A Review of the Clinical Utility of Intravascular Ultrasound to Treat Left Main Coronary Artery Disease

Abstract Background:

Revascularisation of left main coronary artery (LMCA) disease is technically complex procedure. Since publication of the results of the SYNTAX study, there has been particular interest in percutaneous revascularisation of the LMCA. It is becoming clear that percutaneous revascularisation of LMCA disease requires appropriate lesion preparation and carefully optimised stenting in order to offer patients a treatment option as effective as coronary artery bypass grafting. For this reason intravascular imaging, and especially intravascular ultrasound, is becoming a key procedural step in LMCA percutaneous coronary intervention. In this review paper we analyse the role of intravascular imaging with intravascular ultrasound in LMCA percutaneous coronary intervention, focusing on the main applications in this context from lesion assessment to stent sizing and optimisation.

Keywords: Revascularisation, percutaneous coronary intervention, left main coronary artery, intravascular

Introduction

The left main coronary artery (LMCA) obstruction incidence is relatively unusual in the catheterisation laboratory with isolated LMCA disease observed in only 5% of these cases. 1 Intervention to the LMCA is challenging compared to the treatment of coronary stenosis elsewhere in the coronary tree. First, potential complications occurring during LMCA intervention may rapidly progress towards haemodynamic instability, since the LMCA provides the blood supply to 80 % of the left ventricle in patients with right coronary dominance. 2 Second, disease of the LMCA is difficult to assess angiographically because of the possible lack of a proximal reference. 3 Third, atherosclerosis at the LMCA site is diffuse in many cases, frequently involving bifurcation, making LMCA lesions tougher with a consequent need for appropriate and careful lesion preparation 4

Percutaneous Coronary Intervention or Coronary Artery Bypass Grafting in LMCA Disease

Historically, coronary artery bypass grafting (CABG) represented the treatment of choice, with a well documented prognostic benefit.5 The Synergy between PCI with Taxus and Cardiac Surgery (SYNTAX) study showed that when coronary anatomy complexity is low or relatively low (defined by a SYNTAX score <32) , then CABG and PCI performed well have similar outcomes at 1- and 5-year follow-up, with PCI affected by a higher rate of repeated revascularisation and CABG by an higher rate of stroke.6 These conclusions led to a change in the guidelines, with PCI having a IB indication for LMCA disease when the SYNTAX score is <22 and IIaB when the SYNTAX score is >32 and <22 , with no current indications for PCI in LMCA disease when the SYNTAX score is >32 (IIIB indication)7 Two large recently completed trials (EXCEL and NOBLE) investigated CABG versus 'state of the art PCI' (e.g. with fractional flow reserve (FFR) and IVUS guidance and with second generation drug eluting stent adoption) in patients with LMCA disease and low and intermediate SYNTAX scores (<32) . Amongst the two, EXCEL trial showed non inferiority of PCI in LMCA compared to CABG in terms of MACE at 3-year follow-up but on the contrary, NOBLE did not confirm superiority of PCI over CABG in similar situation.8 So, there is a vital role of HEART team during the decision of this kind of complex revascularisation procedure. So for proving that PCI is as good as CABG; there has to be careful patient selection, procedural planning and the best available technology. The use of intravascular imaging, and usually IVUS, is highly recommended during elective LMCA intervention; as per IIa indication in the European guidelines.7

Applicability of IVUS in defining LMCA Anatomy and Plaque Distribution

LMCA assessment can be done by Coronary angiography, but it gives two dimensional picture and can be inaccurate if there is eccentric distribution of the atherosclerotic plaque or complex anatomy with tortuous overlapping segments of the coronary tree. Moreover, a coronary angiogram does not give insight into the vessel wall characteristics. In ostial LMCA involvement, for example, the coronary angiogram can be difficult to interpret, requiring the operator to rely on indirect signs of such as pressure damping during LMCA intubation or lack of contrast dye spilling back during selective injection. Conversely, IVUS is an accurate technique for assessment of both lumen and wall characteristics. The higher tissue penetration of ultrasound compared to infrared light gives better result than optical coherence tomography (OCT).

