In vitro hydrodynamic comparison of mitral valve prostheses at high flow rates

The effect of decellularisation on the real time mechanical fatigue of porcine aortic heart valve roots

Decellularised heart valve roots offer a promising option for heart valve replacement in young patients, having the potential to remodel and repair. Replacement heart valves have to undergo billions of opening and closing cycles throughout the patient’s lifetime. Therefore, understanding the effect of cyclic loading on decellularised heart valve roots is important prior to human implantation. The aim of this preliminary study was to investigate the influence of low concentration sodium dodecyl sulphate (SDS) decellularisation treatment on the in vitro real time mechanical fatigue of porcine aortic heart valve roots under physiological real time cyclic loading conditions. This required a specific real time in vitro method to be developed, since previous methods relied on accelerated testing, which is non-physiological, and not appropriate for valve replacement materials that exhibit time dependent characteristics. The effects of the real time fatigue on hydrodynamic function and mechanical properties of the heart valve roots were assessed. The mechanical fatigue of decellularised porcine aortic heart valve roots (n = 6) was assessed and compared to cellular porcine aortic heart valve roots (n = 6) in a modified Real time Wear Tester (RWT) at a physiological frequency and under cyclic pressure conditions for a maximum of 1.2 million cycles. Periodically, the heart valve roots were removed from the RWT to assess the influence of cyclic loading on valve competency (static leaflet closure). At the end of testing further hydrodynamic performance parameters were ascertained, along with determination of leaflet material properties. A real time mechanical fatigue assessment method was developed and applied; with two cellular and two decellularised porcine aortic leaflets in different heart valve roots showing tears in the belly region. The decellularised aortic heart valve roots exhibited comparative functionality to the cellular heart valve roots under in vitro static and pulsatile hydrodynamic conditions. However, the material properties of the decellularised aortic leaflets were significantly altered following cyclic fatigue assessment and showed increases in elastin and collagen phase slopes and ultimate tensile strength compared to the cellular porcine aortic leaflets in the circumferential direction. This preliminary study demonstrated that low concentration SDS decellularised porcine aortic heart valve roots can withstand physiological cyclic deformations up to 1.2 million cycles in a RWT whilst maintaining their overall hydrodynamic function and leaflet mechanical properties. This is the first full report of preclinical mechanical fatigue assessment of decellularised porcine aortic heart valve roots under physiological real time conditions.

A new polymer valve for mechanical circulatory support systems

A new low-cost artificial heart valve with easily modified dimensions according to its application was developed using segmented polyurethane (SPU). It consists of a convex frame and a concave membrane which floats in the center of the frame while working. The hydrodynamic performance of the polymer valve was compared with that of the Björk-Shiley mechanical valve using a mock circulatory testing system. The hydrodynamic performance of this valve was superior to the Björk-Shiley mechanical valve. The valve was applied to a ventricular assist device (VAD) developed in our institute. In vivo performances of these systems were tested using mongrel dogs. During the experiments, there were no complications related to malfunction of the valve. At postmortem examinations, no thrombus formation was found on the valve surface, and no embolus was detected in the kidneys. We believe this valve could prove a very useful alternative for valves of mechanical circulatory support systems (MCSS) such as VAD and TAH.

Echocardiographic assessment of prosthetic mitral valves in children

AIMS

We studied how Doppler-derived hemodynamic parameters in children change as the relative prosthetic mitral valve (PMV) size decreases with somatic growth and evaluated the diagnostic utility of the parameters for detecting PMV obstruction in children.

METHODS AND RESULTS

We reviewed 26 echocardiographic examination results of 15 mechanical bileaflet PMVs in 12 children. The median age at echocardiographic examination was 6.6 (0.6-18.1) years. The PMV functioned normally in 24 examinations but was obstructed due to thrombosis in two cases. PMV sizes ranged between 16 and 25 mm, which were standardized to body surface area (BSA) at the examination with z-score calculations. We assessed the peak E velocity, mean pressure gradient (PG), and pressure half time (PHT) of the transprosthetic flow, the velocity-time integral (VTI) ratio of the PMV inflow to the left ventricular outflow, and the BSA-indexed effective orifice area (iEOA) of the PMV calculated with the continuity equation. Linear regression analysis revealed statistically significant correlations between all parameters of normally functioning PMVs and the PMV size z-scores (Pearson correlation coefficients: peak E velocity, -0.68; mean PG, -0.71; PHT, -0.82; VTI ratio, -0.76; iEOA, 0.79). Compared with the predictive values derived from the regression equations, the VTI ratio and iEOA exceeded ± 2 standard errors in both patients with obstructive PMVs.

CONCLUSION

To assess PMV function in children, Doppler-derived hemodynamic parameters should be compared with their predictive values based on relative PMV sizes. The deviation of the VTI ratio and iEOA from their predictive values may indicate prosthetic obstruction.

Inertance estimation in a lumped-parameter hydraulic simulator of human circulation

Pulsatile mock loop systems are largely used to investigate the cardiovascular system in vitro. They consist of a pump, which replicates the heart, coupled with a lumped-parameter hydraulic afterload, which simulates vasculature. An accurate dimensioning of components is required for a reliable mimicking of the physiopathological behavior of the system. However, it is not possible to create a component for the afterload inertance, and inertance contributions are present in the entire circuit. Hence, in the literature, inertance is neglected or qualitatively evaluated. In this paper, we propose two quantitative methods (Maximum-likelihood estimation (MLE) and Bayesian estimation) for estimating afterload inertance based on observed pressure and flow waveforms. These methods are also applied to a real mock loop system. Results show that the system has an inertance comparable with the literature reference value of the entire systemic circulation, and that the expected variations over inlet average flow and pulse frequency are in general confirmed. Comparing the methods, the Bayesian approach results in higher and more stable estimations than the classical MLE.

