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. Author manuscript; available in PMC: 2017 Mar 1.
Published in final edited form as: Int J Cardiovasc Imaging. 2015 Oct 22;32(3):461–467. doi: 10.1007/s10554-015-0792-x

Blood Flow Characteristics in the Ascending Aorta after TAVI compared to Surgical Aortic Valve Replacement

Ralf Felix Trauzeddel *, Ulrike Löbe †, Alex J Barker ‡,§, Carmen Gelsinger †, Christian Butter †, Michael Markl ‡,§, Jeanette Schulz-Menger *, Florian von Knobelsdorff-Brenkenhoff *
PMCID: PMC4886550  NIHMSID: NIHMS780744  PMID: 26493195

Abstract

Purpose

Ascending aortic blood flow characteristics are altered after aortic valve surgery, but the effect of transcatheter aortic valve implantation (TAVI) is unknown. Abnormal flow may be associated with aortic and cardiac remodeling. We analyzed blood flow characteristics in the ascending aorta after TAVI in comparison to conventional stented aortic bioprostheses (AVR) and healthy subjects using time-resolved three-dimensional flow-sensitive cardiovascular magnetic resonance imaging (4D-flow MRI)

Methods

Seventeen patients with TAVI (Edwards Sapien XT), 12 with AVR and 9 healthy controls underwent 4D-flow MR (spatial / temporal resolution 2.1×2.4×2.2mm3 / 38.4ms) of the ascending aorta. Target parameters were: severity of vortices and helices (semiquantitative grading from 0=none to 3=severe) and the local distribution of systolic wall shear stress (WSSsystole).

Results

AVR revealed significantly more extensive vortices and helices than TAVI (p=0.042 and p=0.002) and controls (p<0.001 and p=0.001). TAVI showed significantly more extensive vortices than controls (p<0.001). Both TAVI and AVR revealed marked blood flow eccentricity (64.7% and 66.7%, respectively), whereas controls showed central blood flow (88.9%). TAVI and AVR exhibited an asymmetric distribution of WSSsystole in the mid-ascending aorta with local maxima at the right anterior aortic wall and local minima at the left posterior wall. In contrast, controls showed a symmetric distribution of WSSsystole along the aortic circumference.

Conclusions

Blood flow was significantly altered in the ascending aorta after TAVI and AVR. Changes were similar regarding WSSsystole distribution, while TAVI resulted in less helical and vertical blood flow.

Keywords: magnetic resonance imaging, aorta, aortic valve replacement, shear stress, transcatheter aortic valve implantation, 4D-flow

Introduction

Transcatheter aortic valve implantation (TAVI) has become an accepted method for treating patients with severe aortic stenosis who are not eligible or at high risk for conventional surgical aortic valve replacement (AVR) [1-3]. Previous studies have demonstrated that changes in the geometry of the aortic valve such as bicuspid valves or AVR result in altered blood flow patterns and parameters [4,5]. These abnormalities may be associated with aortic remodeling and increased cardiac afterload [6,7].. As there is little knowledge about the effect of TAVI on the global patterns of blood flow in the ascending aorta, the aim of this study was to analyze the blood flow characteristics in the ascending aorta after TAVI in comparison to AVR with a stented aortic bioprosthesis and healthy controls using time-resolved, three-dimensional, flow-sensitive magnetic resonance (4D-flow MRI). This technique allows for the visualization of complex blood flow in the form of helices and vortices as well as quantifying local flow velocities and WSS [8]. We hypothesize that both AVR and TAVI will result in altered hemodynamics compared to controls.

Materials and Methods

Study sample

The local ethics committee approved the study. Written informed consent was obtained from all individuals. Twenty-three consecutive patients with TAVI with an Edwards Sapien XT® prosthesis were prospectively enrolled. This study focused on the Edwards Sapien XT® as at the time of study enrollment this was the preferred device in the cooperating TAVI center. The data of 12 patients with stented bioprostheses and 9 healthy controls were retrospectively analyzed from a previous study [5]. The characteristics of the study participants are summarized in Table 1.

Table 1.

Baseline characteristics of the study participants.

