Impact of arteriovenous fistula on flow states in the evaluation of aortic stenosis among ESKD patients on dialysis

INTRODUCTION

An arteriovenous fistula (AVF) in patients with end-stage kidney disease (ESKD) can influence flow states. We sought to evaluate if assessment of aortic stenosis (AS) by transthoracic echocardiographic (TTE) differs in the presence of AVF compared to other dialysis accesses in patients on dialysis.

METHODS

We identified consecutive ESKD patients on dialysis and concomitant AS from a single center between January 2000 and March 2021. We analyzed TTE parameters of AS severity (velocities, gradients, aortic valve area [AVA]) and hemodynamics (cardiac output [CO], valvuloarterial impedance [Zva]) and compared AS parameters in patients with AVF versus other dialysis access.

RESULTS

The cohort included 94 patients with co-prevalent ESKD and AS; mean age 66 years, 71% male; 43% Black, 24% severe AS. Dialysis access: 53% AVF, 47% others. In the overall cohort, no significant differences were noted between AVF versus non-AVF in AVA/CO/Zva, but with notable subgroup differences. In mild AS, CO was significantly higher in AVF versus non-AVF (6.3 vs. 5.2 L/min; p = .04). In severe AS, Zva was higher in the AVF versus non-AVF (4.6 vs. 3.6 mm Hg/mL/m2 ). With increasing AS severity in the AVF group, CO decreased, coupled with increase in Zva, likely counterbalancing the net hemodynamic impact of the AVF.

CONCLUSION

Among ESKD patients with AS, TTE parameters of flow states and AS severity differed in those with AVF versus other dialysis accesses and varied with progression in severity of AS. Future longitudinal assessment of hemodynamic parameters in a larger cohort of co-prevalent ESRD and AS would be valuable.

Impact of Aortic Valve Regurgitation on Doppler Echocardiographic Parameters in Patients with Severe Aortic Valve Stenosis

BACKGROUND

Diagnosing severe aortic stenosis (AS) depends on flow and pressure conditions. It is suspected that concomitant aortic regurgitation (AR) has an impact on the assessment of AS severity. The aim of this study was to analyze the impact of concomitant AR on Doppler-derived guideline criteria. We hypothesized that both transvalvular flow velocity (maxV_AV) and the mean pressure gradient (mPG_AV) will be affected by AR, whereas the effective orifice area (EOA) and the ratio between maximum velocity of the left ventricular outflow tract and transvalvular flow velocity (maxV_LVOT/maxV_AV) will not. Furthermore, we hypothesized that EOA (by continuity equation), and the geometric orifice area (GOA) (by planimetry using 3D transesophageal echocardiography, TEE), will not be affected by AR.

METHODS AND RESULTS

In this retrospective study, 335 patients (mean age 75.9 ± 9.8 years, 44% male) with severe AS (defined by EOA < 1.0 cm²) who underwent a transthoracic and transesophageal echocardiography were analyzed. Patients with a reduced left ventricular ejection fraction (LVEF < 53%) were excluded (n = 97). The remaining 238 patients were divided into four subgroups depending on AR severity, and they were assessed using pressure half time (PHT) method: no, trace, mild (PHT 500-750 ms), and moderate AR (PHT 250-500 ms). maxV_AV, mPG_AV and maxV_LVOT/maxV_AV were assessed in all subgroups. Among the four subgroups (no (n = 101), trace (n = 49), mild (n = 61) and moderate AR (n = 27)), no differences were obtained for EOA (no AR: 0.75 cm² ± 0.15; trace AR: 0.74 cm² ± 0.14; mild AR: 0.75 cm² ± 0.14; moderate AR: 0.75 cm² ± 0.15, p = 0.998) and GOA (no AR: 0.78 cm² ± 0.20; trace AR: 0.79 cm² ± 0.15; mild AR: 0.82 cm² ± 0.19; moderate AR: 0.83 cm² ± 0.14, p = 0.424). In severe AS with moderate AR, compared with patients without AR, maxV_AV (p = 0.005) and mPG_AV (p = 0.022) were higher, whereas EOA (p = 0.998) and maxV_LVOT/maxV_AV (p = 0.243) did not differ. The EOA was smaller than the GOA in AS patients with trace (0.74 cm² ± 0.14 vs. 0.79 cm² ± 0.15, p = 0.024), mild (0.75 cm² ± 0.14 vs. 0.82 cm² ± 0.19, p = 0.021), and moderate AR (0.75 cm² ± 0.15 vs. 0.83 cm² ± 0.14, p = 0.024). In 40 (17%) patients with severe AS, according to an EOA < 1.0 cm², the GOA was ≥ 1.0 cm².

