Lyes Kadem

Impact of systemic hypertension on the assessment of aortic stenosis

Objective: To determine the effect of systemic arterial hypertension on the indices of aortic stenosis (AS) severity. Methods: A severe supravalvar AS was created in 24 pigs. The maximum and mean pressure gradients across the stenosis were measured by Doppler echocardiography and by catheterisation. Both echocardiography and catheter data were used to calculate stenosis effective orifice area, energy loss coefficient, and peak systolic left ventricular wall stress. Measurements were taken both at normal aortic pressures and during hypertension induced by banding of the distal thoracic aorta in 14 pigs and by intravenous administration of phenylephrine in 10 pigs. Results: During hypertension, systemic arterial resistance downstream from the stenosis increased greatly (all animals: 71 (40)%), whereas total systemic arterial compliance decreased significantly (−38 (21)%). Hypertension resulted in a moderate increase in effective orifice area (29 (14)%) and energy loss coefficient (25 (17)%) and substantial decreases in catheter gradients (maximum: −40 (20)%; mean: −43 (20)%; peak to peak: −70 (23)%) and Doppler gradients (maximum: −35 (17)%; mean: −37 (16)%). In multivariate analysis, peak to peak gradient was significantly (p < 0.001) related to the energy loss coefficient, mean flow rate, and arterial compliance, whereas maximum and mean catheter gradients were related only to the energy loss coefficient and flow rate. Of major importance, maximum systolic left ventricular wall stress increased greatly during hypertension (43 (23)%). Conclusions: The severity of AS may be partially masked by the presence of coexisting hypertension. The markers of AS severity should thus be interpreted with caution in hypertensive patients and be re-evaluated when the patient is in a normotensive state.

Comparison between cardiovascular magnetic resonance and transthoracic Doppler echocardiography for the estimation of effective orifice area in aortic stenosis.

BackgroundThe effective orifice area (EOA) estimated by transthoracic Doppler echocardiography (TTE) via the continuity equation is commonly used to determine the severity of aortic stenosis (AS). However, there are often discrepancies between TTE-derived EOA and invasive indices of stenosis, thus raising uncertainty about actual definite severity. Cardiovascular magnetic resonance (CMR) has emerged as an alternative method for non-invasive estimation of valve EOA. The objective of this study was to assess the concordance between TTE and CMR for the estimation of valve EOA.Methods and results31 patients with mild to severe AS (EOA range: 0.72 to 1.73 cm2) and seven (7) healthy control subjects with normal transvalvular flow rate underwent TTE and velocity-encoded CMR. Valve EOA was calculated by the continuity equation. CMR revealed that the left ventricular outflow tract (LVOT) cross-section is typically oval and not circular. As a consequence, TTE underestimated the LVOT cross-sectional area (ALVOT, 3.84 ± 0.80 cm2) compared to CMR (4.78 ± 1.05 cm2). On the other hand, TTE overestimated the LVOT velocity-time integral (VTILVOT: 21 ± 4 vs. 15 ± 4 cm). Good concordance was observed between TTE and CMR for estimation of aortic jet VTI (61 ± 22 vs. 57 ± 20 cm). Overall, there was a good correlation and concordance between TTE-derived and CMR-derived EOAs (1.53 ± 0.67 vs. 1.59 ± 0.73 cm2, r = 0.92, bias = 0.06 ± 0.29 cm2). The intra- and inter- observer variability of TTE-derived EOA was 5 ± 5% and 9 ± 5%, respectively, compared to 2 ± 1% and 7 ± 5% for CMR-derived EOA.ConclusionUnderestimation of ALVOT by TTE is compensated by overestimation of VTILVOT, thereby resulting in a good concordance between TTE and CMR for estimation of aortic valve EOA. CMR was associated with less intra- and inter- observer measurement variability compared to TTE. CMR provides a non-invasive and reliable alternative to Doppler-echocardiography for the quantification of AS severity.