Parameter TAVI AVR Controls p-value
n 17 12 9 ---
Sex [females/males] 8/9 4/8 1/8 ---
Age [years] 77 ± 7 76 ± 4 55 ± 16 0.001
Prosthetic types Edwards Sapien XT (n = 17), Porcine: Medtronic Hancock (n = 4), Labcore (n = 1); Bovine: Edwards Perimount (n = 2), Sorin Mitroflow (n = 2), unknown (n = 3) --- ---
Labeled valve size 25.8 ± 2.2 23.2 ± 2.2 --- 0.012
Effective orifice area [cm2] 1.9 ± 0.3 1.5 ± 0.5 4.0 ± 0.8 < 0.001
Effective orifice area index [cm2/m2] 0.9 ± 0.3 0.8 ± 0.2 2.0 ± 0.3 < 0.001
LV* enddiastolic volume [ml] 157.0 ± 63.2 149.9 ± 61.8 139.6 ± 41.4 0.870
LV mass [g| 175.7 ± 59.3 165.2 ± 55.1 129.2 ± 25.3 0.079
LV stroke volume [ml] 87.9 ± 33.1 84.9 ± 32.3 91.4 ± 28.2 0.796
LV ejection fraction [%] 57.2 ± 10.1 58.0 ± 10.9 65.9 ± 6.1 0.038
Ascending aortic diameter [mm] 34.8 ± 3.1 38.5 ± 4.4 30.7 ± 4.8 0.002
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Results are given as mean ± standard deviation or as frequencies. The p-values are tested by the Kruskal-Wallis analysis.

*

LV, left ventricular

Image acquisition protocol

Images were acquired as previously described [5]. All subjects underwent a CMR examination at a 1.5 Tesla MR scanner (Magnetom Avanto, Siemens Healthcare, Erlangen, Germany). A 12-channel anterior body array coil was used for signal reception and the body coil for signal transmission. 4D-flow was acquired using a sagittal oblique volume covering the thoracic aorta. Prospective ECG gating was used with a respirator navigator placed on the lung-liver interface. The following scan parameters were chosen: echo time [TE] = 2.3 ms, repetition time [TR] = 4.8 ms, bandwith = 440 Hz/pixel, acceleration mode GRAPPA with factor 2 to 5, reference lines = 24, flip angle α = 9°, temporal resolution = 38.4 ms, field of view = 400 × 375 mm, matrix = 192 × 158, voxel size = 2.1 × 2.4 × 2.2 mm3, phase encoding direction = anterior-posterior, number of slices = 12 - 26, velocity encoding = 1.5 - 2.5 m/s.

Conventional ECG-gated, breath-held steady-state free-precession (SSFP) cine imaging was performed to quantify left ventricular function and the orifice area of the aortic valve and bioprostheses, respectively [9]. The effective orifice area (EOA) of TAVI could not be determined from SSFP images due to significant artefacts, but was taken from the literature [10,11]. Axial SSFP still images of the thorax were used to estimate the size of the ascending aorta at the level of the pulmonary bifurcation [12].

Processing and analysis of the images

All 4D-flow MRI data were processed as previously described [8]. Briefly, data were corrected for noise, eddy currents and velocity aliasing (MatLab; The MathWorks, Natick, MA, USA) [13]. In a second step, a 3D phase contrast MR angiogram was calculated based on the flow measurements to position the analysis planes and to visualize the blood flow (EnSight, CEI, Apex, NC). Three planes were positioned perpendicular to the longitudinal axis of the aortic wall: at the level of the sinotubular junction (S1), in the ascending aorta at the level of the pulmonary bifurcation (S2), and proximal to the brachiocephalic trunk (S3), as recommended in the literature [14] and previously shown [5]. The position of S1 was selected such that it was high enough to avoid signal artifact caused by the presence the metal in the prosthetic stents. These analysis planes were exported into previously reported software for the segmentation and calculation of the blood flow parameters (MatLab; The MathWorks, Natick, MA, USA) [13].

Left ventricular function quantification and planimetry of the orifice area were achieved by manual segmentation using commercial software (CVI42, Circle Cardiovascular Imaging, Calgary, Canada). Assessment of the aortic diameter was done at the in ascending aorta at the level of the pulmonary bifurcation [12].

Blood flow helicity and vorticity in the ascending aorta

Blood flow patterns were semi-quantitatively evaluated using pathline movies and classified as vortex and helix formation as previously described [5]. In short, helices and vortices were graded in 4 categories: 0 = none, 1 = mild, 2 = moderate, 3 = severe at the mid-ascending aorta [5].