CONCLUSION

In severe AS with moderate AR, the maxV_AV and mPG_AV are significantly affected by AR, whereas the EOA and maxV_LVOT/maxV_AV are not. These results highlight the potential risk of overestimating AS severity in combined aortic valve disease by only assessing transvalvular flow velocity and the mean pressure gradient. Furthermore, in cases of borderline EOA, of approximately 1.0 cm², AS severity should be verified by determining the GOA.

[Consistency of effective orifice area of prosthetic mitral valve estimated using 2-dimensional and 3-dimensional transesophageal echocardiography]

OBJECTIVE

To analyze the consistency of effective orifice area (EOA) of prosthetic mitral valve estimated using 2- dimensional (2D) and 3-dimensional (3D) transesophageal echocardiography (TEE).

OBJECTIVE

This study was conducted among 34 patients undergoing mitral valve replacement surgery in Nanjing First Hospital between March and June in 2019. The diameter of the left ventricular outflow tract (LVOT) measured by 2D-TEE was used to calculate the cross sectional area of LVOT (CSALVOT). In 3D-TEE method, LVOT area was measured directly by planimetry on an enface view. The EOAs of the prosthetic mitral valve were calculated for both methods using the continuity equation. Bland-Altman plot consistency test was used to analyze the consistency between the two sets of EOA results, and linear regression analysis was used to analyze their correlation.

OBJECTIVE

The EOA of the prosthetic mitral valve differed significantly between 2D method and 3D method (2.22±0.71 cm² vs 2.35±0.70 cm², P < 0.001) with a mean difference of -0.14±0.20 cm² and 95% coherence boundaries of (-0.53, 0.25 cm²). The regression equation for EOA-3D and EOA-2D is y=0.27 + 0.94x, showing a good correlation between the two methods.

OBJECTIVE

EOA estimation of the prosthetic mitral valve using 2D and 3D TEE has a good consistency, and the results estimated by the 2D method are slightly lower by about 6% than those by the 3D method.

Comprehensive assessment of the aortic valve in critically ill patients for the non-cardiologist. Part I: Aortic stenosis of the native valve

Aortic stenosis (AS) causes left ventricular outflow obstruction. Severe AS has major haemodynamic implications in critically ill patients, in whom increased cardiac output and oxygen delivery are often required. Transthoracic echocardiography (TTE) plays a key role in the AS severity grading. In this review, we will give an overview of how to use the simplified Bernoulli equation to convert the echo Doppler measured velocities (cm s-1) to AS peak and mean gra-dient (mm Hg) and how to calculate the aortic valve area (AVA), using the continuity equation, based on the principle of preservation of flow. TTE allows quantification of compensatory left ventricular (LV) hypertrophy, assessment of LV systolic function, and determination of LV diastolic function and LV loading. Subsequently, the obtained results from the TTE study need to be integrated to establish the AS severity grading. The pitfalls of echocardiographic AS severity assessment are explained, and how to deal with inconsistency between AVA and mean gradient. The contribution of transoesophageal echocardiography, low-dose dobutamine stress echo (in case of low-flow low-gradient AS), echocardiography strain imaging, cardiac magnetic resonance imaging, cardiac multidetector computed tomography and the relatively new concept of Flow Pressure Gradient Classification to the work-up for aortic stenosis is discussed. Finally, the treatment of AS is overviewed. Elective aortic valve replacement is indicated in patients with severe symptomatic AS. In the ICU, afterload reduction by vasodilator therapy and treatment of pulmonary and venous congestion by diuretics could be considered.

