In vitro stress measurements in the vicinity of six mechanical aortic valves using hot-film anemometry in steady flow

Ventricular flow Dynamic past Bileaflet Prosthetic Heart Valves

To characterise hydrodynamic properties of prosthetic heart valves in mitral position, ultrasonic velocity measurements were performed using a cardiovascular simulator. A Duromedics and a Saint-Jude Medical bileaflet heart valve were tested. The Saint-Jude valve was oriented first in an anatomical position, i.e. the tilt axis parallel to the septal wall, and then in an anti-anatomical position. In the anti-anatomical position, from mid diastole to mid systole, two contrarotative vortices are generated in the ventricle by the interaction between the flow directed by the leaflets downstream from the lateral orifices and the ventricular wall motions. In the anatomical position, the mitral flow penetrates the ventricle principally through the lateral orifice proximal to the septal wall, due to the vortex in the atrial chamber. The mitral inflow then circulates along the septal wall to the apex, and produces a large ventricular vortex during systole. In the anatomical position, the Saint-Jude thus provides a better ventricular washout during this phase. The mitral inflow through the Duromedics in the anti-anatomical position produces two contrarotative vortices in the ventricle, but in the opposite sense than downstream the Saint-Jude valve; the flows that penetrate through the lateral orifices are directed to the ventricular walls and then recirculate to the centre of the ventricle, providing a very fluctuating flow near the apex. Thus, a slight difference in valve design produces a significant difference in the ventricular flow fields.

Integrated strategy for in vitro characterization of a bileaflet mechanical aortic valve

Background

Haemodynamic performance of heart valve prosthesis can be defined as its ability to fully open and completely close during the cardiac cycle, neither overloading heart work nor damaging blood particles when passing through the valve. In this perspective, global and local flow parameters, valve dynamics and blood damage safety of the prosthesis, as well as their mutual interactions, have all to be accounted for when assessing the device functionality. Even though all these issues have been and continue to be widely investigated, they are not usually studied through an integrated approach yet, i.e. by analyzing them simultaneously and highlighting their connections.

Results

An in vitro test campaign of flow through a bileaflet mechanical heart valve (Sorin Slimline 25 mm) was performed in a suitably arranged pulsatile mock loop able to reproduce human systemic pressure and flow curves. The valve was placed in an elastic, transparent, and anatomically accurate model of healthy aorta, and tested under several pulsatile flow conditions. Global and local hydrodynamics measurements and leaflet dynamics were analysed focusing on correlations between flow characteristics and valve motion. The haemolysis index due to the valve was estimated according to a literature power law model and related to hydrodynamic conditions, and a correlation between the spatial distribution of experimental shear stress and pannus/thrombotic deposits on mechanical valves was suggested. As main and general result, this study validates the potential of the integrated strategy for performance assessment of any prosthetic valve thanks to its capability of highlighting the complex interaction between the different physical mechanisms that govern transvalvular haemodynamics.

Conclusions

We have defined an in vitro procedure for a comprehensive analysis of aortic valve prosthesis performance; the rationale for this study was the belief that a proper and overall characterization of the device should be based on the simultaneous measurement of all different quantities of interest for haemodynamic performance and the analysis of their mutual interactions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12938-017-0314-2) contains supplementary material, which is available to authorized users.

Turbulence downstream of subcoronary stentless and stented aortic valves

Regions of turbulence downstream of bioprosthetic heart valves may cause damage to blood components, vessel wall as well as to aortic valve leaflets. Stentless aortic heart valves are known to posses several hemodynamic benefits such as larger effective orifice areas, lower aortic transvalvular pressure difference and faster left ventricular mass regression compared with their stented counterpart. Whether this is reflected by diminished turbulence formation, remains to be shown. We implanted either stented pericardial valve prostheses (Mitroflow), stentless valve prostheses (Solo or Toronto SPV) in pigs or they preserved their native valves. Following surgery, blood velocity was measured in the cross sectional area downstream of the valves using 10MHz ultrasonic probes connected to a dedicated pulsed Doppler equipment. As a measure of turbulence, Reynolds normal stress (RNS) was calculated at two different blood pressures (baseline and 50% increase). We found no difference in maximum RNS measurements between any of the investigated valve groups. The native valve had significantly lower mean RNS values than the Mitroflow (p=0.004), Toronto SPV (p=0.008) and Solo valve (p=0.02). There were no statistically significant differences between the artificial valve groups (p=0.3). The mean RNS was significantly larger when increasing blood pressure (p=0.0006). We, thus, found no advantages for the stentless aortic valves compared with stented prosthesis in terms of lower maximum or mean RNS values. Native valves have a significantly lower mean RNS value than all investigated bioprostheses.