Evaluation of shear stress accumulation on blood components in normal and dysfunctional bileaflet mechanical heart valves using smoothed particle hydrodynamics

Evaluating shear induced hemodynamic complications is one of the major concerns in design of the mechanical heart valves (MHVs). The monitoring of these events relies on both numerical simulations and experimental measurements. Currently, numerical approaches are mainly based on a combined Eulerian-Lagrangian approach. A more straightforward evaluation can be based on the Lagrangian analysis of the whole blood. As a consequence, Lagrangian meshfree methods are more adapted to such evaluation. In this study, smoothed particle hydrodynamics (SPH), a fully meshfree particle method originated to simulate compressible astrophysical flows, is applied to study the flow through a normal and a dysfunctional bileaflet mechanical heart valves (BMHVs). The SPH results are compared with the reference data. The accumulation of shear stress patterns on blood components illustrates the important role played by non-physiological flow patterns and mainly vortical structures in this issue. The statistical distribution of particles with respect to shear stress loading history provides important information regarding the relative number of blood components that can be damaged. This can be used as a measure of the response of blood components to the presence of the valve implant or any implantable medical device. This work presents the first attempt to simulate pulsatile flow through BMHVs using SPH method.

Numerical and experimental investigations of pulsatile blood flow pattern through a dysfunctional mechanical heart valve

Around 250,000 heart valve replacements are performed every year around the world. Due their higher durability, approximately 2/3 of these replacements use mechanical prosthetic heart valves (mainly bileaflet valves). Although very efficient, these valves can be subject to valve leaflet malfunctions. These malfunctions are usually the consequence of pannus ingrowth and/or thrombus formation and represent serious and potentially fatal complications. Hence, it is important to investigate the flow field downstream of a dysfunctional mechanical heart valve to better understand its impact on blood components (red blood cells, platelets and coagulation factors) and to improve the current diagnosis techniques. Therefore, the objective of this study will be to numerically and experimentally investigate the pulsatile turbulent flow downstream of a dysfunctional bileaflet mechanical heart valve in terms of velocity field, vortex formation and potential negative effect on blood components. The results show that the flow downstream of a dysfunctional valve was characterized by abnormally elevated velocities and shear stresses as well as large scale vortices. These characteristics can predispose to blood components damage. Furthermore, valve malfunction led to an underestimation of maximal transvalvular pressure gradient, using Doppler echocardiography, when compared to numerical results. This could be explained by the shifting of the maximal velocity towards the normally functioning leaflet. As a consequence, clinicians should try, when possible, to check the maximal velocity position not only at the central orifice but also through the lateral orifices. Finding the maximal velocity in the lateral orifice could be an indication of valve dysfunction.

Cardiovascular magnetic resonance evaluation of aortic stenosis severity using single plane measurement of effective orifice area

BackgroundTransthoracic echocardiography (TTE) is the standard method for the evaluation of the severity of aortic stenosis (AS). Valve effective orifice area (EOA) measured by the continuity equation is one of the most frequently used stenotic indices. However, TTE measurement of aortic valve EOA is not feasible or not reliable in a significant proportion of patients. Cardiovascular magnetic resonance (CMR) has emerged as a non-invasive alternative to evaluate EOA using velocity measurements. The objectives of this study were: 1) to validate a new CMR method using jet shear layer detection (JSLD) based on acoustical source term (AST) concept to estimate the valve EOA; 2) to introduce a simplified JSLD method not requiring vorticity field derivation.Methods and resultsWe performed an in vitro study where EOA was measured by CMR in 4 fixed stenoses (EOA = 0.48, 1.00, 1.38 and 2.11 cm2) under the same steady flow conditions (4-20 L/min). The in vivo study included eight (8) healthy subjects and 37 patients with mild to severe AS (0.72 cm2 ≤ EOA ≤ 1.71 cm2). All subjects underwent TTE and CMR examinations. EOA was determinated by TTE with the use of continuity equation method (TTECONT). For CMR estimation of EOA, we used 3 methods: 1) Continuity equation (CMRCONT); 2) Shear layer detection (CMRJSLD), which was computed from the velocity field of a single CMR velocity profile at the peak systolic phase; 3) Single plane velocity truncation (CMRSPVT), which is a simplified version of CMRJSLD method. There was a good agreement between the EOAs obtained in vitro by the different CMR methods and the EOA predicted from the potential flow theory. In the in vivo study, there was good correlation and concordance between the EOA measured by the TTECONT method versus those measured by each of the CMR methods: CMRCONT (r = 0.88), CMRJSLD (r = 0.93) and CMRSPVT (r = 0.93). The intra- and inter- observer variability of EOA measurements was 5 ± 5% and 9 ± 5% for TTECONT, 2 ± 1% and 7 ± 5% for CMRCONT, 7 ± 5% and 8 ± 7% for CMRJSLD, 1 ± 2% and 3 ± 2% for CMRSPVT. When repeating image acquisition, reproducibility of measurements was 10 ± 8% and 12 ± 5% for TTECONT, 9 ± 9% and 8 ± 8% for CMRCONT, 6 ± 5% and 7 ± 4% for CMRJSLD and 3 ± 2% and 2 ± 2% for CMRSPVT.ConclusionThere was an excellent agreement between the EOA estimated by the CMRJSLD or CMRSPVT methods and: 1) the theoretical EOA in vitro, and 2) the TTECONT EOA in vivo. The CMRSPVT method was superior to the TTE and other CMR methods in terms of measurement variability. The novel CMR-based methods proposed in this study may be helpful to corroborate stenosis severity in patients for whom Doppler-echocardiography exam is inconclusive.

