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Comparative Study
. 2014 Sep;30(9):1064-72.
doi: 10.1016/j.cjca.2014.04.021. Epub 2014 Apr 25.

Echocardiography underestimates stroke volume and aortic valve area: implications for patients with small-area low-gradient aortic stenosis

Calvin W L Chin  1 , Hwan J Khaw  2 , Elton Luo  2 , Shuwei Tan  2 , Audrey C White  2 , David E Newby  2 , Marc R Dweck  2
Affiliations
Comparative Study

Echocardiography underestimates stroke volume and aortic valve area: implications for patients with small-area low-gradient aortic stenosis

Calvin W L Chin et al. Can J Cardiol. 2014 Sep.

Abstract
in English, French

Background: Discordance between small aortic valve area (AVA; < 1.0 cm(2)) and low mean pressure gradient (MPG; < 40 mm Hg) affects a third of patients with moderate or severe aortic stenosis (AS). We hypothesized that this is largely due to inaccurate echocardiographic measurements of the left ventricular outflow tract area (LVOTarea) and stroke volume alongside inconsistencies in recommended thresholds.

Methods: One hundred thirty-three patients with mild to severe AS and 33 control individuals underwent comprehensive echocardiography and cardiovascular magnetic resonance imaging (MRI). Stroke volume and LVOTarea were calculated using echocardiography and MRI, and the effects on AVA estimation were assessed. The relationship between AVA and MPG measurements was then modelled with nonlinear regression and consistent thresholds for these parameters calculated. Finally the effect of these modified AVA measurements and novel thresholds on the number of patients with small-area low-gradient AS was investigated.

Results: Compared with MRI, echocardiography underestimated LVOTarea (n = 40; -0.7 cm(2); 95% confidence interval [CI], -2.6 to 1.3), stroke volumes (-6.5 mL/m(2); 95% CI, -28.9 to 16.0) and consequently, AVA (-0.23 cm(2); 95% CI, -1.01 to 0.59). Moreover, an AVA of 1.0 cm(2) corresponded to MPG of 24 mm Hg based on echocardiographic measurements and 37 mm Hg after correction with MRI-derived stroke volumes. Based on conventional measures, 56 patients had discordant small-area low-gradient AS. Using MRI-derived stroke volumes and the revised thresholds, a 48% reduction in discordance was observed (n = 29).

Conclusions: Echocardiography underestimated LVOTarea, stroke volume, and therefore AVA, compared with MRI. The thresholds based on current guidelines were also inconsistent. In combination, these factors explain > 40% of patients with discordant small-area low-gradient AS.

Introduction: La discordance entre une surface valvulaire aortique rétrécie (SVA; < 1,0 cm2) et un faible gradient de pression moyen (GPM; < 40 mm Hg) touche un tiers des patients souffrant d’une sténose aortique (SA) modérée ou grave. Nous avons posé l’hypothèse que ceci est grandement dû aux mesures échocardiographiques inexactes de la surface de la chambre de chasse du ventricule gauche (surface de la CCVG) et au volume systolique de même qu’à l’incohérence des seuils recommandés.

Méthodes: Cent trente-trois (133) patients souffrant de SA légère à grave et 33 témoins ont subi une échocardiographie complète et une imagerie cardiovasculaire par résonance magnétique (ICRM). Le volume systolique et la surface de la CCVG ont été calculés à l’aide de l’échocardiographie et de l’ICRM, et les effets sur l’estimation de la SVA et les mesures du GPM ont alors été modelés à l’aide de la régression non linéaire et les seuils cohérents de ces paramètres ont été calculés. Finalement, l’effet de ces mesures modifiées de la SVG et des nouveaux seuils sur le nombre de patients ayant une SA à surface rétrécie et à faible gradient a été examiné.