Development and characterization of acellular porcine pulmonary valve scaffolds for tissue engineering

Currently available replacement heart valves all have limitations. This study aimed to produce and characterize an acellular, biocompatible porcine pulmonary root conduit for reconstruction of the right ventricular outflow tract e.g., during Ross procedure. A process for the decellularization of porcine pulmonary roots was developed incorporating trypsin treatment of the adventitial surface of the scraped pulmonary artery and sequential treatment with hypotonic Tris buffer (HTB; 10 mM Tris pH 8.0, 0.1% (w/v) EDTA, and 10 KIU aprotinin), 0.1% (w/v) sodium dodecyl sulfate in HTB, two cycles of DNase and RNase, and sterilization with 0.1% (v/v) peracetic acid. Histology confirmed an absence of cells and retention of the gross histoarchitecture. Immunohistochemistry further confirmed cell removal and partial retention of the extracellular matrix, but a loss of collagen type IV. DNA levels were reduced by more than 96% throughout all regions of the acellular tissue and no functional genes were detected using polymerase chain reaction. Total collagen levels were retained but there was a significant loss of glycosaminoglycans following decellularization. The biomechanical, hydrodynamic, and leaflet kinematics properties were minimally affected by the process. Both immunohistochemical labeling and antibody absorption assay confirmed a lack of α-gal epitopes in the acellular porcine pulmonary roots and in vitro biocompatibility studies indicated that acellular leaflets and pulmonary arteries were not cytotoxic. Overall the acellular porcine pulmonary roots have excellent potential for development of a tissue substitute for right ventricular outflow tract reconstruction e.g., during the Ross procedure.

A RELIABLE METHOD FOR PROTOTYPING FLEXIBLE PHYSIOLOGIC-LIKE BEHAVING LEFT VENTRICLES FOR STUDYING MITRAL VALVE SURGICAL CORRECTIONS

A reliable and inexpensive method was developed for the realization of flexible ventricle prototypes to be used in a pulse duplicator test circuit for cardiac valve testing, with particular emphasis on the valves in the mitral position. The peculiarity of our ventricular models is their ability to preserve a physiologic-like shape during the entire cardiac cycle. Moreover, the models take into account the function of papillary muscles, thus allowing a realistic evaluation of the whole inlet valve complex (mitral valve, chordae tendineae and papillary muscles). Ventricular sacs were designed by structural analyses, driven by a set of criteria identifying the similarity to natural ventricular conformational changes. The computationally optimized ventricle shapes were used to manufacture the mould for prototypes to be subjected to experimental validation trials. The difference between simulation-predicted and experimentally measured behaviors did not exceed 6.5%, which confirmed reliability of the developed method.

A pulsatile simulator for the in vitro analysis of the mitral valve with tri-axial papillary muscle displacement

PURPOSE

We developed a new pulsatile hydrodynamic simulator for the in vitro testing of mitral valve (MV) samples. The required specifications included a 3D positioning system for the papillary muscles (PMs) that is accurate and simple to manage; measurement of the force exerted by the chordae tendineae on the PMs; and the possibility to visually inspect the MV for kinematic analysis.

METHODS

An atrial/ventricular chamber system was developed. The ventricular chamber housed a tri-axial actuator system that was aligned to a morphometric Cartesian frame, allowing for PM positioning even while tests are running. Each PM holder had an embedded load cell for force measurement. The atrial chamber was designed so as to permit MV visual inspection, maintaining a non-disturbed flow at the sample inlet. The setup was subjected to trials with fresh porcine MVs. Flow and pressure difference across the MVs and PM forces were measured in different MV configurations, with different PM spatial dislocations. High speed video recordings were acquired.

RESULTS

The positioning accuracy was assessed. Tests with MVs showed good usability, even by the non-engineering personnel. The effects of PM displacement on valve function (valve competence and PM forces) was consistent with previously published data, thus confirming the general soundness of the design principles.

CONCLUSIONS

The developed simulator is a promising instrument for performing MV in vitro tests in a precise, well-repeatable manner. The ability to completely adjust the PM position while a test is running boosts the simulator's potential for detailed investigations of the pathological and surgically treated MV.

Opening and closing kinematics of fresh and calcified aortic valve prostheses: An in vitro study

OBJECTIVES

In vitro testing of biologic valves has been performed using only fresh but treated valves suitable for patient implantation. The present study investigates changes in hemodynamic performance and leaflet kinematics in progressively calcified porcine and pericardial aortic valve prostheses.

METHODS

Edwards Perimount Magna (Edwards Lifesciences, Irvine, Calif) (n = 5) and Medtronic Mosaic Ultra (Medtronic Inc, Minneapolis, Minn) (n = 5) heart valves (23 mm) were investigated in an artificial circulation system (70 beats/min, cardiac output 5 L/min). Leaflet kinematics were visualized with a high-speed camera (3000 frames/sec). Valves were then exposed to a calcium-phosphate solution at a constant pulse rate of 300 beats/min for a total of 6 weeks. Repeated testing was performed after 1, 2, 3, 4, and 6 weeks of calcification. The calcification process might not be similar to in vivo performance.

RESULTS

Initially, the Perimount Magna valves demonstrated lower pressure gradients compared with the Mosaic Ultra valves (9.7 +/- 0.36 mm Hg vs 14.0 +/- 1.16 mm Hg), but they showed higher closing volume and leakage flow. Total energy loss was equivalent after 1 week of calcification. Perimount Magna valves calcified significantly faster and more severely, leading to an increase in gradients and closure volume. Leaflet kinematics showed progressively longer opening and closing times for the pericardial valves (closing time Perimount Magna 135 +/- 11 msec vs Mosaic Ultra 85 +/- 9 msec after 6 weeks).

CONCLUSIONS

On the basis of visual inspection, despite the new ThermaFix (Edwards Lifesciences) tissue treatment, the Perimount Magna pericardial valves calcified in vitro faster and more severely than did the Mosaic Ultra porcine valves, which demonstrated a more constant performance throughout the calcification process. Leaflet kinematics showed a progressive prolongation of opening and closing times for pericardial valves, leading to higher closing volume.

