The Closing Behavior of Mechanical Aortic Heart Valve Prostheses

Automatic segmentation and tracking of biological prosthetic heart valves

Purpose: Prosthetic heart valve designs must be rigorously tested using cardiovascular equipment. The valve orifice area over time constitutes a key quality metric which is typically assessed manually, thus a tedious and error-prone task. From a computer vision viewpoint, a major unsolved issue lies in the orifice being partly occluded by the leaflets' inner side or inaccurately depicted due to its transparency. Here, we address this issue, which allows us to focus on the accurate and automatic computation of valve orifice areas. Approach: We propose a segmentation approach based on the detection of the leaflets' free edges. Using video frames recorded with a high-speed digital camera during in vitro simulations, an initial estimation of the orifice area is first obtained via active contouring and thresholding and then refined to capture the leaflet free edges via a curve transformation mechanism. Results: Experiments on video data from pulsatile flow testing demonstrate the effectiveness of our approach: a root-mean-square error (RMSE) on the temporal extracted orifice areas between 0.8% and 1.2%, an average Jaccard similarity coefficient between 0.933 and 0.956, and an average Hausdorff distance between 7.2 and 11.9 pixels. Conclusions: Our approach significantly outperformed a state-of-the-art algorithm in terms of evaluation metrics related to valve design (RMSE) and computer vision (accuracy of the orifice shape). It can also cope with lower quality videos and is better at processing frames showing an almost closed valve, a crucial quality for assessing valve design malfunctions related to their improper closing.

In vitro study of trileaflet polytetrafluoroethylene conduit and its valve-in-valve transformation

OBJECTIVES

Handmade trileaflet expanded polytetrafluoroethylene valved conduit developed using the flip-over method has been tailored for pulmonary valve reconstruction with satisfactory outcomes. We investigated the in vitro performance of the valve design in a mock circulatory system with various conduit sizes. In our study, the design was transformed into a transcatheter stent graft system which could fit in original valved conduits in a valve-in-valve fashion.

METHODS

Five different sizes of valved polytetrafluoroethylene vascular grafts (16, 18, 20, 22 and 24 mm) were mounted onto a mock circulatory system with a prism window for direct leaflets motion observation. Transvalvular pressure gradients were recorded using pressure transducers. Mean and instant flows were determined via a rotameter and a flowmeter. Similar flip-over trileaflet valve design was then carried out in 3 available stent graft sizes (23, 26 and 28.5 mm, Gore aortic extender), which were deployed inside the valved conduits.

RESULTS

Peak pressure gradient across 5 different sized graft valves, in their appropriate flow setting (2.0, 2.5 and 5.0 l/min), ranged from 4.7 to 13.2 mmHg. No significant valve regurgitation was noted (regurgitant fraction: 1.6–4.9%) in all valve sizes and combinations. Three sizes of the trileaflet-valved stent grafts were implanted in the 4 sizes of valved conduits except for the 16-mm conduit. Peak pressure gradient increase after valved-stent graft-in-valved-conduit setting was <10 mmHg in all 4 conduits.

CONCLUSIONS

The study showed excellent in vitro performance of trileaflet polytetrafluoroethylene valved conduits. Its valved stent graft transformation provided data which may serve as a reference for transcatheter valve-in-valve research in the future.

High-frequency operation of pulsatile ventricular assist devices: A computational study on circular and elliptically shaped pumps

Pulsatile positive displacement pumps as ventricular assist devices were gradually replaced by rotary devices due to their large volume and high adverse event rates. Nevertheless, pulsatile ventricular assist devices might be beneficial with regard to gastrointestinal bleeding and cardiac recovery. Therefore, aim of this study was to investigate the flow field in new pulsatile ventricular assist devices concepts with an increased pump frequency, which would allow lower stroke volumes to reduce the pump size. We developed a novel elliptically shaped pulsatile ventricular assist devices, which we compared to a design based on a circular shape. The pump size was adjusted to deliver similar flow rates at pump frequencies of 80, 160, and 240 bpm. Through a computational fluid dynamics study, we investigated flow patterns, residence times, and wall shear stresses for different frequencies and pump sizes. A pump size reduction by almost 50% is possible when using a threefold pump frequency. We show that flow patterns inside the circular pump are frequency dependent, while they remain similar for the elliptic pump. With slightly increased wall shear stresses for higher frequencies, maximum wall shear stresses on the pump housing are higher for the circular design (42.2 Pa vs 18.4 Pa). The calculated blood residence times within the pump decrease significantly with increasing pump rates. A smaller pump size leads to a slight increase of wall shear stresses and a significant improvement of residence times. Hence, high-frequency operation of pulsatile ventricular assist devices, especially in combination with an elliptical shape, might be a feasible mean to reduce the size, without any expectable disadvantages in terms of hemocompatibility.

