Prem A. Midha

The Fluid Mechanics of Transcatheter Heart Valve Leaflet Thrombosis in the Neosinus

Background: Transcatheter heart valve (THV) thrombosis has been increasingly reported. In these studies, thrombus quantification has been based on a 2-dimensional assessment of a 3-dimensional phenomenon. Methods: Postprocedural, 4-dimensional, volume-rendered CT data of patients with CoreValve, Evolut R, and SAPIEN 3 transcatheter aortic valve replacement enrolled in the RESOLVE study (Assessment of Transcatheter and Surgical Aortic Bioprosthetic Valve Dysfunction With Multimodality Imaging and Its Treatment with Anticoagulation) were included in this analysis. Patients on anticoagulation were excluded. SAPIEN 3 and CoreValve/Evolut R patients with and without hypoattenuated leaflet thickening were included to study differences between groups. Patients were classified as having THV thrombosis if there was any evidence of hypoattenuated leaflet thickening. Anatomic and THV deployment geometries were analyzed, and thrombus volumes were computed through manual 3-dimensional reconstruction. We aimed to identify and evaluate risk factors that contribute to THV thrombosis through the combination of retrospective clinical data analysis and in vitro imaging in the space between the native and THV leaflets (neosinus). Results: SAPIEN 3 valves with leaflet thrombosis were on average 10% further expanded (by diameter) than those without (95.5±5.2% versus 85.4±3.9%; P<0.001). However, this relationship was not evident with the CoreValve/Evolut R. In CoreValve/Evolut Rs with thrombosis, the thrombus volume increased linearly with implant depth (R2=0.7, P<0.001). This finding was not seen in the SAPIEN 3. The in vitro analysis showed that a supraannular THV deployment resulted in a nearly 7-fold decrease in stagnation zone size (velocities <0.1 m/s) when compared with an intraannular deployment. In addition, the in vitro model indicated that the size of the stagnation zone increased as cardiac output decreased. Conclusions: Although transcatheter aortic valve replacement thrombosis is a multifactorial process involving foreign materials, patient-specific blood chemistry, and complex flow patterns, our study indicates that deployed THV geometry may have implications on the occurrence of thrombosis. In addition, a supraannular neosinus may reduce thrombosis risk because of reduced flow stasis. Although additional prospective studies are needed to further develop strategies for minimizing thrombus burden, these results may help identify patients at higher thrombosis risk and aid in the development of next-generation devices with reduced thrombosis risk.

How Can We Help a Patient With a Small Failing Bioprosthesis?: An In Vitro Case Study.

OBJECTIVES The aim of this study was to investigate the hemodynamic performance of a transcatheter heart valve (THV) deployed at different valve-in-valve positions in an in vitro model using a small surgical bioprosthesis. BACKGROUND Patients at high surgical risk with failing 19-mm surgical aortic bioprostheses are not candidates for valve-in-valve transcatheter aortic valve replacement, because of risk for high transvalvular pressure gradients (TVPGs) and patient-prosthesis mismatch. METHODS A 19-mm stented aortic bioprosthesis was mounted into the aortic chamber of a pulse duplicator, and a 23-mm low-profile balloon-expandable THV was deployed (valve-in-valve) in 4 positions: normal (bottom of the THV stent aligned with the bottom of the surgical bioprosthesis sewing ring) and 3, 6, and 8 mm above the normal position. Under controlled hemodynamic status, the effect of these THV positions on valve performance (mean TVPG, geometric orifice area, and effective orifice area), thrombotic potential (sinus shear stress), and migration risk (pullout force and embolization flow rate) were assessed. RESULTS Compared with normal implantation, a progressive reduction of mean TVPG was observed with each supra-annular THV position (normal: 33.10 mm Hg; 3 mm: 24.69 mm Hg; 6 mm: 19.16 mm Hg; and 8 mm: 12.98 mm Hg; p < 0.001). Simultaneously, we observed increases in geometric orifice area (normal: 0.83 cm(2); 8 mm: 1.60 cm(2); p < 0.001) and effective orifice area (normal: 0.80 cm(2); 8 mm: 1.28 cm(2); p < 0.001) and reductions in sinus shear stresses (normal: 153 dyne/cm(2); 8 mm: 40 dyne/cm(2); p < 0.001), pullout forces (normal: 1.55 N; 8 mm: 0.68 N; p < 0.05), and embolization flow rates (normal: 32.91 l/min; 8 mm: 26.06 l/min; p < 0.01). CONCLUSIONS Supra-annular implantation of a THV in a small surgical bioprosthesis reduces mean TVPG but may increase the risk for leaflet thrombosis and valve migration. A 3- to 6-mm supra-annular deployment could be an optimal position in these cases.