Hydrodynamic evaluation of kangaroo aortic valve matrices for tissue valve engineering

We evaluated the hydrodynamic performance of kangaroo aortic valve matrices (KMs) (19, 21, and 23 mm), as potential scaffolds in tissue valve engineering using a pulsatile left heart model at low and high cardiac outputs (COs) and heart rates (HRs) of 60 and 90 beats/min. Data were measured in two samples of each type, pooled in two CO levels (2.1 +/- 0.7 and 4.2 +/- 0.6 L/min; mean +/- standard errors on the mean), and analyzed using analysis of variance with CO level, HR, and valve type as fixed factors and compared to similar porcine matrices (PMs). Transvalvular pressure gradient (DeltaP) was a function of HR (P < 0.001) and CO (P < 0.001) but not of valve type (P = 0.39). DeltaP was consistently lower in KMs but not significantly different from PMs. The effective orifice area and performance index of kangaroo matrices was statistically larger for all sizes at both COs and HRs.

Hydrodynamic function of the second-generation mitroflow pericardial bioprosthesis

BACKGROUND

The hydrodynamic function of the smaller size Mitroflow Synergy stented pericardial bioprostheses has been studied in an in vitro fresh tissue aortic root model and compared with previous studies of free-sewn bioprostheses.

METHODS

Three valves of each of the sizes 19, 21, and 23 mm were sutured into fresh tissue aortic roots and tested in a pulsatile flow simulator using two different ventricular input impedance conditions. A high-speed camera was used to study the leaflet opening and closing configurations. Mean pressure difference as a function of root mean square forward flow, effective orifice area, regurgitant volumes, and total energy loss across the valves was measured.

RESULTS

Mean pressure difference with respect to root mean square forward flow decreased as the valve size increased. Thus effective orifice area increased as the valve size increased. The open leaflet configuration images showed that all three sizes of Mitroflow valves had a large circular orifice with minimal open leaflet deformation. All valves closed competently with no visible leakage and no closed regurgitant volume. The Mitroflow valves showed better effective orifice areas compared with previously tested frame-mounted porcine bioprostheses but lower effective orifice areas compared with porcine stentless bioprostheses; however, the open leaflet bending deformation was better than for any of the previously tested bioprosthetic valves.

CONCLUSIONS

The hydrodynamic function of the Mitroflow Synergy stented pericardial bioprosthesis shows potential for good in vivo hemodynamic performance. The good hemodynamic performance combined with relative ease of implantation technique makes the pericardial valve a good valve in the aortic position, particularly in older patients with small annuli.

Hydrodynamic evaluation of three artificial aortic valve chambers

The effect of chamber geometry on the characteristics of turbulent steady flow through a newly designed artificial heart valve, "the jellyfish valve," has been investigated for flow rates matching those of peak systole. Laser Doppler Anemometry (LDA) was employed to determine the velocity and shear stress distributions at various locations downstream of the jellyfish valve. Three geometrically different aortic valve chambers have been investigated: namely, a chamber with sinuses of Valsalva, an ellipsoidal chamber, and a cylindrical chamber. The results of this investigation indicated that the aorta with sinuses of Valsalva model had the highest turbulent shear stresses whereas the ellipsoidal model gave the highest-pressure drops. However, for the various flow rates examined, including the systole peak value of 26 L/min, it appears that the ellipsoidal model displays better hydrodynamic characteristics in terms of shear stress and uniformity of axial velocity distributions downstream of the jellyfish valve.

Dobutamine Stress Test to Evaluate Different Sizes of Prosthetic Aortic Valves

Dobutamine stress testing and Doppler echocardiography were used to assess hemodynamics in 27 patients aged 16 to 54 years with various sizes and types of aortic valve prosthesis. All patients underwent a symptom-limited treadmill exercise test within two days of the dobutamine test. There was no significant difference in ejection fractions and transvalvular gradients at rest and during dobutamine stress between St. Jude Medical, Medtronic-Hall, and Carbomedics valves. Exercise duration did not differ significantly among the different types of valve. When patients were classified by their underlying lesion, those with aortic stenosis and those with aortic insufficiency had similar ejection fractions and transvalvular gradients at rest and during dobutamine stress. The mean and peak transvalvular gradients at rest and during dobutamine stress were significantly different in patients with different valve sizes but the extent of the increase in gradients during stress was not significant. Linear regression analysis revealed that both peak and mean gradients during dobutamine stress could be predicted by the resting gradients. There was a negative correlation between valve size and gradients at rest and during stress, while there was a significant correlation between exercise duration and valve size. Dobutamine stress echocardiography was useful for studying hemodynamics in patients with aortic valve prostheses and the findings show that valvular size was the main determinant of exercise capacity.

Evaluation of prosthetic valve function and associated complications

Significant advances in imaging modalities have occurred to evaluate prosthetic valve function and associated complications. These developments involve predominantly the introduction of Doppler technology for the non-invasive determination of gradients and valve areas and TEE for an improved assessment of valve structure, function, and associated complications. The current role of cinefluoroscopy is mostly to complement TEE in the evaluation of motion of mechanical prosthetic valves in the aortic position. Cardiac catheterization is now rarely needed to assess valve function. Diagnosis of prosthetic valve obstruction can be performed in the majority of cases with transthoracic Doppler echocardiography. Differentiation of valve obstruction from normal valve function in small valves with high flow conditions, however, may be difficult. Because of this and the variability in normal valves among different prostheses, knowledge of the type and size of the implanted valve is essential. Patients and ultrasound laboratories are encouraged to seek and provide this information on a routine basis. Although transthoracic echocardiography is the main diagnostic modality for the serial evaluation of prosthetic valve function, it is important to recognize its limitations in assessing prosthetic mitral regurgitation and evaluating structural abnormalities of prosthetic valves. These are the situations in which TEE has the most impact. A summary of general indications of TEE in prosthetic valves is provided in Table 6. Finally, a baseline transthoracic Doppler study is essential in the overall follow-up and serial evaluation of valve function. For future comparisons, the best indices of valve functions are those obtained for patients as their own control, from a baseline Doppler echocardiographic study performed early after the operation.

