Are we using the right fluid mechanics principles?
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Aortic Valve Stenosis↗
Biomedical subjects
Lyes Kadem
Publications and source records attributed to Lyes Kadem.
Respective impacts of aortic stenosis and systemic hypertension on left ventricular hypertrophy.
It has been reported that 30-40% of patients with aortic stenosis are hypertensive. In such patients, the left ventricle faces a double (i.e. valvular and vascular) pressure overload, which results in subsequent wall volume hypertrophy. From a clinical standpoint, it is difficult to separate the respective contributions of aortic stenosis and systemic hypertension to left ventricular burden and patient's symptoms and thus to predict whether valve replacement would be beneficial. The objective of this theoretical study was therefore to investigate the relative effects of valvular and vascular afterloads on left ventricular hypertrophy. We used a ventricular-valvular-vascular mathematical model in combination with the Arts' model describing the myofiber stress. Left ventricular wall volume was computed for different aortic blood pressure levels and different degrees of aortic stenosis severity. Our simulations show that the presence of concomitant systemic hypertension has a major influence on the development of left ventricular hypertrophy in patients with aortic stenosis. These results also suggest that mild-to-moderate aortic stenosis has a minor impact on left ventricular wall volume when compared with hypertension. On the other hand, when aortic stenosis is severe, wall volume increases exponentially with increasing aortic stenosis severity and the impact of aortic stenosis on left ventricular hypertrophy becomes highly significant.
Aortic Valve Stenosis↗
Projected valve area at normal flow rate improves the assessment of stenosis severity in patients with low-flow, low-gradient aortic stenosis: the multicenter TOPAS (Truly or Pseudo-Severe Aortic Stenosis) study.
BACKGROUND: We sought to investigate the use of a new parameter, the projected effective orifice area (EOAproj) at normal transvalvular flow rate (250 mL/s), to better differentiate between truly severe (TS) and pseudo-severe (PS) aortic stenosis (AS) during dobutamine stress echocardiography (DSE). Changes in various parameters of stenosis severity have been used to differentiate between TS and PS AS during DSE. However, the magnitude of these changes lacks standardization because they are dependent on the variable magnitude of the transvalvular flow change occurring during DSE. METHODS AND RESULTS: The use of EOAproj to differentiate TS from PS AS was investigated in an in vitro model and in 23 patients with low-flow AS (indexed EOA <0.6 cm2/m2, left ventricular ejection fraction < or =40%) undergoing DSE and subsequent aortic valve replacement. For an individual valve, EOA was plotted against transvalvular flow (Q) at each dobutamine stage, and valve compliance (VC) was derived as the slope of the regression line fitted to the EOA versus Q plot; EOAproj was calculated as EOAproj=EOArest+VCx(250-Q(rest)), where EOArest and Q(rest) are the EOA and Q at rest. Classification between TS and PS was based on either response to flow increase (in vitro) or visual inspection at surgery (in vivo). EOAproj was the most accurate parameter in differentiating between TS and PS both in vitro and in vivo. In vivo, 15 of 23 patients (65%) had TS and 8 of 23 (35%) had PS. The percentage of correct classification was 83% for EOAproj and 91% for indexed EOAproj compared with percentages of 61% to 74% for the other echocardiographic parameters usually used for this purpose. CONCLUSIONS: EOAproj provides a standardized evaluation of AS severity with DSE and improves the diagnostic accuracy for distinguishing TS and PS AS in patients with low-flow, low-gradient AS.
Aged↗
What do you mean by aortic valve area: geometric orifice area, effective orifice area, or gorlin area?
