https://orcid.org/0000-0002-4636-3339
Abstract
Recently, increased length of the ascending aorta has been suggested as a possible risk factor for acute type A aortic dissection (ATAAD). Our goal was to identify measurable aortic geometrical characteristics associated with elongation that could differentiate ATAAD from uncomplicated aortic dilation (>45 mm).
In angiographic computed tomography scans performed in 180 patients having cardiac surgery, aortic diameters, root length, length of the ascending aorta at both the centreline and the greater curvature (convexity) and the root-ascending (root-asc) angle (that between the root axis and the axis of the ascending tract) and the ascending-arch (asc-arch) angle (that between the axis of the ascending aorta and the arch axis) were measured and compared among 3 patient groups: normal aorta (diameter < 45 mm), dilation/aneurysm (>45 mm) and ATAAD. Correlations between diameters and angles, diameters and lengths and lengths and angles were analysed; multivariable analysis including geometrical factors was performed to identify independent predictors of ATAAD.
Both patients with aneurysms and patients with ATAAD showed significantly elongated ascending aortas (P < 0.001 vs normal). However, in the aneurysms, the root-asc angle (136° ± 20° vs 147° ± 17°; P < 0.001) and in ATAAD the asc-arch angle were uniquely narrower than that in the normal aorta (116° ± 11° vs 132° ± 19°; P < 0.001). All patients with an ATAAD had an asc-arch angle ≤130°. Both in patients with ATAAD and in those without ATAAD, narrowing of the asc-arch angle was associated with elongation of the root segment (P < 0.001). In multivariable analysis, the asc-arch angle and the total length of the ascending aorta (root + tubular) were significant predictors of ATAAD.
The asc-arch angle is a promising measurement that could help predict aortic dissection along with aortic diameter and length: further verification is warranted.
INTRODUCTION
Acute aortic events, namely rupture and acute dissection, can complicate both syndromic and sporadic aortopathies. The rationale for elective surgery is to prevent those acute complications, and today it is indicated on the basis of a few criteria, mainly the maximal diameter of the vessel and its growth rate [1]. However, the limitations of this approach have long been pointed out: a well-known study from the International Registry of Aortic Dissection showed that the majority of acute type A aortic dissection (ATAAD) events occur with a maximal ascending aortic diameter smaller than 55 mm [2]. Moreover, both the historical recommendation of elective surgery at a diameter of 55 mm that still applies [1] and the above-mentioned study [2] were based on aortic diameters measured after ATAAD had occurred. Indeed, Rylski et al. [3] demonstrated that an abrupt increase of ∼30% in the diameter of the ascending aorta accompanies the development of ATAAD; thus it is likely that the diameter representing significantly increased ATAAD risk has been traditionally overestimated [4]. The ‘aortic size paradox’ argument has been advanced, indicating that in the general population normal diameters or mild dilatations are far more frequent than an overt aortic aneurysm. Therefore, given the pre-existence of mild or moderate dilatation, the relative risk of developing ATAAD should still be considered lower than the relative risk in the presence of severe dilatation [5]. Expanding the indication by simply lowering the cut-off diameter (e.g. to 45 mm) would expose a large portion of the general population to undue operative risk that is not justified by the rarity of the ATAAD event [6].
All of the above considerations have led several groups to explore alternative/adjunctive tools to risk-stratify patients, including aortic indexes to anthropometrics or length of the ascending aorta [7–11]. Historically, Krüger et al. [11] were the first to highlight the possible role of aortic elongation as a risk factor for ATAAD. Authors from Maastricht then investigated both the process of elongation occurring during normal aortic ageing [8] and the predictive value of aortic length towards the occurrence of dissection [9]. The database of the Yale Aortic Institute was reviewed to investigate retrospectively the relation between aortic length and acute aortic events [10]: The estimated risk was found to increase sharply between 11.5 and 12.0 cm and 12.5 and 13.0 cm of aortic length. Elongation of the aorta is accompanied by resulting geometric changes, i.e. curvature and angulation [8]. Our purpose in this study was to depict how the aorta is remodelled with elongation to find possible measurable parameters differentiating the cases evolving with chronic enlargement from those developing acute dissection.