The Effect of Vortex Formation on Left Ventricular Filling and Mitral Valve Efficiency

A new mechanism for quantifying the filling energetics in the left ventricle (LV) and past mechanical heart valves (MHV) is identified and presented. This mechanism is attributed to vortex formation dynamics past MHV leaflets. Recent studies support the conjecture that the natural healthy left ventricle (LV) performs in an optimum, energy-preserving manner by redirecting the flow with high efficiency. Yet to date, no quantitative proof has been presented. The present work provides quantitative results and validation of a theory based on the dynamics of vortex ring formation, which is governed by a critical formation number (FN) that corresponds to the dimensionless time at which the vortex ring has reached its maximum circulation content, in support of this hypothesis. Herein, several parameters (vortex ring circulation, vortex ring energy, critical FN, hydrodynamic efficiencies, vortex ring propagation speed) have been quantified and presented as a means of bridging the physics of vortex formation in the LV. In fact, the diastolic hydrodynamic efficiencies were found to be 60, 41, and 29%, respectively, for the porcine, anti-anatomical, and anatomical valve configurations. This assessment provides quantitative proof of vortex formation, which is dependent of valve design and orientation, being an important flow characteristic and associated to LV energetics. Time resolved digital particle image velocimetry with kilohertz sampling rate was used to study the ejection of fluid into the LV and resolve the spatiotemporal evolution of the flow. The clinical significance of this study is quantifying vortex formation and the critical FN that can potentially serve as a parameter to quantify the LV filling process and the performance of heart valves.

Time-resolved DPIV analysis of vortex dynamics in a left ventricular model through bileaflet mechanical and porcine heart valve prostheses

The performance of the heart after a mitral valve replacement operation greatly depends on the flow character downstream of the valve. The design and implanting orientation of valves may considerably affect the flow development. A study of the hemodynamics of two orientations, anatomical and anti-anatomical, of the St. Jude Medical (SJM) bileaflet valve are presented and compared with those of the SJM Biocor porcine valve, which served also to represent the natural valve. We document the velocity field in a flexible, transparent (LV) using time-resolved digital particle image velocimetry (TRDPIV). Vortex formation and vortex interaction are two important physical phenomena that dominate the filling and emptying of the ventricle. For the three configurations, the following effects were examined: mitral valve inlet jet asymmetry, survival of vortical structures upstream of the aortic valve, vortex-induced velocities and redirection of theflow in abidance of the Biot-Savart law, domain segmentation, resonant times of vortical structures, and regions of stagnantflow. The presence of three distinct flow patterns, for the three configurations, was identified by the location of vortical structures and level of coherence corresponding to a significant variation in the turbulence level distribution inside the LV. The adverse effect of these observations could potentially compromise the efficiency of the LV and result in flow patterns that deviate from those in the natural heart.

Particle Image Velocimetry for Flow Analysis in Longitudinal Planes across a Mechanical Artificial Heart Valve

In the last decades a great number of in vitro studies have been conducted to improve the design and to understand the transvalvular flow patterns under steady-state and pulsating flow conditions. Steady-state tests are useful for studying the flow established upstream and downstream of the valve prosthesis in different flow conditions and, in particular, at the peak flow rate. In the present study, the particle image velocimetry (PIV) technique is employed to visualize the flow patterns in a precommercial model of a bi-leaflet mechanical heart valve prosthesis in a steady-state flow regime. The use of the PIV technique and a convenient test rig provide good conditions with which to investigate the whole flow field upstream and downstream of the valve. Velocity and vorticity maps are obtained for the flow passing through the prosthesis in different axial planes. A new and simple solution for the measurement test chamber is proposed. This innovative approach of observing the flow in staggered planes (other than diametrical planes) allows a flow analysis to be performed upstream and downstream of the valve in regions near the tube wall where it has the maximum potential for injury of the vessel's inner cell layer.