Short-term evaluation of thromboresistance of a poly(ether ether ketone) (PEEK) mechanical heart valve with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC)-grafted surface in a porcine aortic valve replacement model

Improved thromboresistance of mechanical valves is desired to decrease the risk of thromboembolism and thrombosis and reduce the dosage of anticoagulation with a vitamin K antagonist (e.g., warfarin). For several mechanical valves, design-related features are responsible for their improved thromboresistance. However, it remains unclear whether material-related features provide a practical level of thromboresistance to mechanical valves. Here, we studied the effect of a bileaflet valve made of poly(ether ether ketone) (PEEK) with a poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC)-grafted surface (PEEK-g-PMPC). PMPC is a well-known thromboresistant polymeric material. A short-term (<26 h) porcine aortic valve replacement model using neither an anticoagulant nor an antiplatelet agent showed that the PEEK-g-PMPC valve opened and closed normally with an allowable transvalvular gradient. Unlike an untreated PEEK valve, no thrombus formed on the PEEK-g-PMPC valves on gross anatomy examination in addition to the absence of traveled thrombi in the kidney and lung tissues. Material (PEEK-g-PMPC)-related thromboresistance appeared to decrease the risk of thromboembolism and thrombosis for patients with mechanical valves. However, thromboresistance of the PEEK-g-PMPC valve requires improvement because fibrous fouling was still observed on the leaflet. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1052-1063, 2019.

A new ex vivo beating heart model to investigate the application of heart valve performance tools with a high-speed camera

High-speed camera examination of heart valves is an established technique to examine heart valve prosthesis. The aim of this study was to examine the possibility to transmit new tools for high-speed camera examination of heart valve behavior under near-physiological conditions in a porcine ex vivo beating heart model. After explantation of the piglet heart, main coronary arteries were cannulated and the heart was reperfused with the previously collected donor blood. When the heart started beating in sinus rhythm again, the motion of the aortic and mitral valve was recorded using a digital high-speed camera system (recording rate 2,000 frames/sec). The image sequences of the mitral valve were analyzed, and digital kymograms were calculated at different angles for the exact analysis of the different closure phases. The image sequence of the aortic valve was analyzed, and several snakes were performed to analyze the effective orifice area over the time. Both processing tools were successfully applied to examine heart valves in this ex vivo beating heart model. We were able to investigate the exact open and closure time of the mitral valve, as well as the projected effective orifice area of the aortic valve over the time. The high-speed camera investigation in an ex vivo beating heart model of heart valve behavior is feasible and also reasonable because of using processing feature such as kymography for exact analysis. These analytical techniques might help to optimize reconstructive surgery of the mitral valve and the development of heart valve prostheses in future.

Aortic Root Compliance Influences Hemolysis in Mechanical Heart Valve Prostheses: An In-Vitro Study

Mechanical heart valve prostheses are known to activate coagulation and cause hemolysis. Both are particularly dependent on the leaflet dynamics, which in turn depends on the flow field in the aortic root influenced by the aortic root geometry and its compliance. Compliance reduction of large vessels occurs in aging patients, both in those who have atherosclerotic diseases and those who do not. In this study we investigated the correlation between hemolysis and the compliance of the proximal aorta in a novel, pulsatile in vitro blood tester using porcine blood. Two mechanical heart valves, the St Jude Medical (SJM) bileaflet valve and a trileaflet valve prototype (Triflo) were tested for hemolysis under physiological conditions (120/80 mm Hg, 4.5 l/min, 70 bpm) and using two different tester setups: with a stiff aorta and with a compliant aorta. Valve dynamics were subsequently analyzed via high-speed videos. In the tests with the Triflo valve, the free plasma hemoglobin increased by 13.4 mg/dl for the flexible and by 19.3 mg/dl for the stiff setup during the 3-hour test. The FFT spectra and closing speed showed slight differences for both setups. Free plasma hemoglobin for the SJM valve was up by 22.2 mg/dl in the flexible and 42.7 mg/dl in the stiff setup. Cavitation induced by the higher closing speed might be responsible for this, which is also indicated by the sound spectrum elevation above 16 kHz.

