Influence of cardiac flow rate on turbulent shear stress from a prosthetic heart valve

Elevated turbulent shear stresses associated with sufficient exposure times are potentially damaging to blood constituents. Since these conditions can be induced by mechanical heart valves, the objectives of this study were to locate the maximum turbulent shear stress in both space and time and to determine how the maximum turbulent shear stress depends on the cardiac flow rate in a pulsatile flow downstream of a tilting disk valve. Two-component, simultaneous, correlated laser velocimeter measurements were recorded at four different axial locations and three different flow rates in a straight tube model of the aorta. All velocity data were ensemble averaged within a 15 ms time window located at approximately peak systolic flow over more than 300 cycles. Shear stresses as high as 992 dynes/cm2 were found 0.92 tube diameters downstream of the monostrut, disk valve. The maximum turbulent shear stress was found to scale with flow rate to the 0.72 power. A repeatable starting vortex was shed from the disk at the beginning of each cycle.     

Effect of prosthetic mitral valve geometry and orientation on flow dynamics in a model human left ventricle

Pulsatile flow dynamics through bileaflet (St Jude and Duromedics), tilting disc (Bjork-Shiley and Omniscience), caged ball (Starr-Edwards), pericardial (Edwards) and porcine (Carpentier-Edwards) mitral valves in a model human left ventricle (LV) were studied. The model human ventricle, obtained from an in situ diastolic casting, was incorporated into a mock circulatory system. Measurements were made at various heart rates and flow rates. These included the transvalvular pressure drop and regurgitation in percent and cm3 beat-1. The effect of valve geometry and the orientation of the valve with respect to the valve annulus was analyzed using a flow visualization technique. Qualitative flow visualization study indicates certain preferred orientations for the tilting disc and bileaflet valve prostheses in order to obtain a smooth washout of flow in the LV chamber.     

Time-varying effective mitral valve area: prediction and validation using cardiac MRI and Doppler echocardiography in normal subjects

Precise knowledge of the volume and rate of early rapid left ventricular (LV) filling elucidates kinematic aspects of diastolic physiology. The Doppler E wave velocity-time integral (VTI) is conventionally used as the estimate of early, rapid-filling volume; however, this implicitly requires the assumption of a constant effective mitral valve area (EMVA). We sought to evaluate whether the EMVA is truly constant throughout early, rapid filling in 10 normal subjects using cardiac magnetic resonance imaging (MRI) and contemporaneous Doppler echocardiography, which were synchronized via ECG. LV volume measurements as a function of time were obtained via MRI, and transmitral flow values were measured via Doppler echocardiography. The synchronized data were used to predict EMVA as a function of time during early diastole. Validation involved EMVA determination using 1) the short-axis echocardiographic images near the mitral valve leaflet tips, 2) the distance between leaflet tips in the echocardiographic parasternal long-axis view, and 3) the distance between leaflet tips from the MRI LV outflow tract view. Predicted EMVA values varied substantially during early rapid filling, and observed EMVA values agreed well with predictions. We conclude that the EMVA is not constant, and its variation causes LV volume to increase faster than is reflected by the VTI. These results reveal the mechanism of early rapid volumetric increase and directly affect the significance and physiological interpretation of the VTI of the Doppler E wave. Application to subjects in selected pathophysiological subsets is in progress.     

Effect of heart rate on left atrial systolic shortening in the dog.

The relationship between heart rate and left atrial end-diastolic diameter (LAEDD) and left atrial systolic shortening (LASS) was investigated in 12 conscious dogs. Atrial pacing, vagal blockade, isoproterenol, and beta-adrenergic blockade were used to change heart rate and the inotropic state of the atrium. LAEDD decreased linearly as heart rate increased. LAEDD averaged 33.0 mm (+/- 0.6 mm SEM) and decreased by 3.2 mm (+/- 0.4 mm SEM) with a change in heart rate of 50 beats/min. The ratio of LASS/LAEDD decreased as LAEDD decreased with increasing heart rate, but there was less of a reduction in the ratio at the extreme levels of LAEDD change with isoproterenol and vagal blockade. Propranolol reduced LASS at any LAEDD. At lower heart rates the reduction of LASS with pacing could be corrected by returning LAEDD to near control levels with a rapid infusion of fluid. It is concluded that LASS is primarily dependent on LAEDD and the inotropic state of the atrium. At higher heart rates, though, some effect of frequency can be observed. Isoproterenol and vagal blockade (increased contractile state) reduced the dependence of LASS on LAEDD.     

