Temporal variations in effective orifice area during ejection in patients with valvular aortic stenosis

Complete Unsteady One-Dimensional Model of the Net Aortic Pressure Drop

Background:

A large amount of engineering and medical research has been devoted to the assessment of aortic valve stenosis severity in the past decades. The net transvalvular pressure drop has been recognized as one of the parameters that better reflect stenosis effects on left ventricle overload, and its adoption in clinical assessment of stenosis has been proposed. Flow unsteadiness has been shown to have a non-negligible impact on the net drop; however, a simple formulation for net drop calculation that includes not only flow pulsatility but also the effects of valve dynamics is still lacking.

Objective:

The present contribution is hence aimed at developing a complete unsteady one-dimensional model of the net aortic transvalvular pressure drop that just requires non-invasive data to be implemented.

Methods:

Transvalvular flow is described as a jet of incompressible viscous fluid through a circular orifice placed in a concentric rigid circular tube. The classical one-dimensional mass and total head conservation equations are applied. The effective orifice area and transvalvular flow rate are assumed to vary with time throughout the ejection period.

Results:

The model is found to capture pressure drop oscillations occurring when the valve opens/closes and/or leaflets flutter, thanks to the inclusion of valve dynamics effects. The model is also proposed as a numerical tool for the calculation of the instantaneous effective orifice area once net pressure drop and flow rate are known.

Conclusion:

The model may contribute to the improvement of non-invasive aortic stenosis assessment.

Usefulness of cardiovascular magnetic resonance imaging for the evaluation of valve opening and closing kinetics in aortic stenosis

AIMS

The aims of this study were : (i) to determine the feasibility and reproducibility of the measurement of valve kinetic parameters by cardiovascular magnetic resonance (CMR) and (ii) to examine the association between these parameters and markers of a poor prognosis in patients with aortic stenosis (AS).

METHODS AND RESULTS

Eight healthy control subjects and 71 patients with AS (0.60 cm(2) ≤ EOA ≤ 1.90 cm(2)) underwent transthoracic echocardiography (TTE) and CMR. The valve opening slope (OS) and closing slope (CS) were calculated from instantaneous effective orifice area (EOA) curves obtained by CMR. Intra- and inter-observer variability were 4.8 ± 3.9 and 5.0 ± 4.1%, respectively, for OS, 3.8 ± 2.9 and 4.0 ± 3.1% for CS. OS was significantly related to the plasma level of NT-pro-brain natriuretic peptide (BNP) (r = -0.36, P = 0.002), whereas the EOA or gradient were not.

CONCLUSION

This study demonstrates the excellent feasibility and reproducibility of CMR for the measurement of valve kinetic parameters in patients with AS. Larger studies are needed to confirm the incremental prognostic value of these new CMR parameters of aortic valve kinetics in patients with severe AS.

[Functional cardiac MRI for assessment of aortic valve disease]

Aortic valve disease shows a rising incidence with the increasing mean age of Western populations. The detection of hemodynamic parameters, which transcends the mere assessment of valve morphology, has an important future potential concerning classification of the severity of disease. MRI allows a non-invasive and a spatially flexible view of the aortic valve and the adjacent anatomic region, left ventricular outflow tract (LVOT) and ascending aorta. Moreover, the technique allows the determination of functional hemodynamic parameters, such as flow velocities and effective orifice areas. The new approach of a serial systolic planimetry velocity-encoded MRI sequence (VENC-MRI) facilitates the sizing of blood-filled cardiac structures with the registration of changes in magnitude during systole. Additionally, the subvalvular VENC-MRI measurements improve the clinically important exact determination of the LVOT area with respect to its specific eccentric configuration and its systolic deformity.

Clinical utility of Doppler echocardiography in assessing aortic stenosis severity and predicting need for intervention in children

The optimal echocardiographic methodology for predicting need for intervention in children with valvar aortic stenosis (VAS) is not known. We reviewed echocardiograms and catheterization reports of 79 children (aged 9.5 +/- 5.9 years) with isolated VAS. The maximum and mean Doppler-predicted gradients from the apical (MIGAP), MEGAP)) and the suprasternal or right parasternal (MIGHP), MEGHP)) windows were measured. The peak-to-peak catheterization gradient and the intervention (if any) were recorded. All sites and methods of Doppler estimation of VAS gradient correlated in a linear fashion with the invasive gradient (R2 = 0.34-0.50) and with one another (R2 = 0.48-0.86). MIGAP and MIGHP overestimated the invasive gradient in 60% and 86% of patients, whereas MEGAP and MEGHP underestimated the invasive gradient in 94% and 83% of patients, respectively. Age and diameter of the ascending aorta had small but significant effects on the level of agreement. A MIGHP < or = 55 mm Hg predicted no intervention with 100% accuracy, whereas the specificities of a MIGHP > 90 mm Hg, a MEGAP > 50 mm Hg, and a (MIGAP + MIGHP)/2 > 70 mm Hg for intervention were 94%, 100%, and 92%, respectively. The magnitude of overestimation was significantly lower from the apical window. In children with VAS, the best prediction of the catheterization gradient could be based on the average of MIGAP and MIGHP.

Assessment of aortic valve area in aortic stenosis using cardiac magnetic resonance tomography: comparison with echocardiography

BACKGROUND

Cardiac magnetic resonance tomography (CMR) is a new imaging technique capable of imaging the aortic valve with high resolution. We assessed the aortic valve area (AVA) in patients with aortic stenosis (AS) using CMR and compared the results to those obtained by transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE).