A new approach for the evaluation of the severity of coarctation of the aorta using Doppler velocity index and effective orifice area: In vitro validation and clinical implications

Early detection and accurate estimation of COA severity are the most important predictors of successful long-term outcome. However, current clinical parameters used for the evaluation of the severity of COA have several limitations and are flow dependent. The objectives of this study are to evaluate the limitations of current existing parameters for the evaluation of the severity of coarctation of the aorta (COA) and suggest two new parameters: COA Doppler velocity index and COA effective orifice area. Three different severities of COAs were tested in a mock flow circulation model under various flow conditions and in the presence of normal and stenotic aortic valves. Catheter trans-COA pressure gradients and Doppler echocardiographic trans-COA pressure gradients were evaluated. COA Doppler velocity index was defined as the ratio of pre-COA to post-COA peak velocities measured by Doppler echocardiography. COA Doppler effective orifice area was determined using continuity equation. The results show that peak-to-peak trans-COA pressure gradient significantly increased with flow rate (from 83% to 85%). Peak Doppler pressure gradient also significantly increased with flow rate (80-85%). A stenotic or bicuspid aortic valve increased peak Doppler pressure gradient by 20-50% for a COA severity of 75%. Both COA Doppler velocity index and COA effective orifice area did not demonstrate significant flow dependence or dependence upon aortic valve condition. As a conclusion, COA Doppler velocity index and COA effective orifice area are flow independent and do not depend on aortic valve conditions. They can, then, more accurately predict the severity of COA.

Modeling the impact of concomitant aortic stenosis and coarctation of the aorta on left ventricular workload

Coarctation of the aorta (COA) is an obstruction of the aorta and is usually associated with bicuspid and tricuspid aortic valve stenosis (AS). When COA coexists with AS, the left ventricle (LV) is facing a double hemodynamic load: a valvular load plus a vascular load. The objective of this study was to develop a lumped parameter model, solely based on non-invasive data, allowing the description of the interaction between LV, COA, AS and the arterial system. First, a formulation describing the instantaneous net pressure gradient through the COA was introduced and the predictions were compared to in vitro results. The model was then used to determine LV work induced by coexisting AS and COA with different severities. The results show that LV stroke work varies from 0.98J (no-AS; no-COA) up to 2.15J (AS: 0.61cm(2)+COA: 90%). Our results also show that the proportion of the total flow rate that will cross the COA is significantly reduced with the increasing COA severity (from 85% to 40%, for a variation of COA severity from 0% to 90%, respectively). Finally, we introduced simple formulations capable of, non-invasively, estimating both LV peak systolic pressure and workload. As a conclusion, this study allowed the development of a lumped parameter model, based on non-invasive measurements, capable of accurately investigating the impact of coexisting AS and COA on LV workload. This model can be used to optimize the management of patients with COA and AS in terms of the sequence of lesion repair.

Discrepancies between cardiovascular magnetic resonance and Doppler echocardiography in the measurement of transvalvular gradient in aortic stenosis: the effect of flow vorticity

