The principle pathobiology of ACS involves plaque disruption and the consequences. Anatomic factors increase the vulnerability of plaques to rupture, including positive arterial remodeling with a large lipid core and a thin fibrous cap.

The extension of knowledge regarding the atherothrombotic process of ACS is perhaps as important as therapeutic progress. Inflammation now is recognized as central to the dynamic transformation occurring within the coronary vasculature. A variety of inflammatory mediators result in leukocyte (primarily monocytes) infiltration and
degradation (metalloproteinases) of the plaque fibrous cap. Interrelated mechanisms involving inflammatory and thrombotic pathways (such as tissue factor expression by macrophages) lead to thrombus formation and the clinical ACS (4). Embolization of plaque and thrombus leading to microvascular obstruction and myocardial necrosis is characteristic of the ACS patient (5).

The focal event resulting in presentation with an ACS is a manifestation of a systemic process. Markers (CRP, Il-6, serum amyloid A, ICAM-1) can be measured as an index of this widespread inflammatory response (4). This has been confirmed anatomically in the coronary circulation with evidence for neutrophil activation distant from the culprit lesion in patients with UA (6). Multiple complex angiographic lesions and ultrasound-identified plaque ruptures have been revealed in nonculprit vessels after both STEMI and NSTEMI (7,8). The presence of multiple complex plaques is associated with recurrent ischemic events (7).

PCI in the environment of multicentric inflammatory arterial disease and ACS influences procedural results and long-term outcomes. Elevated preprocedural C-reactive protein levels predict early complications and restenosis after PCI (9,10). Embolization also remains a hazard of PCI procedures and may be interrelated to the inflammatory process (5).

PCI generates therapeutic benefits for patients with ACS by relieving angina, stabilizing the acute event, and enhancing the overall prognosis. Considerable technological progress has widened the application of PCI to an expanded, more complex population of patients, with improved safety and durability.

Historically, the procedural success rate of BA in NSTE-ACS is comparable to that in patients with stable angina, but the risk of major complications has been consistently higher. This was particularly evident in the initial National Heart, Lung, and Blood Institute (NHLBI) PTCA Registry (11). The complex lesion morphology present in ACS increases the complication risk (12). The large lipid core of the ACS lesion enhances the risk for thrombogenic complications. Intracoronary thrombus amplifies the risk of abrupt closure and its consequences (13). Yet, the BA procedure success rate was 96.1% for patients with NSTE-ACS in the TIMI IIIB trial (periprocedural infarction 2.7%, emergency bypass surgery 1.4%, death 0.5%) (14). Unstable angina has been identified as a risk factor for restenosis (15).

More contemporary progress in revascularization methods is exemplified by 3,192 patients in the NHLBI Dynamic registry (1997-1999). Although more complex patient and lesion characteristics were present in unstable angina, procedural success without major complication was equivalent to stable angina patients (92.3% versus 91.2%) (16). In-hospital (0.7% versus 0.3%, p = NS) and 1-year (4.5% versus 3.1%, p = NS) mortality was also statistically similar. This progress also is illustrated by patients undergoing PCI at the Mayo Medical Center since 1979 (Table 33.1) (17). Despite conducting the procedure on older and more complex patients, clinical success improved and resulted in fewer complications and a higher event-free survival at 1 year.

The transformation of interventional practice by stenting techniques has improved procedural safety and reduced
the risk of restenosis by 25% to 35%, compared to other angioplasty methods (18). A large (n = 16,811) registry of PCI patients demonstrated a reduction in overall hospital mortality (0.3% versus 0.6%, p = 0.014) and emergency bypass surgery in UA patients (0.2% versus 0.5%, p = 0.007) using stenting compared to BA (19). However, a 13-month follow-up analysis of 23 trials (n = 10,347) comparing stenting to BA in patients with and without ACS revealed no difference in death or MI but a significant decrease in major adverse cardiac events driven entirely by a reduction in target-vessel revascularization (TVR) (20).

Early experience suggested an increased hazard for stent implantation in the presence of intracoronary thrombus and identified unstable angina as a predictor for stent thrombosis (21). However, advances in technique, including adoption of high-pressure deployment and antiplatelet pharmacotherapy, have diminished these concerns. Recent analysis of six coronary stent trials (22) and 4,509 patients at the Mayo Clinic (23) did not identify unstable angina as a risk factor for stent thrombosis.