Assessment of the Björk-Shiley prosthetic valve orifice area in the aortic position

The actual orifice area of a valve is still considered to be a valuable index for assessing prosthetic valve function. Valve orifice area as calculated by Gorlin's formula is, however, not constant but changes in proportion to the transvalvular flow rate. The purpose of the present study was to examine the relationship between orifice area and flow rate across the Björk-Shiley prosthetic valve as calculated by Gorlin's formula, and to modify the formula in a series of patients with the Björk-Shiley prosthetic valve in the aortic position. Fifty-six patients who had received aortic valve replacement with a Björk-Shiley prosthetic valve underwent cardiac catheterization. Prosthetic valve orifice area was calculated by Gorlin's formula and then plotted against flow rate across the valve with respect to valve size. The relationship between orifice area and flow was linear. The discharge coefficient of Gorlin's formula was plotted against flow rate, and a linear correlation was obtained. By substituting Gorlin's formula for an empiric coefficient into the function for transvalvular flow rate, a modified formula that can predict the actual orifice area of the prosthetic valve is obtained.

Application of the continuity equation and valve resistance to the evaluation of St. Jude Medical prosthetic aortic valve dysfunction

Doppler echocardiography was applied to the assessment of patients with surgically documented St. Jude medical aortic valve dysfunction. Derivation of effective orifice area and Doppler velocity index with the continuity equation and calculation of valve resistance accurately differentiated stenotic from regurgitant and normal valves.

Effect of simulated valvular stenoticity on predicted flow area for a bileaflet valve using the Gorlin equation

The possibility that the discharge coefficient (Cd) for a mechanical heart valve (MHV) is affected by valvular stenosis is addressed. A 29 mm bileaflet (Si. Jude Medical) mitral valve is tested on a cardiovascular duplicator (CVD) in its normal state and under two degrees of simulated stenosis. Stenoticity is simulated by bracing the occluders such that full opening is impossible. The pressure drop through the valve is described by a two-term second-order polynomial in flow, from which it is shown that the Cd should be a nonlinear function of the flow rate through the valve. The average difference between measured and calculated areas decreased from 74%, when a constant value of 0.7 was used for the Cd, to 3.7%, when the Cd was a nonlinear function of the flow rate.

In vitro testing of artificial heart valves: comparison between Newtonian and non-Newtonian fluids

The in vitro testing of artificial heart valves is often performed with simple fluids like glycerol solutions. Blood, however, is a non-Newtonian fluid with a complex viscoelastic behavior, and different flow fields in comparable geometries may result. Therefore, we used different polymer solutions (Polyacrylamid, Xanthan gum) with blood-like rheological properties as well as various Newtonian fluids (water, glycerol solutions) in our heart valve test device. Hydrodynamic parameters of Björk-Shiley heart valves with a tissue annulus diameter (TAD) of 21-29 mm were investigated under aortic flow conditions. Major results can be summarized as follows. The mean systolic pressure differences depend on the model fluids tested. Closing time and closing volume are not influenced by the rheological behavior of fluids. These parameters depend on TAD and the pressure differences across the valve. In contrast, rheological behavior has a pronounced influence upon leakage flow and leakage volume, respectively. Results show furthermore that the apparent viscosity data as a function of shear rate are not sufficient to characterize the rheological fluid behavior relevant to hydrodynamic parameters of the heart valves investigated. Therefore, similarity in the yield curves of non-Newtonian test fluids mimicing blood is only a pre-requisite for a suitable test fluid. More information about the viscous and elastic component of the fluid viscosity is required, especially in geometries where a complex flow field exists as in the case of leakage flow.

Doppler echocardiographic assessment with the continuity equation of St. Jude Medical mechanical prostheses in the mitral valve position

Evaluation of the St. Jude Medical (SJM) valve in the mitral position with Doppler echocardiography has usually involved the use of gradients across the valve and the application of the pressure half-time (PHT) method to derive a mitral valve area. The purpose of this study was, first, to determine the normal values of effective orifice areas for the SJM valve in the mitral position using the continuity equation, and second, to evaluate whether this parameter provides an improved assessment of valve function. Accordingly, Doppler echocardiography was performed in 40 patients within 6 weeks after valve replacement. All patients were clinically stable, without evidence of valvular dysfunction or aortic insufficiency. Valve size ranged from 23 to 33 mm and ventricular ejection fraction averaged 54 +/- 13%. Effective orifice area was derived by the continuity equation using stroke volume measured in the ventricular outflow tract, divided by the time-velocity integral of the SJM valve jet, and by PHT. Doppler-derived SJM valve mean gradient averaged 4 +/- 2 mm Hg. Effective area by the continuity equation averaged 1.82 +/- 0.36 cm2 (range 1.03 cm2 for a 23 mm valve to 2.63 cm2 for a 31 mm valve) and was smaller than by PHT (mean 3.10 +/- 0.65 cm2, p = 0.0001; range 1.38 to 4.78 cm2). Areas by both methods were smaller than the actual valve orifice area provided by the manufacturer (4.53 +/- 0.80 cm2, p = 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Improved Doppler assessment of the Bjork-Shiley mitral prosthesis using the continuity equation