BackgroundValve effective orifice area EOA and transvalvular mean pressure gradient (MPG) are the most frequently used parameters to assess aortic stenosis (AS) severity. However, MPG measured by cardiovascular magnetic resonance (CMR) may differ from the one measured by transthoracic Doppler-echocardiography (TTE). The objectives of this study were: 1) to identify the factors responsible for the MPG measurement discrepancies by CMR versus TTE in AS patients; 2) to investigate the effect of flow vorticity on AS severity assessment by CMR; and 3) to evaluate two models reconciling MPG discrepancies between CMR/TTE measurements.MethodsEight healthy subjects and 60 patients with AS underwent TTE and CMR. Strouhal number (St), energy loss (EL), and vorticity were computed from CMR. Two correction models were evaluated: 1) based on the Gorlin equation (MPGCMR-Gorlin); 2) based on a multivariate regression model (MPGCMR-Predicted).ResultsMPGCMR underestimated MPGTTE (bias = −6.5 mmHg, limits of agreement from −18.3 to 5.2 mmHg). On multivariate regression analysis, St (p = 0.002), EL (p = 0.001), and mean systolic vorticity (p < 0.001) were independently associated with larger MPG discrepancies between CMR and TTE. MPGCMR-Gorlin and MPGTTE correlation and agreement were r = 0.7; bias = −2.8 mmHg, limits of agreement from −18.4 to 12.9 mmHg. MPGCMR-Predicted model showed better correlation and agreement with MPGTTE (r = 0.82; bias = 0.5 mmHg, limits of agreement from −9.1 to 10.2 mmHg) than measured MPGCMR and MPGCMR-Gorlin.ConclusionFlow vorticity is one of the main factors responsible for MPG discrepancies between CMR and TTE.

3D pulsatile flow in a curved tube with coexisting model of aortic stenosis and coarctation of the aorta

Coarctation of the aorta is a congenital heart disease defined as an obstruction of the aorta distal to the left subclavian artery (between the aortic arch and descending aorta). It is usually associated with other diseases such as bicuspid and tricuspid aortic stenosis. If the coarctation remains uncorrected it can lead to hypertension, left ventricular failure and aortic dissection. Numerous investigations pointed out that there is a relationship between the genesis and the progression of cardiovascular disease and the locally irregular flow occurring at the diseased zone. Therefore, to examine the relationship between arterial disease and hemodynamics conditions, detailed quantitative studies on flow dynamics in arterial models are clearly inquired. In this study we numerically investigate pulsatile blood flow in a simplified model of the aorta (curved pipe) with coexisting coarctation of the aorta and aortic stenosis. Three severities of aortic stenoses (0.61 cm(2), 1.0 cm(2) and 1.5 cm(2)) coexisting with aortic coarctations (50%, 75% and 90% by area) are investigated. An experimentally validated numerical model from literature is used and baseline results are validated against it. To ensure having a physiologically relevant model using this geometry, flow properties are set so that the Dean number falls in the physiological range for the aorta. The results show that the coexistence of these pathologies significantly modifies the flow in a curved pipe. The maximal velocity is shifted towards the outer wall and can reach values as high as 5m/s just downstream of the coarctation. The wall shear stress distribution is significantly modified compared to the normal, unobstructed case. Finally, a clinically significant pressure gradient is induced by the curvature of the tube (up to 36 mmHg). This can lead to an overestimation of the severity of the coarctation using catheterization.

Non-Invasive Determination of Left Ventricular Workload in Patients with Aortic Stenosis Using Magnetic Resonance Imaging and Doppler Echocardiography

Early detection and accurate estimation of aortic stenosis (AS) severity are the most important predictors of successful long-term outcomes in patients. Current clinical parameters used for evaluation of the AS severity have several limitations including flow dependency. Estimation of AS severity is specifically challenging in patients with low-flow and low transvalvular pressure gradient conditions. A proper diagnosis in these patients needs a comprehensive evaluation of the left ventricle (LV) hemodynamic loads. This study has two objectives: (1) developing a lumped-parameter model to describe the ventricular-valvular-arterial interaction and to estimate the LV stroke work (SW); (2) introducing and validating a new index, the normalized stroke work (N-SW), to assess the global hemodynamic load imposed on the LV. N-SW represents the global hemodynamic load that the LV faces for each unit volume of blood ejected. The model uses a limited number of parameters which all can be measured non-invasively using current clinical imaging modalities. The model was first validated by comparing its calculated flow waveforms with the ones measured using Cardiovascular Magnetic Resonance (CMR) in 49 patients and 8 controls. A very good correlation and concordance were found throughout the cycle (median root mean square: 12.21 mL/s) and between the peak values (r = 0.98; SEE = 0.001, p<0.001). The model was then used to determine SW using the parameters measured with transthoracic Doppler-echocardiography (TTE) and CMR. N-SW showed very good correlations with a previously-validated index of global hemodynamic load, the valvular arterial impedance (), using data from both imaging modalities (TTE: r = 0.82, SEE = 0.01, p<0.001; CMR: r = 0.74, SEE = 0.01, p<0.001). Furthermore, unlike , N-SW was almost independent from variations in the flow rate. This study suggests that considering N-SW may provide incremental diagnostic and prognostic information, beyond what standard indices of stenosis severity and provide, particularly in patients with low LV outflow.