Three-dimensional visualization of velocity fields downstream of six mechanical aortic valves in a pulsatile flow model

Hemodynamic and Anatomic Predictors of Renovisceral Stent-Graft Occlusion Following Chimney Endovascular Repair of Juxtarenal Aortic Aneurysms

PURPOSE

To identify anatomic and hemodynamic changes associated with impending visceral chimney stent-graft occlusion after endovascular aneurysm repair (EVAR) with the chimney technique (chEVAR).

METHODS

A retrospective evaluation was performed of computed tomography scans from 41 patients who underwent juxtarenal chEVAR from 2008 to 2012 to identify stent-grafts demonstrating conformational changes following initial placement. Six subjects (mean age 74 years; 3 men) were selected for detailed reconstruction and computational hemodynamic analysis; 4 had at least 1 occluded chimney stent-graft. This subset of repairs was systematically analyzed to define the anatomic and hemodynamic impact of these changes and identify signature patterns associated with impending renovisceral stent-graft occlusion. Spatial and temporal analyses of cross-sectional area, centerline angle, intraluminal pressure, and wall shear stress (WSS) were performed within the superior mesenteric and renal artery chimney grafts used for repair.

RESULTS

Conformational changes in the chimney stent-grafts and associated perturbations, in both local WSS and pressure, were responsible for the 5 occlusions in the 13 stented branches. Anatomic and hemodynamic signatures leading to occlusion were identified within 1 month postoperatively, with a lumen area <14 mm² (p=0.04), systolic pressure gradient >25 Pa/mm (p=0.03), and systolic WSS >45 Pa (p=0.03) associated with future chimney stent-graft occlusion.

CONCLUSION

Chimney stent-grafts at increased risk for occlusion demonstrated anatomic and hemodynamic signatures within 1 month of juxtarenal chEVAR. Analysis of these parameters in the early postoperative period may be useful for identifying and remediating these high-risk stent-grafts.

Assessment of prosthetic aortic valve performance by magnetic resonance velocity imaging

OBJECTIVES

Magnetic resonance (MRI) velocity mapping was used to evaluate non-invasively the flow profiles of the ascending aorta in normal volunteers and in patients with an aortic (mechanical) valve prosthesis.

BACKGROUND

In patients with artificial aortic valves the flow profile in the ascending aorta is severely altered. These changes have been associated with an increased risk of thrombus formation and mechanical hemolysis.

METHODS

Velocity profiles were determined 30 mm distal to the aortic valve in six healthy volunteers and seven patients with aortic valve replacement (replacement within the last 2 years) using ECG triggered phase contrast MRI. Peak flow, mean flow and mean reverse flow were measured in intervals of 25 ms during the entire heart cycle. Systolic reverse flow, end-systolic closing and diastolic leakage volume were calculated for all subjects.

RESULTS

Peak flow velocity during mid-systole was significantly higher in patients with valvular prosthesis than in normals (mean + SD, 1.9 +/- 0.4 m/s vs. 1.2 +/- 0.03 m/s, P < 0.001) with a double peak and a zone of reversed flow close to the inner (left lateral) wall of the ascending aorta of the patients. Closing volume was significantly larger in patients than in controls (-3.3 +/- 1.2 ml/beat vs. -0.9 +/- 0.5 ml/beat; P < 0.001). There was reverse flow during systole in valvular patients amounting to 15.7 +/- 6.7% of total cardiac output compared to 2.3 +/- 1.2% in controls (P < 0.001). Diastolic mean flow was negative in patients after valve replacement but not in controls (-11.0 +/- 15.2 ml/beat vs. 6.8 +/- 3.2 ml/beat; P < 0.01).

CONCLUSIONS

The following three major quantitative observations have been made in the present study: (1) Mechanical valve prostheses have an increased peak flow velocity with a systolic reverse flow at the inner (left lateral) wall of the ascending aorta. (2) A double peak flow velocity pattern can be observed in patients with bileaflet (mechanical) prosthesis. (3) The blood volume required for leaflet closure and the diastolic leakage blood volume are significantly higher for the examined bileaflet valve than for native heart valves.

Pressure development within a sac-type pneumatically driven ventricular assist device

Intrinsic features of the pumping process of a pneumatically driven ventricular assist device (VAD) and the effects of different types of pneumatic drivers upon its performance were investigated in vitro by analysing the pressure distributions within the device and the motions of the prosthetic valves. It was found that the stretching of the flexible, elastic diaphragm in both late systole and diastole initiates a pressure oscillation which directly affects the timing of the pumping process. The timing was also found to be dependent on the length and stiffness of the cannulae which link the VAD to the model circulation system. During the stretch-induced oscillation in late systole, the VAD housing experiences partial collapse due to fluid momentum effects, which tends to increase the effective stroke volume of the device, and reduce the amplitude of the pressure oscillation. Reducing the rising (falling) rate of driving pressures (dpd/dt) may not necessarily reduce the maximum rate of change of the blood chamber pressure (dpch/dtmax) but may upset the stability of the pumping process. This is because a minimum dpch/dtmax exists, which is determined by the stretch-induced oscillation. In order to minimize dpch/dtmax and to provide the device with a stable working condition, dpd/dt should match the dpch/dtmax.

