Introduction

Aortic valve stenosis (AS) is the most common valvular heart disease in western countries [1]. The extent of aortic valve calcification (AVC), measured by multislice computed tomography (MSCT), correlates well with the degree of AS severity [2, 3], and is an integral part of current guideline recommendations for the management of patients with valvular heart disease [4]. While sex- and severity-related distribution of aortic valve calcification are well-researched [5, 6], very little is known about predictors for (a)symmetrical leaflet calcification, which can be frequently observed in daily clinical practice.

Therefore, we performed an in-depth analysis of calcium distribution patterns and favoring, underlying conditions in patients with severe, tricuspid AS.

Methods

Study population

We retrospectively enrolled 567 patients with severe tricuspid AS, who underwent routine pre-procedural planning for transcatheter aortic valve replacement (TAVR) at the Heart Center Düsseldorf. Patients with prior aortic valve replacement and bicuspid aortic valve were excluded to guarantee comparability between groups. AS severity was defined according to the current European guideline recommendations for the management of patients with valvular heart disease [6]. Patients were separated into an asymmetrical (AC) and a symmetrical (SC) leaflet calcification pattern. In case of AC, participants were further divided by the dominant calcified cusp (non-coronary; NCC, left coronary; LCC and right coronary cusp; RCC). Asymmetrical leaflet calcification was defined as a difference of > 150 Agatston Units (AU) in-between the three leaflets, which is also visibly subsumable.

All patients provided written informed consent for the use of clinical, procedural, and follow-up data for research. The study procedures are in accordance with the Declaration of Helsinki. All necessary ethical oversight was secured: the Local Ethics Committee approved the study protocol (4080) and the study is registered at clinical trials (NCT01805739).

Statistical analysis

Continuous data are described by means with standard deviation, median with upper and lower 95% confidence interval (CI) or interquartile ranges (IQR). Categorical variables are expressed by frequencies and percentages of a whole. Sex-related differences are analysed with the 2-sided Student’s t test for continuous variables and the chi-square test (Fisher’s exact test) for categorical variables. Univariate and multivariate analysis are used to identify dependent and independent predictors for asymmetric and symmetric calcification distribution. Others calcification distribution patterns were excluded due to the small sample size (NCC/LCC; NCC/RCC; LCC/RCC) Only co-variates with a p-value below 0.1 in the univariate analysis qualified for multivariate binary logistic regression. Receiver-operating-characteristic (ROC) curves are described as c-indices (area-under-the-curve) with 95% CI. The data analysis was performed with SPSS (version 22.0, SPSS, Inc., Chicago, IL) and GraphPad Prism (version 6.0, GraphPad Software, San Diego, CA). All statistical tests were 2-sided, and a p-value < 0.05 was considered statistically significant.

MSCT imaging acquisition protocol and three‐dimensional (3D) reconstruction

Pre-procedural cardiac MSCT was routinely performed as native and contrast-enhanced multi-slice CT in all patients. CT data were obtained using a 128-slice, single source CT-scanner with temporal resolution of 150 ms and a collimation of 128 × 0.6 mm (“SOMATOM Definition AS+”, Siemens Healthcare, Forchheim, Germany). Images were taken in accordance with TAVR-related standardized recommendations for CT image acquisition [7]. They were transferred to a dedicated workstation for evaluation (3mensio Structural Heart™, Pie Medical Imaging BV, Maastricht, The Netherlands) and reconstructed in the coronal, sagittal and axial planes. After identification of the virtual annular plane, three hinge points were set on the axial plane, and 3D volume-rendered reconstruction initiated. We assessed the calcium amount of the aortic valve and the upper left ventricular outflow tract (LVOT) within 1 cm below the annulus. The calcification was re-calculated in Agatston Units (AU) for the overall- and separated leaflet calcium burden assessment. Calcium originating from extra-valvular structures, such as the mitral valve annulus, the ascending aorta, and the coronary arteries was cropped.

Results

Baseline characteristics

567 Patients were divided either into an asymmetrical (AC, n = 443; 78.1%) or a symmetrical (SC, n = 124; 21.9%) leaflet calcification pattern. In the AC group, the NCC was the most calcified cusp (n = 238; 57.7%), followed by the RCC (n = 72; 12.7%) and the LCC (n = 58; 10.2%). Dominating NCC/RCC calcification was observed in 6.3% (n = 36) of patients, followed by a pronounced NCC/LCC calcification in 4.8% (n = 27), and prominent LCC/RCC calcification pattern in 2.1% (n = 12) of cases.

SC was more common in females (AC/SC: 49.2% vs. 67.7%; p < 0.0001) and was associated with various baseline characteristics such as immunosuppression (AC/SC: 2.7% vs. 8.7%; p = 0.014) and chronic obstructive pulmonary disease (COPD) (AC/SC: 28.4% vs. 45.2%; p = 0.001). Other comorbidities, rheological factors, and hemodynamic values were comparable between groups.

