Regurgitation of prosthetic heart valves: dependence on heart rate and cardiac output.

Prosthetic heart valves exhibit closure and leakage backflow; however, no well-controlled study to evaluate the influence of factors such as cardiac output and heart rate on backflow has been reported to date. Four clinically used prosthetic aortic valves (size 27 mm)--St. Jude Medical, Björk-Shiley Spherical Disc, Björk-Shiley Convexo Concave, and Starr-Edwards model 1260--were studied in the aortic chamber of a pulse duplication system at heart rates of 50, 80, 110, and 140 beats/min, cardiac output of 2, 4, 6, and 8 liters/min, and mean aortic pressure of 100 mm Hg. Regurgitation was calculated in percentage and found to vary directly with heart rate and inversely with cardiac output. The range of values obtained were 5.5% for the Starr-Edwards model 1260 valve at 110 beats/min and 8 liters/min, to 37.5% for the Björk-Shiley Convexo Concave valve at 140 beats/min and 2 liters/min. Regurgitation was also calculated in milliliters/stroke and ranged from 3.4 ml/stroke for the Starr-Edwards model 1260 valve at 140 beats/min and 2 liters/min, to 17.3 ml/stroke for the Björk-Shiley spherical disc valve at 50 beats/min and 2 liters/min. Regurgitation associated with prosthetic heart valves may present a problem clinically, particularly under conditions of low cardiac output and tachycardia.

In vivo accuracy of two radiographic systems in the detection of Björk-Shiley convexo-concave heart valve outlet strut single leg separations.

OBJECTIVE: Modified cineradiographic systems have been used clinically to detect partially broken outlet struts in normally functioning Björk-Shiley convexo-concave heart valves. Almost all such valves were explanted, presuming that full failure would likely follow. Inasmuch as the clinical setting only rarely permits examination of normally rated valves, the accuracy of radiographic detection cannot be clinically defined. This study uses the clinical radiographic technique in sheep implanted with known-status convexo-concave valves, comparing its accuracy and that of a newly developed, geometric image magnification radiography system. METHODS: Twenty-one sheep with mitral convexo-concave valves were studied on both systems. Five were used for extensive training. When operators were expert with both systems, images of four intact valves and 12 valves with outlet strut single leg separations, along with a seventeenth single leg separation valve used for calibration, were integrated into 112 image sets organized into a balanced incomplete block design for evaluation by eight trained, blinded reviewers. RESULTS: Cineradiography sensitivity was 24% versus 31% for direct image magnification. The odds ratio for detection of single leg separation by direct image magnification versus cineradiography was 2.0 (95% confidence interval, 0.76 to 5.9; p = 0.13). Cineradiography specificity was 93% versus 90% for direct image magnification. Sensitivity and specificity varied markedly by reviewer, with sensitivity ranging from 8% to 55% and specificity from 51% to 100% for the combined technologies. CONCLUSIONS: The data support the need for more intensive training for convexo-concave valve imaging and further investigation of unconventional radiographic technologies. Clinical cineradiography of convexo-concave valves may detect as little as 25% of valves having a single leg separation, underestimating the prevalence of single leg separations and thereby implying more rapid progression to full fracture than is actually the case.

The Björk-Shiley Delrin tilting disc heart valve: historical perspective, design and need for scientific analyses after 25 years.

The Björk-Shiley Delrin (BSD) tilting disc heart valve was first used clinically in 1969. It is estimated that up to 24,000 BSD heart valves were implanted between 1969 and 1981, of which 7,000 may still be implanted in patients. The BSD valve provided a low-profile, quiet prosthesis with excellent hemodynamics. There have been only two reports of mechanical failure due to inlet strut fracture. Recent reports of wear of the Delrin disc combined with wear as observed on explanted BSD valves after 17-20 years in patients stimulated the formation of a Scientific Advisory Panel by Shiley to review the status of the BSD heart valve and make recommendations regarding patients living with BSD heart valves. The studies presented in this Supplement of The Journal of Heart Valve Disease summarize some of the scientific investigations conducted by Shiley and those recommended by this Panel. The material reviewed by the Panel included: (i) clinical publications on BSD heart valves; (ii) seventy-three explanted BSD heart valves returned to Shiley over the past 25 years; (iii) BSD valves subjected to accelerated wear tests; (iv) unimplanted BSD heart valves in Shiley's inventory; (v) in vitro studies of static leakage and pulsatile regurgitation of explanted and unimplanted "control' BSD heart valves; (vi) two Christiansen hip prostheses explanted from patients after five and 13 years; (vii) implant data for the BSD heart valve from the Karolinska Hospital, Stockholm, Sweden; and (viii) test reports for studies and analyses conducted by Shiley for the BSD heart valve. This article gives an overview of and introduction to the studies of the BSD heart valve presented in the following papers.