To assess whether derivation of an effective mitral prosthetic valve area using the continuity equation provides an improved functional assessment of the Bjork-Shiley mitral prosthesis over the pressure half-time method, Doppler echocardiographic studies were performed in 43 patients 12 +/- 7 months following the valve replacement. Effective valve orifice area used as the standard for comparison was determined by a hydraulic formula validated in vitro over a wide range of flow rates. All patients were clinically stable, without evidence of prosthetic dysfunction or aortic regurgitation. Prosthetic mitral valve orifice area determined by the hydraulic formula, by the continuity equation and by pressure half-time method for all prostheses sizes averaged 1.6 +/- 0.46 cm2, 1.83 +/- 0.56 cm2 and 2.34 +/- 0.48 cm2, respectively. Effective valve orifice area by the hydraulic formula had a strong correlation with that derived by the continuity equation (r = 0.86; P < 0.0001; standard error of estimate (S.E.E.), 0.12 cm2), but an insignificant correlation with the area calculated by the pressure half-time method (r = 0.24). Prosthetic mitral valve areas determined by the continuity equation and by pressure half-time method also correlated poorly (r = 0.24). Pressure half-time was affected by heart rate, diastolic filling period, left ventricular fractional shortening and presence of atrial fibrillation (P < 0.001). Thus, using the standard continuity equation to determine the orifice area of the Bjork-Shiley prosthesis in the mitral position provides improved assessment compared with the pressure half-time method.

Mechanical and other problems of artificial valves

This chapter has discussed the design, development, laboratory testing and clinical performance of artificial heart valve replacements. The published material on this subject is extensive and clearly this present chapter represents only a limited selection of the many topics and researchers associated with the production of clinically implantable valves. Any omissions should not be regarded as a criticism, but simply the result of economy of space. The field is still developing and the situation regarding the advantages and disadvantages of the various types of valves as illustrated in Table 3 still obtains notwithstanding the many improvements which have been made with current designs. Valve development is, and will continue to be, closely allied to advances in our understanding of the behaviour of the materials of construction. Ultimately, improved haemodynamic performances of the various valve configurations will result from designs based on data from computerised fluid dynamic analysis combined with finite element stress analysis according to recognised engineering design principles.

Comparative study of the function of the Abiomed polyurethane heart valve for use in left ventricular assist devices

Hydrodynamic testing of the Abiomed polyurethane trileaflet valve has been carried out to establish performance data of valve function. A Medtronic Hall tilting disk, a Carbomedics bileaflet, a Hancock II bioprosthesis and an Abiomed polyurethane trileaflet valve, all size 27 mm, underwent both pulsatile and steady-flow hydrodynamic testing. Results of the variation of pressure difference with RMS pulsatile flow and steady flow, and effective orifice area, showed that the Abiomed valve had significantly poorer opening characteristics than the tissue valve and the two mechanical valves. The Abiomed valve's performance was seen to be related to its construction and manufacture. This study highlights some of the problems associated with the design and development of synthetic trileaflet heart valve prostheses.

Computer-controlled in vitro model of the human left heart

The decision for surgical intervention in the treatment of stenosis and for regurgitation of the mitral valve demands an objective and quantitative evaluation of the severity of mitral valve disease. The availability of ultrasound techniques capable of analysing flow velocities across valves and to produce representative images of valve orifices has increased the interest in the hydraulics of cardiac valves. To isolate and study the determinants of transmitral flow, an in vitro model of the human left heart was built. From the model it is possible to differentiate the influence of the different determinants of left heart performance on transmitral flow: preload, compliance of the left atrium and ventricle, peripheral resistance (afterload) and heart rate. The mechanical part of the model consists of a reservoir connected to an elastic closed circuit (Latex pulmonary veins, left atrium, left ventricle and aortic arch) with replaceable mitral and aortic valves. The electronic part of the model drives and controls the hydraulic part, allowing the independent regulation and monitoring of left atrial and left ventricular pressures p, volumes V and 'pV-loops' throughout the cardiac cycle at different cardiac rhythms. Left atrial filling pressure and aortic resistance are variable in a controlled fashion. Echo-Doppler study of the mitral valve and the transmitral valve flow is possible both from an atrial and a ventricular window in the model. This technical note describes the model.

Exercise evaluation of prosthetic heart valves by Doppler echocardiography: comparison with catheterization studies

Although valve replacement remains the cornerstone of treatment for critical heart valve dysfunction, problems including thromboembolism, infection, and primary failure of the prosthesis remain. Resting studies of valve hemodynamics are sometimes insufficient to reveal valve dysfunction. Early studies using cardiac catheterization focused on changes in prosthetic function seen with various types of exercise or drug-induced stress. These studies suffered from an inability to adequately stress catheterized patients and were cumbersome to the patient and the investigator. With the introduction of Doppler echocardiography, however, studies could be performed after significant exercise with low risk and increased ease. Using echocardiography, reports of exercise-induced changes in aortic and mitral valve hemodynamics have appeared. Over 600 patients have been studied using Doppler echocardiography. In the aortic position, all prostheses studied have a mild peak instantaneous gradient (18-26 mmHg) at rest, which increases with exercise (35-63 mmHg). No significant differences between the four models of mechanical prostheses studied are found. The gradients achieved with exercise do not appear to be related to the heart rate achieved or duration of exercise. Smaller prostheses are associated with larger gradients; however, the correlation was not strong. All mitral valve prostheses studied are also mildly stenotic at rest (range of mean gradients 2.3-7.1 mmHg) and become moderately stenotic with exercise (range 5.1-16.5). Although the lowest gradients are seen with St. Jude Medical and Medtronic Hall prostheses, their gradients are not significantly less than with other valves.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative study of the hydrodynamic function of the CarboMedics valve

The hydrodynamic function of each size of the CarboMedics valve was assessed in a pulsatile flow simulator. The mean pressure difference with respect to forward flow, regurgitant volumes, and total energy loss across each valve were analyzed. The results for the 23-mm aortic and 29-mm mitral CarboMedics valves were compared with those for the St. Jude Medical and Björk-Shiley Monostrut valves. Results showed good hydrodynamic function for each CarboMedics valve, although the pressure difference and total energy loss across the 19-mm aortic valve was high. The hydrodynamic function of the CarboMedics valve was comparable with that of the St. Jude Medical valve. Both valves showed similar leakage volumes, which were significantly larger than that for the Björk-Shiley Monostrut valve. On account of this the total energy loss in the Björk-Shiley valve was significantly less than that for the bileaflet valves in the aortic position. Concern remains for the continuing presence of high closed-valve regurgitation in the bileaflet valves.