PATIENTS AND METHODS
Ethical statement
The present study conforms to the Declaration of Helsinki. It was approved by our institutional review board for human research (December 2018, prot. 8/2018): patients gave consent to use their clinical data for research purposes. Specific consent to use the data for this study was waived by our institutional review board, due to the retrospective, observational nature of the investigation.
Study population
All electrocardiographic-gated computed tomography (CT) angiography (CTA) scans of the thoracic or thoraco-abdominal aorta performed in patients admitted to our adult cardiac surgery unit between August 2018 and March 2020 were reviewed for eligibility for the present study. These included preoperative CTA scans routinely performed in all patients scheduled for transcatheter aortic valve replacement, preoperative CTA scans of patients undergoing surgery for ascending aortic aneurysm or ATAAD, other chest CT scans performed for any reason in patients admitted to the cardiac surgery ward or seen in the outpatient clinic. Exclusion criteria were conditions that could alter the native orientation and angulation of the ascending tract: aneurysm of the arch or descending aorta, type B aortic dissection, previous aortic endovascular graft placement, previous cardiac surgery, paracardiac or para-aortic masses, iatrogenic ATAAD, chronic type A aortic dissection (Supplementary Material, Fig. S1). The resulting 180 eligible patients were divided into a normal aorta group (maximum diameter ≤ 45 mm, no sign of dissection: 79 patients), an aortic aneurysm group (maximum diameter > 45 mm, no sign of dissection: 66 patients) and an ATAAD group (regardless of maximum diameter: 35 patients).
Image analysis
In all CT scans of the aorta, using multiplanar reconstruction, the outer-outer diameter was remeasured independently in duplicate by 2 operators (A.D.C. and A.S.R.) using common DICOM viewers including the OsiriX (Pixmeo Ltd, Geneva, Switzerland) and Horos (Horosproject.org), at the following standardized levels in planes orthogonal to the vessel’s centreline: root (at the level of the maximum bulging of the sinuses, commissure-to-sinus, average of the 3 values); sinotubular junction (STJ); mid-ascending (at the level of the pulmonary trunk centreline); and proximal arch [at the most proximal point of the brachiocephalic trunk (BCT) take-off]. In the curved multiplanar reconstruction modality, using the centreline method, the following curvilinear aortic length parameters were obtained (Fig. 1): root length at the centreline (from the ventriculoaortic junction to the STJ), tubular tract of the ascending aorta at the centreline (from the STJ to the origin of the BCT), tubular tract of the ascending at the convexity, modifying the method reported in [11] by using the curved multiplanar reconstruction and total length of the ascending (root + tubular tract at the centreline). The ratio of the ascending length at the convexity to the ascending length at the centreline was calculated as an index of the longitudinal extent of contour deformation of the greater curvature (deformation index): of 2 hypothetical dilatations with the same maximal diameter, a more diffuse, cylindrical dilatation would have a greater deformation index than a more localized, fusiform one (Supplementary Material, Fig. S2).
(A) In curvilinear multiplanar reconstruction (a viewing modality in which sagittal view and coronal view are displayed in the upper left and right images, respectively; axial view and rectilinear reconstruction of the measured aorta in the lower left and right images, respectively), the lengths of the root and ascending aorta were measured with the centreline method; (B) curvilinear multiplanar reconstruction was used also to measure the length of the tubular tract of the ascending aorta at its convexity (greater curvature or right anterolateral aspect).
The anatomy of the aortic valve and the supra-aortic vessels was assessed in the same CTA scans to define the presence of bicuspid aortic valve (BAV) and bovine arch variants [12], conditions known to be associated with increased risk of ATAAD.
Two novel parameters quantifying the proximal aortic curvature were introduced, based on the concept that the elongation of the aorta, whether physiological (ageing) or pathological, implies changes in the angulation among its segments [13], namely the root, the tubular ascending and the proximal arch. Using a 2-dimensional thick slab mode with volume rendering, in the standard coronal view, the following 2 angles were considered: the ‘root-asc’ angle, i.e. that between the projection of the axis of the ventriculoaortic junction cross-sectional plane and the projection of the axis of the ascending aorta cross-section at the level of the pulmonary trunk centreline; and the ‘asc-arch’ angle, i.e. that between this latter axis and the projection of the axis of the arch cross-section at the most proximal point of the BCT take-off. The cross-sectional planes were defined manually as landmark lines, and the Cobb’s angle plug-in was used to obtain an automatic measurement of the angle between their axes [13] (Fig. 2 and Video 1).
