Introduction

In the dilated AA, altered blood flow velocities and displaced flow patterns have been reported (1). In patients with dilated AA with tricuspid aortic valve (TAV), total WSS values are lower as compared to those with non-dilated AA (2-4). The aim of the present study was to detect 4D flow MRI characteristics that are related to AA dilatation by comparing subjects with dilated AA to non-dilated AA in case of tricuspid aortic valve.

Methods

This prospective study included consecutive patients who were scheduled for follow-up due to dilated AA (diameter ≥ 42 mm) and who had TAV (n=20). The control group included 20 healthy volunteers with TAV without AA dilatation (AA diameter < 42 mm). The baseline characteristics of the study subjects are shown in Table 1. Anatomic images and 4D flow data were acquired using Siemens MAGNETOM Aera, 1.5 T (Siemens GmbH, Erlangen, Germany) scanner. Flow was evaluated in 10 planes (Fig. 1) at different levels of the thoracic aorta. Every plane was divided into six segments; 0-point is in the inner curvature of aorta (Fig. 1B) and first segment located anticlockwise. Main parameters under interest were flow displacement (FD), wall shear stress (WSS), regurgitation fraction, maximum velocity and flow, and backward velocity.

Results

In the dilated AA group, blood flow was displaced to outer curvature. FD was most prominent in planes 3 and 4 (Table 2). In plane 3, the median FD was 4.5 % in the dilated AA group compared to 2.0 % in the non-dilated AA group (p<0.001). WSS was higher on the lateral side of the aorta where blood flow was displaced, being 1.3-times higher on the side of the aortic wall that was closest to the center of flow as compared to the opposite side (p<0.01). WSS proved to be more circumferential in dilated AAs in comparison to non-dilated AAs. The fraction between circumferential WSS and total WSS (WSSC / WSS) was higher in the dilated AAs when measured from the inner curvature of AA in planes 1–4 and in the whole aortic ring in plane 5 (Table 3). In the dilated AA group, the backward flow fraction to forward flow was increased in planes 3–10 as compared to the non-dilated AA group (median 3.2 %, range 1.5–5.3 % vs. median 0.5 %, range 0.1–0.9 %, p<0.001). The maximum fraction was detected in plane 7 in the middle of the aortic arch, being 5.0 % in the dilated AA group as compared to 1.0 % in the non-dilated AA group (p<0.001). The maximum backward flow velocities were higher in the dilated AA group (37.1±13.5 cm/s) than in the non-dilated AA group (25.7±9.2 cm/s, p<0.01) in planes 1, 5, 6 and 7.

Discussion

The present study with TAV patients highlights that flow displacement in the aorta leads to increased WSS to the displaced side. In patients with dilated AAs, the fraction between circumferential and total WSS was higher in the inner curvature of the proximal AA and in the whole ring of the distal aorta when compared to the non-dilated AA group. This indicates that the flow pattern is helical throughout the whole dilated AA. Visual streamline inspection of our data shows that helical flow associates with a dominance of circumferential WSS (Fig. 1 and 2) which is in agreement with the published literature (7). In contrast, axial WSS is dominant in normal laminar flow (7).

Conclusion

Aortic flow is displaced in dilated AA compared to non-dilated AA. The flow displacement associates to the increase in WSS on the side of the displacement. This data might be used for the risk stratification of patients with AA dilatation in the future.

Acknowledgements

S. Petteri Kauhanen: Personal grant from the will of Oiva Vaittinen and the Rättimäki Cardiovascular Society

References

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