Hermann Wendt

In vitro results of a new minimally invasive aortic valve resecting tool

BACKGROUND Aortic valve replacement (AVR) using extracorporeal circulation is currently the treatment of choice for symptomatic aortic stenosis. However, patients with multiple high-risk comorbid conditions may benefit from reduced ECC time by a simplified and faster resection in conjunction with quick sutureless valve implantation. METHODS A prototype of a new minimally invasive aortic valve resection tool equipped with rotating and foldable Nitinol cutting edges was designed. Commercially available aortic valve bioprostheses were artificially calcified (group 1: moderate calcified, n=8, group 2: severely calcified, n=8). In vitro resection was performed using a 21mm cutting blade. Resection time (RT), maximum turning moment (MTM) and number of required rotations (NR) were measured. Furthermore, particle generation during the process of cutting was obtained and quantified. RESULTS Aortic valve cutting could be obtained without any complications in all cases. Cutting process resulted in a RT of 15.5+/-3s in group 1 compared to 34.9+/-15s in group 2 (p=0.005), MTM was 3+/-0.6Nm in group 1 compared to 3.5+/-0.6Nm in group 2 (p=0.068) and NR were 30.6+/-2.3 in group 1 compared to 48.1+/-15.5 in group 2 (p=0.007). Particle generation was 1.77+/-0.17g in group 1 compared to 1.41+/-0.44g in group 2 (p=0.047). CONCLUSIONS These first in vitro results confirm feasibility and accelerated aortic valve resection within 30s. This new concept holds promise for very fast AVR in combination with insertion of sutureless aortic valve prosthesis, targeting for ischemic times less than 10min in the open heart situation. Finally, resection and percutaneous AVR within 1min in the beating heart situation is envisioned.

NiTinol-based cutting edges for endovascular heart valve resection: First in-vitro cutting results

Machining of shape memory alloys based on Nitinol (NiTi) creates difficulties due to its ductility and severe strain hardening. In this experiment, different cutting edges and grinding parameters were tested to optimize cutting results on NiTi-based blades intended for endovascular heart valve resection. The cutting procedure was performed using two counter-rotating circular NiTi blades of different diameter. A rotating/punching process should be performed. Different shapes (glazed, waved, and saw tooth), different grinding techniques (manual, manual grinder, and precise milling cutter) and additionally various velocities (50 and 200 rpm) were tested on specific test specimens. Cutting forces were measured and cutting quality was examined using digital microscopy. Preliminary tests with rotating blades showed superior results using cutting edges for the punching process (150 N vs. 200 N; n=7). In a second step special test specimens were tested. Maximum cutting-force was 265 N±20 N (mean±SD; n=7). Subsequently different shapes were tested at 50 and 200 rpm using the rotating/punching method regarding alternate grinding techniques. Cutting forces were 27 N±7.7 N for glazed blades (n=7) at 50 rpm and 18 N±4.7 N at 200 rpm, waved blades (n=7) required a maximum force of 18 N±5 N at 50 rpm and 11 N±3.3 N at 200 rpm, whereas saw tooth blades (n=7) needed 17 N±12.7 N at 50 rpm and 9 N±1.2 N at 200 rpm. Precise cutting quality was only seen when using glazed blades sharpened under accurate conditions with a high-speed milling cutter. Although shape memory alloys based on Nitinol are difficult to process, and well-defined grinding parameters do not exist, acceptable results can be reached using high-speed milling cutters. Best cutting quality can be observed by using glazed blades, performing a rotating/punching process at high velocities. Lower cutting forces can be observed by using other shape-types, however this leads to lower cutting quality. Therefore, further investigations on blade-machining and velocity-testing seem to be necessary to create optimal cutting results.

Comparison of flow dynamics of Perimount Magna and Magna Ease aortic valve prostheses

Abstract The aim of the present study was to evaluate and compare the in vitro and flow dynamics of the Magna (MB) and the Magna Ease aortic valve bioprosthesis (MEB) within the ascending aorta. A 2D-particle-image-velocimetry (2D-PIV) study was performed to compare the flow dynamics induced by each pericardial Carpentier-Edwards Magna and Magna Ease aortic valve prosthesis in the aortic flow field directly behind the valve. Both prostheses (diameter 23 mm) were placed inside an artificial aorta under pulsatile flow conditions (70 Hz and 70 ml stroke volume). The flow field was evaluated according to velocity, shear strength, and vorticity. Both prostheses showed a jet flow type profile with a maximum velocity of 0.97±0.09 m/s for MB and 0.83±1.8 m/s for MEB. Flow fields of both valves were similar in acceleration, peak flow deceleration and leakage phase. Maximum shear strength was 20,285±11,774 l/s2 for MB and 17,006±8453 l/s2 for MEB. Vorticity was nearly similar for counterclockwise and clockwise rotation in both prostheses, but slightly higher with MB (251±41 l/s and -250±39 l/s vs. 225±48 l/s and -232±48 l/s). The point-of-interest (POI)-analysis revealed a higher velocity for left-sided aortic wall compared to right-sided at MB (0.12±0.09 m/s vs. 0.18±0.10 m/s, p<0.001), but was consistent at MEB (0.09±0.05 m/s vs. 0.08±0.04 m/s, p=0.508), respectively. Velocity, shear strength and vorticity in an in vitro test set-up are lower with MEB compared to MB, thus resulting in improved flow dynamics with a similar flow field, which might have a positive influence on blood rheology and potential valve degeneration.