Fractional-order heat conduction models from generalized Boltzmann transport equation

The relationship between fractional-order heat conduction models and Boltzmann transport equations (BTEs) lacks a detailed investigation. In this paper, the continuity, constitutive and governing equations of heat conduction are derived based on fractional-order phonon BTEs. The underlying microscopic regimes of the generalized Cattaneo equation are thereafter presented. The effective thermal conductivity κ_eff converges in the subdiffusive regime and diverges in the superdiffusive regime. A connection between the divergence and mean-square displacement 〈|Δx|²〉 ∼ t^γ is established, namely, κ_eff ∼ t^γ-1, which coincides with the linear response theory. Entropic concepts, including the entropy density, entropy flux and entropy production rate, are studied likewise. Two non-trivial behaviours are observed, including the fractional-order expression of entropy flux and initial effects on the entropy production rate. In contrast with the continuous time random walk model, the results involve the non-classical continuity equations and entropic concepts. This article is part of the theme issue 'Advanced materials modelling via fractional calculus: challenges and perspectives'.

Aortic valve area using computed tomography-derived correction factor to improve the validity of left ventricular outflow tract measurements

AIMS

Given the inherent inaccuracies stemming from the assumption that the left ventricular outflow tract (LVOT) is circular, this study aimed to improve the accuracy of transthoracic echocardiography (TTE)-based aortic valve area (AVA) calculation using continuity equation (CE) by introducing a correction factor (CF) derived from multidetector computed tomography angiography (MDCTA) images and validate it in aortic stenosis (AS) patients.

METHODS AND RESULTS

This retrospective study used MDCTA images of 400 patients for modeling and 403 TTE dataset for validation. Echocardiographic parasternal long-axis view was modeled using MDCTA, and LVOT diameter (D1) was measured. Direct planimetry of LVOT area was performed and subsequently converted into a theoretical circle. The assumed circle (D2) diameter was derived, and D2/D1 was calculated and termed as the CF. The CF was 1.13, and it improved the agreement between MDCTA- and TTE-derived LVOT areas and correlation between AVA and peak velocity, mean pressure gradient, and velocity ratio. In discordant subgroups of severe AS, the CF reclassified patients to moderate AS in 40% in the low flow (LF), low gradient (LG), and low ejection fraction (EF) group; 53% in the LF, LG, and normal EF group; and 68% in the LF, high gradient, and normal EF group.

CONCLUSIONS

CF of 1.13 derived from MDCTA improved the accuracy of TTE-derived LVOT area and AVA and improved correlation with hemodynamic variables in AS patients. Reclassification of AS patients using CF may have clinical applicability for patient selection for early intervention.

Impact of left ventricular outflow tract flow acceleration on aortic valve area calculation in patients with aortic stenosis

Objective

Due to its circular shape, the area of the proximal left ventricular tract (PLVOT) adjacent to aortic valve can be derived from a single linear diameter. This is also the location of flow acceleration (FA) during systole, and pulse wave Doppler (PWD) sample volume in the PLVOT can lead to overestimation of velocity (V₁) and the aortic valve area (AVA). Therefore, it is recommended to derive V₁ from a region of laminar flow in the elliptical shaped distal LVOT (away from the annulus). Besides being inconsistent with the assumptions of continuity equation (CE), spatial difference in the location of flow and area measurement can result in inaccurate AVA calculation. We evaluated the impact of FA in the PLVOT on the accuracy of AVA by continuity equation (CE) in patients with aortic stenosis (AS).