Assessment of prosthetic aortic valve performance by magnetic resonance velocity imaging

OBJECTIVES

Magnetic resonance (MRI) velocity mapping was used to evaluate non-invasively the flow profiles of the ascending aorta in normal volunteers and in patients with an aortic (mechanical) valve prosthesis.

BACKGROUND

In patients with artificial aortic valves the flow profile in the ascending aorta is severely altered. These changes have been associated with an increased risk of thrombus formation and mechanical hemolysis.

METHODS

Velocity profiles were determined 30 mm distal to the aortic valve in six healthy volunteers and seven patients with aortic valve replacement (replacement within the last 2 years) using ECG triggered phase contrast MRI. Peak flow, mean flow and mean reverse flow were measured in intervals of 25 ms during the entire heart cycle. Systolic reverse flow, end-systolic closing and diastolic leakage volume were calculated for all subjects.

RESULTS

Peak flow velocity during mid-systole was significantly higher in patients with valvular prosthesis than in normals (mean + SD, 1.9 +/- 0.4 m/s vs. 1.2 +/- 0.03 m/s, P < 0.001) with a double peak and a zone of reversed flow close to the inner (left lateral) wall of the ascending aorta of the patients. Closing volume was significantly larger in patients than in controls (-3.3 +/- 1.2 ml/beat vs. -0.9 +/- 0.5 ml/beat; P < 0.001). There was reverse flow during systole in valvular patients amounting to 15.7 +/- 6.7% of total cardiac output compared to 2.3 +/- 1.2% in controls (P < 0.001). Diastolic mean flow was negative in patients after valve replacement but not in controls (-11.0 +/- 15.2 ml/beat vs. 6.8 +/- 3.2 ml/beat; P < 0.01).

CONCLUSIONS

The following three major quantitative observations have been made in the present study: (1) Mechanical valve prostheses have an increased peak flow velocity with a systolic reverse flow at the inner (left lateral) wall of the ascending aorta. (2) A double peak flow velocity pattern can be observed in patients with bileaflet (mechanical) prosthesis. (3) The blood volume required for leaflet closure and the diastolic leakage blood volume are significantly higher for the examined bileaflet valve than for native heart valves.

Intraluminal recording of cross-sectional blood velocity distribution of human ascending aorta by ultrasound Doppler technique

A pulsed Doppler ultrasound technique was used for mapping two-dimensional blood velocity profiles in the human ascending aorta during open-heart surgery. An electronic position-sensitive device was constructed and linked to an intraluminal 10 MHz Doppler ultrasound probe. From a plane perpendicular to the central direction of blood flow, velocity mapping was performed covering the entire cross-section of the ascending aorta 6-7 cm above the valve. This method is based on a sequential sampling of velocity from continuously changing locations during a stable haemodynamic period; typically velocity points are recorded from 150-300 beats. Further processing transformed data to suit a previously developed velocity distribution model for normal blood flow in the human ascending aorta, based on multi-regression analyses. In this model, the time series of data from consecutive beats were computed into an average two-dimensional profile described through one cardiac cycle. This method allows high spatial resolution (1.5 mm), in addition to the high-frequency response (200 Hz) of the modified ultrasound Doppler meter. Together with the advantage of velocity directionality and minimal time interventions, this makes the method well suited for studies on normal flow conditions as well as flow velocity distribution distal to different heart valve prostheses.

Late chronic hemolysis after valve replacement for aortic stenosis. Relation to residual hypertrophy and impaired left ventricular function

The relationship between intravascular hemolysis induced by aortic valve prosteses and patient status/left ventricular (LV) function (radionuclide cardiography) was examined in 63 patients of 65 who were alive ten to seventeen years after valve replacement (1965-1973) for aortic stenosis. Serum-lactic dehydrogenase (LDH) exceeded upper reference limit in 62 patients and S-haptoglobin (HAPTO) was reduced in 62. One patient with normal LDH had reduced HAPTO and elevated plasma-hemoglobin. Anemia was noted in 4 patients (6%). S-LDH was higher in men than in women (p less than 0.05), in patients with increased ECG hypertrophy score than in those with a normal score (p less than 0.05), in patients with NYHA class II-III than in those with class I (p less than 0.05), in patients with abnormal LV function than in those with a normal radionuclide study (p less than 0.05), in patients with a pathologic Q wave in the ECG than in those without (p less than 0.05), and in patients with a Starr Edwards cloth-covered (SECC) prosthesis than in those with other types (p = 0.07). ECG hypertrophy score correlated directly with LDH (r = 0.33, p = 0.008) and inversely with LV ejection fraction (r = -0.57, p less than 0.0001), peak ejection rate (r = -0.47, p less than 0.0001), and peak filling rate (r = -0.41, p less than 0.001). Multiple linear regression analysis revealed that LDH was accounted for by ECG hypertrophy score (p = 0.001), SECC prosthesis (p = 0.04), and male gender (p = 0.05). Hypertrophic malfunctioning left ventricles may be responsible for higher degrees of turbulent flow characteristics in the vicinity of prosthetic valves in the aortic position and, by inference, explain the increased tendency toward hemolysis in these patients.