This video shows the method employed for measurement of the asc-arch angle in a patient with non-dilated ascending aorta. Note that the asc-arch angles in the study were defined as the supplements of the angles measured by the tool.
An example of ascending-arch angle measurement using Cobb’s angle method. The landmarks of the ascending aorta at the centreline of the pulmonary trunk and the proximal arch at the origin of the brachiocephalic artery were manually traced; then the angle between the 2 axes of those landmarks was calculated automatically.
Statistical analyses
All continuous study variables were tested for normality of distribution and, if normal, they were consistently presented as mean ± the standard deviation and analysed using parametric methods. Categorical variables were tested by the χ2 and the Fisher’s exact test. For angle measurements, good interobserver and intraobserver reproducibility was ascertained in an initial sample of 50 patients by calculating the coefficient of variation (standard deviation of the differences, expressed as percentage of the mean). Comparisons among the 3 groups (normal aorta, aneurysm and ATAAD) were performed by one-way analysis of variance (ANOVA) with the Bonferroni post hoc analysis. Comparisons between pairs of patient groups (e.g. aneurysm vs ATAAD, asc-arch ≤ 130° vs asc-arch > 130°) were performed by the unpaired t-test. Correlations between diameters and lengths as well as between lengths (or other variables) and angulations were tested by the Pearson’s r correlation coefficient. A multivariable multinomial logistic regression analysis was performed predicting patient belongingness to the ATAAD group: To avoid multicollinearity issues, several regression models were run, first including all covariates significantly associated with ATAAD in univariate analysis (P < 0.05) and then serially removing the less strong predictor from each group of collinear variables and rerunning a regression analysis. This process was repeated until only 1 factor (e.g. STJ diameter) from each group of collinear factors (e.g. the diameters at all levels of measurement) was still among the covariates inserted. SPSS software version 24.0 was used for the analysis (SPSS, IBM Corp., Armonk, NY, USA).
RESULTS
Aortic diameters and lengths in normal, aneurysmal and dissecting aortas
Patients with normal, aneurysmal and dissecting aortas were comparable in terms of the main clinical characteristics, except for age (69 ± 13 vs 63 ± 13 vs 62 ± 11 years, respectively; P = 0.005 by ANOVA; Table 1). When we compared aneurysm and ATAAD patients, age did not differ significantly (P = 0.58; by t-test). Both aortic diameters and lengths were significantly greater in the disease groups compared to the normal aorta group; there was no significant difference between the aneurysm and the ATAAD group for any diameter (Fig. 3). Patients with ATAAD had longer roots than the patients with aneurysms (26 ± 4.6 vs 23 ± 6.5 mm; P = 0.024), the length of the tubular ascending tract being not significantly different (Fig. 3).
(A) Bar graph showing the mean diameter at the various levels of measurements in the 3 study groups. (B) Bar graph showing the mean aortic lengths in the 3 study groups (*P = 0.024; Bonferroni post hoc; all other differences were not significant; standard deviation in parentheses). ATAAD: acute type A aortic dissection; BCA: brachio-cephalic artery; STJ: sinotubular junction.
