The Left Main Bifurcation Angle and Changes Throughout the Cardiac Cycle: Quantitative Implications for Left Main Bifurcation Stenting and Stents

Abstract: Objective. The objective of this study was to quantify the left main (LM) bifurcation angles and their changes throughout the cardiac cycle. Background. LM stenting is an accepted alternative to coronary artery bypass grafting. However, the LM bifurcation has great anatomic variability. Three-dimensional angles and their cyclic changes are important for coronary stenting. Methods. Patients undergoing coronary computed tomography angiography (CCTA) for chest pain were scanned and analyzed in three-dimensional views for left main-left anterior descending (LM-LAD), left main-left circumflex (LM-LCX), and left anterior descending-left circumflex (LAD-LCX) angles and their cyclic changes. Calculations and assessment of angles, angular variability, and how these angles change throughout the cardiac cycle were analyzed. Results. Forty-four patient scans were analyzed. The median end-diastolic LM-LCX angle was 130˚ and the LAD-LCX was 74˚. Median end-systolic angle for the LM-LCX was 133˚, and LAD-LCX was 69˚. Large differences were found across all three absolute angles (LM-LCX, LAD-LCX, LM-LAD). Marked variability also occurred in how these angles changed throughout the cardiac cycle. Conclusions. LM bifurcation geometry in patients shows marked absolute angle variability, as does diastolic-systolic angle movement. LM bifurcation stents should accommodate wide interpatient bifurcation angles at rest for both the LM-LAD and LM-LCX angles.

J INVASIVE CARDIOL 2015;27(9):401-404. Epub 2015 May 15

Key words: left main stenting, left main bifurcation, left main PCI

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Left main (LM) stenting is now an accepted alternative to coronary artery bypass grafting (CABG) in selected patients. The LM bifurcation represents a challenge for coronary stents because of its anatomy, and its movement during the systolic-to-diastolic phase transition is complex, both in absolute angle and dynamic deformation. Few studies have examined the absolute three-dimensional (3D) angles and their changes in living patients.^1-3 Coronary computed tomography angiography (CCTA) provides a 3D method for accurately measuring this motion. We utilized CCTA in living patients to determine the 3D angle changes between the LM, left anterior descending (LAD), and left circumflex (LCX) arteries.

Elucidation and assessment of the bifurcation angles will yield design parameters for stenting, both in understanding absolute anatomy and also in how these angles change throughout millions of cardiac cycles. These data could enable stent designs to be tested in benchtop settings or via finite element analysis, and result in design optimization for LM stents to ensure optimal clinical safety.

Methods

Study population. Scans were obtained from patients undergoing multislice CCTA for clinical indications, all of whom presented with chest pain in an inpatient or outpatient setting. No patient had severe LM or other coronary artery disease.

CTA scan protocol. Prior to CT scan, patients with heart rate (HR) >60 beats/min were given oral metoprolol 50-100 mg at 1 hour prior to scanning to maximize image quality. Additional intravenous (IV) metoprolol 5-40 mg was given immediately before scanning if HR remained >60 beats/min. Sublingual nitroglycerin 0.4 mg was administered shortly before scanning.

All patients were scanned using a dual-source 64-slice CT scanner (Siemens). Scans were done at 120 kV tube voltage, and isotonic IV contrast (Omnipaque 350) was administered through an 18 gauge peripheral IV. Scans were initiated by contrast bolus tracking at the level of the ascending aorta. All scans were prospectively electrocardiographically (ECG)-gated in a sequential mode.

Image reconstruction. Images were reconstructed with slice thickness of 0.6 mm in overlapping increments of 0.3 mm to limit slice misregistration. A medium-sharp reconstruction kernel (B30f) was typically applied. Cardiac phases were reconstructed every 5% between 0%-100% throughout the full cardiac cycle.