Despite the advantages of stenting over BA, restenosis within bare metal stents (BMS) remains a vexing problem. Although intravascular brachytherapy is an effective treatment for in-stent restenosis, a new era has begun with the stent-based local delivery of agents that inhibit neointimal hyperplasia. Impressive reductions in angiographic in-segment restenosis (70% to 75%) and target-lesion revascularization (73% to 77%) have been realized for both currently U.S. Food and Drug Administration (FDA)-approved drug-eluting stents (Taxus [paclitaxel] and Cypher [sirolimus]) (24,25). Concerns exist regarding delayed endothelial healing and other drug effects that may increase the risk of thrombosis within the stent. Patients with ACS were included in investigations of both agents, but randomized trials specifically for this group are lacking. However, the Rapamycin-Eluting Stent Evaluated at Rotterdam Cardiology Hospital (RESEARCH) registry reported a similar 30-day major adverse cardiac event (MACE) rate (6.1% versus 6.6%, p = 0.8) for sirolimus stent (n = 198) versus BMS (n = 301) patients. The stent thrombosis rate was 0.5% using sirolimus and 1.7% using BMS (p = 0.4) (26). Nevertheless, recommendations regarding implantation technique (adequate stent expansion, ample lesion coverage, avoidance of margin dissection) and strict adherence to prolonged antiplatelet therapy may be more critical for patients with ACS.

The merits of a routine “invasive” approach for NSTE-ACS include prompt anatomic definition and expedient revascularization to mitigate the early risk of recurrent ischemic events. In contrast, a “conservative” recurrent ischemia-driven approach avoids potentially unnecessary invasive procedures. The ongoing debate regarding these divergent practice methods generated five large (n >1,000) randomized trials since 1989.

Distinct variations are present in these trials (Table 33.2) that reflect advances in medical therapy and the process of revascularization. Both the TIMI-IIIB and Veterans Affairs Non-Q-wave Infarction Strategies (VANQWISH) trials were conducted before stents and GP IIb/IIIa inhibitors were widely available (27,28). In TIMI-IIIB, no difference in the incidence of death or MI was noted at 6 weeks or 1 year. However, index hospitalization was shorter and rehospitalization was reduced at 1 month using the invasive approach. In the VANQWISH trial of patients with NSTEMI, the invasive arm was associated with higher rates of death and nonfatal infarction during hospitalization (7.8% versus 3.2%, p = 0.007) and at 1 year (24% versus 18.6%, p = 0.05). Mortality also was higher at 1 year in the invasive group (12.6% versus 7.9%, p = 0.025), but survival curves converged at 23 months. Notably, the surgical mortality rate in the invasive arm was 11.6%, compared with 3.4% in the conservative group. No patient died undergoing angioplasty in the invasive group (28). In both trials, a <15% difference was observed in the revascularization rates of the two strategies (27,28).

In contrast, three subsequent trials have shown the superiority of an early revascularization strategy by incorporating more contemporary interventional techniques, including the employment of stents in the majority of patients undergoing PCI. However, differences occur in these trials, particularly related to adjunctive antithrombotic therapy, the timing of angiography, and the relative revascularization rates.

The Fragmin and Fast Revascularization during Instability in Coronary Artery Disease (FRISC-II) trial was the first to demonstrate an invasive strategy advantage. Although there was an early hazard for MI in the invasive group, this risk was subsequently lower than in the conservative group. At 6 months, a significant decrease in death or infarction was noted in the invasive group (9.4% versus 12.1%, p = 0.031) (29). At 1 year, the risk of death (2.2% versus 3.9%, p = 0.016) and infarction (8.6% versus 11.6%, p = 0.15) was significantly lower using an invasive approach (30). These differences remain significant at 2-year follow-up (31). The small proportion (9%) of patients in the conservative group revascularized at initial hospitalization rose to 43% (78% in the invasive arm) at 1 year. Although stents were utilized frequently, only 10% received a GP IIb/IIIa inhibitor.

The TACTICS TIMI-18 trial incorporated uniform treatment with GP IIb/IIIa inhibition (tirofiban for 48 hours or until 12 hours post-PCI) into an examination of early angiography and revascularization (4 to 48 hours), compared to a conservative approach (3). At 6 months, the primary endpoint of death, infarction, or rehospitalization for ACS was reduced using the early invasive strategy
(15.9% versus 19.4%, p = 0.025). No early hazard was noted in the invasive group, perhaps reflecting the benefits of GP IIb/IIIa inhibition and/or earlier revascularization when compared with previous trials.