Evaluation of disc retention of Björk-Shiley Delrin heart valves.

BACKGROUND AND AIMS OF THE STUDY: Impact wear grooves have been observed in the occluder discs of clinically explanted Björk-Shiley Delrin (BSD) heart valves due to repetitive closing impacts against the inlet strut. We examined whether this impact wear could increase the disc-to-strut gap, which could in turn affect the retention of occluder disc. MATERIALS AND METHODS: Three BSD heart valves with deep wear grooves were tested in a left ventricular assist device set-up simulating dynamic conditions that would facilitate the dislodgement. RESULTS: The disc was found to be rotating within the flange during the testing and no embolization of the occluder disc occurred in any of the BSD valves tested. The Björk retention limit and the physical retention limit were calculated for each valve. The Björk retention limit is defined as (disc well depth)-(disc-to strut gap)-(1/2 outlet strut wire diameter) and the physical retention as (disc well depth)-(disc-to strut gap). CONCLUSION: The disc retention would not be affected by the impact wear grooves as long as the disc-to-strut gap did not exceed the physical retention limit.

A comparison of wear patterns in vivo and in vitro for Björk-Shiley Delrin heart valve discs.

MATERIALS AND METHODS: Wear patterns for 33 explanted Björk-Shiley Delrin heart valve discs with known implant durations were compared and contrasted with the wear patterns for 17 discs from accelerated wear tested (AWT) valves. Previous test reports and the current study provided data for the maximum depth of wear grooves for a total of 56 explanted BSD valve discs with implant durations up to 22.4 years. RESULTS: The discs of the explanted BSD valves showed, in general, more than one type of wear pattern. Eighteen out of 33 exhibited a full indent groove on the inflow side and 24 of the 33 discs showed well indents on the outflow side. Only two discs showed evidence of non-concentric edge wear. All maintained their original surface polish outside of these areas. The data indicated that wear patterns in the case of the AWT discs were more pronounced, and in some discs were different in type from those of discs from explanted valves. For example, no cracks, fractures or perforations of the Delrin discs were observed, or reported, for the explanted valves, while in the 17 AWT valves there were three cracked discs, four disc fractures, three disc well perforations and three perforating edge indents. CONCLUSION: It appears that the Shiley AWT conditions were more severe and imposed greater loads than those produced by patients with BSD valves implanted.

The effect of left ventricular dP/dt on the in vitro dynamics of the Björk-Shiley Convexo-Concave mitral valve.

Left ventricular (LV) dP/dt is considered an important hemodynamic factor influencing the dynamics of mechanical heart valve prostheses. LV dP/dt is dependent on patient factors including age, cardiac activity, health, and medication. The objective of this study was to determine the effect of LV dP/dt on the closing dynamics of mechanical heart valve prostheses in the mitral position. Eight instrumented 29 mm Björk-Shiley Convexo-Concave (BSCC) heart valves were tested in the pulse duplicator of the Helmholtz Institute. The valves had miniature strain gages mounted at the base of the outlet strut to measure impact loads at closure. Closing velocities were measured with a "light gate" device which was triggered by the closing leaflet. Physiologic pressure and flow waveforms were generated by a computer-controlled hydraulic drive unit. LV dP/dt was varied from 500 to 4000 mmHg/s simulating a wide range of physiologic conditions. It was found that the closing velocity was almost linearly related to LV dP/dt. At 4000 mmHg/s, closing velocities ranged from 1.5 to 2.0 m/s. Impact loads increased monotonically with LV dP/dt and closing velocity. In some valves, impact loads reached 2800 g at LV dP/dt of 4000 mmHg/s, and closing velocities of 2.0 m/s.