Patient characteristics in the overall study population and in the 3 groups
| Overall | Normal aorta (N = 79) | Aneurysm (N = 66) | ATAAD (N = 35) | P-valuea | P-valueb | |
|---|---|---|---|---|---|---|
| Age (years) | 65 ± 13 | 69 ± 13 | 63 ± 13 | 62 ± 11 | 0.005 | 0.58 |
| Sex (male) | 68% | 63% | 73% | 69% | 0.48 | 0.81 |
| Height (cm) | 166 ± 16 | 164 ± 21 | 168 ± 11 | 172 ± 10 | 0.14 | 0.34 |
| Weight (kg) | 82 ± 23 | 83 ± 31 | 84 ± 21 | 77 ± 10 | 0.55 | 0.36 |
| Hypertension | 81% | 73% | 82% | 94% | 0.29 | 0.54 |
| Bicuspid aortic valve | 25% | 20% | 33% | 20% | 0.16 | 0.17 |
| Bovine arch | 19% | 16% | 20% | 23% | 0.70 | 0.80 |
| Root phenotypec | 15% | 31% | 12% | 12% | 0.23 | 0.60 |
| Overall | Normal aorta (N = 79) | Aneurysm (N = 66) | ATAAD (N = 35) | P-valuea | P-valueb | |
|---|---|---|---|---|---|---|
| Age (years) | 65 ± 13 | 69 ± 13 | 63 ± 13 | 62 ± 11 | 0.005 | 0.58 |
| Sex (male) | 68% | 63% | 73% | 69% | 0.48 | 0.81 |
| Height (cm) | 166 ± 16 | 164 ± 21 | 168 ± 11 | 172 ± 10 | 0.14 | 0.34 |
| Weight (kg) | 82 ± 23 | 83 ± 31 | 84 ± 21 | 77 ± 10 | 0.55 | 0.36 |
| Hypertension | 81% | 73% | 82% | 94% | 0.29 | 0.54 |
| Bicuspid aortic valve | 25% | 20% | 33% | 20% | 0.16 | 0.17 |
| Bovine arch | 19% | 16% | 20% | 23% | 0.70 | 0.80 |
| Root phenotypec | 15% | 31% | 12% | 12% | 0.23 | 0.60 |
Analysis of variance across the 3 groups.
T-test of aneurysm versus ATAAD.
Only in patients with aortic diameter >40 mm.
ATAAD: acute type A aortic dissection.
Patient characteristics in the overall study population and in the 3 groups
| Overall | Normal aorta (N = 79) | Aneurysm (N = 66) | ATAAD (N = 35) | P-valuea | P-valueb | |
|---|---|---|---|---|---|---|
| Age (years) | 65 ± 13 | 69 ± 13 | 63 ± 13 | 62 ± 11 | 0.005 | 0.58 |
| Sex (male) | 68% | 63% | 73% | 69% | 0.48 | 0.81 |
| Height (cm) | 166 ± 16 | 164 ± 21 | 168 ± 11 | 172 ± 10 | 0.14 | 0.34 |
| Weight (kg) | 82 ± 23 | 83 ± 31 | 84 ± 21 | 77 ± 10 | 0.55 | 0.36 |
| Hypertension | 81% | 73% | 82% | 94% | 0.29 | 0.54 |
| Bicuspid aortic valve | 25% | 20% | 33% | 20% | 0.16 | 0.17 |
| Bovine arch | 19% | 16% | 20% | 23% | 0.70 | 0.80 |
| Root phenotypec | 15% | 31% | 12% | 12% | 0.23 | 0.60 |
| Overall | Normal aorta (N = 79) | Aneurysm (N = 66) | ATAAD (N = 35) | P-valuea | P-valueb | |
|---|---|---|---|---|---|---|
| Age (years) | 65 ± 13 | 69 ± 13 | 63 ± 13 | 62 ± 11 | 0.005 | 0.58 |
| Sex (male) | 68% | 63% | 73% | 69% | 0.48 | 0.81 |
| Height (cm) | 166 ± 16 | 164 ± 21 | 168 ± 11 | 172 ± 10 | 0.14 | 0.34 |
| Weight (kg) | 82 ± 23 | 83 ± 31 | 84 ± 21 | 77 ± 10 | 0.55 | 0.36 |
| Hypertension | 81% | 73% | 82% | 94% | 0.29 | 0.54 |
| Bicuspid aortic valve | 25% | 20% | 33% | 20% | 0.16 | 0.17 |
| Bovine arch | 19% | 16% | 20% | 23% | 0.70 | 0.80 |
| Root phenotypec | 15% | 31% | 12% | 12% | 0.23 | 0.60 |
Analysis of variance across the 3 groups.
T-test of aneurysm versus ATAAD.
Only in patients with aortic diameter >40 mm.
ATAAD: acute type A aortic dissection.
The correlations were tested between the root diameter and its length, between the tubular ascending tract diameter and its length (both at the centreline and at the convexity) and between the total length of the root plus the tubular and the maximal diameters, in the 3 patient groups. All those correlations were significant (Supplementary Material, Fig. S3): The lowest Pearson’s r values were observed in the aneurysm group, indicating that the degree of elongation accompanying dilatation can be more variable than in the other 2 groups.