In patients with AC, a lower wedge pressure (AC/SC PCWP: 16.7 ± 8.1 mmHg vs. 20.5 ± 5.1 mmHg; p = 0.005), a smaller aortic valve area (AC/SC AVA: 0.7 ± 0.2 cm2 vs. 0.8 ± 0.2 cm2; p = 0.009), and higher transvalvular gradients (AC/SC mean pressure gradient: 62.2 ± 23.2 mmHg vs. 54.2 ± 23.2 mmHg; p = 0.001) were observed. Overall, patients with AC have an overall increased calcification burden (AC/SC: 2208 AU [1188–2906] vs. 1143 [495–1641]; p < 0.001) an larger aortic root dimensions as compared to SC patients. Further baseline information are displayed in Table 1.

Table 1 Patients’ clinical and functional characteristics
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Univariate and multivariate predictors for calcification distribution patterns

Symmetrical calcification

Multivariate analysis depicted the presence of COPD (OR 2.15 [1.26–3.65], p = 0.005), an LVOT calcification < 25AU (OR 1.81 [1.09–3.00], p = 0.021), a mean gradient below 36 mmHg (OR 1.77 [1.03–3.05], p = 0.039), and an annulo-apical angulation above 67° (OR 1.68 [1.00–2.80], p = 0.049) as predictive for a SC pattern. However, c-statistics—even when combined—only showed a moderate correlation (Table 2).

Table 2 Discrimination performance (ROC and AUC statistics)
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Asymmetric calcification with dominating NCC calcification

Univariate analysis depicted the presence of malignancy, male gender, the absence of a porcelain aorta, previous coronary artery bypass grafting, and a larger aortic anatomy as possible influencing factors for dominant NCC calcification. Multivariate analysis identified the absence of a porcelain aorta (OR 2.03 [1.07–3.86], p = 0.031) and previous coronary artery bypass grafting (OR 1.95 [1.14–3.32], p = 0.0014) as independent predictors for a pronounced NCC-calcification. However, c-statistics—even combined—remained only in a poor range (Table 2).

Asymmetric calcification with dominating LCC calcification

Prominent LCC-calcification was linked to a porcelain aorta, a smaller aortic valve area, higher pressure gradients and pronounced LVOT calcification. Multivariate analysis offered no independent predictor for dominating LCC-calcification in patients with AC pattern.

Asymmetric calcification with dominating RCC calcification

Concerning RCC calcification univariate analysis identified, inter alia, the absence of a permanent pacemaker (PPM) at baseline, an annulus ellipticity index < 1.22, larger dimension of the sinus of Valsalva, a greater RCA-to-RCC leaflet ratio (RCA/RCC > 1.43) and an annulo-apical angulation < 67° as potential predictors.

In a multivariate analysis the absence of PPM at baseline (OR 6.01 [1.40–25.78], p = 0.016), an annulus ellipticity < 1.22 (OR 2.78 [1.55–4.97], p = 0.001), an RCA/RCC leaflet ratio > 1.43 (OR 2.04 [1.15–3.65], p = 0.016), and an annulo-apical angulation < 67° (OR 1.98 [1.11–3.55], p = 0.022) proved to be independent predictive factors for prominent RCC calcification. c-Statistics remained only in a moderate range (Table 2). A graphical illustration of the read-out is given in Fig. 1. Detailed results of uni- und multivariate regression analysis can be found in the Supplement (Table 3).

Fig. 1
figure 1

Calcification distribution and independent predictors. The amount of calcium of the aortic valve and surrounding structures were assessed and re-calculated in Agatston Units (AU) for overall- and separated leaflet calcium burden. Symmetrical and asymmetrical leaflet calcification with the dominant calcified cusp (non coronary cusp-magenta; left coronary cusp-blue; right coronary cusp-yellow) are displayed with independent predictors—identified by multivariate analysis. CABG coronary artery bypass grafting, COPD chronic obstructive pulmonary disease, dPmean mean pressure gradient, PPM permanent pacemaker, Ratio RCA/RCCb ratio of the right coronary artery to the base of the right coronary cusp

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Discussion

To our knowledge, this is the first study with a systematic and in-depth analysis of AVC distribution, which includes comorbidities, hemodynamic parameters, and anatomical pre-dispositions of patients with severe tricuspid AS undergoing TAVR.

Our retrospective study revealed that:

  • A symmetrical calcification pattern is more frequently observed in females who have narrow aortic root dimensions.

  • Cumulative leaflet calcification is higher in patients with asymmetrical leaflet calcification patterns.

  • Independent predictors for prominent calcification of single aortic valve cusps vary widely and have only poor to moderate predictive value:

  • Independent predictors for symmetrical leaflet calcification are COPD, a less pronounced LVOT calcification, lower mean pressure gradients, and a more horizontal aorta.

  • Independent predictors for pronounced NCC calcification are previous CABG and the absence of a porcelain aorta.

  • Independent predictors for a prominent RCC calcification are the absence of a PPM, a less elliptical annulus index, a higher RCA/RCC ratio, and a less horizontal aorta.