Theoretical and practical differences between the Gorlin formula and the continuity equation for calculating aortic and mitral valve areas

Although the Gorlin formula and the continuity equation are both used to calculate valvular areas in the clinical situation, there have been few comparisons of the 2 methods. Mathematically, it can be shown that both formulas are derived from similar hydrodynamic principles which basically give a measure of the physiologic or effective area occupied by flow. However, the Gorlin formula contains errors in formulation and incorporates a constant that purports to give a measure of the anatomic rather than of the effective area of the valve. If both formulas are applied to the same hemodynamic data from aortic and mitral bioprostheses studied in a pulse duplicator system, the Gorlin formula constantly yields results 1 to 2% higher than the continuity equation for aortic valves and 12 to 13% higher for mitral valves. For any given type and size of prosthesis, the areas calculated by either formula increase linearly in relation to increasing pressure and flow (up to 20% for aortic valves and up to 35% for mitral valves). It is concluded that the Gorlin formula and the continuity equation are both pressure- and flow-dependent and are primarily related to the effective area occupied by flow rather than to the anatomic area of the valve. The 2 methods yield consistently different results due to differences in mathematical formulation. Such factors are important to consider when interpreting valve area calculations clinically.

Doppler echocardiographic assessment of the St. Jude Medical prosthetic valve in the aortic position using the continuity equation

To test whether the continuity equation can be applied to the noninvasive assessment of prosthetic aortic valve function, Doppler echocardiography was performed in 67 patients (mean age, 58 +/- 14 years) within 10 +/- 6 days after valve replacement with St. Jude Medical valves. All patients were clinically stable and without evidence of valve dysfunction. Valve size ranged from 19 to 31 mm, and ejection fraction ranged from 30% to 75%. With the parasternal long-axis view, the left ventricular outflow diameter measured just proximal to the prosthetic valve correlated well with valve size (r = 0.92). Doppler-derived maximal gradients ranged from 9 to 71 mm Hg. Effective prosthetic aortic valve area by the continuity equation ranged between 0.73 cm2 for a 19-mm valve and 4.23 cm2 for a 31-mm valve. With analysis of variance, effective orifice area differentiated various valve sizes (p less than 10(-14)) better than did gradients alone (p = 0.003) and correlated better with actual valve orifice area (r = 0.83 versus - 0.40). A Doppler velocity index, the ratio of peak velocity in the left ventricular outflow to that of the aortic jet, averaged 0.41 +/- 0.09 and was less dependent on valve size (r = 0.43). Thus, the continuity equation can be applied to the assessment of prosthetic St. Jude valves in the aortic position. By accounting for flow through the valve, it provides an improved assessment over the sole use of gradients in the evaluation of prosthetic valve function.

Flow characteristics of bioprosthetic heart valves

A review of the in vivo and in vitro fluid dynamic performance of three bioprosthetic heart valves is presented. Data on Hancock porcine valves (standard models 242 aortic and 342 mitral and modified orifice model 250 aortic), Carpentier-Edwards porcine valves (model 2625 aortic and 6625 mitral), and the Ionescu-Shiley pericardial valve are reviewed. These valves were chosen because of their past or present popularity in clinical use and because of the variation in fluid dynamic performance reported by different investigators. The flow parameters that are reported include in vivo and in vitro mean pressure drop, cardiac output or cardiac index, regurgitant volume, effective orifice area, and performance index. These data provide a framework for differentiation of normal and abnormal bioprosthetic valve function.

Influence of orifice geometry and flow rate on effective valve area: an in vitro study

Fluid dynamics suggests that orifice geometry is a determinant of discharge properties and, therefore, should influence empiric constants in formulas (such as the Gorlin formula) to calculate stenotic valve area. An in vitro study utilizing a model of transmitral flow was conducted to investigate how the discharge coefficient changes with 1) orifice eccentricity (ratio of long to short diameter), 2) absolute area, 3) the presence of a nozzle-like inlet, and 4) varying flow. Twenty-three orifices with areas varying between 0.3 and 2.5 cm2 and eccentricities from 1:1, or circular, to 5:1, or elliptic, were tested. The calculated discharge coefficients ranged between 0.675 and 0.93. For a given area, the discharge coefficient decreased by a mean value (+/- SD) of 5.5 +/- 1.3% between circular orifices and 5:1 ellipses. Discharge coefficients increased by a mean of 8.9 +/- 3.5% from 0.3 to 2.5 cm2 area within each eccentricity class. A gradually tapering inlet (nozzle) raised the discharge coefficient by 8.8 +/- 3.9%, leading to a discharge coefficient between 0.81 and 0.93 for round orifices. The discharge coefficient did not change appreciably with flow. The concept of the discharge coefficient and its role in assessing restrictive orifices in general by hydraulic formulas (for example, the Gorlin and pressure half-time calculations) are discussed.

Limitations of Doppler ultrasound in the assessment of the function of prosthetic mitral valves

Pressure half time has been assumed to be a relatively flow-independent measure of orifice area, but it may also be influenced by atrial and ventricular factors. Pressure half time and peak left ventricular inflow velocity were measured by continuous wave Doppler ultrasound in 164 patients with normally functioning Carpentier-Edwards, Björk-Shiley, and Starr-Edwards mitral prostheses. Pressure half time was shorter in the Björk-Shiley than in the other value types and peak transmitral velocity was highest in the Starr-Edwards prostheses. These differences, however, were partly explained by coexistent differences in transmitral flow. Filling time accounted for 19% and stroke volume for 15% of the variance in pressure half time compared with only 5.6% for prosthetic design and 0.4% for annulus diameter when each of these variables was considered alone. The design of the prosthesis explained 18% of the variance in peak transmitral velocity, while cardiac output and annulus diameter did not contribute significantly. With Doppler ultrasound it is impossible to define reliable normal ranges for prosthetic function independently of atrial and ventricular function. Formulas for orifice area based on peak transmitral velocity and flow seem more likely to reflect the behaviour of normally functioning prostheses than those based on pressure half time.