A new tool for the resection of aortic valves: In-vitro results for turning moments and forces using Nitinol cutting edges.

The use of minimally invasive techniques for aortic valve replacement (AVR) may be limited for severely calcified and degenerated stenotic aortic valves. A quick resection leaving a defined geometry would be advantageous. Therefore, a new minimally invasive resection tool was developed, using rotating foldable cutting edges. This report describes the first experimental in-vitro results of measuring turning moment and forces during cutting of test specimens. Nitinol cutting edges were mounted on a simplified version of the resection instrument. The instrument shaft was combined with an exchangeable gear (1:3.71 vs. 1:5.0), and an exchangeable screw thread for accurate feed motion (0.35 mm or 0.5 mm) was implemented. Furthermore, the option of an added stabilisation body (SB) to prevent strut-torsion during cutting was tested. Tests were performed upon specially designed test specimens, imitating native calcified aortic valves. Resection was successful in all 60 samples (12 samples for each of the five configurations). Mean resection time ranged from 18.7±1.0 s (gear 1:3.71, screw thread 0.5, with SB) to 29.3±4.6 s (gear 1:5, screw thread 0.35, with SB), mean maximum turning moment ranged from 2.1±0.2 Nm (gear 1:3.71, screw thread 0.35, with SB) to 2.8±0.4 (gear 1:5, screw thread 0.35, with SB), mean maximum force from 36.0±11.3 N (gear 1:3.71, screw thread 0.35, with SB) to 56.3±10.5 N (gear 1:3.71, screw thread 0.5, without SB) and mean number of required rotations from 41.3±2.9 (gear 1:3.71, screw thread 0.5, with SB) to 59.1±3.7 (gear 1:3.71, screw thread 0.35, without SB). In summary, the positive influence of the stabilisation body could be shown. Combining the right parameters, it is possible to limit maximum cutting forces to Fmax<50 N and maximum turning moments to Mmax< 3.0 N.

Development and in vitro characterization of a new artificial flow channel.

To date, cardiac valve diseases are considered as a major public health problem and most frequently, the aortic valve is affected. To treat high-risk patients, catheter-based techniques have been developed recently, avoiding open heart surgery and/or cardiopulmonary bypass. Although these sophisticated and rapidly emerging catheter-based technologies do allow a minimally invasive treatment option of high-risk patients on the one hand, further developments and in vitro testing under physiological conditions are necessary, on the other hand, in order to further optimize them for clinical routines. Therefore, we present the concept of a new multifunctional flow channel, offering (i) the possibility of transapical access; (ii) the simulation of physiological flow conditions; and (iii) the evaluation of the fluid flow by 2D particle image velocimetry within a wide range of parameters.

The investigation of systolic and diastolic leaflet kinematics of bioprostheses with a new in-vitro test method

Abstract Objectives: We aimed to investigate leaflet kinematics of bioprostheses with a novel high-speed imaging method. Material and methods: High-speed-imaging (1000Hz) was used to evaluate leaflet kinematics of the Carpentier-Edwards Perimount Magna (PM) and Magna Ease (PME) aortic bioprostheses. Both prostheses (diameter 23 mm) were placed inside a model aorta under pulsatile flow conditions. Frequencies (F) and different stroke volumes (S) were simulated. Maximum aortic valve area (AVA), total ejection time (TET), rapid valve opening time (RVOT) and rapid valve closing time (RVCT) as well as opening (OS) and closing (CS) speeds were evaluated. Results: Both bioprostheses showed different results dependent on flow conditions. The test setup was capable of identifying small AVA-differences between both valves (235 vs 202 mm², F60/S60; 272 vs 207 mm²; F70/S80), as well as differences in OS and CS (2.36 vs 1.62 mm²/ms; 2.97 vs 2.44 mm²/ms, F80/S60). TET was comparable (638 vs 645 ms F60/S60; 341 vs 343 ms, F90/S60), while results for RVOT and RVCT were equal, and dependent on frequency and stroke volume. Conclusions: The novel evaluation method is sensitive to detect differences between valves, although differences were found to be small. PM has a larger visible AVA associated with higher opening and closing speeds in contrast to PME.

The Fluid Dynamical Performance of the Carpentier-Edwards PERIMOUNT Magna Ease Prosthesis

The aim of the present in vitro study was the evaluation of the fluid dynamical performance of the Carpentier-Edwards PERIMOUNT Magna Ease depending on the prosthetic size (21, 23, and 25 mm) and the cardiac output (3.6–6.4 L/min). A self-constructed flow channel in combination with particle image velocimetry (PIV) enabled precise results with high reproducibility, focus on maximal and local peek velocities, strain, and velocity gradients. These flow parameters allow insights into the generation of forces that act on blood cells and the aortic wall. The results showed that the 21 and 23 mm valves have a quite similar performance. Maximal velocities were 3.03 ± 0.1 and 2.87 ± 0.13 m/s; maximal strain Exx, 913.81 ± 173.25 and 896.15 ± 88.16 1/s; maximal velocity gradient Eyx, 1203.14 ± 221.84 1/s and 1200.81 ± 61.83 1/s. The 25 mm size revealed significantly lower values: maximal velocity, 2.47 ± 0.15 m/s; maximal strain Exx, 592.98 ± 155.80 1/s; maximal velocity gradient Eyx, 823.71 ± 38.64 1/s. In summary, the 25 mm Magna Ease was able to create a wider, more homogenous flow with lower peak velocities especially for higher flow rates. Despite the wider flow, the velocity values close to the aortic walls did not exceed the level of the smaller valves.