Impact of bicuspid aortic valve and bovine arch on aortic length
When considering the whole study population, the presence of BAV was associated with greater ascending tubular tract diameter and length compared to patients with a tricuspid aortic valve (diameter: 48 ± 11 vs 43 ± 10 mm; P = 0.005; length at the centreline: 86 ± 15 vs 80 ± 14 mm; P = 0.03; length at the convexity: 118 ± 21 vs 107 ± 20 mm; P = 0.002). The presence of a bovine arch in the overall study population was not associated with an increase in diameters and lengths.
When considering only the patient subgroup with a normal aorta, in addition to a larger (38 ± 4 vs 35 ± 4 mm; P = 0.031) and longer tubular ascending aorta (100 ± 12 vs 92 ± 13 mm at the convexity; P = 0.028), patients with BAV also had an elongated root compared to patients with tricuspid aortic valve (22 ± 3 vs 20 ± 4 mm; P = 0.044). In the subgroup of patients with an aneurysmal aorta, a bovine arch was associated with greater root diameter compared to patients without a bovine arch (44 ± 8 vs 39 ± 5 mm; P = 0.004). All other diameter and length comparisons in subgroups showed no significant impact of BAV and bovine arch.
Geometry of aortic elongation in aneurysms and dissections
The 2 angles root-asc (P = 0.001 by ANOVA) and asc-arch (P < 0.001 by ANOVA) significantly differed among the patient subgroups (Fig. 4). The mean root-asc angle was significantly narrower in patients with an aneurysm than in the other 2 groups, whereas the mean asc-arch angle was significantly narrower only in patients with ATAAD. Intraobserver and interobserver variability was 2.1% and 2.7% (root-asc), 2.7% and 3.0% (asc-arch), respectively.
Bar graph showing mean root-asc and asc-arch angles in the 3 study groups. Analysis of variance showed significant differences across the groups for both (root-asc: P = 0.001; asc-arch: P < 0.001). Significant differences between group pairs determined by Bonferroni post hoc analysis are annotated; standard deviations are in parentheses. asc-arch: ascending-arch; root-asc: root-ascending.
Narrowing of the ascending arch angle
The mean value of the asc-arch angle in the whole study population was 130° ± 18°. There was no significant correlation between the asc-arch angle and the aortic diameter at the tubular ascending tract (r = −0.13; P = 0.09) and only weak correlations with diameters at other levels (root: r = −0.23, P = 0.003; STJ: r = −0.34, P < 0.001; origin of the BCT: r = −0.23, P = 0.003; maximum diameter: r = −0.19, P = 0.013). There was a significant inverse correlation of the asc-arch angle with root length (r = −0.43; P < 0.001), ‘deformation index’ (r = −0.39; P < 0.001), length of the tubular ascending tract at the convexity (r = −0.22; P = 0.005) and total length (root + tubular: r = 0.18; P = 0.026).
In Fig. 5, the maximum aortic diameter of each study patient is plotted against the respective asc-arch angle. The maximum diameter in 29% of the patients with ATAAD fell below the 45-mm diameter threshold; in 49%, below 50 mm; and in 66%, below 55 mm. No patient with ATAAD had an asc-arch angle >130° compared with more than half of the patients in the other 2 groups.
Scatterplot of the asc-arch angle against the maximum aortic diameter. The red horizontal line indicates the mean asc-arch angle in the whole population (130°); the grey vertical lines indicate different cut-offs of the aortic diameter (45 mm; 50 mm; 55 mm). asc-arch: ascending-arch; ATAAD: acute type A aortic dissection.
The ascending arch angle in patients with no dissection
The condition of the narrow asc-arch angle was then further explored to determine its associated features in the non-dissected patient groups (n = 145). Patients with an asc-arch angle narrower than 130° were taller (168 ± 11 vs 157 ± 22 cm; P = 0.031) and had a longer root segment (24 ± 6 vs 20 ± 5 mm; P < 0.001), larger root-asc angle (154° ± 13° vs 133° ± 18°; P < 0.001) and a trend towards greater root diameter (37 ± 7 vs 34 ± 5 mm; P = 0.05) and shorter ascending tubular tract as measured at the centreline (77 ± 13 vs 82 ± 14 mm; P = 0.05), compared to those with a larger asc-arch angle. The ‘deformation index’ of the tubular tract was in fact significantly greater in patients with an asc-arch angle narrower than 130° (1.39 ± 0.1 vs 1.29 ± 0.1; P < 0.001) (Supplementary Material, Fig. S2).