Prediction of stenotic valve orifice area: an in vitro study on a bioprosthesis

The Gorlin equation for the hemodynamic assessment of valve area is commonly used in cardiac catheterization laboratories. A study was performed to test the prediction capabilities of the Gorlin formula, as well as those of the Aaslid and Gabbay formulas for the effective orifice area of a porcine valve of varying degrees of stenosis. Pressure gradient, flow, and valve opening area measurements were performed on Carpentier-Edwards porcine valve prostheses (made stenotic by suturing at the commissures) mounted in the aortic position of an in vitro pulse duplicator. With the known valve orifice area, a discharge coefficient was computed for each of the three orifice area formulas. After some theoretical considerations, it was proposed that the discharge coefficient would be a function of the flow rate through the valve. The discharge coefficient was observed to increase with increasing systolic flow rate. An empirical relationship of the discharge coefficient as a linear function of the systolic flow rate was determined through a regression analysis, with a different relationship for each orifice area formula. Using this relationship in the orifice area formulas improved the accuracy of the prediction of the effective orifice area with all three formulas performing equally well.

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.

Effect of systolic flow rate on the prediction of effective prosthetic valve orifice area

The Gorlin equation for the hemodynamic assessment of valve area is commonly used in cardiac catheterization laboratories. A study was performed to test the prediction capabilities of the Gorlin formula as well as the Aaslid and Gabbay formula for the effective orifice area of prosthetic heart valves. Pressure gradient, flow, and valve opening area measurements were performed on four 27 mm valve prostheses (two mechanical bileaflet designs, St. Jude and Edwards-Duromedics, an Edwards pericardial tissue valve, and a trileaflet polyurethane valve) each mounted in the aortic position of an in vitro pulse duplicator. With the known valve orifice area, a different discharge coefficient was computed for each of the four valves and three orifice area formulas. After some theoretical considerations, it was proposed that the discharge coefficient would be a function of the flow rate through the valve. All discharge coefficients were observed to increase with increasing systolic flow rate. An empirical relationship of discharge coefficient as a linear function of systolic flow rate was determined through a regression analysis, with a different relationship for each valve and each orifice area formula. Using this relationship in the orifice area formulas improved the accuracy of the prediction of the effective orifice area with all three formulas performing equally well.

Hydrodynamic function of ten prosthetic heart valves in the aortic position

The hydrodynamic function of five mechanical, four pericardial and one porcine size 23 mm aortic prosthetic valves has been analysed by measuring pressure differences, regurgitation and energy losses in our pulsatile flow test rig. St Jude Medical, Hancock pericardial and Mitral Medical pericardial valves had the lowest pressure drop. Calculations of total energy loss showed that the Bjork-Shiley monostrut had similar overall function to the above valves, while the Ionescu-Shiley Standard, Ionescu-Shiley Low Profile, Omniscience and Duromedics valves had slightly greater total energy losses. The Bjork-Shiley spherical disc and Wessex Medical porcine valve had the greatest energy losses. The function of the newer designs of pericardial valves compared well with the mechanical valves currently available.

Five-year follow-up of Hancock pericardial valves: management of premature failure

The durability and function of bovine pericardial valves are dependent upon design, preservation, patient factors (age, sex), and site of valve implantation. In 1983, a shelf recall of all Hancock bovine pericardial valves (HPV) was instituted by the manufacturer. This report represents the results of an organized 5-year follow-up in a hospital Prosthetic Valve Registry of 129 HPV implanted in 122 patients (79 males, 43 females) between May 1982 and April 1985 using echo Doppler and careful clinical evaluation. Mean age was 56 +/- 15 years. There were 81 AVR, 33 MVR, 7 DVR, and 1 TVR. Concomitant coronary bypass was performed in 38 patients (31%). Surgery was on a redo basis in 25 patients (20%), urgent in 14 (11%), and for SBE in 8 patients (7%). There were seven hospital deaths (5.7%). Mean follow-up was 44 months (maximum 66 months) for 114 patients (99% complete), representing 417 patient years. There have been 20 late deaths (18%), of which 7 were directly valve related. Linearized frequency of major events (per pt-yr) was: thromboembolism, 1.6%; anticoagulant related hemorrhage, 0.8% (1 late death); prosthetic valve endocarditis 1.3%; primary tissue failure, 5.8%. Patient symptomatology was a more accurate predictor of bioprosthetic failure requiring reoperation than echo Doppler studies, which were completed in 74 of 97 patients examined during scheduled follow-up visits. Twenty-four of the 96 patients (25%) have required re-replacement at a mean interval of 44 months (27-59 months) from initial implantation. This was due to vertical shear starting at the top of the strut anchoring commissural attachments in every case.(ABSTRACT TRUNCATED AT 250 WORDS)

The Hancock pericardial xenograft: incidence of early mechanical failures at a medium-term follow-up