Aortic valve area can be measured by cardiac catheterization, Doppler echocardiography, or imaging planimetry to assess aortic stenosis severity. These diagnostic techniques provide the Gorlin area, the effective orifice area (EOA) and the geometric orifice area (GOA), respectively. The differences between these three parameters depend mainly on the valve inflow shape and cross-sectional area of the ascending aorta. Because the values obtained may differ noticeably in the same patient, they may lead to different estimations of stenosis severity depending on the measurement method used. It is therefore essential to be aware of the underlying fundamentals on which these parameters are based. The aim of this state-of-the-art report was to clarify these hemodynamic concepts and to underline their clinical implications. Because planimetry only provides GOA and does not characterize the flow property, this method should preferably not be used to assess stenosis severity. The most appropriate parameters for this purpose are the Gorlin area and the energy loss coefficient (E(L)Co), which corresponds to the EOA adjusted for aortic cross-sectional area. From a hemodynamic viewpoint, Doppler E(L)Co and Gorlin area both reflect the fluid energy loss induced by aortic stenosis, and describe better the increased overload imposed on the left ventricle. Although the Gorlin area and Doppler E(L)Co are equivalent, the latter parameter has the advantage of being measurable non-invasively using Doppler echocardiography.
Aortic Valve↗
Value and limitations of peak-to-peak gradient for evaluation of aortic stenosis.
BACKGROUND AND AIM OF THE STUDY: In patients with aortic stenosis (AS), it has been reported that the transvalvular pressure gradients (APs) may be reduced or even abolished in the presence of concomitant arterial hypertension, but the mechanisms underlying this phenomenon remain unclear. The study aim was to: (i) examine the relationship between systemic arterial hemodynamics and the peak-to-peak (deltaP(PtoP)), peak deltaP and mean deltaP; and (ii) propose and validate a new formula for the non-invasive estimation of the deltaP(PtoP) and of the peak left ventricular systolic pressure (LVSP) using Doppler echocardiography. METHODS: Two fixed stenoses (geometric orifice area 1.0 and 1.35 cm2) and one bioprosthesis (effective orifice area (EOA) 1.2 cm2) were tested in a mock flow circulation model. Systemic vascular resistance (R) was increased from 1,500 to 3,300 dyne.s/cm5, and systemic arterial compliance (C) was decreased from 2.9 to 0.9 ml/mmHg, while transvalvular flow was held constant. RESULTS: Neither C nor R had any significant impact on EOA, peak deltaP and mean deltaP. deltaP(PtoP) was decreased markedly, however, when C was reduced (bioprosthesis: -15 mmHg (-69%); orifice 1.35 cm2: -24 mmHg (-30%); cm2: (-13%)). Subsequently, an equation was proposed to predict deltaP(PtoP) from EOA, mean deltaP, and C measured by Doppler echocardiography. LVSP calculated by adding the predicted deltaP(Ptop) to systolic arterial pressure (SAP) was compared with LVSP measured directly in a dataset of 24 pigs with experimentally induced AS. There was a strong agreement between the estimated and measured LVSP (r = 0.97; mean absolute error 5 +/- 5 mmHg). CONCLUSION: deltaP(Ptop) should not be used to evaluate AS severity because, as opposed to peak and mean deltaPs, it is highly influenced by C. The new non-invasive method proposed in this study to estimate the LVSP may be useful for obtaining a more accurate estimate of global LV afterload in patients with AS.
Animals↗
Flow-dependent changes in Doppler-derived aortic valve effective orifice area are real and not due to artifact.