Blood flow eccentricity in the ascending aorta

Blood flow was semi-quantitatively graded as central or mild eccentric or marked eccentric as previously described [5,15]. A central flow was characterized if the flow occupied the majority of the vessel lumen. A mild eccentric flow occupied two-thirds to one-third of the vessel and a marked eccentric flow occupied one-third or less of the vessel.

Wall shear stress in the ascending aorta

Quantification of systolic WSS (WSSsystole; unit N/m2) was performed for 8 regional segments along the aortic circumference for each analysis plane S1-S3 as previously described [17]. Regional WSS was averaged over three time points.

Statistical analysis

Analysis of the data was performed using SPSS 22 (IBM, Armonk, US). Graphics were created using PRISM 5 (GraphPad Software Inc, San Diego, California, US) and plug-in software for MatLab. Categorical data are expressed as percentages, continuous data as mean ± standard deviation (SD). The three groups (TAVI, AVR, controls) were compared using a Kruskal-Wallis test. WSSsystole was tested on a regional basis for significance using a paired two-tailed t-test for equal or unequal variances (depending on a two sample F-test). In case of significance, a Mann-Whitney-U test was added. Statistical significance was set at a probability level of < 0.05.

Results

Baseline characteristics

Table 1 summarizes the baseline characteristics of the study participants. Six patients were excluded from further analysis due to extensive respiratory motion and thus inefficient respiratory navigator. Healthy controls were significantly younger than patients with AVR (p=0.002) and TAVI (p=0.001) and had significantly larger EOA (p<0.001) and EOA index (p<0.001). They also had a smaller aortic diameter than AVR (p=0.003) and TAVI (p=0.021). AVR and TAVI were not different regarding age (p = 0.347), but AVR hat larger aortic diameter (p=0.018) and TAVI had larger EOA (p=0.027), but did not differ significantly from AVR regarding the EOA index (p=0.060).

Blood flow patterns

All examinations resulted in diagnostic image quality. Local artifacts occurred in the proximity of the prosthetic stents similarly for TAVI and AVR. The most proximal analysis plane was therefore positioned at the sinotubular level to warrant sufficient distance from the artifact. Representative blood flow patterns of the three groups are shown in figure 1. Figure 2 shows a comparison between the qualitative grades for vortex and helix severity. AVR revealed significantly more severe helices and vortices than TAVI and controls, whereas TAVI only had significant more vortices, but not helices, than normal subjects.

Figure 1.

Figure 1

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Representative blood flow patterns in the ascending aorta as illustrated with pathlines (TAVI, transcatheter aortic valve replacement; AVR, aortic valve replacement; EOA, effective orifice area; EF, ejection fraction; EDV, enddiastolic volume; AoD, ascending aortic diameter

Figure 2.

Figure 2

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Comparison of the severity of vortices (left) and helices (right)

Blood flow eccentricity

Figure 3 summarizes the grading of the blood flow eccentricity. Controls predominantly exhibited central flow. In contrast, 11/17 TAVI patients had a markedly eccentric flow and the remaining 6 showed mildly eccentric flow. Similarly, 8/12 AVR recipients had markedly eccentric flow and the remaining 4 were mildly eccentric. AVR and TAVI differed significantly concerning the mean value of eccentricity from healthy subjects (p<0.001). TAVI and AVR did not differ significantly (p = 0.777)

Figure 3.

Figure 3

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Prevalence of qualitatively graded blood flow eccentricity.

Wall shear stress

The distribution of WSSsystole in the ascending aorta is illustrated in figure 4. Compared to controls, TAVI revealed significantly higher WSSsystole at the right anterior segment at the mid-ascending aorta, and significantly lower WSS at the mid-ascending aorta and distal ascending aorta in the right-posterior, posterior, left-posterior and left segments. Similarly, AVR showed significantly lower WSSsystole compared to healthy controls at the posterior segments at S2 and S3 and at the left segment at S1. At the right anterior segment at the mid-ascending aorta, AVR showed elevated WSSsystole compared to healthy controls but which were not statistically significant. TAVI and AVR did not differ significantly regarding WSSpeak in all analysis planes.

Figure 4.

Figure 4

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Distribution of the peak wall shear stress in the ascending aorta. R=right, A=anterior, L=left, P=posterior. * indicates p < 0.05 and ** indicates p < 0.01

Discussion

In this pilot study, we used 4D-flow MRI to assess the blood flow in the ascending aorta after TAVI in comparison to AVR with stented bioprostheses as well as healthy controls. TAVI and AVR revealed significantly higher regional blood flow patterns in the form of helices and vortices as well as WSSsystole and a more eccentric blood flow than controls.