Multivariable predictors of acute type A aortic dissection
In multivariable multinomial regression analysis predicting whether the patient belongs to the ATAAD group (Table 2), the most significant independent predictor was the asc-arch angle, but the total length of the aorta (root plus tubular) also emerged, with a Nagelkerke pseudo-R2 of the model of 0.67.
Multivariable multinomial regression analysis results
| Reference group = normal aorta | B | OR | 95% CI | P-value |
|---|---|---|---|---|
| asc-arch angle | −0.05 | 0.95 | 0.91–0.99 | 0.017 |
| Total length (root + tubular) | 0.12 | 1.12 | 1.06–1.19 | <0.001 |
| STJ diameter | 0.27 | 1.31 | 1.14–1.50 | <0.001 |
| Reference group = aneurysm | B | OR | 95% CI | P-value |
| asc-arch angle | −0.07 | 0.93 | 0.89–0.97 | <0.001 |
| Reference group = normal aorta | B | OR | 95% CI | P-value |
|---|---|---|---|---|
| asc-arch angle | −0.05 | 0.95 | 0.91–0.99 | 0.017 |
| Total length (root + tubular) | 0.12 | 1.12 | 1.06–1.19 | <0.001 |
| STJ diameter | 0.27 | 1.31 | 1.14–1.50 | <0.001 |
| Reference group = aneurysm | B | OR | 95% CI | P-value |
| asc-arch angle | −0.07 | 0.93 | 0.89–0.97 | <0.001 |
Dependent variable = acute type A aortic dissection.
asc: ascending; CI: confidence interval; OR: odds ratio; STJ: sinotubular junction.
Multivariable multinomial regression analysis results
| Reference group = normal aorta | B | OR | 95% CI | P-value |
|---|---|---|---|---|
| asc-arch angle | −0.05 | 0.95 | 0.91–0.99 | 0.017 |
| Total length (root + tubular) | 0.12 | 1.12 | 1.06–1.19 | <0.001 |
| STJ diameter | 0.27 | 1.31 | 1.14–1.50 | <0.001 |
| Reference group = aneurysm | B | OR | 95% CI | P-value |
| asc-arch angle | −0.07 | 0.93 | 0.89–0.97 | <0.001 |
| Reference group = normal aorta | B | OR | 95% CI | P-value |
|---|---|---|---|---|
| asc-arch angle | −0.05 | 0.95 | 0.91–0.99 | 0.017 |
| Total length (root + tubular) | 0.12 | 1.12 | 1.06–1.19 | <0.001 |
| STJ diameter | 0.27 | 1.31 | 1.14–1.50 | <0.001 |
| Reference group = aneurysm | B | OR | 95% CI | P-value |
| asc-arch angle | −0.07 | 0.93 | 0.89–0.97 | <0.001 |
Dependent variable = acute type A aortic dissection.
asc: ascending; CI: confidence interval; OR: odds ratio; STJ: sinotubular junction.
DISCUSSION
The starting hypothesis of the present study was that elongation of the ascending aorta, recently identified as a possible risk factor for ATAAD in aortopathies [10], implies changes in its geometry that can be appreciated and quantified, for risk stratification purposes, as angulations among its segments [13]. It is believed that aortic elongation may be pathogenetically linked to ATAAD through increased deformational wall stress in the longitudinal direction, different from the circumferential stress increase caused by dilatation: indeed, intimal tears in ATAAD are usually horizontal with respect to the axis of the vessel, suggesting that they might result from failure of tissue strength in the longitudinal direction [14]. Our goal was to define a CT scan measurement that could be a promising candidate, once validated in further studies, as a marker of the evolution of the aortopathy towards the occurrence of ATAAD. Although they are preliminary, the results suggest that the asc-arch angle, i.e. the frontal projection of the angulation between the elongated ascending aorta and the proximal arch, could be such a measurement.