Noninvasive assessment of the stenotic mitral valve orifice by two-dimensional echocardiography

Two-dimensional echocardiographic imaging of the mitral valve orifice was attempted in 26 patients with isolated mitral stenosis. The intention was to examine further the clinical usefulness and limitations of this technique for estimating the severity of mitral stenosis. Technically adequate recordings of the mitral orifice were obtained in 20 patients (77%). Mitral valve area calculated from echocardiography compared favorably to the valve area derived from cardiac catheterization with the use of the Gorlin formula (r = 0.95). The average difference between the two methods was 0.109 cm(2). Two-dimensional echocardiography does provide clinically useful data for predicting the degree of mitral stenosis in the majority of patients provided that critical technical limitations are recognized.     

Effect of wedging on the flow characteristics past tilting disc aortic valve prosthesis

To evaluate the efficacy of implanting a tilting disc aortic valve prosthesis in an angulated (wedged) supra-annular position, an in vitro experimental study was performed. The aortic valve prosthesis was mounted in an axi-symmetric valve chamber in a wedged position and incorporated in a mock circulatory system. Measurements were obtained on the transvalvular pressure gradient, percent regurgitation as well as velocity profiles and turbulent normal stresses distal to the valve. Our results showed that there was no significant reduction in the pressure gradient in mounting a larger sized valve in the wedged supra-annular position. On the other hand, the percent regurgitation increased with increase in heart rate and wedge angle. The valve failed to function properly above 110 beats min-1 at any wedge angle with the normal flow rate. The velocity profiles also showed significant changes with an increase in the turbulent normal stress with increase in wedge angle. Hence our study suggests that implanting the tilting disc prosthesis in a wedged supra-annular position in the aorta is not advisable.     

Insights from in-vitro flow visualization into the mechanism of systolic anterior motion of the mitral valve in hypertrophic cardiomyopathy under steady flow conditions.

Hypertrophic obstructive cardiomyopathy is a heart disease characterized by a thickened interventricular septum which narrows the left ventricular outflow tract, and by systolic anterior motion (SAM) of the mitral valve which can contact the septum and create dynamic subaortic obstruction. The most common explanation for SAM has been the Venturi mechanism which postulates that septal hypertrophy, by narrowing the outflow tract, produces high velocities and thus low pressure between the mitral valve and the septum, causing the valve leaflets to move anteriorly. This hypothesis, however, fails to explain why SAM often begins early in systole, when outflow tract velocities are low or negligible or why it may occur in the absence of septal hypertrophy. The goal of this study was therefore to investigate an alternative hypothesis in which structural abnormalities of the papillary muscles act as a primary cause of SAM by altering valve restraint and thereby changing the geometry of the closed mitral apparatus and its relationship to the surrounding flow field. In order to test this hypothesis, an in vitro model of the left ventricle which included an explanted human mitral valve with intact chords and papillary muscle apparatus was constructed. Flow visualization was used to observe the ventricular flow field and the mitral valve geometry. Displacing the papillary muscles anteriorly and closer to each other, as observed clinically in patients with cardiomyopathy and obstruction produced SAM in the absence of septal hypertrophy. Flow could be seen impacting on the upstream (posterior) surface of the leaflets; such flow is capable of producing form drag forces which can initiate and maintain SAM.(ABSTRACT TRUNCATED AT 250 WORDS)     

Protein estimation by the product of integrated peak area and flow rate

A convenient method for protein estimation is described, making use of uv detectors and peak integrators that are standard equipment on modern high-performance liquid chromatographs to determine the product of integrated peak area and flow rate of eluting protein at 214 nm (AF214). We demonstrate that AF214 is proportional to the amount of eluted protein and describe two approaches for calibrating the integrator, by quantitative amino acid analysis and by determining the elution yield of a known amount of applied protein, allowing direct estimation of protein from AF214. Both approaches yield similar results. The basis for the method is that, for virtually all proteins, absorbance at 214 nm is dominated by the summed contributions from the peptide groups. More accurate estimates can be made when the amino acid composition of the eluting protein is known, since this permits a correction to be made for contributions of amino acid side chains to absorbance at 214 nm. Comparison of AF214 estimates for proteins from the small (30 S) subunit of the Escherichia coli ribosome with those obtained by Bradford analysis shows the latter to give somewhat higher values.     

Arterial bifurcation flows--effects of flow rate and area ratio.

Velocity profiles and surface shear rates, for three model symmetrical bifurcations made of glass from dimensions based on the arterial system, were investigated. The models studied had area ratios of 0.75, 1.02, and 1.29, with a common included angle of 75 degrees. Area ratio and parent tube flow rate were the two independent variables evaluated. Measurements were made with a tracer particle technique using cinephotography. Velocity profiles had their highest values on the inside, and lowest values on the outside, of the branch. Flow symmetry existed in the plane perpendicular to the plane of the bifurcation. Surface shear rates remained well above the daughter-tube developed values, between two and six diameters downstream from the carina. Shear rates below the daughter-tube developed value were found on the outside wall between the carina and two daughter-tube diameters downstream. Vortex-like flow was absent in this region for the 0.75 area ratio branch and was found above 900 Reynolds number in the 1.29 area ratio branch. The disturbed flow described by others in this region may not contain vortex-like streamlines for the physiologically important 0.75 area ratio.     