Both stented bioprostheses and TAVI consist of biological material mounted on a stent. Despite this similarity, AVR resulted in more distinct helices and vortices than TAVI. This may be attributed to the lower EOA of the AVR cohort in this study, but may also reflect differences in stent design. Whereas the TAVI device is fixed passively at the calcified aorta wall, stented bioprostheses contain a sewing ring that may be an unfavorable obstacle within the blood flow, even if the implantation is done principally completely supra-annularly. The latter aspect is supported by generally lower pressure gradients of TAVI compared to AVR reported in the literature [18,19]. Despite this potential advantage of TAVI, TAVI - as expected - also led to an abnormal blood flow pattern compared to healthy controls. At least for bicuspid aortic valves, a correlation of blood flow pattern and aortic growth rate has been shown [6]. Furthermore, novel studies that link blood flow pattern to energy loss suppose an association of blood flow pattern and cardiac afterload [20,7]. Hence, future studies have to evaluate whether the altered blood flow of TAVI and AVR has impact on aortic and left ventricular remodeling as well. Both TAVI and stented bioprostheses revealed an eccentric distribution of blood flow velocities in the ascending aorta compared to healthy controls, who exhibited a physiological central flow. An eccentric flow is associated with regional elevation of WSS [4,20,6]. This is hypothesized to contribute to an increase of the aortic diameter and aneurysm formation [21] and may be associated with a increased viscous and turbulent energy losses [7,20]. TAVI and AVR revealed a similar eccentric distribution of WSS along the aortic circumference, which was significantly elevated and depressed in focal regions. This asymmetry clearly differed from healthy controls. Local abnormalities in WSS are thought to stimulate aneurysm formation [22,23]. In a recent study using computational fluid-dynamic analysis, WSS has been shown to be regionally elevated at the site of ascending aortic aneurysm formation [24]. Furthermore, studies reported that the WSS reached a critical value at aneurysm diameters of 6cm [25] - a cutoff, which is known be associated with a high risk of aortic dissection. Hence, it is notable that both TAVI and AVR lead to a WSS profile that imparts a regional hemodynamic abnormality at the aortic wall, which is suspected to increase the chance of an adverse vascular event. Whether the resultant WSS patterns are relevant for patients with calcified aortic stenosis, who typically have thickened and stiff aortic walls, is unclear. However, in patients with aortic regurgitation and thinned aortic wall, who may receive AVR or even TAVI in the future, this prior knowledge regarding the impact of AVR and TAVI may prove useful.

Limitations

i) The control group differed from the intervention groups regarding age, orifice area and aortic dimensions. Age has a known influence on the hemodynamics in the ascending aorta as well as the aortic diameter [26-28]. Therefore, this study has to be interpreted as a pilot study. ii) It is known that 4D-flow underestimates the true WSS [24]. However, as we chose the same technique in all subjects, the relative differences are comparable. iii) The orifice area of the TAVI prostheses was taken from literature, as direct measurement was infeasible due to artifacts. iv) This cross-sectional study was designed as a hypothesis generating pilot study, thus the enrollment was limited to small numbers. To examine the influence of the blood flow characteristics on the ascending aorta, a longitudinal study is required.

Conclusion

This pilot study demonstrated that TAVI and AVR with a stented bioprosthesis lead to altered blood flow characteristics in the ascending aorta compared to healthy controls, with more intense flow eccentricity and regional elevation of wall shear stress.

Acknowledgements

We thank our technicians Kerstin Kretschel, Evelyn Polzin and Denise Kleindiest for performing the CMR scans. We also thank our study nurses Elke Nickel-Szczech and Annette Koehler as well as the other working group members for their assistance in realizing the study.

Funding: This study was partly funded by the National Institutes of Health (NIH K25HL119608, R01HL115828)

Abbreviations

EOA

Effective orifice area

AVR

aortic valve replacement

SD

Standard deviation

SSFP

steady-state free-precession

TAVI

Transcatheter aortic valve implantation

WSS

Wall shear stress

4D-flow

Time-resolved three-dimensional flow-sensitive cardiovascular magnetic resonance imaging

Footnotes

Conflict of Interest: The authors declare that they have no conflict of interest.

Compliance with Ethical Standards

All human studies have been approved by the local ethics committees and have therefore been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. All persons gave their informed consent prior to their inclusion in the study.

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