/IIIa inhibitors is recommended.

Coronary artery bypass graft
In cases of acute closure due to left main coronary artery injury or coronary artery perforation, installation of intra-aortic balloon counterpulsation is necessary and emergency coronary artery bypass grafting (CABG) should be considered. A long dissection which does not resolve with stenting also needs CABG.

Definition
Definitions of stent thrombosis (ST) range from "angiographically proven to "clinically suspected" ST with the inclusion of myocardial infarction involving the target vessel to unexplained death (within 30 days). Although the first definition has a well-defined mechanism limited to selected patients undergoing angiography at the time of ST, there is concern for underestimation of the true incidence of ST. On the other hand, the other broader definitions include events potentially related to disease progression, life threatening arrhythmias, myocardial infarction of non-culprit lesions, and non-cardiac sudden death and therefore overestimate the true incidence. Accounting for these limitations, an academic research consortium (ARC) proposed a new standardised definition of ST (Table 5). 67 It is based on 2 principles: level of certainty that ST is underlying mechanism of the adverse event and the time of the adverse event relative to the index procedure.
ST can be classified based on the time of adverse event ( Figure 5). Early ST refers to the first 30 days after stent implantation and is further stratified into acute (<24 hours) and subacute (24 hours to 30 days). Late ST is time between 1 month and 1 year. Very late ST means beyond 1 year. The rationale of this classification is to account for different pathophysiological mechanisms that may be at work at various times.

Definite ST
Definite stent thrombosis is diagnosed when either angiographic or pathological confirmation is present

Incidence
In the bare metal stent (BMS) era, most of ST was early ST and very late ST was extremely rare, although several cases of late ST were reported. 73,74 Early ST is encountered with a similar or even somewhat lower frequency after drug-eluting stent (DES) compared with BMS. A metaanalysis of 6 studies comparing BMS with sirolimus-eluting stent (SES) reported that early rates of ST were 0.5% with SES and 0.6% with BMS, respectively (P=0.55). 68 A pooled analysis of 5 trials comparing BMS with paclitaxel-eluting stent (PES) revealed early ST was 0.5% in PES and 0.6% in BMS, respectively (P=0.51). 69 Although there had been some reported cases of late ST during the BMS era, this was not a clinical concern for most. According to recent a meta-analysis, no differences existed in the incidence of late ST between DES and BMS (0.2% versus 0.3%, 95% CI: 0.35-2.84; P=1.00). 70 In another meta-analysis of 9 trials comparing SES and PES, no significant differences were detected for up to 1 year of follow-up ( Figure 6)

Clinical sequelae
The reason why ST attracts attention is that it is associated with much higher mortality compared to other complications. Moreover, ST may be responsible for late complications of MI, including heart failure, arrhythmias, or mechanical complications. The impact of ST depends upon the myocardial area at risk, its viability, the degree of instantly recruitable collaterals, and the availability of rapid reperfusion therapy. The mortality rate varies depending on the definition of ST and follow-up duration (7-45%). [73][74][75][76][77] Most ST patients experience myocardial infarction (>66%) with no differences between DES and BMS. 71

Risk factors
ST is a multifactorial problem related to the stent itself, procedural factors, response to antiplatelet drugs, and lesion factors ( Table 6). Many cases of early ST are caused by the procedure itself such as the presence of residual dissections or stent underexpansion. Poor response to antiplatelet drugs is also a documented cause of ST. 78 Individual or ethnic differences have been reported and it has been suggested that several genetic polymorphisms are related to this drug resistance. Discontinuation of antiplatelet drugs is one of the most important predictors of ST. Patients noncompliance is the main problem and discontinuation due to dental procedures, surgical procedures or bleeding is also an important predisposing factor for late and very late ST. 76

Hypersensitivity reaction with extensive vasculitis
Virmani and colleagues 79 first described a case of local hypersensitivity reaction with extensive vasculitis of the intima, media, and adventitia consisting predominantly of lymphocytes and eosinophils in a patient suffering very late DES thrombosis. Histopathological analysis of an autopsy case revealed aneurysmal dilatation of the vessel wall within the stented segment with incomplete stent apposition and thick fibrin thrombus between the stent and the arterial wall. Most hypersensitivity cases reported to the Food and Drug administration after DES implantation were attributed to the DES itself, especially the polymer coating.

