Pressure recovery distal to a stenosis: potential cause of gradient "overestimation" by Doppler echocardiography

Doppler ultrasound is currently being widely applied to measure intracardiac pressure gradients noninvasively. In comparative invasive studies, it is generally assumed that pressure is effectively uniform distal to the stenosis. As the poststenotic jet expands, however, its velocity decreases, and pressure is recovered to the extent permitted by turbulence, so that the measured gradient will be lower if the distal catheter is positioned downstream from the vena contracta. This can lead to apparent Doppler "overestimation" of the pressure gradient because of this phenomenon of pressure recovery. This study demonstrates that pressure recovery can be important in a variety of clinical settings studied by in vitro models. Although most prominent in streamlined tunnels modeled after the obstruction in patients with hypertrophic cardiomyopathy, these effects are important even for central stenoses at physiologic flow rates. Because precise catheter position is not always known or controlled, these findings suggest an important advantage for Doppler gradient estimation, because it provides the maximal gradient at the vena contracta, which determines the load on the proximal chamber.     

Computational simulations of mitral regurgitation quantification using the flow convergence method: Comparison of hemispheric and hemielliptic formulae

Mitral regurgitation results from the incomplete closure of the mitral valve, and the noninvasive diagnosis of this disease remains an important clinical goal. In this study, steady flow computer simulations were used to evaluate flow convergence method for flow rate estimation. The hemispheric and hemielliptic formulae were compared for accuracy in the presence of complicating factors such, as ventricular confinement, orifice shape, and aortic outflow. Results showed that in the absence of aortic outflow and ventricular confinement, there was a plateau zone where the hemispheric formula approximated the true flow rate, independent of orifice shape. However, in the presence of complicating factors such as aortic outflow and ventricular confinement, there was no clear zone where the hemispheric formula could be applied. The hemielliptic formula, however, worked in, all cases, regardless of chamber size or magnitude of aortic outflow. Therefore, application of the hemielliptic formula shold be considered in future clinical studies.     

In vitro velocity measurements down strem from the lonescu-Shiley aortic bioprosthesis in steady and pulsatile flow

Velocity measurements were made in vitro using laser Doppler anemometry (LDA) downstream from an lonescu-Shiley (IS) bioprosthetic aortic heart valve. Velocity measurements were made in both steady and pulsatile flow. A systematic, flow mapping approach to the measurement methodology showed that the IS valve generated a large jetlike flow constriction. The acceleration ratio, defined as the maximum mean velocity for the IS valve divided by that for no valve obstructing the flow, was as high as 2·4 for steady flow and 2·6 for pulsatile flow. It was concluded that the IS valve generated a flow quite unlike that observed by other in vestigators for the natural human aortic valve, after which the leaflet design of the IS valve was modelled. In addition, a comparative analysis of steady and pulsatile results was undertaken. It was found that the pulsatile flow results for the systolic ejection interval could be divided into three phases, denoted early, mid, and late systole, as defined by the flow structure at the data plane location. Only during midsystole were the pulsatile flow results approximated by the steady flow results. Also, it was found that the magnitude of the flow disturbance measured in steady flow tended to be an upper bound on that measured for pulsatile flow.     

Bileaflet Aortic Valve Prosthesis Pivot Geometry Influences Platelet Secretion and Anionic Phospholipid Exposure

The clinical histories of the Medtronic Parallel (MP) and St. Jude Medical (SJM) Standard valves suggest pivot geometry influences the thrombogenic characteristics of bileaflet prostheses. This work studied the effects of various pivot geometries on markers of platelet damage in a controlled, in vitro apparatus. The Medtronic Parallel valve, two St. Jude Medical valves, and two demonstration prostheses were used to study the effects of bileaflet pivot design, gap width, and size on platelet secretion and anionic phospholipid expression during leakage flow. A centrifugal pump was used to drive blood through a circuit containing a bileaflet prosthesis. Samples were taken at set time intervals after the start of the pump. These samples were analyzed by cell counting, flow cytometry, and enzyme-linked immunosorbant assay. No significant differences were observed in platelet secretion or anionic phospholipid expression between experiments with the SJM 27 Standard regular leaker, the SJM 20 regular leaker, and the MP 27 valves. Significant differences in platelet secretion and anionic phospholipid expression were observed between a SJM 27 Standard regular leaker and a SJM 27 high leaker valve. These studies suggest that leakage gap width within bileaflet valve pivots has a significant effect on platelet damage initiated by leakage flow. © 2001 Biomedical Engineering Society. PAC01: 8719Uv, 8719Tt, 8380Lz, 8768+z     