IVUS imaging does not require contrast injection to clear the lumen of blood. Although OCT is superior in assessment of distal LMCA assessment, IVUS should probably still be considered the first-line imaging method for LMCA ostium assessment. Before wide use of fractional flow reserve, IVUS was used to define the degree of stenosis in intermediate LMCA disease. Minimal lumen area (MLA) used as a parameter to determine when intervention to the LMCA could be safely deferred and initial cut-off was MLA ≤9.0 mm. After the start of use of FFR studies, this threshold was progressively lowered for deferring intervention, until the currently-accepted value of 6 mm. IVUS gives a better plan for PCI to the LMCA and can be integrated with FFR in assessing the indication for revascularisation when moderate to significant LMCA disease is present in the left anterior descending (LAD) or left circumflex (LCx) branch of the LMCA bifurcation. IVUS helps to unmask the potential to underestimate the true LMCA stenosis using FFR when significant concomitant proximal LAD or proximal LCx disease is present.10 IVUS can assess LMCA disease contribution to the overall the ischaemic burden when the FFR measured in the less diseased branch is in a 'grey zone' between 0.85 and 0.81 and could help the operator decide on the indication for LMCA revascularisation.

IVUS pullback should be performed on both branches of the LMCA bifurcation, which will produce information about the distribution of atherosclerosis at the site of the bifurcation. LMCA disease frequently involves both branches of the bifurcation, even when angiography appears to be 'normal'. Understanding the exact distribution of plaque burden at the bifurcation is clearly important when deciding whether a provisional stenting strategy can be applied to treat LMCA disease or a two stent strategy should be considered upfront.

A:           Critical LMCA disease with its bifurcation involvement.

B:           PCI done by DK Crush technique with final kissing balloon.

C:           IVUS checked to show good area achieved in LMCA, LAD, LCX and Polygon of Confluence;

D:           Pictorial diagram of summary

Qualitative Characterisation of LMCA Disease

IVUS can image the deeper layers of the arterial wall, assessment of plaque composition at the site of the LMCA. It is very common to have calcium in the lesion of LMCA and must be treated by rotational atherectomy to prepare the lesion. In this context IVUS defines the extension of calcium by the high backscattering signal with posterior shadowing11 IVUS can show the circular and longitudinal extension of the calcific arch, depth of the calcific component

Stent Sizing

IVUS can provide information about true vessel dimensions in order to facilitate stent sizing. LMCA has large calibre and also generally the mismatch in diameter between the LMCA and LMCA-bifurcation branches, stent sizing in LMCA PCI can be tricky using angiography alone. But IVUS can provide a more detailed definition of the lumen, with stent-sizing that takes into account not just one single diameter (as in angiography), but three diameters, namely the maximum, minimum and mean diameter. Also it provides better identification of proximal and distal landing zones that are free from disease.

Stent Optimisation

It is the main indication for IVUS in LMCA PCI. Large registries suggest a long-term mortality benefit in patients undergoing IVUS-guided compared to angio-guided PCI to the LMCA12

IVUS can detect Stent mal-apposition, stent under- expansion, geographical miss and large uncovered stent- edge. IVUS-assisted LMCA PCI can be associated with a lower rate of target lesion revascularisation and stent thrombosis. Stent under-expansion is the main predictor of stent failure. So, Kang et al. have proposed a minimum area that should be covered in each segment of the LMCA bifurcation after stenting (>5 mm at the ostium of the LCx; >6 mm at the ostium of the LAD; >7 mm at the polygon of confluence; and >8 mm at the LMCA).

Measurement of distal reference diameter calculates stent diameter and lesion length can calculate stent diameter. The MLA (minimum lumen area) and % diameter stenosis must be met to ensure stent expansion accurately. IVUS is done to both LAD and LCX vessel to see the true bifurcation type and extent of plaque and its characteristics.

Provisional single stent strategy from LMCA to LAD is done with POT. A second stent strategy should be done if there is sub-opimal angiographic LCX result and FFR is <0.8. Under this condition, second stent is put by ant method of bifurcation stent strategy; kissing balloon inflation is done and then final IVUS check is done to see if targeted MLA is achieved after final POT. It is important to recognise whether intervention on the side branch can be avoided. IVUS has a role in decision making for this. If carina shift is identified as the underlying mechanism, then kissing balloon inflation could be enough to reshape the carina and restore normal flow in both limbs of the bifurcation. Conversely, if plaque shift is detected then the operator may switch from the provisional single-stent strategy to a two-stent procedure.

Figure 02 & 03

Figure 3: Intended IVUS guided treatment plan for bifurcation lesion of LMCA of Madina classification 1,1,0.

Conclusion

IVUS has become a standard part of the PCI procedure for the treatment of LMCA disease. Improving the consistency of its application is likely to improve stenting technique and results, and consequently patient outcomes.

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