Thoracic endovascular stent graft repair (TEVAR) for treating type B aortic dissections (TBAD) : a hemodynamic and morphologic perspective

Abstract

TEVAR has been used extensively to treat TBAD. The principle of treatment involve placement of a stent graft in the true lumen to cover the primary tear, thereby excluding the false lumen. Success depends on a combination of factors: reduction of false lumen pressure and perfusion, thrombosis of the false lumen, and remodeling of the aorta leading to eventual healing. The long-term goals are to prevent continuous growth of the false lumen, reinterventions, and aneurysm rupture. The success of TEVAR depends on a combination of factors, including the blood flow and pressure in the two aortic lumens, and remodeling is a dynamic process. Much controversy exists regarding the ideal timing of TEVAR, its efficacy in effecting complete false lumen exclusion, the long-term durability of the repair, and the fate of the aortic size. The objective of this thesis is to examine the morphological and hemodynamic changes within the aortic lumens after TEVAR, using a combination of ex-vivo animal models and computational tomography analysis. The residual pressure of the true and false lumens in TBAD models was studied. Volumetric analyses of CT scan of patients were compared. The ultimate goals are to determine if it is beneficial to treat type B dissections early, and to determine long-term morphological results.

In ex-vivo hemodynamic study, 28 fresh porcine aortas models were created to simulate three different pathological scenarios of TBAD: model A represented pre-treated TBAD; model B represented post-treated TBAD with patent false lumen; and model C represented chronic stage of post-treated TBAD with false lumen thrombosis. True lumen and false lumen pressure differences were compared between the three models. Pressure effect was successfully reduced by 30% in model C in comparison with the other two models. No hemodynamic parameters were significantly different between model A and model B.

Aortic remodeling parameters were volumetrically analyzed and compared between two groups of patients who underwent endo-grafting for uncomplicated TBAD (group A) and dissecting aneurysms (group B). Modern DIOCM processing workstations and software were used to reconstruct thoracic aorta with serial CT scans. The true lumen, false lumen, thrombus and aortic size were measured volumetrically. Stent graft migration and area of inlet and outlet were also quantified. There were progressive migration and continuous expansion of the stent graft on patients in both groups. Favorable aortic remodeling was observed in most. One fourth of all patients demonstrated aortic volume increase at 36 months. However, there was no difference between group A and group B in terms of stent graft re-shaping and aortic remodeling.

In conclusion, Aortic remodeling after TEVAR in treating TBAD is a continuous process. There were no significant differences between chronic dissections and dissecting aneurysms in all morphological parameters. Treating chronic dissections before aneurysm formation does not seem to have a morphologic advantage. Sealing of primary entry tear with introducing thrombosis could significantly reduce false lumen pressure. However, the remaining pressure accumulations should be considered to reduce by further excluding distal reentry tears in those patients who undergo progressive false lumen expansion after TEVER.