Echocardiography Underestimates Stroke Volume and Aortic Valve Area: Implications for Patients With Small-Area Low-Gradient Aortic Stenosis

Background

Discordance between small aortic valve area (AVA; < 1.0 cm²) 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 (LVOT_area) 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 LVOT_area 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 LVOT_area (n = 40; −0.7 cm²; 95% confidence interval [CI], −2.6 to 1.3), stroke volumes (−6.5 mL/m²; 95% CI, −28.9 to 16.0) and consequently, AVA (−0.23 cm²; 95% CI, −1.01 to 0.59). Moreover, an AVA of 1.0 cm² 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 LVOT_area, 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.