Methods

CE-based AVA calculations were performed in patients with AS once with PWD-derived velocity time integral (VTI) in the distal LVOT (VTI_LVOT) and then in the PLVOT to obtain a FA velocity profile (FA-VTI_LVOT) for each patient. A paired sample t-test (P < 0.05) was conducted to compare the impact of FA-VTI_LVOT and VTI_LVOT on the calculation of AVA.

Result

There were 46 patients in the study. There was a 30.3% increase in the peak FA-VTI_LVOT as compared to the peak VTI_LVOT and AVA obtained by FA-VTI_LVOT was 29.1% higher than obtained by VTI_LVOT.

Conclusion

Accuracy of AVA can be significantly impacted by FA in the PLVOT. LVOT area should be measured with 3D imaging in the distal LVOT.

A comparison of two forms of the continuity equation in the Trifecta bovine pericardial aortic valve

AIM

To compare the classical and simplified form of the continuity equation in small Trifecta valves.

METHODS

This is a retrospective analysis of post-operative echocardiograms performed for clinical reasons after implantation of Trifecta bioprosthetic valves.

RESULTS

There were 60 patients aged 74 (range 38-89) years. For the valves of size 19, 21 and 23mm, the mean gradient was 11.3, 10.7 and 9.7mmHg, respectively. The effective orifice areas by the classical form of the continuity equation were 1.4, 1.7 and 1.9cm(2), respectively. There was a good correlation between the two forms of the continuity equation, but they were significantly different using a t-test (P<0.00001). Results using the classical form were a mean 0.11 (s.d. 0.18)cm(2) larger than those using the simple formula.

CONCLUSION

Haemodynamic function of the Trifecta valve in the small aortic root is good. There are significant differences between the classical and simplified forms of the continuity equation.

A Critical Evaluation of the Influence of the Dark Exchange Current on the Performance of Dye-Sensitized Solar Cells

The influence of the thickness of the nanostructured, mesoporous TiO₂ film on several parameters determining the performance of a dye-sensitized solar cell is investigated both experimentally and theoretically. We pay special attention to the effect of the exchange current density in the dark, and we compare the values obtained by steady state measurements with values extracted from small perturbation techniques. We also evaluate the influence of exchange current density, the solar cell ideality factor, and the effective absorption coefficient of the cell on the optimal film thickness. The results show that the exchange current density in the dark is proportional to the TiO₂ film thickness, however, the effective absorption coefficient is the parameter that ultimately defines the ideal thickness. We illustrate the importance of the exchange current density in the dark on the determination of the current-voltage characteristics and we show how an important improvement of the cell performance can be achieved by decreasing values of the total series resistance and the exchange current density in the dark.

Moderate Aortic Stenosis and Coronary Artery Bypass Grafting: Clinical Update for the Perioperative Echocardiographer

Incidental aortic stenosis in the setting of coronary artery bypass surgery may be a perioperative challenge. The accurate assessment of the degree of aortic stenosis remains an important determinant. Although severe aortic stenosis is an indication for valve replacement, current guidelines advise a balanced approach to the management of moderate aortic stenosis in this setting. Multiple factors should be considered in a team discussion to balance risks versus benefits for the various management options in the given patient. The rapid progress in aortic valve technologies also offer alternatives for definitive management of moderate aortic stenosis in this setting that will likely become even safer in the near future.

Modeling tree crown dynamics with 3D partial differential equations

We characterize a tree's spatial foliage distribution by the local leaf area density. Considering this spatially continuous variable allows to describe the spatiotemporal evolution of the tree crown by means of 3D partial differential equations. These offer a framework to rigorously take locally and adaptively acting effects into account, notably the growth toward light. Biomass production through photosynthesis and the allocation to foliage and wood are readily included in this model framework. The system of equations stands out due to its inherent dynamic property of self-organization and spontaneous adaptation, generating complex behavior from even only a few parameters. The density-based approach yields spatially structured tree crowns without relying on detailed geometry. We present the methodological fundamentals of such a modeling approach and discuss further prospects and applications.