Qualitative coronary artery angle analysis. Multiplanar image reformatting constructed a plane containing the LM bifurcation and all vessel (LM, LAD, LCX) centerlines. Validated commercial software (Vitrea v. 4.1; Vital Images) was used to define coronary artery centerlines. Segments at least 1 cm in length along the vessels were used to measure bifurcation angles. Figure 1 shows a 3D volumetric reconstruction of the LM bifurcation anatomy.

Statistical analysis. Descriptive statistics were calculated as means and standard deviations for continuous variables. When continuous variables had skewed distributions, data were summarized as median and interquartile range. Shapiro-Wilk tests were used to determine if variables were normally distributed. Means comparison by t-tests or Wilcoxon signed-rank tests assessed statistical significance of continuous variables. A P-value of ≤.05 was considered significant and all P-values listed are two sided. All statistical calculations and plots were done with Stata 11.1 (Stata Corp).

Results

CCTA images from 44 patients were examined. Bifurcation angles showed wide variability for minimum and maximum angles. End-diastolic angle ranges (min-max) were LM-LAD (108˚-178˚), LM-LCX (74˚-170˚), and LAD-LCX (25˚-158˚), with similar broad variability at end-systole (Table 1). Table 2 shows how the angles changed with cardiac motion, mean angle change LM-LAD (1˚), LM-LCX (-3˚), and LAD-LCX (3˚), as shown in Figures 2A and 2B. Since the mean changes showed a wide angular spread, including negative and positive changes, we also calculated absolute values and ranges for these angular changes (Table 3). These showed mean LM-LAD angle of 6˚ (2˚-8˚), mean LM-LCX angle of 7˚ (3˚-9˚), and mean LAD-LCX angle of 7˚ (3˚-9˚).

These values demonstrate significant variability between absolute angles and substantial differences in minimum and maximum values. They also show profound dissimilarity in coronary anatomy across patients and great variability of cardiac motion on angular change.

Discussion

This study examined the LM coronary bifurcation angle in three dimensions. Two-dimensional invasive coronary angiography has limitations for accurately assessing coronary angle due to vessel overlap and foreshortening, and may not accurately represent the true bifurcation angle. CT angiography and its 3D visualization are more accurate than invasive angiography.^2,4

Our results are relevant because they augment and extend recent studies^5-11 suggesting that LM bifurcation angles are important for stenting technique, which in turn may correlate with major adverse cardiac event (MACE) rate. Bifurcation stent techniques can create difficulties during deployment, increasing restenosis and target lesion revascularization rates.^12-14

Few data exist regarding the clinical importance of the LM bifurcation angle change. However, as novel stents and materials (eg, thinner struts, new strut designs, novel materials such as bioabsorbable polymers) become clinically used, it is easy to speculate that shear stress due to wide angle variability could result in strut fractures. The cardiac-cycle angular variation may provide stent engineers a target with which to test the novel stents in benchtop cycling and assure that no fracture occurs.

Several stenting techniques are used for LM stenting, including crush, culotte, kissing-balloon, and T-stenting.^5,7,10,11 Data suggest that minimizing MACE may depend on matching stent technique to coronary bifurcation angle.¹¹ For example, T-stenting is considered optimal for LM bifurcation angles >70˚ because it provides complete coverage of the bifurcation.¹¹ Crush stenting may be more effective for angles <50˚ or <70˚.^9,10 Culotte stenting may be acceptable for many bifurcation angles, but this method can be time consuming and produces a high metal concentration in the proximal portion of the bifurcation.¹¹

A bench study replicating the LM bifurcation and its curvature found a positive correlation between the LM-LCX angle and the expansion ratio of the LCX stent.⁸ The LM-LCX angle was more important than the LAD-LCX angle with the crush technique, since small LM-LCX angles restricted stent apposition at the ostium.⁸ These studies highlight the LM-LCX bifurcation angle as the key for adequate stent deployment at the LCX ostium. We found the LM-LCX angle medians were 133˚ for systole and 130˚ for diastole, so that crush-stenting may not be optimal for typical LM-LCX angles; therefore, T-stenting or culotte techniques are better suited for such LM-LCX lesions.