Delayed healing and dysfunctional endothelialization
Another possible explanation is delayed healing and endothelial dysfunction. Delayed healing manifested by persistent fibrin deposition and incomplete reendothelialization emerged as an important discriminator between BMS and DES. 80 Physiological evidence of dysfunctional endothelium comes from studies assessing vasomotion 6 months after DES implantation. 81 Table 6. Multifactorial Origin of ST

Incomplete stent apposition
Incomplete stent apposition resulting from positive arterial remodeling or stent underexpansion, and penetration of the stent into a necrotic core leads to ST.

Patient and lesion selection
PCI is not essential in all angina patients. The COURAGE trial, a randomization study comparing PCI and medical therapy in carefully selected patients with stable angina, revealed that there were no significant differences in mortality, acute myocardial infarction, and rehospitalization for acute coronary syndrome. 83 It is therefore appropriate to consider medical therapy to the initial treatment option in stable angina patients with relatively low risk. In determining of use of BMS or DES, the risk of restenosis, the probability of bleeding The need for non-cardiac surgical procedures may arise after recent DES implantation. ST can arise after antiplatelet therapy withdrawal, especially within 12 months after PCI. In a study of 103 stent patients undergoing noncardiac surgery, an alarming 5% mortality rate and 45% complication rate were noted. 84 It should be determined whether the surgical procedure can be postponed beyond 12 months after stenting or whether dual antiplatelet therapy can be maintained throughout the perioperative period. However if it is not possible to delay surgery beyond 12 months, balloon angioplasty or BMS implantation can be considered instead of DES implantation.

Antiplatelet therapy
The important of dual antiplatelet maintenance after PCI cannot be emphasized strongly enough. In a registry, 14% discontinued thienopyridine therapy within 30 days after discharge. 85 Predictors of premature thienopyridine discontinuation were older age, lower socioeconomic status, preexisting cardiovascular disease, and lack of discharge instructions or cardiac rehabilitation referral. Mortality was about 10 times higher and rehospitalization was almost twice as high in patients without thienopyridine therapy.
The optimal duration of dual antiplatelet therapy after DES implantation is not well established.
The AHA/ACC guidelines are not based upon multicenter trials. Dual antiplatelet therapy is recommended at least 1 month for BMS implantation whereas for DES implantation, adherence to a 12 month regimen is recommended. 86 It is also important that surgeons and dentists are advised not to automatically discontinue antiplatelet therapy but rather to consult first with the patient's cardiologist.

Technique
Attention to technical details also may improve results when PCI is performed with DES. Optimal deployment of stents by full expansion throughout their entire length should be ensured and residual dissections should be avoided. Intravascular ultrasound or optical coherence tomography is helpful to avoid stent malapposition. Multiple stenting in bifurcation lesions should be limited to only cases where really needed.

Development of new DES
Because the polymer coatings of DES were suspected to be responsible for some of ST, biodegradable polymers 87 and polymer-free DES 88 were developed. Another approach banks on drugs with improved healing properties such as antibodies capturing CD34+ endothelial progenitor cells 89 or antithrombotic substances 90 applied to the stent surface. Biodegradable stents, fully disintegrated in the body over a long period of time, have been recently developed. 91 The Titan-TINOX stent (Hexacath, Rueil-Malmaison, France) is made of stainless steel coated with a Titanium nitride oxide (TNO) compound. The coating minimizes the leakage of metal residues, mostly nickel, from the metal stent into the arterial wall and, to some extent, attenuates electrical conductivity. Thus, the device was designed to enhance endothelialization and decrease the rate of stent-related thrombosis and restenosis. Stent coating with TNO reduced angiographic and ultrasonic measures of restenosis compared with stainless steel control stents of otherwise identical design in the prospective, randomized, multicenter trial. 92