The Effect of Valve-in-Valve Implantation Height on Sinus Flow

Valve-in-valve transcatheter aortic valve replacement (VIV-TAVR) has proven to be a successful treatment for high risk patients with failing aortic surgical bioprostheses. However, thrombus formation on the leaflets of the valve has emerged as a major issue in such procedures, posing a risk of restenosis, thromboembolism, and reduced durability. In this work we attempted to understand the effect of deployment position of the transcatheter heart valve (THV) on the spatio-temporal flow field within the sinus in VIV-TAVR. Experiments were performed in an in vitro pulsatile left heart simulator using high-speed Particle Image Velocimetry (PIV) to measure the flow field in the sinus region. The time-resolved velocity data was used to understand the qualitative and quantitative flow patterns. In addition, a particle tracking technique was used to evaluate relative thrombosis risk via sinus washout. The velocity data demonstrate that implantation position directly affects sinus flow patterns, leading to increased flow stagnation with increasing deployment height. The particle tracking simulations showed that implantation position directly affected washout time, with the highest implantation resulting in the least washout. These results clearly demonstrate the flow pattern and flow stagnation in the sinus is sensitive to THV position. It is, therefore, important for the interventional cardiologist and cardiac surgeon to consider how deployment position could impact flow stagnation during VIV-TAVR.

On the Mechanics of Transcatheter Aortic Valve Replacement.

Transcatheter aortic valves (TAVs) represent the latest advances in prosthetic heart valve technology. TAVs are truly transformational as they bring the benefit of heart valve replacement to patients that would otherwise not be operated on. Nevertheless, like any new device technology, the high expectations are dampened with growing concerns arising from frequent complications that develop in patients, indicating that the technology is far from being mature. Some of the most common complications that plague current TAV devices include malpositioning, crimp-induced leaflet damage, paravalvular leak, thrombosis, conduction abnormalities and prosthesis-patient mismatch. In this article, we provide an in-depth review of the current state-of-the-art pertaining the mechanics of TAVs while highlighting various studies guiding clinicians, regulatory agencies, and next-generation device designers.

The hemodynamic effects of acute aortic regurgitation into a stiffened left ventricle resulting from chronic aortic stenosis

Acute aortic regurgitation (AR) post-chronic aortic stenosis is a prevalent phenomenon occurring in patients who undergo transcatheter aortic valve replacement (TAVR) surgery. The objective of this work was to characterize the effects of left ventricular diastolic stiffness (LVDS) and AR severity on LV performance. Three LVDS models were inserted into a physiological left heart simulator. AR severity was parametrically varied through four levels (ranging from trace to moderate) and compared with a competent aortic valve. Hemodynamic metrics such as average diastolic pressures (DP) and reduction in transmitral flow were measured. AR index was calculated as a function of AR severity and LVDS, and the work required to make up for lost volume due to AR was estimated. In the presence of trace AR, higher LVDS had up to a threefold reduction in transmitral flow (13% compared with 3.5%) and a significant increase in DP (2-fold). The AR index ranged from ∼42 to 16 (no AR to moderate AR), with stiffer LVs having lower values. To compensate for lost volume due to AR, the low, medium, and high LVDS models were found to require 5.1, 5.5, and 6.6 times more work, respectively. This work shows that the LVDS has a significant effect on the LV performance in the presence of AR. Therefore, the LVDS of potential TAVR patients should be assessed to gain an initial indication of their ability to tolerate post-procedural AR.