Incidence and outcome of prosthetic valve endocarditis after transcatheter aortic valve replacement in the Netherlands

Background

Transcatheter aortic valve replacement (TAVR) is increasingly being used as an alternative to conventional surgical valve replacement. Prosthetic valve endocarditis (PVE) is a rare but feared complication after TAVR, with reported first-year incidences varying from 0.57 to 3.1%. This study was performed to gain insight into the incidence and outcome of PVE after TAVR in the Netherlands.

Methods

A multicentre retrospective registry study was performed. All patients who underwent TAVR in the period 2010–2017 were screened for the diagnosis of infective endocarditis in the insurance database and checked for the presence of PVE before analysis of general characteristics, PVE parameters and outcome.

Results

A total of 3968 patients who underwent TAVR were screened for PVE. During a median follow-up of 33.5 months (interquartile range (IQR) 22.8–45.8), 16 patients suffered from PVE (0.4%), with a median time to onset of 177 days (IQR 67.8–721.3). First-year incidence was 0.24%, and the overall incidence rate was 0.14 events per 1000 person-years. Overall mortality during follow-up in our study was 31%, of which 25% occurred in hospital. All patients were treated conservatively with intravenous antibiotics alone, and none underwent a re-intervention. Other complications of PVE occurred in 5 patients (31%) and included aortic abscess (2), decompensated heart failure (2) and cerebral embolisation (1).

Conclusion

PVE in patients receiving TAVR is a relatively rare complication and has a high mortality rate.

     

Diastolic gradient of pressure and the effective artificial mitral valve section area in terms of its dysfunction

A seven-year experience of treatment of 126 patients with mitral heart disease who were implanted monocuspid MIKS and bicuspid MEDINZh-2 and ROSKARDIKS prostheses (Russia) is presented. The comparative assessment of hemodynamic efficiency and the analysis of the rate of the occurrence of dysfunction of these mechanical prostheses revealed that the MIKS and MEDINZh-2 implants have advantages of hemodynamic characteristics over ROSKARDIKS, despite the priority of the standard size. It was shown that the initially low diastolic pressure gradient on the mitral valve prosthesis and the initially larger area of the prosthetic effective mitral valve aperture are of crucial importance for preventing valve complications and reducing the number of open heart reoperations.     

A theoretical description of blood flow through the mitral orifice

A nonlinear differential equation describing the Doppler velocity profile for blood flow through the mitral valve has been derived. This equation is based on fluid dynamics and a simple, but comprehensive model of atrial and ventricular mechanics. A numerical solution to the equation is described and provides excellent agreement with Doppler velocity curves obtained clinically. One important result of the theory is that in patients with mitral stenosis, the slope of the clinically observed straight-line descent of the velocity profile is proportional to the mitral orifice area and inversely proportional to the atrioventricular compliance.     

A three-dimensional insight into the complexity of flow convergence in mitral regurgitation: adjunctive benefit of anatomic regurgitant orifice area

Mitral effective regurgitant orifice area (EROA) using the flow convergence (FC) method is used to quantify the severity of mitral regurgitation (MR). However, it is challenging and prone to interobserver variability in complex valvular pathology. We hypothesized that real-time three-dimensional (3D) transesophageal echocardiography (RT3D TEE) derived anatomic regurgitant orifice area (AROA) can be a reasonable adjunct, irrespective of valvular geometry. Our goals were to 1) to determine the regurgitant orifice morphology and distance suitable for FC measurement using 3D computational flow dynamics and finite element analysis (FEA), and (2) to measure AROA from RT3D TEE and compare it with 2D FC derived EROA measurements. We studied 61 patients. EROA was calculated from 2D TEE images using the 2D-FC technique, and AROA was obtained from zoomed RT3DE TEE acquisitions using prototype software. 3D computational fluid dynamics by FEA were applied to 3D TEE images to determine the effects of mitral valve (MV) orifice geometry on FC pattern. 3D FEA analysis revealed that a central regurgitant orifice is suitable for FC measurements at an optimal distance from the orifice but complex MV orifice resulting in eccentric jets yielded nonaxisymmetric isovelocity contours close to the orifice where the assumptions underlying FC are problematic. EROA and AROA measurements correlated well (r = 0.81) with a nonsignificant bias. However, in patients with eccentric MR, the bias was larger than in central MR. Intermeasurement variability was higher for the 2D FC technique than for RT3DE-based measurements. With its superior reproducibility, 3D analysis of the AROA is a useful alternative to quantify MR when 2D FC measurements are challenging.