Percutaneous Coronary Intervention for Unprotected Left Main Coronary Artery Stenosis

Key Points

Coronary angiography has limitations in assessing left main coronary artery (LMCA) disease and guiding treatment. The complementary use of coronary physiology and imaging modalities is helpful to improve outcomes following LMCA stenting.
Long-term prognosis following ostial or shaft LMCA stenting is excellent.
When treating the distal LMCA bifurcation, the interventionalist should understand the advantage and disadvantage of each stenting strategy. Whatever percutaneous coronary intervention (PCI) strategy is selected, achieving sufficient minimal stent cross-sectional area after LMCA stenting is of paramount importance to prevent in-stent restenosis and adverse clinical outcomes.
PCI offers early safety advantages, whereas coronary artery bypass grafting (CABG) offers greater durability with respect to freedom from repeat revascularization.
The optimal choice of revascularization modality for LMCA disease should take into account the specific circumstances of each patient, patient preferences, and suitability of PCI or CABG.

introduction

Left main coronary artery (LMCA) disease was first described by James Herrick in 1912 in patients dying of cardiogenic shock after acute myocardial infarction (MI). Clinically significant LMCA disease has been found in 3% to 5% of all patients who undergo coronary angiography and in 10% to 30% of patients who undergo bypass surgery. Owing to the large area of jeopardized myocardium, LMCA disease was associated with high morbidity and mortality, and thus coronary artery bypass grafting (CABG) has been the standard revascularization strategy. However, over several decades, there has been a considerable evolution in the field of percutaneous coronary intervention (PCI). Remarkable advancements in stent devices, technical refinement, and adjunctive medical therapy have led to improved PCI outcomes for unprotected LMCA disease. With a widespread use of drug-eluting stents (DES), PCI for LMCA lesion has become technically more feasible and associated with favorable long-term clinical outcomes. Recently, several clinical trials using first- and second-generation DES found similar survival rates after PCI and CABG. Thus unprotected LMCA stenting has become more generalized as primary revascularization strategy.

Anatomy and Pathology

The LMCA arises from the left aortic sinus just below the sinotubular junction of the aortic root. In approximately two-thirds of patients, the LMCA bifurcates into the left anterior descending (LAD) and left circumflex (LCX) arteries; in one-third of patients, the LMCA trifurcates into the LAD, LCX, and ramus intermedius. The LMCA supplies, on average, 75% of the left ventricle. Examination of 100 autopsy cases found that the LMCA had an average length of 10.8 mm (±5.2 mm, range 2 to 23 mm), an average diameter of 4.9 (±0.8) mm, with an average branches angle of 86.7 degrees (±28.8, range 40 to 165 degrees). The anatomic portion of the LMCA stenosis is divided into three anatomic regions: the ostium, midshaft, and distal bifurcation. Histologically, the ostial portion resembles the aorta, being rich in aortic smooth muscle cells and elastic fibers. The distal bifurcation is the part of the LMCA most susceptible to the development of an atherosclerotic lesion because of low shear flow disturbance. Especially in the bifurcation, the lateral wall—that is, the wall opposite the flow divider—is the most frequent site of atherosclerotic plaque accumulation, whereas the flow divider (the bifurcation carina) is usually spared because of high shear stress.

Several etiologies contribute to LMCA stenosis ( Table 24.1 ). The most frequent cause is atherosclerosis. The pathologic plaque composition of LMCA disease varies from the pathologic intimal thickening to thin-cap fibroatheroma with or without plaque rupture. In cases of minimal LMCA disease, the most frequent underlying plaque type is pathologic intimal thickening (64%), followed by fibroatheroma with early or late core (17%). However, most lesions with significant LMCA stenosis, defined as more than 50% narrowing, showed more complex plaques, such as fibroatheromas with late core, thin-cap fibroatheroma, surface ruptures, fissures, and intraplaque hemorrhage. In addition, intravascular ultrasound (IVUS) analysis demonstrates the diffuse nature of LMCA disease that extends from the LMCA to the LAD in 90% of patients and to the LCX in 62% of patients.

TABLE 24.1

Causes of Left Main Coronary Artery Stenosis

Atherosclerosis
Nonatherosclerotic causes
Idiopathic causes
- Irradiation
- Takayasu arteritis
- Syphilitic aortitis
- Rheumatoid arthritis
- Aortic valve disease
- Kawasaki disease
- Cardiac surgery or transcatheter aortic valve implantation
- Injury after left main coronary intervention

Definition of Significant Left Main Coronary Artery Stenosis

Traditionally, an angiographic stenosis diameter of 50% has been considered the cutoff for significant LMCA stenosis. Angiography, however, has limitations in assessing lesion morphology and the true luminal size of the LMCA. In addition, noninvasive testing is often not helpful due to clinical and technical limitations. The intermediate LMCA stenosis can be directly assessed by fractional flow reserve (FFR), with an FFR below 0.80 considered a significant LMCA stenosis. Previous studies found patients with an insignificant FFR can be safely treated medically with comparable outcomes to those undergoing CABG. In case of LMCA with downstream stenoses in the LAD and/or LCX, FFR measured across the LMCA stenosis will be increased leading to underestimation of the lesion significance. However, in vivo study found that unless downstream stenosis is very significant, its impact is clinically negligible.