PneumoniaCheck: A Device for Sampling Lower Airway Aerosols

The pathogens causing pneumonia are difficult to identify because a high quality specimen from the lower lung is difficult to obtain. A new specimen collection device is designed to collect aerosol specimens selectively from the lower lung generated during deep coughing. The PneumoniaCheck device utilizes a separation reservoir and Venturi valve to segregate contents from the upper and lower airways. The device also includes several specially designed features to exclude oral contaminants from the sample and a filter to collect the aerosolized pathogens. Verification testing of PneumoniaCheck demonstrates effective separation of upper airway gas from the lower airway gas (p <0.0001) and exclusion of both liquid and viscous oral material (p<0.0001) from the collection chamber. The filters can collect 99.9997% of virus and bacteria sized particles from the sampled lower lung aerosols. The selective collection of specimens from the lower airway may aid in the diagnosis of specific pathogens causing pneumonia.

Response by Sharma et al to Letter Regarding Article, “The Fluid Mechanics of Transcatheter Heart Valve Leaflet Thrombosis in the Neosinus”

Tang and colleagues raise 3 pertinent issues related to our study1 and its related findings. First, they seek clarification regarding the prevalence of observed leaflet thrombosis across the different inflation parameters quoted in a prior study2 arising from a similar data pool. In addition, they question the potential suggestion that nominal inflation should be avoided given the presence of thrombus in valves expanded <100%. Although the data from this study originated from the same registry as the previously quoted study by Kazuno et al,2 there is no way to determine which, if any, of the 39 patients in that study overlap with the 72 cases in our study, as all data were deidentified. This study is a purely mechanistic …

Characterization of aortic root geometry in transcatheter aortic valve replacement patients

The study aimed to characterize the geometry of the aortic root pre‐ and post‐transcatheter aortic valve replacement (TAVR) and investigate differences in pre‐ and post‐TAVR anatomy.

Validation of Cardiac Output as Reported by a Permanently Implanted Wireless Sensor

The CardioMEMS heart failure (HF) system was tested for cardiac output (CO) measurement accuracy using an in vitro mock circulatory system. A software algorithm calculates CO based on analysis of the pressure waveform as measured from the pulmonary artery, where the CardioMEMS system resides. Calculated CO was compared to that from reference flow probe in the circulatory system model. CO measurements were compared over a clinically relevant range of stroke volumes and heart rates with normal, pulmonary hypertension (PH), decompensated left heart failure (DLHF), and combined DHLF + PH hemodynamic conditions. The CardioMEMS CO exhibited minimal fixed and proportional bias.

Mechanical Advantage of a Compliant Mechanism and Significant Factors Affecting it, Using the Pseudo-Rigid-Body Model Approach

Although work related to mechanical advantage of compliant mechanisms has been presented almost two decades ago, unlike many rigid-body mechanism systems, this performance measure has seldom been used. In great part, the reasons are attributed to, one, the relatively recent development of and a lack of familiarity with this technology and, two, the complexity of the understanding and evaluation of mechanical advantage of compliant systems. In an effort to simplify the evaluation, this work uses the pseudo-rigid-body model (PRBM) of a compliant mechanism, along with traditional notions of power conservation and angular velocity ratios using instant centers. As a first step, the inherent compliance in the mechanism is neglected in determining its mechanical advantage, followed by considerations to optimize its structural configuration for enhancing its mechanical advantage. The PRBM methodology, which offers us a way to estimate the characteristic compliance of the mechanism, now enables its inclusion in determining the mechanical advantage of the compliant mechanism. Two significant factors affecting it are i) the structural configuration of the PRBM, and ii) the energy stored in compliant elements of the mechanism. Several case studies are presented, which suggest that minimizing the latter contribution relative to that of an optimized structural configuration may improve the mechanical advantage of a compliant mechanism. Nonetheless, its effect on the mechanical advantage cannot be neglected.Copyright