OBJECTIVES: We sought to determine whether the flow-dependent changes in Doppler-derived valve effective orifice area (EOA) are real or due to artifact. BACKGROUND: It has frequently been reported that the EOA may vary with transvalvular flow in patients with aortic stenosis. However, the explanation of the flow dependence of EOA remains controversial and some studies have suggested that the EOA estimated by Doppler-echocardiography (EOA(Dop)) may underestimate the actual EOA at low flow rates. METHODS: One bioprosthetic valve and three rigid orifices were tested in a mock flow circulation model over a wide range of flow rates. The EOA(Dop) was compared with reference values obtained using particle image velocimetry (EOA(PIV)). RESULTS: There was excellent agreement between EOA(Dop) and EOA(PIV) (r2 = 0.94). For rigid orifices of 0.5 and 1.0 cm2, no significant change in the EOA was observed with increasing flow rate. However, substantial increases of both EOA(Dop) and EOA(PIV) were observed when stroke volume increased from 20 to 70 ml both in the 1.5 cm2 rigid orifice (+52% for EOA(Dop) and +54% for EOA(PIV)) and the bioprosthetic valve (+62% for EOA(Dop) and +63% for EOA(PIV)); such changes are explained either by the presence of unsteady effects at low flow rates and/or by an increase in valve leaflet opening. CONCLUSIONS: The flow-dependent changes in EOA(Dop) are not artifacts but represent real changes in EOA attributable either to unsteady effects at low flow rates and/or to changes in valve leaflet opening. Such changes in EOA(Dop) can be relied on for clinical judgment making.
Aortic Valve↗
Analytical modeling of the instantaneous maximal transvalvular pressure gradient in aortic stenosis.
In presence of aortic stenosis, a jet is produced downstream of the aortic valve annulus during systole. The vena contracta corresponds to the location where the cross-sectional area of the flow jet is minimal. The maximal transvalvular pressure gradient (TPG(max)) is the difference between the static pressure in the left ventricle and that in the vena contracta. TPG(max) is highly time-dependent over systole and is known to depend upon the transvalvular flow rate, the effective orifice area (EOA) of the aortic valve and the cross-sectional area of the left ventricular outflow tract. However, it is still unclear how these parameters modify the TPG(max) waveform. We thus derived an explicit analytical model to describe the instantaneous TPG(max) across the aortic valve during systole. This theoretical model was validated with in vivo experiments obtained in 19 pigs with supravalvular aortic stenosis. Instantaneous TPG(max) was measured by catheter and its waveform was compared with the one determined from the derived equation. Our results showed a very good concordance between the measured and predicted instantaneous TPG(max). Total relative error and mean absolute error were on average 9.4+/-4.9% and 2.1+/-1.1 mmHg, respectively. The analytical model proposed and validated in this study provides new insight into the behaviour of the TPG(max) and thus of the aortic pressure at the level of vena contracta. Because the static pressure at the coronary inlet is similar to that at the vena contracta, the proposed equation will permit to further examine the impact of aortic stenosis on coronary blood flow.
Aortic Valve↗
Reduced systemic arterial compliance impacts significantly on left ventricular afterload and function in aortic stenosis: implications for diagnosis and treatment.
OBJECTIVES: We sought to determine to what extent systemic arterial compliance (SAC) might impact on afterload and left ventricular (LV) function in patients with aortic stenosis (AS). BACKGROUND: Although AS and reduced SAC may often coexist in the same patient, their relative impact on LV function is not well understood. METHODS: Systemic arterial compliance was calculated as the ratio of stroke volume index to arterial pulse pressure in 208 patients with at least moderate AS. As a measure of global afterload, we calculated the valvulo-arterial impedance (Zva), which theoretically accounts for the effects of both AS and SAC. RESULTS: Patients were divided into four groups: group 1, moderate AS and normal SAC (n = 77; 37%); group 2, moderate AS and low SAC (n = 50; 24%); group 3, severe AS and normal SAC (n = 45; 22%); and group 4, severe AS and low SAC (n = 36; 17%). The prevalences of LV diastolic and systolic dysfunction were 60% and 6% in group 1, 86% and 12% in group 2, 82% and 16% in group 3, and 94% and 31% in group 4. In multivariate analysis excluding Zva, energy loss index and SAC were both independent predictors of LV dysfunction, but when Zva was entered into the analyses, it became the only hemodynamic variable to be independently associated with LV dysfunction. CONCLUSIONS: Reduced SAC is a frequent occurrence in elderly patients with AS, where it independently contributes to increased afterload and decreased LV function. Systemic arterial compliance should be taken into consideration when evaluating these patients with regard to diagnosis and treatment.