The Hancock pericardial xenograft has been used in our Institution since August 1981 as an alternative to porcine bioprostheses. Up to July 1984, 97 Hancock pericardial xenografts have been implanted in 84 patients; of 76 operative survivors with a mean age of 55.2 +/- 13 years (range 13-75 years), 50 had undergone aortic valve replacement, 16 mitral valve replacement and 10 mitral-aortic valve replacement. Follow-up ranged from 0.5 to 5.2 years with a cumulative duration of 239 patient/years and is 99% complete. Actuarial survival is 92% +/- 4% for patients with aortic valve replacement and 84% +/- 10% for patients with mitral valve replacement at 5 years, and 77% +/- 14% for those with mitral-aortic valve replacement at 4 years. Thromboembolic episodes occurred in 2 patients (1 after aortic and 1 after mitral valve replacement). The actuarial freedom from emboli is 100% for patients with mitral-aortic valve replacement at 4 years, and 96% +/- 3% for patients with aortic and 93% +/- 6% for patients with mitral valve replacement at 5 years. Reoperation was performed in 13 patients (9 aortic, 2 mitral and 2 mitral-aortic valve replacements), because of endocarditis in 3 (2 aortic and 1 mitral valve replacement), paravalvular leak in 1 (aortic valve replacement), and primary tissue failure in 9 (6 aortic, 1 mitral and 2 mitral-aortic valve replacements). Actuarial freedom from primary tissue failure is 72% +/- 9% for aortic and 83% +/- 8% for mitral Hancock pericardial xenografts at 5 years. Eleven xenografts explanted because of primary tissue failure were studied pathologically.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of Björk-Shiley prosthetic valves by real-time two-dimensional Doppler echocardiographic flow mapping

We studied the value of two-dimensional Doppler echocardiographic color flow mapping for identifying normal transvalve flow profiles and valve malfunction in 20 patients with Björk-Shiley prosthetic valves. Seven patients had Björk-Shiley prosthetic valves in the aortic position alone, seven in the mitral position, and six had prosthetic valves in both the aortic and mitral positions. In 10 patients with normally functioning mitral valves, the ratios of the maximal major and minor Doppler-imaged orifice flow diameters to the valve ring diameters were 25 +/- 3% (mean +/- SD) and 24 +/- 3%, respectively, similar to values reported in in vitro studies. No mitral regurgitation was found in these patients by two-dimensional Doppler echocardiographic flow mapping or by spectral Doppler. Of the 10 clinically normal aortic Björk-Shiley valves, no valvular regurgitation was found by color flow mapping, whereas mild aortic regurgitation was found in two patients with the use of spectral Doppler. Malfunction of six valves was documented in five patients and was confirmed by cardiac catheterization and/or surgery. These included one case of focal fibrous ingrowth involving primarily the minor orifice of a mitral prosthetic valve, one case of mitral valve prosthetic thrombosis with decreased major and minor orifice flow diameters and valvular regurgitation, and four cases of paravalvular regurgitation involving prosthetic valves in the aortic position (three patients) and the mitral position (one patient). Two-dimensional Doppler echocardiographic flow mapping provides new observations that may aid in identifying Björk-Shiley prosthetic valve malfunction. By localizing precisely the site of prosthetic stenosis or regurgitation, it may also assist in defining the cause of valve malfunction.

Evaluation of normal prosthetic valve function by Doppler echocardiography

Previous investigations have suggested that Doppler echocardiography is useful in detecting dysfunction in aortic (AVR) and mitral prostheses (MVR). However, to diagnose abnormalities, the spectrum of normal velocities through these valves must be established. Therefore, we used Doppler echocardiography to study 100 patients with 105 prosthetic valves that had no clinical evidence of valve dysfunction 9 +/- 8 days postoperatively. There were 66 Carpentier-Edwards (C-E), 23 St. Jude (S-J), and 16 Ionescu-Shiley (I-S) valves. In 70 AVR, the peak instantaneous gradient was 26.4 +/- 8.2 Hg, mean systolic gradient was 15.6 +/- 5 mm Hg, and gradients varied inversely with valve size, although differences were significant only when comparing the smallest vs the largest valve sizes (p less than or equal to 0.03). Peak instantaneous gradients greater than 36 mm Hg occurred only in AVR size 23 or smaller. There were no significant differences in gradients among C-E, S-J, and I-S AVR. In 35 MVR, mean gradient was 6.9 +/- 2.3 mm Hg and valve area was 2.7 +/- 0.8 cm2; neither varied significantly with valve size. However, S-J MVR group had smaller mean gradients and larger effective valve area than C-E bioprosthetic MVR (p = 0.01 and p = 0.05, respectively). Regurgitation was more common in AVR (26%) than in MVR (9%), p = 0.04, although all instances were mild and clinically silent. We conclude that normal AVR and MVR of a given size and type have a predictable range of Doppler echocardiographic parameters. Doppler evidence of mild regurgitation is a frequent finding in normal AVR and MVR.(ABSTRACT TRUNCATED AT 250 WORDS)

Flow characteristics of four commonly used mechanical heart valves

The in vivo and in vitro fluid dynamic performance of 4 mechanical heart valves was reviewed: Starr-Edwards silicon-rubber ball valves (models 1200/1260 aortic and 6120 mitral valves), Björk-Shiley tilting disc valves (standard spherical model, modified and unmodified convexo-concave [60 degrees and 70 degrees C-C] models), the Medtronic-Hall (Hall-Kaster) tilting disc valve and the St. Jude Medical bileaflet valve. These valves were chosen because of their past or present popularity in clinical use and because they encompass most of the basic mechanical valve designs used during the past 2 decades. The flow measurements reported include in vivo and in vitro mean pressure drop, cardiac output or cardiac index, regurgitant volume, effective orifice area and performance index.

Development and use of a remote-controlled mitral valve

A remote-controlled mitral valve was designed and constructed to occlude the mitral or tricuspid orifice at any time in the cardiac cycle. It is used to study ventricular properties in the anesthetized dog by controlling ventricular filling, atrial properties by controlling atrial emptying and interaction of the two chambers by uncoupling them. The device can produce transient or steady-state perturbations in filling volume which make possible studies of intrinsic control of cardiac output. When filling volume is transiently reduced, stroke volume and end-systolic volume are reduced. A compensatory increase in stroke volume of the next cycles occurs due to increased ventricular preload and reduced afterload. The compensation continues until the lost stroke volume is regained, at which time the system returns to its previous steady state.