A quantitative method for thein vitro study of sounds produced by prosthetic aortic heart valves Part II: an experimental,comparative study of the sounds produced by a normal and simulated-abnormal Starr-Edwards series 2400 aortic prosthesis

A comparative study was made of the sounds produced by a normal Starr-Edwards 2400 aortic valve prosthesis with those produced by the same valve but having a simulated overgrowth at the apex of the struts. Comparisons were made over the entire cardiac cycle for time and amplitude, power-density spectra, power-distribution spectra, power-distribution surfaces associated with individual valves, and three-dimensional power-distribution-difference surface. Power-density spectra were compared for portions of the cycle corresponding to the opening, systolic, and closing sounds of the valve. Physical parameters of an acoustical model were estimated from the power-density spectra. The results showed that each comparison gave information pertinent to the simulated malfunction. Opening. systolic and closing sounds, respectively, were different for each valve. The opening sound of the abnormal valve displayed a much lower frequency. Systolic sounds for the two valves were similar in frequency, but the normal valve produced more total power for this sound. The closing sound of the abnormal valve occurred later than that of the normal valve. These differences were more clearly seen when viewed in the frequency domain.     

Assessment of the accuracy of color Doppler flow mapping by digital image analysis

An in vitro steady flow experiment was performed in order to test the accuracy of velocity measurements obtained through color Doppler flow mapping (CDFM). Using the American Society of Echocardiography (ASE) flow phantom, low (maximum velocity = 60 cm/sec), medium (maximum velocity = 300 cm/sec) and high (maximum velocity = 600 cm/sec) speed accelerating flow fields, in which multiple aliases were visible, were imaged. A fully automatic computer algorithm was used to unwrap the aliases and to convert the CDFM to digital velocity. Packet size and wall filter frequency on the ultrasound machine were varied and the measured velocity compared to the true velocity. The results show that the velocity obtained in this way from the CDFM is very accurate at the low and medium velocities, but for the high velocity the turbulence is too intense to obtain an accurate result. There was no marked difference between the data for different packet sizes or wall filter settings.     

Quantification of Mitral and Tricuspid Regurgitation Using Jet Centerline Velocities: An In Vitro Study of Jets in an Ambient Counterflow

A method for quantifying mitral and tricuspid regurgitant volume that utilizes a measure of jet orifice velocity U(0) - m/sec), a distal centerline velocity (U(m) - m/sec), and the intervening distance (X - cm) was recently developed; where jet flow rate (Q(cal) - L/min) is calculated as Q(cal) = (U(m)X)(2)/(26.46U(o)). This method, however, modeled the regurgitant jet as a free jet, whereas many atrial jets are counterflowing jets because of jet opposing intra-atrial flow fields (counterflows). This study concentrated on the feasibility of using the free jet quantification equation in the atrium where ambient flow fields may alter jet centerline velocities and reduce the accuracy of jet flow rate calculations. A 4-cm wide chamber was used to pump counterflows of 0, 4, and 22 cm/sec against jets of 2.3, 4.8, and 6.4 m/sec originating from a 2-mm diameter orifice. For each counterflow-jet combination, jet centerline velocities were measured using laser Doppler anemometry. For free jets (no counterflow), flow rate was calculated with 98% mean accuracy. For all jets in counterflow, the calculation was less accurate as: (i) the ratio of jet orifice velocity to counterflow velocity decreased (U(o)/U(c), where U(c) is counterflow velocity), i.e., the counterflow was relatively more intense, and (ii) centerline measurements were made further from the orifice. But although counterflow lowered jet centerline velocities beneath free jet values, it did so only significantly in the jet's distal portion (X/D > 16, i.e., >16 orifice diameters from the origin of the jet). Thus, the initial portion (X/D < 16) of a jet in counterflow behaved essentially as a free jet. As a result, even in significant counterflow, jet flow rate was calculated with >93% accuracy and >85% for jets typical of mitral and tricuspid regurgitation, respectively. Counterflow lowers jet centerline velocities beneath equivalent free jet values. This effect, however, is most significant in the distal portion of the jet. Therefore, regurgitant jets, although not classically free because of systolic atrial inflow or jet-induced intra-atrial swirling flows, will decay in their initial portions as free jets and thus are candidates for quantification with the centerline technique. (ECHOCARDIOGRAPHY, Volume 13, July 1996)     