Dynamics of Björk-Shiley Convexo-Concave mitral valves in sheep.

The outlet strut of Björk-Shiley (BSCC) Convexo-Concave heart valves has fractured in some implanted valves resulting in disc escape and emergency reoperation or death. The closing dynamics of BSCC heart valves was studied in situ to determine the forces acting on the outlet strut during valve closure. BSCC valves with strain gages attached to the outlet strut were implanted in the mitral position in sheep. Impact forces were measured under various circumstances including anesthetized, sedentary, and exercise conditions. Left ventricular (LV) pressures were recorded simultaneously via catheter-tip pressure transducers. Impact forces increased with the 1/3 power of left ventricular pressure rise and the square root of LV pressure at valve closure. Good agreement between the in vivo measurements and theoretical models was observed. Based on this study, it is concluded that sheep provides a good in situ model for the testing of mechanical mitral valves.

Velocity of closure of Björk-Shiley Convexo-Concave mitral valves: effect of mitral annulus orientation and rate of left ventricular pressure rise.

The purpose of this study was to determine analytically the hemodynamic factors that affect the closing velocity of the disc of Björk-Shiley convexo-concave (BSCC) prosthetic mitral valves. The motion of the BSCC disk was modelled by Newton's second law written in the form of a second order differential equation which expressed the instantaneous angle of the disc with respect to the valve ring as a function of the instantaneous pressure drop across the mitral valve, delta P(t), and the angle of the pressure gradient vector acting upon the disc during closure. The disc closes in response to the negative pressure drop created by the crossover of left atrial and left ventricular (LV) pressures. The rate of closure depends on the rate of development of the pressure drop across the valve, d delta P/dt, which is largely dependent upon the rate of change of left ventricular pressure during isovolumic contraction, LV dP/dt. The closure rate is also strongly dependent on the initial angle of the pressure drop vector with respect to the disc. The disc was predicted to reach its highest velocity at the moment of impact, based on the Runge-Kutta solution. Modelling suggests that a high LV dP/dt during valve closure or distorted LV geometry, causing the angle between the fully open disc and the pressure drop vector to shift, will cause the valve to have a high velocity at the moment of impact and may produce high impact loads.

Low frequency analysis of opening sound for detection of single leg separation of Björk-Shiley Convexo-Concave heart valves.

The aim of the study was to evaluate the potential of low frequency analysis of valve opening sounds in order to detect the presence of single leg separation of the outlet strut of Björk-Shiley convexo-concave (BSCC) valves. Single leg separation is believed to precede outlet strut fracture. Phonocardiograms (PCG) of 28 patients with BSCC mitral valves were recorded and filtered to limit the frequency bandwidth between 90 and 1400 Hz (-20 dB). Twenty-four patients had BSCC valves that were presumably unimpaired and four patients had BSCC valves with single leg separation. Spectral analysis consisted of computing the power spectrum for 30 opening sounds for each patient using a fast Fourier transform (FFT) algorithm on an IBM-PC compatible computer. For each patient, the power spectra of the selected opening sounds were averaged and normalized in amplitude on a 0-100% linear scale. A ratio of the high to the low frequency area of the averaged power spectra was computed and evaluated as a diagnostic feature to identify the valves with single leg separation. Six cut-off frequencies serving to distinguish between high and low frequency areas of the spectra were tested: 400, 450, 500, 550, 600, and 650 Hz. The results showed that the high-to-low frequency area ratios tested were all able to identify valves with single leg separation. The correct classification performance was 96.4%. The sensitivity to detect valves with single leg separation was 75% and the specificity was 100%. The positive predictive value was 100% and the negative predictive value 96%. The study confirmed the potential of low frequency analysis of valve opening sounds to detect single leg separation of the outlet strut of BSCC valves implanted in humans.

Pressure distribution near the occluders and impact forces on the outlet struts of Björk-Shiley convexo-concave valves during closing.