Turbulent stresses downstream of porcine and pericardial aortic valves implanted in pigs

Because late valve-related complications such as hemolysis and thromboembolic events are considered related to flow disturbances caused by the inserted valve, velocity fields downstream of aortic valve prostheses were studied in pigs. Acute hemodynamic evaluation of size 25-mm porcine and pericardial aortic valve prostheses 1 diameter downstream of the valve ring was performed using dynamic three-dimensional visualization of velocity profiles and spatial distribution of turbulence. Point blood velocity signals obtained with a 1-mm hot-film anemometer needle probe were used to compute Reynolds normal stresses (RNS) by calculation of the turbulent velocity energy of the axial velocity component in the systole. The porcine valves caused a skewed velocity and turbulence profile revealing mean spatial systolic RNS at 70 nm-2 +/- 35 nm-2 (+/- SD). The spatial maximum RNS was 275 +/- 139 nm-2. Corresponding values for the pericardial valves were 20 +/- 11 nm-2 and 72 +/- 46 nm-2. The pericardial valves revealed plug-shaped velocity profiles and turbulent profiles with slightly higher RNS values at the stent posts. From a hemodynamic point of view, these acute studies indicate superiority of the pericardial valves compared to the porcine valves. The turbulent stresses found in this study are of a magnitude that may cause blood corpuscular and endothelial damage.

Three-dimensional visualization of velocity fields downstream of six mechanical aortic valves in a pulsatile flow model

Velocity fields downstream of 27 mm Björk-Shiley Standard, Björk-Shiley Convex-Concave, Björk-Shiley Monostrut, Hall-Kaster (Medtronic-Hall), St. Jude Medical and Starr-Edwards Silastic Ball aortic valves were studied in a pulsatile mock circulation. Stroke volume was 70 cm3 and frequency 71 min-1 and 88 min-1. Fluid velocity was measured by a catheter mounted hot-film anemometer probe in a glycerol water mixture one and two diameters downstream of the aortic valve. Velocity fields were dynamically visualized by a three-dimensional technique and revealed qualitative independence of frequency. All profiles were flat in the acceleration phase of systole. From peak systole and throughout the systolic deceleration phase profiles characteristic of the individual valves appeared. The pivoting and tilting disc valves caused a skewed velocity profile with highest velocities downstream of the major orifice and lowest velocities downstream of the minor orifice. The differences between the three investigated Björk-Shiley valves were remarkable. The St. Jude Medical valve generated velocity peaks downstream of the two major orifices and the central slit, and lower velocities in the hinge areas. A rather flat profile with central hollowing was seen downstream of the Starr-Edwards Ball valve. All velocity profiles were more or less dampened two diameters downstream.

Turbulent stress measurements downstream of six mechanical aortic valves in a pulsatile flow model

In a pulsatile flow model aortic Björk-Shiley Standard, Convex-Concave and Monostrut valves were investigated together with the Hall-Kaster (Medtronic-Hall), St Jude Medical and Starr-Edwards Silastic Ball valve using hot-film anemometry. Three-dimensional visualization of average systolic Reynolds normal stresses (RNS) reflected the design of the valves. Mean average RNS were used for comparison of the fluid dynamic performance along with Velocity Energy Ratio (VER100) and Turbulence Energy Ratio (TER) as a relative turbulence intensity for pulsatile flow. Mean average RNS ranged from 13.2 to 37.6 Nm-2 for all the valves with the highest levels for the Björk-Shiley Standard and Starr-Edwards Ball valve and lowest values for the St Jude Medical valve and with the Hall-Kaster (Medtronic-Hall), Björk-Shiley Convex-Concave and Monostrut valves in between.