James D. Thomas

Age related development of atherosclerotic plaque stress: A population-based finite-element analysis

BackgroundIn order to identify those age-related factors in the development of coronary atherosclerosis that would affect the stability of the plaque system, we have developed idealized, finite-element, cross-sectional models of the arterial wall and associated lesions, derived from population-based data. MethodsThe physical development and morphology of coronary plaques was documented in the Pathobiological Determinants of Atherosclerosis in Youth histological study. Using this database, finite-element analysis models were created for five age groups (15–19, 20–24, 25–29 and 30–34 years) and for the 25 largest lesions. Cosmos (Structural Research, Inc., Los Angeles, California, USA) was used to create and analyze the models. ResultsThe area of greatest stress shifted from the intima opposite the lesion in the 15–19 years age group to the edge of the cap and adjacent healthy tissue in the later age groups. Increasing age had a strong positive correlation with the shoulder stress level (r = 0.95) and the per cent stenosis correlated well with shoulder stress (r = 0.99, P < 0.002). Increasing the cap stiffness from a soft cap to a fibrous cap in the 30–34 year age group model resulted in a localized increase in shoulder surface stress by 10%. A calcified cap increased this shoulder surface stress by 30%. ConclusionsThis finite-element analysis of the population-based data shows that the increase in stress appears to be closely related to the impaired load-bearing capability of the lipid pool that develops with age. The shoulder area of the lesion has been shown to be the location of most of the plaque fractures. Coronary Artery Dis 9:13–19

Left ventricular diastolic filling with an implantable ventricular assist device: beat to beat variability with overall improvement

OBJECTIVESnWe studied the effects of left ventricular (LV) unloading by an implantable ventricular assist device on LV diastolic filling.nnnBACKGROUNDnAlthough many investigators have reported reliable systemic and peripheral circulatory support with implantable LV assist devices, little is known about their effect on cardiac performance.nnnMETHODSnPeak velocities of early diastolic filling, late diastolic filling, late to early filling ratio, deceleration time of early filling, diastolic filling period and atrial filling fraction were measured by intraoperative transesophageal Doppler echocardiography before and after insertion of an LV assist device in eight patients. A numerical model was developed to simulate this situation.nnnRESULTSnBefore device insertion, all patients showed either a restrictive or a monophasic transmitral flow pattern. After device insertion, transmitral flow showed rapid beat to beat variation in each patient, from abnormal relaxation to restrictive patterns. However, when the average values obtained from 10 consecutive beats were considered, overall filling was significantly normalized from baseline, with early filling velocity falling from 87 +/- 31 to 64 +/- 26 cm/s (p < 0.01) and late filling velocity rising from 8 +/- 11 to 32 +/- 23 cm/s (p < 0.05), resulting in an increase in the late to early filling ratio from 0.13 +/- 0.18 to 0.59 +/- 0.38 (p < 0.01) and a rise in the atrial filling fraction from 8 +/- 10% to 26 +/- 17% (p < 0.01). The deceleration time (from 112 +/- 40 to 160 +/- 44 ms, p < 0.05) and the filling period corrected by the RR interval (from 39 +/- 8% to 54 +/- 10%, p < 0.005) were also significantly prolonged. In the computer model, asynchronous LV assistance produced significant beat to beat variation in filling indexes, but overall a normalization of deceleration time as well as other variables.nnnCONCLUSIONSnWith LV assistance, transmitral flow showed rapidly varying patterns beat by beat in each patient, but overall diastolic filling tended to normalize with an increase of atrial contribution to the filling. Because of the variable nature of the transmitral flow pattern with the assist device, the timing of the device cycle must be considered when inferring diastolic function from transmitral flow pattern.

Noninvasive assessment of diastolic intraventricular pressure gradients using color Doppler M-mode echocardiography

Diastolic intraventricular gradients, while previously observed are now hypothesized to play a critical role in diastolic filling and function. However the clinical ability to monitor intraventricular pressure gradients is not normally possible. Doppler echocardiographic M-mode analysis may provide a noninvasive estimate of this important information. The Euler equation, which describes the pressure-velocity relationship along an inflow streamline, is used to compute the diastolic pressure gradient within the ventricle. In a novel experimental setup, the invasive pressures at multiple locations within the ventricular chamber are acquired simultaneously with noninvasive color Doppler M-mode images providing spatiotemporal velocity distributions during diastolic filling. Techniques have been developed to calculate pressure gradients using digitally processed spatiotemporal velocity distributions obtained from the color Doppler M-mode image. These results have been compared with the direct invasive pressure measurements.

Three-dimensional shape, deformation, and motion analysis of mitral annuli using transesophageal echocardiographic data

Deformation and motion of the Mitral Annulus (MA) is closely related to the left ventricular function. Measurement and visualization of the characteristic parameters in 3D will help in understanding the relationship. Data for this study was acquired from patients undergoing transesophageal echocardiographic examination with the transducer aligned along the axis roughly perpendicular to the annuli, and rotated automatically to cover 360 degrees. ECG gated images were acquired at 24 angles for each phase of the cardiac cycle. The annuli hinge points were identified from each echo image and the annuli reconstructed. The parameters measured to characterize the annuli were: (1) area of projection, (2) non- planarity, (3) excursion of annulus centroid, (4) change in the annulus orientation. We validated the method using a wire loop shaped in the form of a saddle and a planar rubber ring imaged in a water bath at different orientations. Four MAs were reconstructed using this method. Two were patients with dilated cardiomyopathy (DCM) and two were patients with normal ventricular function. The change in parameters was measured from systole to diastole. Percentage change in area (29% vs. 16%) and excursion (8 mm vs. 3 mm) were much larger for normals than for patients. While, changes in non-planarity (20%) and orientation (6 deg) were similar. These preliminary results show that MA parameters do reflect the abnormality, and could be used for diagnosis and prognosis of patients with bad ventricles.

