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a Department of Cardiothoracic Surgery, LBI for Cardiosurgical Research, Medical University of Vienna, Vienna, Austria
b Department of Biomedical Engineering, Medical University of Vienna, Vienna, Austria
c Center for Biomedical Research, Medical University of Vienna, Vienna, Austria
Accepted for publication September 22, 2007.
* Address correspondence to Dr Zimpfer, Department of Cardiothoracic Surgery, University of Vienna, Wahringer Guertel 18-20, Vienna, A-1090, Austria (Email: daniel.zimpfer{at}meduniwien.ac.at).
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Abstract |
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Methods: The entire thoracic aortic aorta including the supraaortic branches was harvested from 12 adult pigs and an intimal tear was artificially created. The aortic annulus was then sewn into a silicon ring of a driving chamber. The distal aorta was connected to tubing with adjustable resistance elements. The circulation was driven by a hydraulic motor piston pump to mimic aortic flow and pressure. After creating a dissection by elevating the systolic aortic pressure to 180 mm Hg, a 2- x 2.6-cm covered stent graft was inserted through the brachiocephalic trunk using a specially designed delivery system. Stent graft placement was performed under continuous ultrasound control.
Results: The longitudinal length of the created ascending aortic dissection was 1.8 ± 0.39 cm. Ultrasound studies revealed successful deployment of the stent graft and closure of the false lumen in all 12 cases. Diameter and area of the true lumen increased from 0.52 ± 0.15 cm to 2.54 ± 0.36 cm (p < 0.05) and from 0.78 ± 0.27 cm2 to 5.13 ± 1.35 cm2 (p < 0.05), respectively. The circumference of the true lumen increased from 4.50 ± 0.52 cm to 7.96 ± 1.2 cm (p < 0.05). Ultrasound studies also revealed uncompromised function of the aortic valve in all cases. No dislodging of stent grafts was observed.
Conclusions: Given ideal anatomy, experimental stent graft placement for ascending aortic dissection is feasible and achieves complete closure of the false lumen.
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Introduction |
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In multimorbid and elderly patients, stent graft treatment of acute as well as chronic diseases of the descending thoracic and abdominal aorta has reduced rates of mortality and morbidity [4–7]. Reduction of mortality is attributed to the less invasive nature of the procedure. Nevertheless, the long-term durability of this approach is matter of ongoing discussion [5].
Endovascular treatment of ascending aortic diseases has so far only been reported as successful in single cases [8, 9]. The reason for this is the highly complex anatomy of the ascending aorta caused by the close relation to the aortic valve, the coronary ostia, and the supraaortic branches. The landing zones that are crucial for the success of stent graft treatment are therefore limited. It remains to be clarified if closure of the false lumen can be successfully achieved. The present study evaluated the feasibility of endovascular stent graft treatment of acute aortic type a dissections in a porcine in vitro model.
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Material and Methods |
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TopAbstractIntroductionMaterial and MethodsResultsCommentReferences |
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A previously described model for induction of descending aortic dissections was adapted for creating ascending aortic dissections [10]. An aortotomy was performed, and an intima dissector was used to create a 0.5-cm-long intimal tear. The aortotomy was closed, and the aortic annulus was sewn into a silicon ring and placed into the artificial circulatory circuit.
Circulator Circuit
A previously described modular hydraulic model was used to mimic aortic flow and pressure conditions [11]. Briefly, the circulation is driven by a piston pump (Vivitro, Victoria, British Columbia, Canada) that provides a computer-controlled flow pattern. A rubber tube, a damping fiber element, and an adjustable resistor mimicked peripheral arterial impedance (Fig 1).
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Experimental Setup
After the artificial intimal tear was created, the aorta was mounted into the artificial circulatory circuit. Air was removed, and the mock circulation was started. Pressure was steadily elevated using peripheral resistant elements to a target pressure of 200/160 mm Hg at constant flow rate (3.5 L/min). After reaching the target pressure, 5 minutes of mock circulation at fixed pressure and flow (200/160 mm Hg; 3.5 L/min) were performed to promote the aortic dissection. Length of dissection, diameter, and area of true and false lumen were assessed using ultrasound imaging. Pressure and flow were not changed for ultrasound assessments.
The pressure was then lowered to 80/60 mm Hg by reducing peripheral resistance. Flow was kept constant (3.5 L /min). A covered custom-made (Le Maitre Vascular, Burlington MA) 2- x 2.6-cm stent graft was advanced through the brachiocephalic trunk using a specially designed delivery system. Ultrasound guidance (side port, 1-MHz probe) was used to direct and exactly deliver the stent graft with the proximal end at the sinotubular junction. After stent graft delivery, pressure was again steadily elevated to 200/160 mm Hg at constant flow of 3.5 L/min. After a second 5-minute period of mock circulation, completion ultrasound studies were performed.
The whole aorta was retrieved immediately after the experiments, and visual inspection was performed for competence of aortic valve, integrity of the coronary ostia, coverage of the dissection lamella, and integrity of the separation of the supraaortic vessels.
Definition of Success
Success of the procedure was defined as complete exclusion of the false lumen with no residual flow in color Doppler ultrasound, competence of the aortic valve (in ultrasound and upon visual inspection), integrity of the coronary ostia, complete coverage of the dissection lamella, integrity of the separation of the supraaortic vessels, and no signs of stent graft dislodgement.