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Asymmetrical vs. symmetrical AVC distribution

Koshkelashvili et al. retrospectively analyzed 318 non-contrast axial chest CT scans of subjects aged over 65 years in an all-comers cohort. They could also show that the LCC was frequently the most calcified cusp [8]. The study was not performed in patients solely presenting with AS but provided important information on race-related differences in early calcification patterns. In our study, race-differences were not considered to be relevant since over 99% of the study population were Caucasian. Overall, the number of studies concerning AVC patterns is limited since AVC is pre-dominantly examined in total and semi-quantitatively during pre-procedural planning for TAVR [9, 10]. It has already been shown that AVC distribution is linked to important clinical endpoints such as the occurrence of conduction disturbances, the risk of annular rupture, coronary occlusion or paravalvular leakage [11,12,13,14]. Especially asymmetric calcification, possibly leading to increased cardial forces on the NCC and RCC and adjacent structures such as the intramembranous septum, and pronounced LCC calcification are relevant variables for a permanent pacemaker implantation post TAVR [15, 16].

Current studies do not provide insight in pre-dispositioning factors, which are linked to the presented calcification pattern in patients with severe tricuspid AS. Furthermore, fibrosis is an essential factor in degenerative AS, leading to higher-grade stenosis under non-severe AVC-thresholds, especially in women. Sex-related differences in the progression and clinical phenotypes of aortic valve stenosis are well researched. High AVC is more likely in men and shows a strong correlation with the severity of AS in both sex [6]. Woman have a slower progression rate of AS but lower calcium levels are already sufficient to create the same hemodynamic impairment as in men with higher AVC load [17]. This might be explained by the fact that women are more prone to higher levels of valvular fibrosis and dense connective tissue at the same degree of hemodynamic relevant aortic stenosis severity as compared to their male counterparts [18]. Since the SC group primarily consists of women, the lower aortic valve gradients and the lower overall AVC load may be explained by the aforementioned facts.

Shear forces, aortic root entrance angles, and flow-patterns may also play an important role in AVC distribution and should be further analyzed. A horizontal aorta, the extreme form of increased aortic root angulation and the annulo-apical entrance angles, is known to limit maneuverability of self-expandable devices [19, 20] but also means a potential shear force trigger, probably leading to flow-dependent calcification. However, flow is pre-dominantly linked to myocardial function. Surprisingly, neither cardiac output, higher-grade reduced left ventricular ejection fraction or other hemodynamic characteristics in this context took a particular influence on calcification distribution, supposing that calcification may be more significantly linked to aortic flow characteristics.

Dominant leaflet calcification in patients with AC

Permanent ventricular pacing was shown to be associated with alterations in regional myocardial and coronary perfusion [21] and may, therefore, be linked to altered calcification patterns. Coronary artery disease was no dependent or independent predictor in our analysis, but the distribution of coronary artery disease and the distance of the coronary arteries might have an impact on AVC patterns through altered sinus perfusion and connected ostial calcification areas. Effacement of the sinuses may impair coronary flow and increase the mechanical stress and thus structural degeneration of the aortic valve leaflets [22]. In bicuspid valves, deterioration of aortic blood flow or eccentric flow is well-known to increase radial pressure and shear stress on the aortic wall [23], also causing enhanced dilatation of the aortic root, closing the circle towards aortic entrance angles and a horizontal aorta in tricuspid valves. A sub-analysis of the TAVI-WIN Registry [24] could show that increased calcium volumes of the RCC were an independent predictor for new pacemaker implantation after TAVR, whereas increased calcium volume of the NCC had a protective effect. However, findings are highly contradictive throughout current literature [14, 25]. Pathophysiologically, it remains unclear how different calcification patterns are created: in the initiation phase of disease, endothelial damage allows lipid infiltration and subsequent inflammation. In the propagation phase pro-fibrotic pathways and microcalcification smooth the way for further calcification [26]. Biomechanical studies could link high strain to the formation of calcific noduli and disease progression [27, 28]. Therefore, variable cusp geometries and congestive differences in length may also contribute to different calcification patterns, rather then underlying comorbidities.

Limitations

This is only a single-center analysis and limited to the retrospective quality of available data, which is also reflected by c-statistics. Furthermore, several important factors, like pressure recovery and ventriculo-arterial impedance, were not analyzed. This study of pre-disposing factors for different calcification patterns lacks translational value and does not influence clinical decision making.

Future directions

This is the first study trying to determine predictive factors for different types of aortic calcification patterns. Predictive factors vary widely throughout baseline characteristics and only showed poor to moderate correlation. This study may encourage clinicians to perform quantitative calcium assessment measurements in pre-TAVR MSCTs to create further evidence in this field.

Conclusions

Data from this retrospective analysis indicate that SC occurs more frequently in female patients and narrow aortic root anatomies, whereas cumulative leaflet calcification seems to be higher in AC patterns. Independent predictors for SC are COPD, a less pronounced LVOT calcification, lower mean pressure gradients and a more horizontal aorta. However, the correlation of different baseline characteristics with certain calcification patters were only in poor to moderate range. This is the first study addressing predictors for different calcification patterns, which are known to impact clinical outcomes of patients undergoing TAVR.