In this study, the LAD-LCX bifurcation angle increased in the majority of patients (67%) during the cardiac cycle (Figure 3); these results were similar to those of Girasis et al.¹ Further investigation into this phenomenon is warranted, as it suggests that patients might be grouped based on coronary angle shifts during the cardiac cycle, and could determine which stent techniques yield better results by the bifurcation angle motion, especially regarding the takeoff angle of the LCX.

The question of whether bifurcation angle plays a substantive role in patient outcome is unanswered, since few 3D data exist for LM bifurcation angles. Our study suggests one possible reason, specifically high variability in absolute angles, and angle changes across patients.

The numbers derived above both for absolute LM angles and for changes in those angles within the cardiac cycle provide solid estimation for future LM bifurcation stent designs. Most importantly, the LM-LAD angle at both end-systole and end-diastole has a variability of roughly 68˚ (174˚-108˚) between the maximum and minimum values measured. However, the mean variation of this angle throughout the cardiac cycle across the entire patient group is small (about 6˚). While this 6˚ change might be considered “minimal,” this angle in reality corresponds to substantial stent movement. For example, the tip of a 3.0 x 15 mm stent being moved on a fulcrum at 6˚ moves a total of 1.5 mm. Such a stent at its distal end is being moved by fully half of its diameter. Moreover, the distance moved is variable depending on the point along the length of the stent, creating substantial metal shear within the stent. This clearly has implications for the possibility of stent fracture, considering that the stent may undergo 80 million cycles in its implant life.

The true variability between two standard projections of fluoroscopic coronary angiography are highly variable depending on the angles under consideration. This study demonstrated the perceived limitation of coronary angiography and superiority of CT, which provides true 3D angle capability. For example, in any angiographic right anterior oblique flat or mildly caudal view, the parent angle change between the LCX and LAD coronary arteries is minimal; however, the proper angiographic view would be impossible to achieve, as it would go directly through the patient’s head. CT, on the other hand, would show substantial parent angle change in this position.

The variability of the LM-LCX angle is about 115˚, although the cardiac cycle variability is also small (about 7˚). Taken together, this study suggests that LM bifurcation stents should expect wide interpatient angles for both the LM-LAD and LM-LCX, but would likely not see large angulation changes. This cardiac cycle variability will likely be even less since stent stiffness will probably somewhat impair the cardiac cycle-based bending of the stent.

Stent failure and presumed negative clinical endpoints will relate to the ability of LM stents to conform to a wide range of angles, although cyclic changes with the cardiac cycle will likely be mild.

Study limitations. One limitation of our study is that we did not examine patients with severe LM or other coronary artery disease. We speculate that the presence of distal disease should not affect the LM angles as they are measured, but might affect the systole-diastole variation throughout the cardiac cycle. This concept is an interesting hypothesis to test in future studies.

Conclusion

This study quantified in 3D bifurcation angles for the LM-LAD, LM-LCX, and LAD-LCX during the cardiac cycle using CCTA. All three angles had marked variations across patients in both absolute angle and angle, but milder changes from systole to diastole with cardiac motion. Whether this variability and angle information plays a role in patient outcomes is suggested, but unproven. If true, knowledge of individual angles and cyclic changes may make 3D measurements useful, since changes are small and likely do not present a challenge to future specific LM stent designs.

References

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From the ¹Minneapolis Heart Institute Foundation at Abbott Northwestern Hospital, Minneapolis, Minnesota; and ²ArraVasc Limited, Galway, Ireland.

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. The authors report no conflicts of interest regarding the content herein.

Manuscript submitted March 13, 2014, provisional acceptance given April 3, 2014, final version accepted November 7, 2014.

Address for correspondence: Robert S. Schwartz, MD, Minneapolis Heart Institute Foundation at Abbott Northwestern Hospital, 920 East 28th Street, Suite 620, Minneapolis, MN 55407. Email: robschwartzmd@gmail.com