Stent Graft and Delivery System
The stent grafts and delivery system were custom made (Le Maitre Vascular). We used a self-expanding nitinol stent graft with polytetrafluoroethylene (diameters, 2 x 2.6 cm). The delivery system consisted of an inner an outer sheath. The stent grafts were designed for single use, but the delivery system was reusable. The stent graft was crimped before each experiment to fit between the inner an outer sheath of the delivery system. Upon delivery, the outer sheath of the delivery system was pulled back, and the stent graft thereby expanded.
Statistical Analysis
Data are reported as mean ± SD. Continuous variables were compared using the Student t test after testing for normality of distribution. Categoric variables were compared using the 
2 test or Fisher test as appropriate. Two-sided values of p < 0.05 were considered significant. The study data were analyzed using SPSS 11.0 software (SPSS Inc, Chicago, IL).
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Results |
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Comment |
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As has been previously demonstrated in descending and abdominal aortic disease, stent graft treatment of ascending aortic dissections may evolve as an alternative to conventional surgical repair with circulatory arrest [4–6]. Patients with advanced age and other comorbidities, who currently face high perioperative mortality and morbidity, may especially benefit from a less-invasive treatment option that avoids sternotomy, cardiopulmonary bypass, and circulatory arrest [1, 2].
Experience gained in the descending and abdominal aorta cannot, however, be extrapolated to the ascending aorta owing to the highly complex functional anatomy of the ascending aorta with a close relationship of coronary ostia, aortic valve, and separation of supraaortic vessels. An experimental model and experimental experience is therefore necessary before stent graft treatment of ascending aortic dissections is evaluated in the clinical setting in a larger patient population. Arising questions that should be addressed by an experimental model are feasibility of closure of the false lumen, sufficiency of landing zones, and risk of interference with aortic valve, coronary ostia, and supraaortic branches.
With the present in vitro model, we have addressed some of the pressing questions about stent graft treatment of ascending aortic dissections. Stent graft placement was possible in all specimens without compromising aortic valve function or covering coronary ostia. Uncovered stent grafts or only partly covered stent grafts may further reduce the risk of covering coronary ostia. Nevertheless, it is uncertain if uncovered and only partially covered stent grafts are as effective as covered ones, and further experiments with different stent graft designs are warranted.
Covering the primary entry tear is the most crucial point of the procedure. In the present study, entry tears were at or immediately above the sinotubular junction with no observed retrograde dissections. We were able to identify entry tears in all cases using ultrasound. In the clinical setting, ultrasound imaging in the form of transesophageal echocardiography (TEE) also provides continuous visualization of the stent graft and its relation to the aortic valve and the sinotubular junction, and additional angiography will exactly display the takeoff of the coronary ostia. Another crucial point is exact visualization of the true and false lumen. We believe that this will be possible using a combination of ultrasound imaging (TEE) and angiography.
In the experimental setting, perfusion pressure was lowered upon stent graft deployment and no dislodging of stent grafts was observed. In the clinical setting, in addition to lowering perfusion pressure, it will be necessary to minimize cardiac output during stent graft deployment using, for example, adenosine or rapid ventricular pacing to avoid dislodging of the stent graft upon deployment.
We were able to close the false lumen with no residual flow in all cases. This may answer the arising question of sufficiency of landing zones in the ascending aorta. A conclusion about the likelihood of endoleak formation cannot be drawn from this finding, however. We did not observe coverage of separation of supraaortic branches. Separation of supraaortic branches will not pose a problem in the clinical setting if there is sufficient knowledge of the individual’s anatomy and a selection of suitable stent grafts.
Extrapolating our experimental experience to clinical reality, those dissections with an entry tear at or above the sinotubular junction, with no retrograde dissection into the coronary sinuses, will be amenable to stent graft treatment. Whether the dissection has to be limited to the ascending aorta is uncertain. Coverage of the primary entry tear should stop the pathophysiologic process independent of the extension of the dissection. Significant dissection-related aortic valve insufficiency will exclude the option of stent graft placement. As we have previously reported, stent graft treatment of ascending aortic dissections is feasible given an ideal anatomy [9].
The primary limitation of the present study is the relatively short length of the ascending aortic dissection and that the dissection was limited to the ascending aorta, which is not what happens in more than 90% of clinical cases. This might be explained by the relatively short period of mock circulation and because we used healthy aortas free from atherosclerosis and significant enlargement. Still, after creation of a small artificial intimal tear, we observed propagation of the dissection in all experiments, which underlies that we really created aortic dissections. Nevertheless, we observed no retrograde dissections. Water was used as the perfusate to limit the complexity of the mock circulatory circuit. We did not perform any angiographic studies in the present experiments because we had optimal access for ultrasound studies that enabled us to directly visualize the whole aorta. Furthermore, no histologic examinations were performed because we believe that success of an endovascular procedure can only be determined during mock circulation.
In conclusion, given ideal anatomy, experimental stent graft treatment of ascending aortic dissection is feasible. Aortic valve function, coronary ostia, and separation of supraaortic branches remain uncompromised. Complete closure of the false lumen with no residual flow can be achieved.
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References |
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