rtMRI provided excellent visualization of the aortic prosthesis implantation mounted on both stent types. The active wires were a superb indicator of the valve orientation in MRI. The passive markers on the SE stents were also help to identify the valve orientation. These markers were somewhat difficult to visualize via MRI when the stents were fully crimped but became more apparent as the stents were deployed.
We recorded the procedure time and deployment time for using BE stent and SE stent on 22 survival animals. We also performed t-test to determine the significance of the differences on the procedure time and deployment time between two groups. Total procedure time was 37 and 31 minutes for using BE stent and SE stent respectively. The total procedure time for both stents were not significantly different (p=0.12). The time from introduction of the prosthesis into the trocar to deployment the stent is fully expanded (deployment time) was 74±18 seconds (mean ± std. dev.) and 60±14 seconds (mean ± std. dev.) respectively. This deployment time was significantly shorter for the SE stent (p=0.027). Once the BE prosthesis was in place, the balloon is first partially inflated by using normal saline mixed 100:1 with an MR contrast agent Gd-DTPA (Magnavist, Berlex Inc, Montville, NJ), the position is re-confirmed to be ideal and the balloon is then fully expanded and the stented prosthesis completely deployed. The procedures using BE prosthesis therefore took a slightly longer time because of this time for staging the balloon-inflation and the difficulty in orienting the valve knowing that once the balloon was completely inflated there was no margin to allow for adjustment. When the balloon was inflated to deploy the BE stent there was a significant change in the blood pressure and a concomitant decrease in the cardiac output. This hemodynamic compromise was seen even with partial inflation of the balloon. There was no significant blood pressure drop observed during the SE prosthesis implantation. Even partially deployed the SE stent was not as obstructive as the combination of balloon and stent for the BE device. Blood can flow through a partially deployed SE stent but even a partially filled balloon for a BE stent is obstructive and had a negative impact on the hemodynamic stability of the animal. Additionally the inability to easily control deployment with the BE stent resulted in coronary obstruction and death. 4 out of 20 of the animals in acute feasibility study with BE prosthesis implanted died of coronary obstruction.
Post-placement images were acquired to confirm the positions of the prostheses and assess the valvular and heart function. Gated cine MRI revealed excellent myocardial function after valve implantation in both long- and short-axis views for animals in whom the valves were appropriately positioned. The phase-contrast CINE MR images confirmed good systolic flow with excellent valve leaflet opening and no evidence of turbulence, diastolic regurgitant flow, or paravalvular leak. First-pass perfusion studies [24
] demonstrated adequacy of myocardial blood flow after valve placement in all animals following successful deployment. The perfusion results confirmed adequacy of blood flow at the tissue level, indicating proper valve positioning with respect to the coronary ostia.
Among 11 animals implanted with a BE prosthesis, 3 died between 1 and 2 weeks postoperatively of respiratory complications. Among 11 animals implanted with SE prosthesis, 3 animals died within one month postoperatively due to late pneumothorax or pneumonia, 1 died of pneumonia at day 35. None of the survivors of the initial implantation died of late coronary obstruction. Hemodynamic measurements of ventricular and valvular parameters of 15 animals based on 6 months follow-up echocardiograms (and confirmed by MRI scans) are presented in . There is no significant difference between the animals with the BE prosthesis and those with the SE prosthesis. The degree of insufficiency of the aortic valve and the mitral valve at 6 months after the procedure were measured and are shown in . More aortic and mitral regurgitation was seen with the BE prosthesis but this trend was not statistically significant.
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 (more ...)
Aortic valve and mitral valve regurgitation of the animals 6 months after implantation as assessed by echocardiography.
Post-mortem pathologic analysis, after sacrifice at 6 months, verified that the implanted both BE and SE prosthesis appeared in place in the aortic root. The prosthetic commissures were incorporated with neointimal growth continuous with the native leaflet commissures. The average strut fractures for the platinum iridium BE stent was 5.0±3.1 (mean ± std. dev.), while the average fractures for the nitinol SE stent was 1.6±2.5 (mean ± std. dev.)(p=0.046). There was no particular pattern of strut fractures observed. In eight animals with BE stent, 2 animals had fractures at the level of the proximal crowns; 3 animals had fractures at the level of the distal crowns, and 3 animals had fractures scattered throughout the stent. In seven animals with SE stent, 1 animal had fracture at the level of the proximal crowns, 1 animal had fractures at the level of distal crowns, and 2 animals had fractures throughout the stent, 3 animals did not have fractures. The fractures are due to the stent material fatigue and the expansion, contraction, torsion between the aorta and the stent. We performed a t-test to determine the significance of the differences in the number of strut fractures between the two groups. The nitinol SE stent has elasticity and its geometry design permit to handle the torsion better, while the BE stent has no elasticity and the material is relatively soft.
Some limitations were only observed in BE stent cases at the time of sacrifice. In one animal, the stent was collapsed/compressed on itself at its mid-section. Malapposed stent struts at either ends were observed in five animals. In one animal, a prosthesis cusp appeared collapsed. All of these were due to uneven deployment of the BE prosthesis. On the other hand in the SE cases, transverse sections from the device (prosthetic aortic valve) showed a widely patent implant lumen with circular, symmetrical shape maintaining tight apposition of the stent frame to the aortic wall in all the animals with the SE prosthesis. Representative radiographs of the BE stented prosthesis and the SE stented prosthesis are shown in . Autopsy confirmation of BE and SE prosthetic valve location after 6 months implantation is seen in .
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) (more ...)
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, (more ...)