Evaluation of Dynamic Intimal Flap Movement Using IVUS in Patients with Type B Aortic Dissections Before and after TEVAR

Objective: Evaluation and characterization of intimal flaps within the visceral aorta before and after thoracic endovascular stent graft placement in Stanford type B aortic dissections (TBAD). Methods: We evaluated 19 patients with TBAD: 11 chronic and 8 acute (less than 14 days). Intravascular Ultrasound (IVUS) recorded aortic flap movement at the level of the superior mesenteric artery during one RR-interval of the ECG. Flap movement was record before and after stent-graft placement. We defined and evaluated the following; intimal flap as the flap movement index (FMI) and flap area index (FAI) with measurements of the minimum and maximum aortic diameters (ADmin, ADmax), minimum and maximum true lumen diameters (TDmin, TDmax), as well as true lumen areas (TAmin, TAmax). Flap movement index was defined as [(TDmax/ADmax-TDmin/ADmin)]/TDmin/ADmin x 100 (%). The flap area index was defined as TAmax-TAmin/TAmin x100. Results are reported as the mean +/- the standard deviation. Significant P values are less than 0.05. Results: Mortality and SCI rates were 0%. All patients were successfully treated with complete thrombosis of the false lumen of the thoracic aorta at 1-month follow-up as shown by CTA. There was nearly complete remodeling of the thoracic aorta. There was an immediate and significant increase in the true lumen diameters and areas following stent graft repair in the visceral aorta. Following thoracic stent graft repair, the flap movement index and flap area index were reduced from 19.63 ± 2.3% to 10.66 ± 1.9% (P=.0001) and 95.65 ± 21 % to 18.85 ± 4.2% (P=.0001), respectively. Conclusion: Thoracic aortic stent grafting has become the treatment of choice for patients with complicated TBAD. This is the first study to evaluate and characterize the dynamic aortic flap movement of the visceral aorta in patients with TBAD immediately before and after thoracic stent graft repair using IVUS.


Introduction
Since its initial use in treating TBAD was reported in 1999, thoracic endovascular aortic repair (TEVAR) has evolved as the treatment of choice for complicated descending thoracic aortic dissections [1][2][3][4][5][6]. Endovascular repair in TBAD attempts to reestablish the pre-dissection hemodynamics of the aorta by covering the proximal entry tear and stimulating aortic remodeling [7]. Numerous studies have demonstrated aortic remodeling after TEVAR for TBAD [8,9]. Aortic remodeling, characterized by expansion of the true lumen and thrombosis and healing of the false lumen, is associated with the best outcomes following TEVAR [8,[10][11][12].
The immediate effects of TEVAR, however, are less frequently described. In an aortic dissection, the septum is of varying thickness and mobility. It is also sensitive to changes in the hemodynamic pressures within the aorta, often contributing to dynamic stenosis or occlusion of branch vessels. The placement of a thoracic stent

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graft has the potential to immediately improve flow within the true lumen and reduce dynamic obstruction caused by the intimal flap. The purpose of this study was to describe the intimal flap movement at the level of the visceral aorta intra-operatively prior to and following the deployment of the endograft in patients with TBAD.

Analysis of IVUS Data
The video recordings of the aortic and aortic flap movement were played on a DICOM viewer (Rubo Medical, Aerdenhout, The Netherlands) to evaluate the movement throughout a single cardiac cycle. A Scion PCI Frame Grabber (Scion Corporation, Frederick, MD, USA) was used to capture still images, which were then analyzed using ImageJ software to measure diameters and luminal areas.
The minimum (at diastole) and maximum (at systole) total aortic lumen diameter and area as well as the minimum and maximum true lumen diameter and area were measured. Measurements were obtained from the inner wall to the inner wall, the lumen was then bounded and the total areas of the aorta and of the true lumen were assessed quantitatively in square centimeters. Each

Statistical Analysis
Measures of diameter and area are described as mean ± standard deviation. Difference in area and diameters from systole to diastole were evaluated using a Student t-test for paired data.
Differences in measurements before and after TEVAR were evaluated using analysis of variance for repeated measurements.
Analyses of measurement method comparison data according to Bland and Altman were performed to analyze repeatability and to compare measurements by two observers (there was no significant difference between the measurements of the two observers at any point during this study). A P value less than 0.05 was considered statistically significant.

Results
Of the 19 patients with TBAD treated with TEVAR, 11 were chronic (>2 weeks), and 8 acute (<2 weeks). All patients were treated with the Valiant thoracic stent graft. Coverage in all patients was proximally from zone 2 (coverage of the left subclavian artery), with a distal extent down to 2 cm above the celiac artery.

Discussion
Since 1999, when the first papers describing the use of endografts to treat complicated type B dissections were published, TEVAR has been found to be a safe and effective alternative to open repair. It has been also shown to provide improved 5-year outcomes when compared to optimal medical therapy alone [1][2][3]6]. The primary goal of TEVAR during treatment of TBAD is to exclude blood flow to the false lumen. Persistent flow within the false lumen, especially partial thrombosis of the false lumen, is an important risk factor for aortic enlargement, and false lumen thrombosis is associated with improved outcomes [13][14][15][16]. TEVAR promotes false lumen thrombosis and remodeling by covering the primary entry tear and expanding the true lumen. On the other hand, continued false lumen patency did not affect the true lumen volume percentage after TEVAR (46.7% to 47.7%) [18]. It is our practice to maximize the coverage of the thoracic aorta in patients with TBAD, since remodeling may not occur in segments that are not stented [19]. In this study, we evaluated the visceral segment of the aorta, which often remains untreated and has fenestrations present, allowing continued flow into the false lumen. In this study, we were able to clearly demonstrate that with maximum thoracic stent graft coverage to just above the celiac artery, there is a significant increase in the true lumen diameter and volume even in the untreated visceral segment of the aorta.
Compression of the true lumen can be dramatic, especially in acute IVUS clearly demonstrated that there is a significant increase in true lumen diameter and volume immediately after TEVAR. We were also able to dynamically study the aortic intimal flap with the use of IVUS and to assess the immediate effects of intervention on flap mobility. We found a significant decrease in the movement