Velocity profiles in the ascending aorta in pigs: axial development and influence of changes in left ventricular contraction pattern

Earlier studies using hot-film anemometry in pigs have revealed skewed tangentially rotating velocity profiles in the ascending aorta during systole. The reason for this phenomenon has been postulated to be caused by the left ventricular contraction pattern. Therefore, the aim of this study was to investigate the influence of the left ventricular contraction pattern on the velocity fields in the ascending aorta of pigs. We used a 10 MHz perivascular pulsed Doppler ultrasound system to measure point blood velocities at two axial locations over the entire cross sectional area in the ascending aorta of 90 kg pigs. The axial component of the velocity profiles was visualized dynamically by computerized 3-dimensional animation techniques. Changing left ventricular contraction patterns were accomplished by reversible occlusion of either the left anterior descending or right posterior descending coronary artery. The axial development of the systolic rotating and skewed velocity profiles in the ascending aorta was described. The appearance of the systolic velocity profiles were virtually unaffected by changes in left ventricular contraction pattern.

Near Velocity Field Downstream Prosthetic Valves in Aortic Position

Using a cardiovascular simulator to duplicate in vitro the flow conditions through valves in aortic position, bidimensional velocity maps very near the valve are reconstructed, from an ultrasonic 8 Mhz doppler system, in an elastic model of the ascending aortic arch. Three mechanical heart valves representative of the different types of commercial models (a tilting disc, a ball in cage and a two-leaflet valve) and a new bileaflet prototype were investigated. From examination of the velocity field, it is possible to define the main characteristics of the valve wake and to observe the development of negative velocities associated with regurgitant flows. From a comparison with tests in rigid tubes, the role played by the arch elasticity is analysed.

Experimental and numerical analyses of the steady flow field around an aortic Björk-Shiley standard valve prosthesis

To develop a numerical method for the description of the flow field around a Björk-Shiley (BS) standard valve prosthesis in aortic position, detailed experimental measurements and numerical calculations are performed under steady flow conditions. The experiment was conducted at Reynolds numbers up to 800. In order to perform LDA measurement of velocity in the vicinity of the valve with a curved sinus boundary, a mixture of oil and kerosine was used as the fluid which exactly matches the refractive index of the perspex aortic model. The velocity profiles at six positions in the vicinity and downstream of the valve were measured, including both axial and radial velocity components. The results show very clearly the existence of two nearly symmetric spiral vortex streams downstream of the valve. There is no recirculation area in the aorta downstream and also no obvious stagnation area in the minor orifice region near the valve when Re less than or equal to 800. Theoretically, the flow field of a BS valve is simulated by the flow pattern around a circular plate with an angle of incidence to the approaching stream. The numerical calculations were carried out by means of a 2-D model using the FEM together with the penalty function method. The maximum Reynolds number is 700. The results agree with the experimental results in the plane of symmetry when the Reynolds number is small. However, as the Reynolds number increases, the difference becomes evident. Our conclusion is that the steady flow field of a BS valve is completely 3-dimensional, featured by two spiral vortices. It cannot be simulated exactly by 2-D numerical calculations. To get more detailed and complete information about the flow field of this valve, 3-D numerical calculations are needed.

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.

Turbulent stresses downstream of porcine and pericardial aortic valves implanted in pigs

Because late valve-related complications such as hemolysis and thromboembolic events are considered related to flow disturbances caused by the inserted valve, velocity fields downstream of aortic valve prostheses were studied in pigs. Acute hemodynamic evaluation of size 25-mm porcine and pericardial aortic valve prostheses 1 diameter downstream of the valve ring was performed using dynamic three-dimensional visualization of velocity profiles and spatial distribution of turbulence. Point blood velocity signals obtained with a 1-mm hot-film anemometer needle probe were used to compute Reynolds normal stresses (RNS) by calculation of the turbulent velocity energy of the axial velocity component in the systole. The porcine valves caused a skewed velocity and turbulence profile revealing mean spatial systolic RNS at 70 nm-2 +/- 35 nm-2 (+/- SD). The spatial maximum RNS was 275 +/- 139 nm-2. Corresponding values for the pericardial valves were 20 +/- 11 nm-2 and 72 +/- 46 nm-2. The pericardial valves revealed plug-shaped velocity profiles and turbulent profiles with slightly higher RNS values at the stent posts. From a hemodynamic point of view, these acute studies indicate superiority of the pericardial valves compared to the porcine valves. The turbulent stresses found in this study are of a magnitude that may cause blood corpuscular and endothelial damage.

Turbulent stress measurements downstream of six mechanical aortic valves in a pulsatile flow model

In a pulsatile flow model aortic Björk-Shiley Standard, Convex-Concave and Monostrut valves were investigated together with the Hall-Kaster (Medtronic-Hall), St Jude Medical and Starr-Edwards Silastic Ball valve using hot-film anemometry. Three-dimensional visualization of average systolic Reynolds normal stresses (RNS) reflected the design of the valves. Mean average RNS were used for comparison of the fluid dynamic performance along with Velocity Energy Ratio (VER100) and Turbulence Energy Ratio (TER) as a relative turbulence intensity for pulsatile flow. Mean average RNS ranged from 13.2 to 37.6 Nm-2 for all the valves with the highest levels for the Björk-Shiley Standard and Starr-Edwards Ball valve and lowest values for the St Jude Medical valve and with the Hall-Kaster (Medtronic-Hall), Björk-Shiley Convex-Concave and Monostrut valves in between.