Aged↗
Discrepancies between catheter and Doppler estimates of valve effective orifice area can be predicted from the pressure recovery phenomenon: practical implications with regard to quantification of aortic stenosis severity.
OBJECTIVES: We sought to obtain more coherent evaluations of aortic stenosis severity. BACKGROUND: The valve effective orifice area (EOA) is routinely used to assess aortic stenosis severity. However, there are often discrepancies between measurements of EOA by Doppler echocardiography (EOA(Dop)) and those by a catheter (EOA(cath)). We hypothesized that these discrepancies might be due to the influence of pressure recovery. METHODS: The relationship between EOA(cath) and EOA(Dop) was studied as follows: 1) in an in vitro model measuring the effects of different flow rates and aortic diameters on two fixed stenoses and seven bioprostheses; 2) in an animal model of supravalvular aortic stenosis (14 pigs); and 3) based on catheterization data from 37 patients studied by Schöbel et al. RESULTS: Pooling of in vitro, animal, and patient data showed a good correlation (r = 0.97) between EOA(cath) (range 0.3 to 2.3 cm(2)) and EOA(Dop) (range 0.2 to 1.7 cm(2)), but EOA(cath) systematically overestimated EOA(Dop) (24 +/- 17% [mean +/- SD]). However, when the energy loss coefficient (ELCo) was calculated from EOA(Dop) and aortic cross-sectional area (A(A)) to account for pressure recovery, a similar correlation (r = 0.97) with EOA(cath) was observed, but the previously noted overestimation was no longer present. CONCLUSIONS: Discrepancies between EOA(cath) and EOA(Dop) are largely due to the pressure recovery phenomenon and can be reconciled by calculating ELCo from the echocardiogram. Thus, ELCo and EOA(cath) are equivalent indexes representing the net energy loss due to stenosis and probably are the most appropriate for quantifying aortic stenosis severity.
Animals↗
Independent contribution of left ventricular ejection time to the mean gradient in aortic stenosis.
BACKGROUND AND AIMS OF THE STUDY: Transvalvular mean pressure gradients (MPG) are important in the evaluation of aortic stenosis, but surprisingly they often differ in patients having similar valve effective orifice area (EOA) and stroke volume (SV). The study aim was to determine if these differences could be explained by variations in left ventricular ejection time (LVET). METHODS: A pulse duplicator system with a constant SV of 75 ml and incremental increases of LVET from 250 to 450 ms was used to measure MPG by Doppler echocardiography in three fixed stenoses (0.5, 1.0 and 1.5 cm2). The same variables were also measured at rest in 192 patients with isolated aortic stenosis (EOA <1.5 cm2) as well as during stress in a subgroup of 24 patients. RESULTS: In vitro, the increase in LVET produced marked decreases of MPG ranging from -40 mmHg (-45%) for the 0.5-cm2 stenosis to -22 mmHg (-61%) for the 1.5-cm2 stenosis. In vivo, MPG measured by Doppler correlated strongly (R2 = 0.83) with the MPG predicted by the formula: MPGpred [SV/(50xEOAxLVET)]2, and on this basis the relative contributions of EOA, SV and LVET to the variance of MPG were found to be 36, 34 and 13%, respectively. During stress, the contribution of LVET to the increase in MPG was variable, but was sometimes as important as that of SV. CONCLUSION: LVET may significantly and independently influence MPG in aortic stenosis. Clinically, variations of up to 15 mmHg in MPG may be observed uniquely on the basis of a change in duration of LVET, and hence the MPG cannot be used as a stand-alone parameter for serial evaluations or for comparisons of aortic stenosis severity between patients. A correction of MPG for LVET (in ms) such as MPGc = MPGx(LVET/300)2 might be helpful for rendering comparisons of MPG more meaningful in patients with aortic stenosis.
Adolescent↗