BACKGROUND AND AIMS OF THE STUDY: An in vitro study of the mechanics of closure of Björk-Shiley convexo-concave (BSCC) valves is presented in order to investigate the mechanics of outlet strut fracture reported in a small fraction of the implanted valves. MATERIALS AND METHODS: Four BSCC 29 mm valves instrumented with strain gages on the outlet strut legs were mounted in the mitral position of an axisymmetric flow chamber of a mock pulsatile flow loop. Measurements of the pressure field in the vicinity of the occluder, closing velocity of the occluder tip in the major orifice, and the impact force between the occluder and outlet strut at the instant of valve closure were obtained at a range of physiologic flow rates. RESULTS: The results indicated an uneven pressure distribution on the occluder associated with a tendency for the occluder to over-rotate and induce loads on the outlet struts. The impact loads on the outlet struts were asymmetric with load on one leg being larger than the other by up to 25%. These results are consistent with single leg separation preceding outlet strut fracture in most of the valve failures reported. Orientation of the valve with respect to the mitral orifice (major orifice towards the top or bottom) did not significantly affect the loads on the outlet strut. A significant variation in the impact loads of the four valves was measured for identical experimental conditions suggesting that valve specific factors influence outlet strut loads. CONCLUSIONS: This study provided an understanding of the cause-effect relationship between valve dynamics and outlet strut fracture.

Surface and edge wear of Björk-Shiley Delrin heart valve discs.

BACKGROUND AND AIMS OF THE STUDY: Wear of Björk-Shiley Delrin (BSD) heart valve discs is known to have occurred in some patients, possibly contributing to increased regurgitation. This paper specifically addresses surface and edge wear that have been observed on some discs of explanted BSD valves after implant durations up to 22.4 years. METHODS: The wear patterns have been documented using either photographic or scanning electron microscopic methods for 42 out of 73 explanted BSD valve discs. The remainder of the 73 discs were not available for analysis. RESULTS: One form of surface wear found on 18 out of 42 of the Delrin discs was the concentric wear of mild abrasive origin along the surface near the disc edge due to contact with the inlet and outlet struts. In five instances, surface anomalies were observed, primarily in the areas of high velocity blood flow. This paper also describes two Delrin discs with non-concentric edge wear patterns: (a) one which appears to be due to fatigue micro-chipping and abrasive wear of the disc of a 20 year BSD explant, which had fibrous tissue ingrowth, causing abnormal rotation of the disc during valve closure, and (b) a second one which is thought to have been caused by a cutting action of the knife-like stub of one inlet strut leg which had separated. Cross-sectional analyses of two explanted BSD discs, with full indent grooves on the inflow side, indicated that the Delrin material was primarily compressed under these wear grooves, rather than removed by abrasion. Hardness profiles indicated that the Delrin 150 microns below the surface was harder and would tend to prevent further deformation. A simple model describing the compound impact (impact with sliding) phenomenon is introduced to explain abrasive wear found on some explanted BSD discs. CONCLUSION: Based on the studies here and reports in the literature, the BSD heart valve appears to present a design which provides many years of service and, when wear occurs, it occurs in a manner that provides easily recognized clinical symptoms, which allow time for diagnosis and treatment.

Strut fracture mechanisms of the Björk-Shiley convexo-concave heart valve.

Investigations of convexo-concave (C/C) valve outlet strut fractures (OSFs) were initially confounded by knowledge that the strut was subject to bending forces in arresting the opening disc. Pulse duplicator studies subsequently showed that closing loads were all born by the inlet strut, along with an understandable focus on the nature of the welds, where most fractures occurred. As observations of explanted valves accumulated, certain features pointed to unusual closing loads that might be contributory factors, but these hypothetical forces could not be verified. Epidemiological extrapolations and case-matched control studies have shown that certain valve and patient characteristics were each associated independently with increased OSF risk, leading to clinically valuable risk stratification, but little additional understanding of why OSFs continued to occur. Detection of the causative, highly transient (< 0.5 ms), outlet-strut-tip impacts due to closing disc over-rotation that have almost ten times the force of disc opening, and the capability of inducing leg-base bending stresses beyond the strut wire's fatigue endurance limit had to await the development of computer-controlled pulse duplicators and strut-leg strain gaging. Exercised young animals easily achieved such strut loading, but most human patients would probably have more difficulty. The actual OSF mechanism is a long-term, valve-patient interaction that requires the concurrence of susceptible valve geometry and sufficient ventricular contractility potential to develop the isovolumic, high dP/dt needed for forceful disc over-rotation. Critical strut tip loading must then occur often enough to fatigue fracture both strut legs within the patient's lifetime with the valve.