In vitro assessment of pressure recovery through St. Jude heart valve prostheses

Pressure recovery has recently been reported as a possible mechanism for the apparent overestimation of transvalvular pressure gradients by Doppler echocardiography across St. Jude heart valve prostheses when implanted in the aortic position. We have studied the effect of mitral and aortic configurations on downstream pressure recovery in different sized St. Jude valves. Pressure recovery in the central and side orifice was significantly higher for prostheses in the aortic position than for a mitral configuration (p<O.OOl). Pressure recovery in the side orifice was most effected by the downstream geometry which permitted the most accurate Doppler derived pressure gradients to be obtained in the mitral position across the side orifice (Doppler catheter = 2.5 t 1.3 Hg). INTRODUCTION Doppler echocardiography has become a standard method to evaluate the function of native and prosthetic heart valves. This technique allows for the assessment of structural integrity, valvular regurgitation and valvular stenosis. Doppler velocity measurements are used to determine pressure gradients noninvasively using the simplified Bernoulli equation (Ap = pv2/2, where ~p is computed in Pascals from the measured Doppler velocity, v I d s ] , and the density of blood [ 1050 kg/m3]). This method has been validated for native valvular stenosis; however, controversy remains about the accuracy of this method to measure pressure drop across prosthetic heart valves. Discrepancies between Doppler and catheter gradients have been reported for different types of mechanical prostheses and may be multifactorial in origin. Apparent overestimation of pressure gradients by Doppler appears to be most striking for bileaflet prostheses such as the St. Jude heart valve and is likely secondary to pressure recovery downstream of the prosthesis[ 11. This has only been evaluated for prostheses in a simulated aortic position. Since recovery of pressure downstream critically depends on the geometry of the outflow diffuser, we hypothesized that pressure recovery would be different for prosthetic valves in aortic and mitral positions. The aim of this study is to evaluate pressure recovery and accuracy of Doppler gradients across St. Jude valve prostheses mounted in mitral and aortic positions using in vitro flow studies and computational fluid dynamics simulations. r o exDenments: Three different S t . METHODS Jude prosthetic valves (sizes: 21,23 and 25 mm) were mounted in an in vitro flow model. For each valve size, three distal geometries were examined: outflow tubes with diameters 26 and 31 mm were used to simulate the ascending aorta; outflow discharging directly into the distal chamber approximated a mitral valve configuration. Steady state flow was produced ranging from 270 to 455 ml/sec while driving pressure was kept constant. For the 23 mm valve four different driving pressures were evaluated with and without the 26 mm outflow tube. Pressure measurements were obtained using a high-fidelity pressure t ransducer (Millar, Houston, TX). The pressure catheter was connected to a microprocessor controlled infusion pump (Harvard Apparatus, South Natick, M A ) and slowly pulled back through the central and the side orifice at 0.8 /sec. The pressure waveform was digitized at 1000 Hz over 100 s (8 cm distance). Simultaneous continuous wave Doppler velocity spectra were obtained using a Sonos 1500 echocardiograph (Hewlett-Packard, Andover, M A ) . By directing the ultrasound beam, velocities in the central and side orifices could be interrogated separately. 0-7803-1377-1/93

Instantaneous diastolic transmitral pressure differences from color Doppler M mode echocardiography.

3.00 01993 IEEE 91 1 Islmulation: Fluid dynamics computations were performed using commercially available finite difference software (Fluent Inc., Hanover, NH). The Navier-Stokes equations were solved on a 53000 node grid simulating steady state flow through a 25 mm St. Jude valve mounted in an aortic and mitral position. Pressure profiles were reconstructed along a streamline through the central and the side orifices. ANALYSIS The maximum catheter pressure gradient was measured for each hydrodynamic condition as the difference between the pressure measured 1.5 cm proximal to the valve and the minimum pressure recorded. The net pressure drop was measured between the proximal pressure and the pressure measured 6.5 cm downstream from the valve. The amount of pressure recovered downstream was expressed as a percentage of the maximal pressure drop. The Doppler derived pressure gradients were obtained using the simplified Bernoulli equation and were compared with catheter gradients using linear regression analysis. RESULTS and DISCUSSION The figure at the right shows the pressure profiles through the central and side orifice from the numerical simulation (dashed) and the in vitro experiment (solid) for a 25 mm valve. This shows that a deep pressure well occurs at the entrance of the central orifice with gradual pressure recovery downstream. Pressure drop in the side orifices occurs fur ther downstream and was less pronounced. These findings were similar in both numerical simulations and experimental data. For valves in the aortic position (26 mm),, as much as 58.5 k 3.4% of the initial pressure drop occurring in the central orifice is recovered downstream ( 3 3 . 6 & 1.5% of the total pressure drop is recovered before flow leaves the valve leaflets and an additional 24.8 k5.0% is recovered in the aorta). Total pressure recovery was significantly higher in valves with a small aorta compared to the larger aorta (31 mm) (49.5 f 5.8%, p<O.OOl) and forvalves in the mitral position (34.8 f 5.646, p<O.OOl). For the mitral valves, the initial pressure drop in the central orifice is recovered entirely in between the valve leaflets. Overall, pressure recovery in the side orifices was significantly lower than in the central orifice (p<O.OOl). For aortic valves with the smaller aorta 45.9 f 5.0% of the pressure drop in the side orifice was recovered, whereas only 19.2 f 6.1% of the pressure was recovered in the mitral valves (p<O.OOl).