Effects of leaflet geometry on the flow field in three bileaflet valves when installed in a pneumatic ventricular assist device

Our group is currently developing a pneumatic ventricular assist device (PVAD). In this study, in order to select the optimal bileaflet valve for our PVAD, three kinds of bileaflet valve were installed and the flow was visualized downstream of the outlet valve using the particle image velocimetry (PIV) method. To carry out flow visualization inside the blood pump and near the valve, we designed a model pump that had the same configuration as our PVAD. The three bileaflet valves tested were a 21-mm ATS valve, a 21-mm St. Jude valve, and a 21-mm Sorin Bicarbon valve. The mechanical heart valves were mounted at the aortic position of the model pump and the flow was visualized by using the PIV method. The maximum flow velocity was measured at three distances (0, 10, and 30 mm) from the valve plane. The maximum flow velocity of the Sorin Bicarbon valve was less than that of the other two valves; however, it decreased slightly with increasing distance it the X-Y plane in all three valves. Although different bileaflet valves are very similar in design, the geometry of the leaflet is an important factor when selecting a mechanical heart valve for use in an artificial heart.

Effects of mechanical valve orifice direction on the flow pattern in a ventricular assist device

We have been developing a pneumatic ventricular assist device (PVAD) system consisting of a diaphragm-type blood pump. The objective of the present study was to evaluate the flow pattern inside the PVAD, which may greatly affect thrombus formation, with respect to the inflow valve-mount orientation. To analyze the change of flow behavior caused by the orifice direction (OD) of the valve, the flow pattern in this pump was visualized. Particle image velocimetry was used as a measurement technique to visualize the flow dynamics. A monoleaflet mechanical valve was mounted in the inlet and outlet ports of the PVAD, which was connected to a mock circulatory loop tester. The OD of the inlet valve was set at six different angles (OD = 0 degrees, 45 degrees, 90 degrees, 135 degrees, 180 degrees, and 270 degrees, where the OD opening toward the diaphragm was defined as 0 degrees ) and the pump rate was fixed at 80 bpm to create a 5.0 l/min flow rate. The main circular flow in the blood pump was affected by the OD of the inlet valve. The observed regional flow velocity was relatively low in the area between the inlet and outlet port roots, and was lowest at an OD of 90 degrees. In contrast, the regional flow velocity in this area was highest at an OD of 135 degrees. The OD is an important factor in optimizing the flow condition in our PVAD in terms of preventing flow stagnation, and the best flow behavior was realized at an OD of 135 degrees.

An Experimental Study of Steady Flow Patterns of a New Trileaflet Mechanical Aortic Valve

Hemodynamic research shows that thrombosis formation is closely tied to flow field turbulent stress. Design limitations cause flow separation at leaflet edges and the annular valve base, vortex mixing downstream, and high turbulent shear stress. The trileaflet design opens like a physiologic valve with central flow. Leaflet curvature approximates a completely circular orifice, maximizing effective flow area of the open valve. Semicircular aortic sinuses downstream of the valve allow vortex formation to help leaflet closure. The new trileaflet design was hemodynamically evaluated via digital particle image velocimetry and laser-Doppler anemometry. Measurements were made during peak flow of the fully open valve, immediately downstream of the valve, and compared with the 27-mm St. Jude Medical (SJM) bileaflet valve. The trileaflet valve central flow produces sufficient pressure to inhibit separation shear layers. Absence of downstream turbulent wake eddies indicates smooth, physiologic blood flow. In contrast, SJM produces strong turbulence because of unsteady separated shear layers where the jet flow meets the aortic sinus wall, resulting in higher turbulent shear stresses detrimental to blood cells. The trileaflet valve simulates the physiologic valve better than previous designs, produces smoother flow, and allows large scale recirculation in the aortic sinuses to help valve closure.