Discussion

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DR. STARNES: Chairman, ladies and gentlemen. At first glance, computational fluid dynamic analysis of the forces exerted upon stent grafts seems a bit challenging and daunting in terms of the complex physics equations involved. Dr. Cheng and his colleagues, however, have taken a simplistic approach to the measurement of forces exerted upon stent grafts in the thoracic location. More specifically, they have attempted to characterize the forces exerted in a dissection model by introducing CT measurements pre and post endovascular management of aortic dissection.

The authors have provided us with some valuable information. Namely, that, (1) the drag forces exerted upon stent grafts in the thoracic location increase dramatically with increasing diameter. In this study an increase in graft diameter of 62%, that is, from 26 mm to 42 mm, correlated with an increase in drag forces of 160%, with the highest calculated drag force in this series being 30 N, which is fivefold higher than the mean force exerted upon infrarenal bifurcated grafts; (2) the mere presence of curvature matters more than the degree of curvature. Stated more plainly, substantial drag force is exerted on a graft placed transversely in the arch versus those placed in a rather straight portion within the descending aorta; and (3) stent graft length has minimal impact on the drag force exerted within the stent graft.

I have the following three series of questions for the authors:

Number one, your analysis was based on CT scans of 12 actual patients presenting with uncomplicated type B aortic dissections. Can you tell us more about these patients? What was the vintage of the aortic dissections? We know that the septa tend to be leather-like in chronic dissections. Were these all acute or a mixture of acute and chronic? And how did the age of the dissection affect drag force and aortic remodeling? More importantly, in followup of at least 12 months, how many of the patients actually demonstrated migration and did this correlate as you suggest, with drag force?

Number two, currently the only stent graft in the US approved by the FDA for implantation in the thoracic aorta is the WL Gore TAG endoprosthesis. All of your thoracic stent grafts were Cook TX2's that have positive fixation in the form of barbs. In your opinion, is positive fixation mandatory in the thoracic region based on the momentum forces that you have shown to be exerted upon these grafts?

And finally, this brings up the issue of managing uncomplicated type B aortic dissections. Are we to understand that you manage all dissections with an endovascular approach? Current literature suggests that roughly 30% of type B dissections progress to aneurysmal degeneration; however, in our series we have observed a smaller number to progress and the majority of these patients have a benign clinical course with medical management alone. What is your rationale for treating these patients?

I would like to thank the authors again for an elegant manuscript and fine presentation and the Society for the opportunity to discuss this paper.

DR. CHENG: Thank you very much, Ben, for the very insightful comments.

The patients were a mix between acute, subacute and chronic dissections. These represent our earliest experiences with a series of patients with more than 12 months of followup. I believe 7 endografts were performed in the subacute stage meaning that they are done within one month of the dissection, 2 patients were treated in the acute stage, and 3 patients for dissecting aneurysms. It is not within the confines of this study to investigate the relationship of remodeling and the age of the dissecting flap and we have found no correlation between the timing of the procedure and remodeling in our series.

Two of our patients had shown some degree of stent graft migration over the length of the followup period with one significantly. It may be a coincidence that both of these patients had very large diameter grafts placed in their aorta – both were 42–mm grafts. However, the position of the stent graft on the aorta is dependent on the drag forces as well as the fixation forces. The fixation forces are determined by a lot of variables like the length and the health of the landing zone, and graft oversizing, the length of the stent graft which provides friction, and whether there is any distal attachment or bare stents. Without looking at these fixation forces we cannot comment on whether the migration is a result of increasing drag forces. It is also possible that an increase in migration of the stent graft could result in an actual expansion of the diameter of the graft and therefore further increase in the drag forces, but these are all speculations.

Our experience is mainly with the Cook TX2 graft. I believe previous bench top studies on abdominal grafts have shown that the grafts with hooks and suprarenal fixation would be able to sustain a higher distraction force. I believe the distraction force required to dislodge an abdominal graft is in the order of 24 N. Since the result of this study showed that there is a significantly higher drag force in the aortic arch, I believe the use of added fixation is certainly highly desirable.

Concerning the indication of treating patients with type B dissection, I believe that it is still controversial. We certainly do not treat all the patients with endovascular stent grafting. In acute situations, we only treat patients with branch vessel compromise and end organ ischemia or in a patient with severe pain. In subacute situations we would offer this to patients when the aortic false lumen remained patent, especially if the aortic diameter exceeded 4 cm. We do this in 3 to 4 weeks. In a chronic situation we only do aneurysms. We will not treat a chronic stable type B dissection.