IVUS is another useful tool to determine the significance of LMCA stenosis. Jasti and colleagues reported that a minimal luminal area (MLA) of 5.9 mm ² had the highest sensitivity and specificity (93% and 95%, respectively) for determining a significant LMCA stenosis, compared with FFR as the gold standard. Park and colleagues more recently addressed 112 patients with isolated intermediate LMCA stenosis who underwent preinterventional IVUS and FFR measurements to determine the IVUS MLA criteria that correspond to an FFR below 0.80. They found the IVUS MLA value within the LMCA that best predicted FFR below 0.80 was less than 4.5 mm ² . In addition, in contrast to non-LMCA stenosis, the positive predictive value of IVUS-measured MLA less than 4.5 mm ² is high, at 84%. Thus, if an FFR measurement is not feasible or unreliable, IVUS MLA criteria can be used.

Stenting Techniques

Ostial and Shaft Lesion

Stenting of LMCA ostial and shaft lesion can be performed safely if meticulous guiding technique and careful hemodynamic monitoring are performed. Coaxial alignment of the guiding catheter is important to minimize ostial injury and to ensure proper positioning of the stent. As long as coaxial alignment is maintained, it is usually easy to advance and position the balloon or stent at the target lesion. Once the balloon or stent is properly positioned, the guiding catheter can be gently retracted 1 to 2 cm into the aorta with gentle forward pressure on the device catheter. The ostial LMCA lesion is dilated and stented with the guide tip positioned in the aortic sinus. The proximal stent edge is positioned very slightly outside the ostium and is expanded against the aortic wall. After deployment of the stent, the stented segment is further dilated with high-pressure balloon inflations an to achieve an optimal stent cross-sectional area. Balloon inflations are brief (<30 seconds) to avoid prolonged global ischemia and ischemia-related complications. Aortoostial coverage may not be mandatory for treatment of disease limited to the shaft of the LMCA.

Bifurcation Lesion

PCI for LM bifurcation is technically demanding and has been associated with a high rate of adverse clinical events, and thus should be restricted to experienced interventionalists who understand the risk/benefit ratio of the percutaneous approaches.

Selection of Left Main Coronary Artery Bifurcation Treatment Strategy

Based on nonrandomized studies and extrapolations of non-LMCA bifurcation trials, the provisional one-stent approach has been considered as a preferred strategy over the elective two-stent technique. However, because of the large myocardium territory supplied by the LCX, there is a possibility of acute circulatory collapse after main vessel stenting. In addition, a provisional one-stent approach for LMCA bifurcation with angiographic significant LCX ostial disease found a higher risk of early target-vessel MI and target-vessel revascularization at 1 year. Therefore, the presence of significant disease in the LCX ostium is regarded as an important factor in choosing a stenting strategy. Table 24.2 summarizes the selection criteria for stenting strategies based on the anatomic features involving the LMCA bifurcation. The provisional one-stent approach is preferred for LM bifurcations with insignificant stenosis at the ostial LCX or a nondominant left coronary system. By contrast, the elective two-stent technique is preferred in patients with significant LCX ostial stenosis with a dominant left coronary system. When there is angiographic ambiguity of LCX ostium, direct imaging of LCX ostium using IVUS may prevent circumflex occlusion or unnecessary complex bifurcation intervention.

TABLE 24.2

Favorable or Unfavorable Anatomic Features for Single-Stent Crossover Stenting in the Treatment of Unprotected Left Main Coronary Artery Stenosis

From Moussa ID, Colombo A, eds. Tips and Tricks in Interventional Therapy of Coronary Bifurcation Lesions . New York: Informa Healthcare; 2010:135.

	Anatomic Features
Favorable	Insignificant stenosis at the ostial LCX with Medina classification 1,1,0 or 1,0,0 Diminutive LCX with <2.5 mm in diameter right-dominant coronary system Wide angle with LAD No concomitant disease in LCX Focal disease in LCX
Unfavorable	Insignificant stenosis at the ostial LCX with Medina classification 1,1,1; 1,0,1; or 0,1,1 Large size of LCX with ≥2.5 mm in diameter left-dominant coronary system Narrow angle with LAD Concomitant disease in LCX Diffuse disease in LCX

LAD, Left anterior descending coronary artery; LCX, left circumflex coronary artery.

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