METHODS

Forty-two patients (36% female, 71 +/- 8 years) symptomatic for AS underwent TTE followed by TEE to determine the AVA; the continuity equation was used with TTE and the planimetry technique with TEE. In 26 of these patients, the AVA was additionally obtained by CMR planimetry.

RESULTS

The mean AVA derived by TTE, TEE and CMR were 0.74 +/- 0.27, 0.87 +/- 25 and 0.97 +/- 0.30 cm(2), respectively. The mean absolute differences in AVA were 0.13 +/- 0.19 cm(2) for TTE vs. TEE, 0.21 +/- 0.25 cm(2) for TTE vs. CMR and 0.05 +/- 0.11 cm(2) for CMR vs. TEE.

CONCLUSION

There is a good agreement between CMR and the echocardiographic determination of the AVA. If multicenter, large-scale studies confirm these observations, CMR could serve as a noninvasive alternative to TTE/TEE for the assessment of AVA in AS.

Calcific aortic stenosis: an update

Calcific aortic stenosis in the elderly is the number one cause of surgical valve replacement in the US and Europe. The incidence of calcific aortic stenosis is increasing as the general age of the population increases. For many years, rheumatic heart disease was the main cause of aortic valve disease. Over the last half century, however, there has been a change from a rheumatic etiology to a 'degenerative' mechanism because of the increase in access to health care in developed countries and the increasing age of the population in the US and Europe. For many years 'degenerative' aortic stenosis was thought to be caused by the passive accumulation of calcium on the surface of the aortic valve leaflet. Recent studies have demonstrated, however, that the etiology of aortic valve disease has a similar pathophysiology to that of vascular atherosclerosis, and that the treatment of this disease could be similar to that of chronic vascular atherosclerosis. This Review will discuss our current understanding of the pathophysiology, risk factors, cellular mechanisms, diagnosis and finally, future medical therapies for calcific aortic stenosis.

Impact of blood pressure on the Doppler echocardiographic assessment of severity of aortic stenosis

OBJECTIVE

To investigate the impact of blood pressure (BP) on the Doppler echocardiographic (Doppler-echo) evaluation of severity of aortic stenosis (AS).

METHODS

Handgrip exercise or phenylephrine infusion was used to increase BP in 22 patients with AS. Indices of AS severity (mean pressure gradient (DeltaP(mean)), aortic valve area (AVA), valve resistance, percentage left ventricular stroke work loss (% LVSW loss) and the energy loss coefficient (ELCo)) were measured at baseline, peak BP intervention and recovery.

RESULTS

From baseline to peak intervention, mean (SD) BP increased (99 (8) vs 121 (10) mm Hg, p<0.001), systemic vascular resistance (SVR) increased (1294 (264) vs 1552 (372) dynexs/cm(5), p<0.001) and mean (SD) transvalvular flow rate (Q(mean)) decreased (323 (67) vs 306 (66) ml/s, p = 0.02). There was no change in DeltaP(mean) (36 (13) vs 36 (14) mm Hg, p = NS). However, there was a decrease in AVA (1.15 (0.32) vs 1.09 (0.33) cm(2), p = 0.02) and ELCo (1.32 (0.40) vs 1.24 (0.42) cm(2), p = 0.04), and an increase in valve resistance (153 (63) vs 164 (74) dynexs/cm(5), p = 0.02), suggesting a more severe valve stenosis. In contrast, % LVSW loss decreased (19.8 (6) vs 16.5 (6)%, p<0.001), suggesting a less severe valve stenosis. There was an inverse relationship between the change in mean BP and AVA (r = -0.34, p = 0.02); however, only the change in Q(mean) was an independent predictor of the change in AVA (r = 0.81, p<0.001).

CONCLUSIONS

Acute BP elevation due to increased SVR can affect the Doppler-echo evaluation of AS severity. However, the impact of BP on the assessment of AS severity depends primarily on the associated change in Q(mean), rather than on an independent effect of SVR or arterial compliance, and can result in a valve appearing either more or less stenotic depending on the direction and magnitude of the change in Q(mean).

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.

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.

Analytical modeling of the instantaneous pressure gradient across the aortic valve

Aortic stenosis is the most frequent valvular heart disease. The mean systolic value of the transvalvular pressure gradient (TPG) is commonly utilized during clinical examination to evaluate its severity and it can be determined either by cardiac catheterization or by Doppler echocardiography. TPG 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 ascending aorta. However it is still unclear how these parameters modify the TPG waveform. We thus derived a simple analytical model from the energy loss concept to describe the instantaneous TPG across the aortic valve during systole. This theoretical model was validated with orifice plates and bioprosthetic heart valves in an in vitro aortic flow model. Instantaneous TPG was measured by catheter and its waveform was compared with the one determined from the transvalvular flow rate, the valvular EOA and the aortic cross-sectional area, using the derived equation. Our results showed a very good concordance between the measured and predicted instantaneous TPG. The analytical model proposed and validated in this study provides a comprehensive description of the aortic valve hemodynamics that can be used to accurately predict the instantaneous transvalvular pressure gradient in native and bioprosthetic aortic valves. The consideration of this model suggests that: (1) TPG waveform is exclusively dependent upon transvalvular flow rate and flow geometry, (2) the frequently applied simplified Bernoulli equation may overestimate mean TPG by more than 30% and (3) the measurement of ejection time by cardiac catheterization may underestimate the actual ejection time, especially in patients with mild/moderate aortic stenosis and low cardiac output.