Pathogenetic concepts: relevance of anatomy to stress and failure
ATAAD arises from failure of aortic wall resistance to mechanical forces that develop from intravascular pressure and flow; wall strength can be impaired by altered extracellular matrix composition and structure [15]. Both the degenerative changes in the medial layer of the aortic wall and the consequent macroscopic dimensional changes, namely aortic dilatation and elongation, are observed both in the ageing process and non-specifically, more marked or simply earlier, in aortopathies [8, 9, 16]. The curvature of the ascending aorta has been suggested by mathematical models to play a fundamental role in determining the magnitude and distribution of aortic wall stress [17]. In a small recent study, a strong correlation was observed between the geometry of the aorta (as reconstructed from predissection CT images) and the distribution of wall stress: The site of the entry tear corresponded to the predissection area of maximal computed wall stress [18]. Thus, aortic elongation can indicate tissue pathological weakness, and curvature changes can cause abnormal stress: the 2 factors when combined may increase the risk of ATAAD. It has been shown that an aortic length cut-off of 98 mm (tubular tract only) had a seven-fold greater sensitivity (28%) for ATAAD risk prediction compared to the 55 mm aortic diameter cut-off (4%). However, 28% sensitivity is still modest in absolute terms [9]. Further improvement may come from the implementation of other geometrical factors, like angles defining the curvature of the elongated aorta.
Two different modes of aortic elongation
Although the preceding pathogenetic concepts are not new, research on ATAAD risk did not focus on the curvilinear length of the aorta and other 3-dimensional-based measurements until the general confidence in the methods of multiplanar CT reconstruction increased [9, 13]. Gode et al. [13] measured, as a metric of ascending aortic curvature, the angle between the axis of the valvular plane and the axis of the cross-sectional plane of the aortic arch at the level of the brachiocephalic artery: aneurysms of the ascending aorta showed a mean angle of 93° and ATAADs of 76°. In the present series, however, using a more standardized method of measurement, e.g. projection in the coronal plane for all patients instead of rotating the 3-dimensional image to find the narrowest diameter [13], the respective values for that angle were 91° versus 86° (P = 0.068) with no significant difference between aneurysms and ATAADs. Therefore, considering a third landmark plane (the section of the tubular ascending at the level of the pulmonary trunk centreline), the root-asc angle and the asc-arch angle were introduced. The root is in fact often horizontalized in markedly elongated aneurysms of the tubular tract of the ascending aorta. Of note, the typical elongation occurring with ageing implies transition from type I arch configuration (in which the 3 branches arise at the same axial level) to type II (the BCT origin is displaced at a lower level than the other 2 branches) or type III (both BCT and left carotid origins are lower than the left subclavian) [8, 12]. Prompted by concomitant and predominant elongation of the descending portion, the arch unfolds anteriorly and towards the right; as a consequence, the tubular ascending elongates ‘downwards’, consistent with narrowing of the root-asc angle. To simplify, it might be said that the mode of elongation in the aneurysm group was similar to the physiological elongation observed with ageing. Conversely, a narrowing of the asc-arch angle (in patients with and without ATAAD) was associated with elongation and dilatation of the root segment. When the root elongates, it is prevented from assuming a horizontal position, and the ascending aorta elongates ‘upwards’ at the expense of its angulation with the proximal arch (Supplementary Material, Fig. S4). The resulting configuration might imply excessive longitudinal stress in the ascending tract, predisposing the patient to ATAAD development, a hypothesis to be verified in further studies. So far, at least in the setting of BAV, the association between ‘root phenotype’ dilatation and risk of ATAAD has been suggested by a considerable amount of evidence [19, 20], whereas in aortopathies associated with tricuspid aortic valve this question has not yet been addressed. However, it is notable that the large study by Wu et al. [10], in which BAV patients represented only 20%, found significant association of the total ascending length (root + tubular) with acute aortic events. Nonetheless, those authors did not verify selectively the contribution of elongation of the root in this association. Indeed, practically, especially in aneurysmal aortas, the total ascending length could be a better measurement to consider than root length or tubular length alone because a dilated STJ can be difficult to identify, making it a poorly reliable landmark.
Cause or effect?