A working cardiac valve phantom for radiographic assessment of prosthetic heart valves.

RATIONALE AND OBJECTIVES: A working valve phantom (WVP) that both exercises the valve occluder and simulates movements of the mitral annulus is described. It was designed to develop a method for radiographic detection of a single broken leg of the two-legged Björk-Shiley convexo-concave (C/C) heart valve outlet strut. METHODS: The WVP consists of a pneumatically driven left ventricular assist device immersed in 22 cm of water. Left ventricular assist device annulus movements are generated by systolic turgor and diastolic relaxation of the aortic outflow graft within limits set by the holding fixture design. RESULTS: WVP images were comparable in attenuation, valve motion, and diagnostic sensitivity to clinical C/C valve images and were effective in assessing leaflet excursions in another valve model. Techniques developed in the WVP have proved successful in the clinical detection of C/C valves that have a single broken leg but that show normal function in all other tests. CONCLUSION: The WVP can be a useful tool for developing refined radiographic assessments of prosthetic heart valves.

A bovine model for detecting high intensity transient signals originating from mechanical heart valves.

In patients with mechanical heart valves (MHVs), transcranial Doppler methods commonly detect high intensity transient signals (HITS) representing microemboli. These microemboli, which are presumably gaseous, may cause stroke and cognitive deterioration. A bovine model was therefore developed for studying the relationship between mitral MHV induced HITS and potential etiogenic factors. We placed an 18 mm, 4 MHz Doppler probe in the brachiocephalic artery to detect MHV induced microbubbles at baseline (rest) and under 9 other conditions. To elucidate the gas composition (CO2 or N2) of the microbubbles, we administered 1%, 3%, and 5% CO2, and 100% O2. To determine effect of the heart rate, we paced the heart at 120, 160, and 180 bpm. To alter the myocardial contractility, we gave dobutamine and esmolol. Two independent, blinded observers counted the HITS from recorded doppler spectra. HITS were defined by an initial unidirectional spectral deviation, a signal power of >8 dB relative to the background power, and lack of a cyclic appearance. The electrocardiogram, aortic and LV pressures, and LV dP/dt were obtained telemetrically. The calves were studied 4 to 6, 8 to 10, and 12 to 14 weeks postoperatively, after which the animals were sacrificed at an approximate 4 month study duration, and a postmortem evaluation of the heart and the main viscera was performed. In all, 27 HITS recordings were made in 10 calves. Myocardial contractility was the only factor to significantly affect HITS frequency; the heart rate and blood gas concentrations had minimal effect on HITS frequency. Our bovine model will be useful for assessing valve designs, as well as the mechanism of HITS, the composition of the microemboli, and their possible pathophysiologic effects on the kidneys and brain.

Time-frequency analysis of the first heart sound: Part 3: Application to dogs with varying cardiac contractility and to patients with mitral mechanical prosthetic heart valves.

The cone-kernel distribution (CKD) is first applied to the analysis of the intracardiac and the thoracic first heart sound (S1) of dogs in various cardiac contractile states, and secondly to the S1 of patients with mitral mechanical prosthetic heart valves. The CKD of native S1 in dogs shows that the dominant components of S1 are generally concentrated in a band at around 50 Hz with a horizontal flat or a semi-lunar shape, independently of the myocardial contractile state. There is no significant systematic rising frequency component. The instantaneous frequency of S1 shows a good cross-correlation with the time derivative of the left ventricular pressure (dP/dt), but the maximum frequency is not proportional to the maximum of dP/dt. The CKD of S1 in patients with mitral mechanical prosthetic heart valves showed a pulse-like component with a high-frequency bandwidth, which is distinct from the low constant-frequency components of S1 produced by native heart valves.