Résultats: Comparativement à l’ICRM, l’échocardiographie a sous-estimé la surface de la CCVG (n = 40; −0,7 cm2; intervalle de confiance [IC] à 95 %, −2,6 à 1,3), les volumes systoliques (−6,5 ml/m2; IC à 95 %, −28,9 à 16,0) et, conséquemment, la SVA (−0,23 cm2; IC à 95 %, −1,01 à 0,59). De plus, une SVA de 1,0 cm2 correspondait à un GPM de 24 mm Hg selon les mesures échocardiographiques et de 37 mm Hg après la correction des volumes systoliques issus de l’ICRM. Selon les mesures traditionnelles, 56 patients avaient une SA à surface rétrécie et à faible gradient. À partir des volumes systoliques issus de l’ICRM et des seuils révisés, une réduction de la discordance de 48 % a été observée (n = 29).

Conclusions: L’échocardiographie a sous-estimé la surface de la CCVG, le volume systolique et, par conséquent, la SVA comparativement à l’ICRM. Les seuils des lignes directrices actuelles étaient également incohérents. Combinés, ces facteurs expliquent la raison pour laquelle > 40 % des patients souffrent d’une SA dont la surface rétrécie et le faible gradient sont discordants.

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Figures

Figure 1
Figure 1
Estimation of left ventricular outflow tract (LVOT) area using echocardiography and magnetic resonance imaging. (A) The LVOT diameter was measured at the aortic cusp insertion points (red arrows) in the parasternal long axis view. The LVOT area was estimated from the diameter measured. (B) The stroke volume was calculated as the difference between end-diastolic and end-systolic volumes. Planimetry of the endocardial borders (red contours in end-diastolic and end-systolic frames) was performed including the papillary muscles and minor trabeculations in volume measurements during both phases of the cardiac cycle. Left ventricular mass was calculated by multiplying the total end-diastolic myocardial volume (green and red contours in the end-diastolic frame) by the specific gravity of the myocardium (1.05 g/mL). Papillary muscles and minor trabeculations were excluded in mass measurements, with care taken to avoid right ventricular trabeculations. (C) Planimetry of the LVOT area in the coaxial short axis view on cardiovascular magnetic resonance imaging at mid-systole.
Figure 2
Figure 2
Stroke volume correlation and Bland-Altman analysis. Doppler stroke volume correlated weakly with magnetic resonance imaging (MRI) stroke volume (A), with a fixed bias and wide limits of agreement (B). In 40 patients, stroke volume was calculated using planimetered left ventricular outflow tract area on MRI and Doppler left ventricular outflow tract flow (MRI-Doppler). This approach demonstrated excellent correlation with MRI stroke volume (C), without significant bias (D).
Figure 3
Figure 3
Left ventricular outflow tract (LVOT) area correlation and Bland-Altman analysis. Although LVOT area estimated using echocardiography demonstrated a moderate correlation with planimetered LVOT area on magnetic resonance imaging (A), the echocardiographic LVOT area underestimated the planimetered area with wide limits of agreement (B).
Figure 4
Figure 4
Aortic valve area corrleation and Bland-Altman analysis. Aortic valve area estimated using Doppler stroke volume and magnetic resonance imaging-derived stroke volume demonstrated poor agreement and significant underestimation (A), despite excellent correlation (B).
Figure 5
Figure 5
Relationship between aortic valve area and mean pressure gradient. The aortic valve area was calculated from the continuity equation using Doppler stroke volume. An aortic valve area of 1.0 cm2 corresponded to a mean pressure gradient of 24 mm Hg (A). Correcting these values using the magnetic resonance imaging stroke volume, an aortic valve area of 1.0 cm2 corresponded to a mean pressure gradient of 37 mm Hg (B).
Figure 6
Figure 6
Reclassification of aortic stenosis severity. Using traditional echocardiographic measurements and the recommended severity cutoffs established in current guidelines (A), 56 patients had discordant small-area low-gradient aortic stenosis. Twenty patients were reclassified to concordant nonsevere aortic stenosis when cardiovascular magnetic resonance imaging stroke volume was used to estimate aortic valve area (B). A further 7 patients were reclassified as having concordant severe disease using the revised thresholds of 1.0 cm2 and 37 mm Hg (C). The corresponding pie charts show the flow states in patients with discordant small-area low-gradient aortic stenosis (stroke volume estimated using cardiovascular magnetic resonance imaging).
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