Few previous studies have examined the changes in aortic geometry occurring instantaneously as ATAAD develops [3, 21, 22]. Besides sounding the alarm about the actual aortic diameter at which aortas can dissect, one of those studies also found that the increase in aortic length was negligible (∼5%) compared to the acute change in diameter (20–30%) [3]. Wu et al. [10] reported the acute change in length to be even smaller, 2.7%. Their result could explain at least in part the better sensitivity claimed for aortic length compared to aortic diameter in predicting ATAAD [9]; however, no prospective study has ever confirmed this assumption. Predissection CT scans were not available for the present study but, based on the association of ascending aorta angulation with aortic elongation, it can be postulated that, similarly to aortic length, the asc-arch angle is not affected by significant acute changes during ATAAD development. Therefore, narrowing of the asc-arch angle is likely to be a pre-existing feature of the aortic geometry that predisposes to ATAAD. Notably, in 2 patients with iatrogenic ATAAD who were therefore excluded from the analysis (dissection developing during transcatheter procedures, no underlying aortopathy), the post-dissection asc-arch angles were 135° and 151° (Fig. 6). Although they are only anecdotal, these cases further suggest that the narrowing observed in non-iatrogenic ATAADs is not an effect of ATAAD itself.
Two cases of iatrogenic acute type A aortic dissection showed ascending-arch angles >130°, suggesting that the ascending-arch angle narrowing observed in non-iatrogenic acute type A aortic dissections may be a cause rather than an effect of dissection.
The strong correlation of the asc-arch angle with root length and the deformation index in patients without ATAAD suggest that root elongation and a more longitudinally extended deformation at the aortic convexity are the morphological characteristics determining a narrow asc-arch angle.
Limitations
Apart from the lack of predissection CT scans, discussed previously, another limitation of the present study is the relatively low number of patients included. Data collection is currently continuing in order to further analyse the association of a narrow asc-arch angle with ATAAD. It must also be noted that this study was hospital-based. This fact may represent a bias in relation to the description of the angulation of the aorta in the normal setting, and a population-based study would be needed for this purpose. The correlation between aortic length and the patient’s height is controversial: it was poor in Krüger’s studies [11, 23], whereas Wu et al. [10] reported a moderately good correlation, with a Pearson’s r = 0.34. We did not present measurements in this study indexed to height or other anthropometrics because none of the indexing methods are common practice today; nevertheless, when repeating the analysis after indexing the aortic length and angles to body height, the main results were all confirmed (data not shown). Given the relatively small population, at this stage of the still ongoing research, we tried to keep the number of variables in the analysis as low as possible; however, the possible influence of other factors on aortic geometric changes remains to be assessed, including aortic valve function, which will be implemented in the analysis in our next larger study.
CONCLUSIONS
Aortic diameter >55 mm has been demonstrated to be a specific but poorly sensitive predictor of ATAAD [2, 7]; an asc-arch angle <130° appears to be a highly sensitive but less specific predictor. Although further studies on larger series are needed, to include an adequate number of predissection CT scans and possibly develop a predictive model, this parameter seems promising as a possible synergistic adjunct to aortic diameter in future risk stratification for ATAAD.
Conflict of interest: none declared.
Author contributions
Alessandro Della Corte: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Supervision; Visualization; Writing—original draft. Antonino Salvatore Rubino: Conceptualization; Data curation; Investigation; Methodology; Visualization; Writing—review & editing. Antonio Pio Montella: Data curation; Investigation. Ciro Bancone: Data curation; Investigation. Federica Lo Presti: Data curation; Investigation; Visualization; Writing—review & editing. Denise Galbiati: Data curation; Investigation; Visualization. Giovanni Dialetto: Data curation; Investigation; Methodology. Marisa De Feo: Data curation; Supervision.
Reviewer information
European Journal of Cardio-Thoracic Surgery thanks Tobias Krüger, Piergiorgio Tozzi and the other, anonymous reviewer(s) for their contribution to the peer review process of this article.
Presented at the 34th Annual Meeting of the European Association for Cardio-Thoracic Surgery, Barcelona, Spain, 8–10 October 2020.
REFERENCES
ABBREVIATIONS
- ANOVA
Analysis of variance
- ATAAD
Acute type A aortic dissection
- BAV
Bicuspid aortic valve
- BCT
Brachiocephalic trunk
- CT
Computed tomography
- CTA
Computed tomography angiography
- STJ
Sinotubular junction
