Animal procedures were conducted according to contemporary National Institutes of Health guidelines, in 19 naive Yorkshire swine (43 ± 9 kg). Eight animals were used for technique development and 11 were kept alive and assigned to up to 3 months of follow-up. Animals were pre-treated with aspirin and amiodarone. Anesthesia was maintained with inhaled isoflurane and mechanical ventilation.
Before LV access, a subxiphoid pericardial drainage catheter (8.3-F, Boston Scientific, Natick, Massachusetts) was placed under x-ray fluoroscopy, and the femoral artery and vein were accessed percutaneously for monitoring (12
Active interventional equipment
For cardiac puncture, an 18-gauge, MRI-compatible (nickel-titanium-chromium) needle was customized for “active profiling” (visibility along its entire length during MRI) by integrating a loop coil along the needle shaft. Characteristic spacing between needle marker coils assisted in distinguishing the needle shaft from the tip.
The LV puncture site was closed with a commercial 6-mm muscular ventricular septal defect (VSD) occluder (AGA Medical, Plymouth, Minnesota). Unmodified devices were used in initial technique development, and modified devices (replacing the microscrew stainless steel with titanium to minimize MRI artifacts) () were implanted in the survival cohort.
Ex Vivo MRI of Muscular VSD Occluder
A custom delivery cable for the VSD occluder was developed to enhance visibility and reduce MRI artifacts. It incorporated a loopless antenna and had its steel mating microscrew replaced with titanium. The antenna was tuned so that MRI signal was maximal when the attached occluder achieved the fully deployed state before release.
Active devices were connected to a separate MRI receiver channel, and a device-related signal was independently colored and overlaid on real-time images (13
Percutaneous LV access
All procedures were performed under real-time MRI guidance, with multiple continuously updated slices ().
An apical puncture trajectory was selected in consideration of the following: 1) avoiding bony structures of the chest; 2) the shortest distance to the heart; 3) avoiding coronary arteries; 4) the desired LV needle entry target, whether apical or para-apical; 5) avoiding injury to intracavitary structures (papillary muscles and interventricular septum); and 6) further intracavitary trajectories toward aortic or mitral valve structures.
An “active” 18-gauge needle was advanced from a selected thoracic skin location and, based on the position of the needle microcoils, the final trajectory was planned, and the needle was advanced into the LV cavity. An 18-F sheath (Cook Medical, Bloomington, Indiana) was delivered over a 0.035-inch Nitrex guidewire (ev3, Plymouth, Minnesota). A second, backup nitinol guidewire was also positioned alongside the sheath.
Heparin was administered to achieve an activated clotting time ≥250 s immediately following sheath placement and during and after closure to “stress” the test of immediate hemostasis.
Percutaneous LV closure
Percutaneous LV closure was performed with the muscular VSD occluder, under real-time MRI guidance. In an effort to avoid pericardial entrapment by the occluder, saline was instilled into the pericardial space guided by real-time MRI immediately before device deployment temporarily to separate the 2 pericardial layers (“permissive pericardial tamponade”) (12
). A 6-mm VSD occluder was deployed using standard techniques (14
). Finally, the instilled pericardial fluid was aspirated.
Magnetic resonance imaging
Interventions were performed in a 1.5-T MRI scanner (Espree, Siemens, Erlangen, Germany) using standard surface body receive coils. The wide (70-cm) bore allows operator access to the chest of large animals that we predict would translate to patients. MRI was used to guide all interventional procedures and for geometry and function measurements of the LV, pleural and pericardial fluid assessment, and myocardial scar.
For interventional procedures, we used real-time balanced steady state free precession (repetition time/echo time: 3.67/1.23 ms; flip angle: 45°; field of view: 340 × 255 mm; matrix: 192 × 144 pixels; slice thickness: 6 mm; and bandwidth: 789 Hz/pixel; parallel imaging and undersampling provided 3 to 5 frames/s). Images were reconstructed and displayed to the operator on a separate real-time workstation (13
Cardiac computed tomography (CT) angiography roadmaps coregistered with real-time MRI
To avoid coronary artery injury, we used a cardiac CT angiography-derived 3-dimensional road map of the coronary tree overlaid on the real-time MRI.
Electrocardiogram-gated cardiac CT angiography was performed in a nonspiral mode on a 320-slice CT scanner (Aquilion ONE, Toshiba Medical Systems, Tokyo, Japan) using 16-cm detector coverage, 100-kV tube voltage, 550-mA tube current, and 350-ms gantry rotation time. The CT surface model and MRIs were manually registered to each other using anatomic landmarks, such as the aortic arch and coronary ostia (3D Slicer, Slicer.org, Brigham and Women’s Hospital, Boston, Massachusetts). After registration to the MRI volume, the coronary volume was imported into the real-time workstation (IFE, Siemens Corporate Research, Princeton, New Jersey) as a 3-dimensional surface overlaid on the real-time MRI images ().
Cardiac Structures To Avoid During Percutaneous LV Access
After closure, animals were observed for 3 h by serial hemodynamics and MRI, with attention to pericardial and pleural fluid, and by radiocontrast angiography. Serum and pericardial hematocrit and (human) cardiac troponin I (i-STAT, Abbott, Princeton, New Jersey) were measured.
Follow-up was pre-specified for days 1 to 3 and 5 and weeks 4 and 12 and included MRI to evaluate device position, LV function, myocardial scar, and pericardial and pleural fluid volume. Euthanasia at pre-specified time points allowed necropsy samples at weeks 1, 4, and 12.
Necropsy examination of the heart, pericardium, and muscular VSD occluder was performed in all. Histological analysis was performed 3 months after the procedure. Sections were examined by light microscopy for the presence of inflammation, thrombus, neointimal formation, and injury following toluidine blue and basic fuchsin staining.
Statistical analysis used GraphPad Software, La Jolla, California). Numerical parameters are reported as mean ± SD or median (interquartile range) as appropriate. Continuous repeated parameters were tested using analysis of variance, 2-tailed paired t test, or Wilcoxon matched-pairs signed-rank test, as appropriate, and p < 0.05 was considered significant.