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

Julio Garcia, Romain Capoulade, Florent Le Ven, Emmanuel Gaillard, Lyes Kadem, Philippe Pibarot, Éric Larose, Julio Garcia, Romain Capoulade, Florent Le Ven, Emmanuel Gaillard, Lyes Kadem, Philippe Pibarot, Éric Larose

Abstract

Background: Valve 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.

Methods: Eight 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 (MPG(CMR-Gorlin)); 2) based on a multivariate regression model (MPG(CMR-Predicted)).

Results: MPGCMR 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. MPG(CMR-Gorlin) and MPGTTE correlation and agreement were r = 0.7; bias = -2.8 mmHg, limits of agreement from -18.4 to 12.9 mmHg. MPG(CMR-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 MPG(CMR-Gorlin).

Conclusion: Flow vorticity is one of the main factors responsible for MPG discrepancies between CMR and TTE.

Figures

Figure 1
Figure 1
Fluid mechanics of the aortic valve. Schematic representation of the system composed of left ventricle, aortic valve and ascending aorta with corresponding static pressure (P) and energy in terms of total pressure (P+4 V2). LVOT indicates left ventricular outflow tract, V indicates LVOT velocity, AOA indicates anatomic aortic area and VC indicates the vena contracta position, cross-sectional area of VC corresponds to the valve effective orifice area. Magnetic resonance velocity measurements at vena contracta (10 mm from the aortic valve) were used to compute dimensionless flow parameters and vorticity magnitude. AAo indicates ascending aorta.
Figure 2
Figure 2
Comparison of mean transvalvular pressure gradients measured by TTE versus by CMR. Panel A shows the Pearson correlation plot for mean transvalvular pressure gradient measured by TTE (MPGTTE) and CMR (MPGCMR). Panel B shows the corresponding Bland-Altman plot. Panel C shows the Pearson correlation plot for mean transvalvular pressure gradient measured by TTE (MPGTTE) and predicted by Gorlin equation using CMR (MPGCMR-Gorlin). Panel D shows the corresponding Bland-Altman plot. Panel E shows the Pearson correlation plot for mean transvalvular pressure gradient measured by TTE (MPGTTE) and predicted model using vorticity and dimensionless stroke volume from CMR (MPGCMR-Predicted). Panel F shows the corresponding Bland-Altman plot.
Figure 3
Figure 3
Effective orifice area and mean transvalvular pressure gradients. Panel A shows the aortic valve effective orifice area and mean transvalvular pressure gradient (MPG) plot using measurements from TTE (MPGTTE) and CMR (MPGCMR). Panel B shows the same plot but using MPG predicted by Gorlin equation and CMR measurements (MPGCMR-Gorlin). Panel C shows the same plot but using MPG predicted using mean vorticity magnitude and dimensionless stroke volume from CMR measurements (MPGCMR-Predicted).

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