Transapical aortic valve replacement under real-time magnetic resonance imaging guidance: experimental results with balloon-expandable and self-expanding stents

Abstract

Objective: Aortic valves have been implanted on self-expanding (SE) and balloon-expandable (BE) stents minimally invasively. We have demonstrated the advantages of transapical aortic valve implantation (tAVI) under real-time magnetic resonance imaging (rtMRI) guidance. Whether there are different advantages to SE or BE stents is unknown. We report rtMRI-guided tAVI in a porcine model using both SE and BE stents, and compare the differences between the stents.

Methods: A total of 22 Yucatan pigs (45-57 kg) underwent tAVI. Commercially available stentless bioprostheses (21-25 mm) were mounted on either BE platinum-iridium stents or SE-nitinol stents. rtMRI guidance was employed as the intraoperative imaging. Markers on both types of stents were used to enhance visualization in rtMRI. Pigs were allowed to survive and had follow-up MRI scans and echocardiography at 1, 3, and 6 months postoperatively.

Results: rtMRI provided excellent visualization of the aortic valve implantation mounted on both stent types. The implantation times were shorter with the SE stents (60 ± 14s) than with the BE stents (74 ± 18s), (p=0.027). The total procedure time was 31 and 37 min, respectively (p=0.12). It was considerably easier to manipulate the SE stent during deployment, without hemodynamic compromise. This was not always the case with the BE stent, and its placement occasionally resulted in coronary obstruction and death. Long-term results demonstrated stability of the implants with preservation of myocardial perfusion and function over time for both stents.

Conclusions: SE stents were easier to position and deploy, thus leading to fewer complications during tAVI. Future optimization of SE stent design should improve clinical results.

Fig 1

St. Jude Toronto SPV valve sewn inside the nitinol self-expanding stent (left), Medtronic Freestyle valve sewn inside the Numed platinum-iridium stent balloon expandable stent (right).

Fig. 2

Selected frames from the real-time MRI displayed within the scan room, showing the deployment of the prosthetic valve mounted on a balloon-expandable stent. A: The position of the aortic valve annulus is obvious. B. The prosthetic valve is advanced into position in the left ventricular outflow track. C. interactive saturation used to enhance visualization of extent of the balloon (filled with dilute Gd-DTPA) inflation. D. The balloon is deflated and the delivery device is removed from the trocar.

Fig 3

Selected frames from the real-time MRI displayed within the scan room, showing the deployment of the self-expanding stented prosthetic valve. a: Stented prosthesis is inserted inside the trocar. b: The stented prosthesis is advanced to the end of the trocar. c: The prosthesis is inserted across the native valve and aligned with coronary ostia and aortic annulus. d,e,f,g: Sequence of the prosthesis expansion. During this period, the prosthesis can be retrieved. h:. The prosthesis is fully deployed. i: The delivery device is removed from the trocar.

Fig 4

Echocardiographic hemodynamic measurements 6 months after implantation. There were 8 animals in Balloon-expandable prosthesis group, 7 animals in Self-expanding prosthesis group. The numbers above the bar are mean ± SD. We performed t-test to determine the significance of the differences between BE group and SE group on ejection fraction (EF), Aortic valve peak gradient (AV peak grad), Aortic valve mean gradient (AV peak grad). The p-values are presented on the bottom of the graph.

Fig 5

Aortic valve and mitral valve regurgitation of the animals 6 months after implantation as assessed by echocardiography.

Fig 6

Radiographs of the hearts with balloon-expandable stent (top) and self-expanding stent (bottom) respectively. In the top row, both anterior and lateral views of the heart show several strut fractures at the level of proximal and distal crowns (arrows) of the balloon-expandable stent. In the bottom row, both views of the heart show an intact self-expanding stent frame without any fractures.

Fig 7

Necropsy results of the hearts with the balloon-expandable prosthetic valve (top) and the self-expanding prosthetic valve (bottom) after 6 month implantation. The top row shows inferior and superior views of the balloon-expandable prosthetic valve leaflets, note several of the stent frame crowns remain uncovered while the annulus of the prosthetic is covered by opaque white tissue with focal brown areas in the region of the anterior mitral leaflet. On the right is the superior view of the prosthetic valve showing good coaptation of the free edges of 3 valve leaflets. The bottom row shows inferior and superior views of the self expanding prosthetic aortic valve, note that the entire stent frame crowns are covered as well as the annulus of the prosthetic valve is covered by opaque white tissue without pannus formation extending into the valve bases. The anterior mitral leaflet is unremarkable. On the right is the superior view of the aortic prosthetic valve showing good coaptation of the free edges of the valve leaflets.