A comparison of the hinge and near-hinge flow fields of the St Jude medical hemodynamic plus and regent bileaflet mechanical heart valves

OBJECTIVE: The most widely implanted prosthetic valves are the mechanical bileaflets, most of which have good forward flow hemodynamics. However, recent clinical experiences illustrate the importance of understanding the flow structures generated within the hinge. The purpose of this study was to evaluate the hinge-flow dynamics of two new variations of a 17-mm St Jude Medical bileaflet valve: the Hemodynamic Plus and the Regent (St Jude Medical, Inc, St Paul, Minn). METHODS: Clinical quality reproductions of the valves were manufactured with clear housings. Laser Doppler velocimetry velocity and turbulent shear stress measurements were conducted within the hinge and thumbnail regions of the valves. RESULTS: In the 17-mm Hemodynamic Plus hinge, a rotating flow structure developed in the inflow pocket during forward flow. During systole, velocities through the hinge pocket reached 0.70 m/s, and the turbulent shear stress reached 1000 dynes/cm(2). In the thumbnail, forward flow velocities ranged from 1.4 m/s to 1.7 m/s. In the 17-mm Regent hinge, a rotating flow structure partially developed in the inflow pocket during forward flow. During systole, velocities through the hinge pocket reached 0.75 m/s, and the turbulent shear stress reached 1300 dynes/cm(2). In the thumbnail, forward flow velocities ranged from 1.0 m/s to 1.3 m/s. CONCLUSIONS: The active leaflet motion through the St Jude Medical hinge creates a washout pattern that restricts the persistence of stagnation zones and thus may be a contributing factor to its successful clinical performance. The hinge and thumbnail flow dynamics of the 17-mm Regent valve are at least equivalent to, and possibly superior to, those of the 17-mm Hemodynamic Plus valve.     

Mitral leaflet geometry perturbations with papillary muscle displacement and annular dilatation: an in-vitro study of ischemic mitral regurgitation

BACKGROUND AND AIM OF THE STUDY: Perturbations of leaflet geometry are the final end point through which left ventricular (LV) ischemia causes incomplete mitral leaflet closure and resultant mitral regurgitation (MR). Geometric inconsistencies observed with valvular or subvalvular structural alterations raise several questions. METHODS: A new in-vitro LV flexible bag model was developed in order to visualize and analyze leaflet geometric changes under simulated pathological ischemic MR conditions. RESULTS: Papillary muscle (PM) displacement and annular dilatation decreased leaflet coaptation length, leading to significant MR. Symmetrical PM displacement shifted the coaptation line towards the leaflet edges and created central gaps along this line. Asymmetric PM displacement generated diametrically uneven coaptation with a tent-shaped leaflet at the tethered PM side, while the leaflet bulged at the opposite side towards the left atrium. CONCLUSION: Leaflet geometry during systole is affected by subvalvular structures. Asymmetric PM displacement, which may occur in regional or acute myocardial infarction, induces irregular deformation of the leaflet's coaptation line and, as a result, MR at the tethered side. Direct visualization of leaflet perturbation under these simulated pathological conditions may promote understanding of mechanisms present in ischemic MR.